EFFECTS OF FREEZE DRYING, REFRACTANCE WINDOW DRYING AND HOT-AIR DRYING ON THE QUALITY PARAMETERS OF AÇAÍ

BY

MARIANA A. PAVAN

THESIS

Submitted in partial fulfillment of the requirements for the degree of Master of Science in Food Science and Human Nutrition

in the Graduate College of the University of Illinois at Urbana-Champaign, 2010

Urbana, Illinois

Adviser:

Associate Professor Hao Feng

ii

Abstract

Açaí is a dark purple berry native to the Amazonian region in Brazil and widely

consumed in South America. The fruit is a major component of the diet and significant to the

local economy. Recently, more attention has been paid to açaí due to its high antioxidant

properties. However, açaí is a highly perishable fruit and should be processed quickly after

harvesting in order to preserve its nutritional and sensory properties as well as its bioactive

compounds.

Drying is one of the processing techniques used to preserve the fruit. Dehydrated

products present high stability and are easy to transport and store. Dried açaí is, currently,

mainly produced by freeze drying. Even though this method results in high quality products, the

high cost associated with capital investment and operation is its major disadvantage. Thus, the

search for alternative and more economic drying methods is of interest. Refractance Window

drying is a novel and promising drying technique, and it has been demonstrated to retain food

quality. Hot-air convective drying is an ancient and inexpensive process. It is the most widely

used drying method in the food industry, although care should be taken to ensure product

quality. The objective of this research was to evaluate the impact of three drying methods -

freeze drying, refractance window drying and hot-air drying - on the quality parameters of açaí

after drying and over a 3 month storage period at 25°C. To examine the potential quality

changes, the following analyses were performed: moisture content, water activity, moisture

working isotherm, glass transition temperature, color, antioxidant capacity, anthocyanin

quantification, and flavor analysis.

The moisture contents of the dried samples were all below the monolayer values. The

water activity values of the açaí immediately after drying and during storage were also low,

indicating relatively good product stability. The Brunauer-Emmett-Teller (BET) and Guggenhein-

Anderson-de Boer (GAB) models showed a good fit to the isotherm data. Differential Scanning

Calorimetry (DSC) thermograms suggested that there exists a subtle glass transition between

50-60°C. DSC analysis also revealed that the lipids present in açaí are liquid at room

temperature. Drying affected both the color and flavor of açaí juice. Dried samples were

featured by a lighter color and lower hue angle and chroma when compared to the juice.

Storage was marked by a decrease in lightness, most likely because of the formation of brown

pigments by Maillard reaction and polymerization of oxidized phenolic compounds. For flavor

analysis, the compounds (E,Z) 2,6-nonadienal, α-ionone and 2-phenylethanol/β-ionone were the

most important odorants identified in açaí samples. The dried açaí had a higher concentration of

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Strecker aldehydes and compounds derived from lipid oxidation, which indicates the occurrence

of those reactions during drying. During storage, lipid oxidation was the most important reaction

that took place in the dried samples. Freeze drying proved to be superior to the other drying

methods, as shown by a best retention of the anthocyanins and antioxidant capacity (ORAC) of

the fruit. Refractance Window dried açaí also exhibited a good retention of anthocyanin content

similar to that of freeze-dried samples and an ORAC value higher than that of the hot-air dried

powders. Anthocyanin content and antioxidant capacity, within each treatment, did not change

over the 3 months of study.

Keywords: açaí, refractance window drying, freeze drying, ORAC, anthocyanins, glass

transition

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With love to my parents, Nizar and Marina, and my brother Felipe

for all their love and for being my motivation to succeed.

And to Marcelo for his unconditional love and support.

Com amor aos meus pais, Nizar e Marina, e ao meu irmão Felipe

por todo o seu amor e por serem minha motivação para triunfar.

E ao Marcelo por seu amor incondicional e todo seu apoio.

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Acknowledgements

First, I would like to thank God for all the blessings I have received. Special thanks to my

adviser, Dr. Hao Feng, for his support, kindness and trust. I would like to express my gratitude

to Dr. Keith Cadwallader, Dr. Shelly Schmidt and Dr. Nicki Engeseth for their time, guidance and

willingness to be part of my committee. I sincerely thank John Jerrel and Dr. Hun Kim for all

their assistance and suggestions on my project. Also, thanks to all my labmates, especially to

Atilio and Bin Zhou for their friendship and encouragement. I must as well thank the members of

Dr. De Mejia, Dr. Cadwallader and Dr. Schmidt’s laboratory, for allowing me to use their

equipment and resources and for their time. I want to thank Gad Yousef, Ning Zhu, Yang Gao,

Joo Won Lee and Li Xu for their help on my research.

I am eternally thankful to my parents Nizar and Marina for always being there for me and

for being an example in my life. I want to thank my brother Felipe for all his love and support. All

my gratitude goes to my boyfriend Marcelo for his unconditional love and understanding and for

all his help in times when I needed him the most. I sincerely thank all my family and friends for

their endless support.

This project was supported by the USDA 1890 Institution Capacity Building Grants

(#2006-05897-00-01,A4873).

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Table of contents

List of Tables ............................................................................................................................vii

List of Figures .......................................................................................................................... viii

Chapter 1: Introduction .............................................................................................................. 1

References ................................................................................................................................ 2

Chapter 2: Literature Review ..................................................................................................... 4

Açaí ........................................................................................................................................... 4

Dehydration Process ................................................................................................................. 9

Water Activity, Moisture Sorption Isotherms, and Glass Transition Temperature ................... 14

Anthocyanins ........................................................................................................................... 21

Flavor Analysis ........................................................................................................................ 24

References .............................................................................................................................. 26

Chapter 3: Effects of freeze drying, Refractance Window drying, and hot-air drying on the quality parameters of açaí ........................................................................................................ 35

Materials and Methods ............................................................................................................ 35

Results and Discussion ........................................................................................................... 44

References .............................................................................................................................. 71

Chapter 4: Conclusions ............................................................................................................ 78

Appendix A: HPLC Chromatograms ....................................................................................... 80

Appendix B: DSC Thermograms ............................................................................................. 83

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List of Tables

Table 1. Nutrient analysis of açaí pulp .......................................................................................... 6

Table 2. Types of processed açaí ................................................................................................. 8

Table 3. Color measurements of açaí juice and powders on day 0 ............................................ 48

Table 4. Estimated GAB and BET parameters ........................................................................... 55

Table 5. Identification of anthocyanins ........................................................................................ 56

Table 6. Odor-active compounds of açaí juice identified by AEDA ............................................. 64

Table 7. Concentration of volatile compounds in açaí juice and powders on day 0 (expressed in

μg/L) .................................................................................................................................... 65

Table 8. Concentration of volatile compounds in freeze, RW and hot-air dried açaí on day 0 and

after 3 months of storage (expressed in μg/L) ..................................................................... 66

Table 9. Concentration of volatile compounds in açaí powders after a 3-month storage

(expressed in μg/L). ............................................................................................................. 67

viii

List of Figures

Figure 1. Bunch of açaí ................................................................................................................. 4

Figure 2. Açaí berries .................................................................................................................... 5

Figure 3. Processing of açaí fruits ................................................................................................ 7

Figure 4. Freeze dryer configuration ........................................................................................... 10

Figure 5. Refractance Window dryer .......................................................................................... 12

Figure 6. Hot air dryer ................................................................................................................. 13

Figure 7. “Stability map” .............................................................................................................. 15

Figure 8. Classification of van der Waals adsorption isotherms ................................................. 17

Figure 9. Sorption isotherm ......................................................................................................... 17

Figure 10. General anthocyanin structure (the flavilium cation) .................................................. 21

Figure 11. Anthocyanin structures in aqueous solutions ............................................................ 23

Figure 12. Effect of different drying methods on moisture content ............................................. 45

Figure 13. Effect of storage on moisture content ........................................................................ 46

Figure 14. Effect of different drying methods on water activity ................................................... 47

Figure 15. Effect of storage on water activity .............................................................................. 47

Figure 16. Representation of sample’s hue ................................................................................ 49

Figure 17. Effect of different drying methods on Lightness ......................................................... 50

Figure 18. Effect of different drying methods on hue angle ........................................................ 50

Figure 19. Effect of different drying methods on chroma ............................................................ 51

Figure 20. Effect of storage on Lightness ................................................................................... 52

Figure 21. Effect of storage on hue angle ................................................................................... 52

Figure 22. Effect of storage on chroma ....................................................................................... 53

Figure 23. Moisture sorption working isotherms of the freeze-dried, RW-dried and hot-air dried

açaí ...................................................................................................................................... 55

Figure 24. HPLC chromatogram for açaí juice on day 0 ............................................................. 56

Figure 25. Effect of different drying methods on anthocyanin content ........................................ 57

Figure 26. Effect of storage on anthocyanin content .................................................................. 58

Figure 27. Effect of different drying methods on antioxidant capacity ........................................ 60

Figure 28. Effect of storage on antioxidant capacity ................................................................... 60

Figure 29. DSC thermogram for freeze-dried açaí ...................................................................... 68

Figure 30. DSC thermogram for freeze-dried açaí oil ................................................................. 69

Figure 31. DSC thermogram for freeze-dried açaí solids ........................................................... 71

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Figure 32. HPLC chromatogram for freeze dried açaí on day 0 ................................................. 80

Figure 33. HPLC chromatogram for RW-dried açaí on day 0 ..................................................... 81

Figure 34. HPLC chromatogram for hot-air dried açaí on day 0 ................................................. 82

Figure 35. DSC thermogram for RW-dried açaí .......................................................................... 83

Figure 36. DSC thermogram for RW-dried açaí oil ..................................................................... 84

Figure 37. DSC thermogram for RW-dried açaí solids ............................................................... 85

Figure 38. DSC thermogram for hot-air dried açaí ...................................................................... 86

Figure 39. DSC thermogram for hot-air dried açaí oil ................................................................. 87

Figure 40. DSC thermogram for hot-air dried açaí solids ........................................................... 88

1

Chapter 1: Introduction

It has been suggested that consumption of fruits and vegetables contributes to the

prevention of many chronic diseases, such as cancer and coronary heart disease. The health

benefits associated with consumption of those foods are mainly attributed to their

phytochemicals, especially polyphenolic compounds (Dauchet et al. 2006; Franceschi et al.

1998; Joshipura et al. 2001). Trends towards healthier lifestyles have resulted in an increase in

consumer’s demand for health-promoting foods and food products, such as fresh fruits and fruit

juices. A report from the United States Department of Agriculture (USDA) showed that the

consumption of fruits and vegetables in U.S. increased 25% from 1977-79 to 1997-99 (Pollack

2001). However, most fruits with potential health benefits, such as wild blueberries, are very

perishable. To find appropriate food preservation methods that retain the bioactive compounds

with relatively low cost is challenging.

Açaí, a fruit of Euterpe oleracea Mart palm tree, has recently received considerable

attention, mainly due to its high antioxidant capacity and potential health benefits. The fruit, a

dark purple berry native to the Amazonian region, has a high nutritional value and is rich in

polyphenolic compounds (Lichtenthaler et al. 2005; Menezes et al. 2008b; Schauss et al. 2006).

However, açaí is highly perishable, being degraded due to enzyme activity, high microbial load

and elevated temperatures and relative humidity in the harvesting/production area (Menezes et

al. 2008a). Therefore, the fruit must be pulped within 24 h after harvest. Fresh pulp or juice,

which has a shelf life of 12 h, should be either frozen or dried in order to increase its shelf life

and preserve its bioactive compounds (Tonon et al. 2009).

Drying is one of the most efficient ways to preserve foods. It reduces product’s water

activity, which inhibits microbial growth and decreases degradative reactions, resulting in higher

stability. Besides, drying results in substantial volume reduction, which facilitates transport and

storage (Marques et al. 2009; Maskan 2001). Dehydrated açaí is currently produced primarily by

freeze drying. Freeze dried products are known to have high quality, since the technique

preserves bioactive compounds, color, texture and flavor. However, due to the use of vacuum

and very low temperatures, freeze drying is the most expensive drying technology (Hammami et

al. 1997; Khalloufi et al. 2003). Therefore, the search for less expensive drying methods, which

are able to result in high quality products, is of interest.

Refractance Window drying is a relatively new drying method introduced by MCD

Technologies (Tacoma, Washington, USA). The technique involves applying the product, as a

2

thin layer, to the top surface of a transparent plastic conveyor belt. Under the plastic sheet, hot

water circulates and it is used to carry thermal energy to the product. The method uses

moderate temperatures and short drying times, which has been reported to result in high quality

products. The technique is especially suitable for liquid and semi-liquid products, such as puree

and juices with particulates (Nindo et al. 2007).

Hot-air drying is the most extensively used drying method. The process consists of

blowing a stream of hot air on top of the product, which results in moisture evaporation.

Although been very common, the method involves high temperatures and long processing

times, which are known to damage nutritional and sensory properties of foods (Ratti 2001). For

comparison purposes hot-air drying is frequently used in drying studies.

The objective of this project was to examine how freeze drying, refractance window

drying and hot-air drying affect the quality of açaí juice. It was also of interest to evaluate the

effects of storage on the quality of the powders and, therefore, dried açaí was stored for 3

months at room temperature, with samples being taken monthly for analysis. Antioxidant

capacity and anthocyanin content were evaluated in order to further understand potential health

benefits of the product. Moisture content, water activity, glass transition temperature and

moisture sorption isotherms were analyzed with the aim of of understanding and predicting

product stability. Color and flavor analyses were performed as possible indicators of consumer’s

acceptability.

The hypothesis of this project was that the quality of the refractance window dried açaí

juice would be comparable to that of the freeze-dried product.

References Dauchet, L., Amouyel, P., Hercberg, S. and Dallongeville, J. 2006. Fruit and vegetable consumption and risk of coronary heart disease: A meta-analysis of cohort studies. J. Nutr. 136, 2588-2593. Franceschi, S., Parpinel, M., La Vecchia, C., Favero, A., Talamini, R. and Negri, E. 1998. Role of different types of vegetables and fruit in the prevention of cancer the colon, rectum, and breast. Epidemiology. 9, 338-341. Hammami, C. and René, F. 1997. Determination of Freeze-drying Process Variables for Strawberries. J. Food Eng. 32, 133-154.

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Joshipura, K.J., Hu, F.B., Manson, J.E., Stampfer, M.J., Rimm, E.B., Speizer, F.E., Cloditz, G., Ascherio, A., Rosner, B., Spiegelman, D. and Willett, W.C. 2001. The effect of fruit and vegetable intake on risk for coronary heart disease. Ann. Intern. Med. 134, 1106-1114+I. Khalloufi, S. and Ratti, C. 2003. Quality deterioration of freeze-dried foods as explained by their glass transition temperature and internal structure. J. Food Sci. 68, 892-903. Lichtenthaler, R., Rodrigues, R.B., Maia, J.G.S., Papagiannopoulos, M., Fabricius, H. and Marx, F. 2005. Total oxidant scavenging capacities of Euterpe oleracea Mart. (Acai) fruits. Int. J. Food Sci. Nutr. 56, 53-64. Marques, L.G., Prado, M.M. and Freire, J.T. 2009. Rehydration characteristics of freeze-dried tropical fruits. LWT - Food Sci. Technol. 42, 1232-1237. Maskan, M. 2001. Kinetics of colour change of kiwifruits during hot air and microwave drying. J. Food Eng. 48, 169-175. Menezes, E.M.S., Rosenthal, A., Sabaa-Srur, A., Camargo, L., Calado, V. and Santos, A. 2008a. High hydrostatic pressure effect on enzyme activity of aç pulp. Cienc. Tecnol. Aliment. 28, 14-19. Menezes, E.M.D.S., Torres, A.T. and Srur, A.U.S. 2008b. Lyophilized acai pulp (Euterpe oleracea, Mart.) nutritional value. Acta Amazon. 38, 311-316. Nindo, C.I. and Tang, J. 2007. Refractance window dehydration technology: A novel contact drying method. Dry Technol. 25, 37-48. Pollack, S.L. 2001. Changing structure of global food consumption and trade. Regmi, A. editor. http://www.ers.usda.gov/publications/wrs011/wrs011h.pdf. Accessed 2010 April 1st. Ratti, C. 2001. Hot air and freeze-drying of high-value foods: A review. J. Food Eng. 49, 311-319. Schauss, A.G., Wu, X., Prior, R.L., Ou, B., Huang, D., Owens, J., Agarwal, A., Jensen, G.S., Hart, A.N. and Shanbrom, E. 2006. Antioxidant capacity and other bioactivities of the freeze-dried Amazonian palm berry, Euterpe oleraceae Mart. (Acai). J. Agric. Food Chem. 54, 8604-8610. Tonon, R.V., Alexandre, D., Hubinger, M.D. and Cunha, R.L. 2009. Steady and dynamic shear rheological properties of açai pulp (Euterpe oleraceae Mart.). J. Food Eng. 92, 425-431.

4

Chapter 2: Literature Review

Açaí General Aspects

Euterpe oleracea Martius is a palm tree native to the Amazonian region, where it can be

abundantly found in the Amazon River estuary (Muñiz-Miret et al. 1996). In Brazil, the state of

Pará is the main producer of this palm, but it can be also found in other Brazilian states,

including Amapá, Maranhão, Mato Grosso, and Tocantins, as well as in other Central and South

American countries, such as Venezuela, Colombia, Ecuador, Suriname, and Panama (De

Sousa 2000).

The tree is a slender, multi-stemmed, monoecious palm that can reach over 30 meters in

height. It is mainly found on estuarine floodplains, slightly less so on perennially flooded areas

and not very common on upland areas (Lewis 2008). It is often known as açaí palm and it is

famous for its hearts of palm and for its fruit, called açaí. Each palm produces 3 or 4 bunches of

açaí per year (Figure 1), each one containing 3 to 6 kg of fruit (Del Pozo-Insfran et al. 2004).

The palm is of great importance in the Amazonian region, since its products are major

components of the diet and of great significance to the local economy (Brondizio et al. 1994).

Figure 1. Bunch of açaí (www.liquidhealth.com.au)

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The heart of palm is the inner core of the tree and it is treated as a delicacy, because of

the intensive labor task required for its harvesting (Bovi et al. 1993). The product is often eaten

in salads or as a filling. Nowadays, 95% of the hearts of palm produced in Brazil come from açaí

palms, since the native trees were devastated without preservation (Tonon 2009).

Açaí is a round-shaped fruit with a diameter ranging from 1.1 to 1.5 cm (Figure 2). Most

of the açaí varieties are green when young and turn into a dark purple color when ripe.

However, some varieties remain green in their mature stage, being called “açaí branco” or white

açaí (Bovi et al. 1993). The seed corresponds to 85% of the fruit total weight and is used for oil

extraction, animal feed or as a fertilizer, among others. The seed is covered by fibrous fibers on

top of which is the edible part (pulp), accounting for 15% of the fruit (Nogueira 2006).

Figure 2. Açaí berries (www.truestarhealth.com)

After being processed into juice, açaí is widely used in smoothies, ice cream, jelly and a

variety of other products. It is estimated that approximately 120,000 liters per day of açaí juice

are consumed only in the city of Belém, Pará state, where the product is part of the staple food

of the population (Carneiro 2000). Until a couple years ago, production of the juice was almost

exclusively intended for the local market. However, recently, the product is becoming more and

more popular in other Brazilian states and internationally. The production of açaí is growing

around 15% per year. In 2008, 540,000 tons of açaí juice were produced in Brazil (Sampaio

2009).

Açaí is considered a fruit of high nutritional value, representing a good source of

proteins, fiber and minerals, mainly potassium and calcium. Interestingly, it also has a high lipid

content, being able to provide approximately 65% of the recommended daily value (Tonon

2009). The oil extracted from açaí juice is rich in monounsaturated (60%) and polyunsaturated

6

(10%) fatty acids, mainly oleic and linoleic acids. Unsaturated fatty acids are said to be

important for cardiovascular diseases prevention (Do Nascimento et al. 2008). Açaí oil has also

a high phenolic content, mainly phenolic acids and procyanidins (Pacheco-Palencia et al. 2008).

Table 1 shows the nutrient analysis for açaí pulp.

Table 1. Nutrient analysis of açaí pulp

Source: Tonon 2009

The fruit’s purple color is due to the presence of anthocyanins, natural pigments that

belong to the flavonoid group. Anthocyanins are powerful antioxidants and may have other

potential benefits for humans, like antiinflammatory and anticarcinogenic properties (Del Pozo-

Insfran et al. 2004).

Açaí fruits are produced throughout the year, with production peaks varying from state to

state. However, fruits harvested during August to December (“dry months”) usually have shown

higher yields and better organoleptic quality (Lichtenthaler et al. 2005) Harvesting is typically

done manually and is an arduous task, since the bunches of fruits may reach 10 – 15 m in

height. The harvester climbs the palm tree and cuts the bunches with a knife. After that, the

bunches are placed on the floor and the fruits are picked and selected by hand. Rotten fruits or

those attacked by insects are removed (De Vasconcelos et al. 2006).

Açaí fruits are perishable and should be processed in 24 hours, maximum, after

harvesting (Tonon et al. 2009b). Their degradation is due to high microbial load, enzymes

(mainly peroxidase and polyphenol oxidase), and elevated temperatures and relative humidity in

the harvesting/production area (Menezes et al. 2008).

A typical processing of açaí fruits (Figure 3) involves, mainly, the following steps (Cohen

et al. 2006):

Analytes AmountDry Weight (%) 15.26Proteins (g/100g DW) 10.05Total Lipids (g/100g DW) 52.64Total Sugars (g/100g DW) 2.96Reducing Sugars (g/100g DW) 2.91Sucrose (g/100g DW) 0.05Fiber (g/100g DW) 25.22Ash (g/100g DW) 3.09

7

- Reception of fruits: after harvesting, the fruits are placed in plastic boxes or baskets, which are

then transported, usually in boats, to processing plants.

- Selection of fruits: during selection, unripe and damaged fruits are removed.

- Pre-washing, Softening, and Washing:

- Pre-washing: fruits are immersed into water to remove dirt;

- Softening: fruits are immersed into water with temperature ranging from room

temperature to 60°C for 10 to 60 minutes. This step is used to soften the fruits,

facilitating pulping, and

- Washing: fruits are then washed with chlorinated water, followed by potable water.

- Blanching: an optional step used to inactivate enzymes, reduce microbial load and stabilize

color. Blanching is done at 80°C for 10 seconds.

- Pulping: açaí pulp is removed. Water is added during pulping to facilitate the process.

- Homogenization: after pulping, pulp is sieved, to remove any dirt remaining, and homogenized.

- Pasteurization: the time/temperature used is 80-85°C / 10sec. After pasteurization, product

can be frozen or dried.

Figure 3. Processing of açaí fruits

Reception of fruits

Selection

Pre-washing, Softening, Washing

Pulping

Homogenizing

Pasteurization

Freezing Drying

Blanching

Addition of water

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According to the Brazilian Regulation there are four types of processed açaí (Ministerio

da Agricultura, Pecuaria e Abastecimento 2000) and they are presented in Table 2:

Table 2. Types of processed açaí

Functional Properties

Recently, much attention has being paid to açaí due to potential health benefits

associated with its high antioxidant capacity and phytochemical composition. The fruit is

abundant in polyphenolic compounds, which are thought to prevent many chronic diseases,

including cancer, hiperlipidemia, cardiovascular, and neurodegenerative diseases (Hogan et al.

2010). Some polyphenolics already identified in açaí include anthocyanins, ferulic acid, gallic

acid, ellagic acid, (-)-epicatechin, (+)-catechin, orientin and procyanidin polymers, among others

(Del Pozo-Insfran et al. 2004; Pacheco-Palencia et al. 2009).

Schauss et al. (2006a) evaluated the antioxidant capacity and other bioactivities of

freeze-dried açaí and reported a high scavenging capacity for superoxide and peroxyl radical

(the highest of any fruit or vegetable reported to date). Mild activity was detected against the

peroxynitrite and hydroxyl radical. Freeze-dried açaí also seemed to inhibit the formation of

reactive oxygen species in freshly purified human neutrophils. The extract appeared to be

effective at very low doses (5μg/mL). In addition, the product was found to be a potential

cyclooxygenase (COX)-1 and COX-2 inhibitor, indicating anti-inflamatory and pain-relieving

properties.

Oxidative damage of brain tissue leads to an increased susceptibility to

neurodegenerative diseases, such as Parkinson’s and Alzheimer’s. The effects of frozen açaí

juice in reducing oxidative stress induced by H2O2 in cerebellum, cerebral cortex, and

hippocampus from rats was studied by Spada et al. (2009). H2O2 caused an increase in lipid

and protein damage in brain tissue from rats. However, when tissues were pretreated with açaí

a significant decrease in lipid and protein damage and in superoxide dismutase and catalase

activities were observed. The reduction of lipid damage was around 48% in the cerebral cortex,

64% in the hippocampus, and 72% in the cerebellum. Protein damage reduction was 55% in the

Type Addition of water % SolidsAçaí Pulp NoThick or special (type A) Yes >14Medium or regular (type B) Yes 11-14Thin or popular (type C) Yes 8-11

9

cerebral cortex, 36% in the hippocampus, and 42% in the cerebellum. The results suggest

possible use of açaí to help prevent the development of age-related neurodegenerative

diseases.

Oliveira de Souza et al. (2009) studied the antioxidant and hypocholesterolemic effects

of açaí pulp in control and in hypercholesterolemic fed rats. The addition of açaí pulp to control

and hypercholesterolemic diets significantly improved antioxidant capacity by decreasing the

level of carbonyl protein (marker of oxidative stress) and increasing the sulfhydryl groups (free

radical scavengers) in the sera of animals fed those diets. Rats fed the hypercholesterolemic –

açaí diet also showed significant reduction of SOD (antioxidant enzyme) activity, compared with

the control rats. This suggested that açaí supplementation reduced oxidative stress, which

diminished the need for protective responses. Also, addition of açaí juice in the

hypercholesterolemic diet reduced cholesterol levels (total and non-HDL) of rats fed that diet.

Dehydration Process

As previously mentioned, açaí is a highly perishable fruit and processing is essential to

preserve its quality and functional properties. Unless the fresh fruit is consumed quickly, it must

be preserved by drying or freezing.

The dehydration process, as defined by Fellows (2000), constitutes “the application of

heat under controlled conditions to remove the majority of the water normally present in a food”.

Removal of water can be done by evaporation or sublimation (in case of freeze drying) (Fellows

2000).

The main goal of dehydrating food products is to extend their shelf life (Vega-Mercado et

al. 2001). Extension of shelf life is achieved by reducing the product’s water activity, which

inhibits or retards microbial growth as well as the occurrence of chemical reactions (Zhang et al.

2006; Mayor et al. 2004). Drying also decreases the weight and bulk of foods, which reduces

packaging, shipping and storage costs (Chou et al. 2001).

An efficient drying process must consider the following (Hawlader et al. 2006):

- Avoid undesirable changes, preserving food quality,

- Operate at optimum conditions, lowering cost,

- Reduce environmental impact (low energy consumption and waste generation).

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Currently, dried açaí is produced mainly by freeze drying. Freeze drying consists of a

low temperature drying process, in which most of the water is removed by sublimation. The

technique involves three main stages (Koroishi et al. 2009):

- Freezing: quickly lowers the product below its freezing temperature to maximize ice content;

- Primary drying: sublimation of ice, at a temperature below freezing and under reduced

pressure (below triple point);

- Secondary drying: removal of unfrozen, bound water by evaporation at a temperature above

0oC.

The water vapor, removed from the product chamber by a vacuum pump, is directed to a

condenser, where it is retained frozen. Figure 4 shows the basic configuration of a freeze dryer.

Ice Condenser

Figure 4. Freeze dryer configuration (Adapted from http://www.rpi.edu/dept/chem-eng/Biotech-Environ/DOWNSTREAM/fig7.htm)

During the freeze drying process, heat is provided to the product in order to sublime the

ice and remove the water. It is important to ensure that the product temperature is below its

glass translation temperature to avoid collapse (Okos et al. 2007). Sublimation starts at the

surface of the product and then moves toward the bottom. During the process, the dry layer acts

as an insulation material and the drying rate slows as the layer thickens (Vega-Mercado et al.

2001).

It is well known that freeze-dried foods have high quality. When water is removed by

sublimation a porous structure with minimal shrinkage remains. Also, the use of low

11

temperatures and vacuum prevents degradative reactions, such as lipid oxidation and

nonenzymatic browning, and preserves nutrients and bioactive compounds. In addition, there is

little or no flavor loss during the process (Marques et al. 2009).

There are a large number of published data showing the superior quality of freeze-dried

products. Yurdugül (2008) compared the quality characteristics of fresh and freeze-dried alpine

strawberries and found no difference between the two products. The researcher evaluated

sugar content, pH, color, vitamin C and anthocyanins, among others, and all the results were

very similar between the fresh and dried samples, indicating that the freeze drying process

preserved the quality of the fruit.

The effect of different drying methods - freeze drying, hot-air drying, vacuum microwave

drying, and combination drying - on phytochemical content and antioxidant activity of Saskatoon

berries was studied by Kwok et al. (2004). Freeze-dried berries showed the greatest retention of

phenolics, anthocyanins and reducing power and the highest antioxidant activity compared to

berries processed by other methods. Results were attributed to the reduced exposure of berries

to heat and oxygen during freeze drying.

Krokida et al. (2006) evaluated the retention of aroma during air and freeze drying of

apples. Three representative compounds of apple flavor were evaluated before, during and after

the drying processes. The researchers found a higher retention of flavor in freeze-dried

samples, attributed to the lower temperatures used in the process.

On the other hand, because of the use of vacuum, sub-zero temperatures, and thermal

energy for sublimation and evaporation (phase change), freeze drying is the most expensive

drying method. Low temperature drying results in slow drying rates and long drying times. In

addition, the low pressure required for sublimation and therefore the use of a vacuum pump, as

well as the refrigeration system for maintaining a low temperature to collect moisture in the form

of ice on the condenser all add cost to the process. As a result, both the capital investment for

purchasing a freeze drying system and the production cost, mainly the high energy

consumption, are high compared to other drying technologies. The process is therefore mainly

employed for very sensitive foods or high value products (Sablani et al. 2007b). Consequently, the search for an alternative drying method, which would result in a

product with quality parameters comparable to those of freeze-dried material, is of great

interest. Refractance Window® (RW) drying is a novel drying process developed by MCD

Technologies (Tacoma, Washington, USA). In this method hot water is used to carry thermal

energy to the product to be dehydrated (Nindo et al. 2003a).

12

The temperature of the water is usually a few degrees below boiling point (96-98°C) and

is kept at that range to avoid formation of bubbles, which would reduce heat transfer. During the

process the temperature of the food is usually around 71°C, far below the circulating water

temperature. Also, the residence time of the food on the equipment is around 3 to 5 min (Nindo

et al. 2004). This relatively low temperature and short residence time reduce product

degradation (Vega-Mercado et al. 2001).

Figure 5 shows the layout of a Refractance Window® dryer. The product is spread as a

thin film on top of a transparent plastic conveyor belt, under which hot water circulates. The food

flows co-currently with the hot water. Heat is transferred through the plastic sheet directly into

the product for drying. At the end of drying the product is cooled down, by moving over cold

water, and separated from the belt by a scraper device (Nindo et al. 2007).

Figure 5. Refractance Window dryer (Abonyi et al. 2002)

Refractance Window drying has shown to be a promising technique. Abonyi et al. (2002)

studied the effect of Refractance Window drying on quality retention of strawberry and carrot

purees. The retention of vitamin C in strawberry purees dried with RW drying was 94% and was

comparable to 93.6% in freeze-dried samples. Also, the color of RW dried strawberry purees

was similar to freeze-dried products. The overall flavor of strawberry purees was altered by RW

drying, with a decrease in fruity and green notes. For carrot purees, the color of RW dried

product was similar to fresh puree. The carotene losses for RW dried puree were 8.7% (total

carotene), 7.4% (α-carotene) and 9.9% (β-carotene) and were similar to losses for the freeze-

dried product.

13

Nindo et al. (2003b) analyzed the effects of different drying methods on the retention of

physical quality and antioxidants in asparagus. RW-dried pureed asparagus was closest in

greenness to freeze-dried product. RW and freeze drying enhanced the total antioxidant activity

of asparagus. RW drying retained the highest amount of ascorbic acid compared to other drying

methods.

The effect of different drying methods and storage on color characteristics of paprika

was studied by Topuz et al. (2009). The processes used were RW drying, freeze drying, hot-air

oven drying and natural convective drying. For reflected color, RW and freeze drying resulted in

products with better characteristics. The less color alterations were attributed to the mild drying

conditions of those methods. RW and freeze drying also resulted in samples with the lowest

browning index, indicating the occurrence of less non-enzymatic browning reaction during those

processes. During storage, a gradual discoloration in all paprika samples was noted.

Based on the results reported by other researchers, it is of interest to analyze how RW

drying would affect the quality of açaí juice and, therefore, this technique was selected for this

research.

Hot-air convective drying is the most widely used process for production of dehydrated

foods (Krokida et al. 2003). This technique consists of exposing the food product to a

continuously circulating stream of hot air, which supplies the thermal energy for vaporization

and also carries away moisture (Ratti 2001). The cabinet dryer consists of a closed cabinet

fitted with metal trays, which can be perforated or not. The product is placed on the trays as a

thin layer, usually 2 – 6 cm deep. Air is first heated and then blown over and/or through each

tray at 0.5 – 5 ms-1 (Fellows 2009). Figure 6 shows the basic layout of a convective cabinet

dryer.

Figure 6. Hot air dryer (Heldman et al. 1997)

14

Despite being very popular, hot-air convective drying uses high temperatures and long

drying times, which can cause serious damage to food color, flavor, nutrients, and texture,

negatively affecting the quality of the final product (Marfil et al. 2008). Nevertheless, for being an

ancient process and extensively used in the food industry, hot air drying is often used in drying

studies for comparison purposes. It was therefore selected for the present research.

Water Activity, Moisture Sorption Isotherms, and Glass Transition Temperature

Water Activity

It is well known that a relationship exists between the water content of foods and their

perishability (Franks 1991). However, different foods with the same moisture content may show

different perishability, implying that the water content, alone, is not a reliable indicator of product

deterioration. This is, partially, due to differences in intensity of association of water molecules

with nonaqueous constituents. In order to reflect the intensity of water association with other

compounds, the “water activity” (aw) concept was developed (Reid et al. 2008).

Water activity can be described as:

(1)

where p is the partial vapor pressure of water above the food, p0 is the vapor pressure of pure

water at the same temperature, and ERH is the equilibrium relative humidity (%) (Labuza et al.

1998).

The water activity concept can be used to assess moisture sorption isotherms, develop

new products, and aid in process design and control, among others (Schmidt 2004). However,

probably, the most important use of the water activity concept is to determine product stability

and shelf life. It has been extensively reported that water activity influences the rate of chemical

reactions, physical changes, and microbial growth/resistance in food products. The “stability

map” (Figure 7), introduced by Labuza et al. (1970), shows the general relationship between

water activity and several food stability phenomena (Nelson et al. 1994).

15

The map illustrates that lipid oxidation occurs rapidly at low water activities, which

means dehydrated foods are susceptible to this reaction. The rate of non-enzymatic browning is

very low at low water activities (Labuza et al. 1970). There is also a critical aw below which no

microorganisms can grow. For most foods, this value is around 0.6 – 0.7 aw (Vos et al. 1974).

Figure 7. “Stability map” (Labuza et al. 1972)

Domínguez et al. (2007) studied the effect of water activity on the stability of macadamia

nuts. For lipid oxidation evaluation, the aw values used were 0.215, 0.436 and 0.628. A higher

peroxide value was observed for samples stored at the lowest and highest aw. The total

oxidation index (TOTOX) was also higher for nuts stored at 0.215 and 0.628 aw. This is in

agreement with the stability map, which shows that lipid oxidation occurs faster at low and high

water activities

Acevedo et al. (2008) analyzed the kinetics of non-enzymatic browning in dried potatoes

through water-solids interactions and water mobility. The non-enzymatic browning rate,

measured at 70°C, showed a maximum value at aw equal to 0.84 and decreased markedly

above this value. At low water activities, an increase in aw accelerated the browning reaction,

probably due to increased molecular mobility. At high water activities water inhibited the reaction

and/or diluted the reactants, causing a decrease in non-enzymatic browning.

The effects of water activity and temperature on growth of Aspergillus niger, A. awamori

and A. carbonarius isolated from different substrates were investigated by Astoreca et al.

(2007). Mycelial growth and lag phase prior to growth were significantly influenced by aw,

temperature and their interactions. For most of the isolates the optimum aw for growth was 0.97,

with optimum temperature of 30°C. Overall growth was reduced up to 50% at 0.93 aw. Minimal

16

aw for growth was 0.85 at 30°C. Lag times at the optimal temperature varied a lot depending on

the water activity. Lag time for A. niger, for example, varied from 2 h at 0.97 aw to 260 h at 0.85

aw.

Sauvageot et al. (1991) studied the effect of water activity on crispness of different

breakfast cereals. Samples were equilibrated over saturated salt slurries with aw varying from 0

to 0.843. Sensory tests and mechanical analysis were used to investigate crispness. All the

products analyzed showed very similar results. In the sensory evaluation, a high crispness

intensity was observed until 0.53 aw. From 0.53 to 0.71 aw, however, there was a rapid decrease

in crispness intensity, followed by a slight decrease of that sensation as aw increased above

0.71. The results of the mechanical analysis were in agreement with the sensory test.

Moisture Sorption Isotherms

Moisture sorption isotherms describe the equilibrium relationship between moisture

content of a food and water activity, at constant temperature (Moreira et al. 2008). Isotherms

provide valuable information for food processing operations, such as drying and packaging.

They are also important for predicting final quality and product stability during storage (Moraga

et al. 2004).

Sorption isotherms can be obtained by rehydrating a dried sample (adsorption isotherm),

drying a wetted sample (desorption isotherm) or by the combination of the two methods

(working isotherm). Brunauer et al. (1940) classified adsorption isotherms into five types, as

showed in Figure 8. Most foods show a sigmoidal shape (type II) isotherm, while foods

containing soluble crystalline molecules, such as crystalline sugars, show a J-shape isotherm

(type III) (Al-Muhtaseb et al. 2002). Several factors influence the position and shape of an

isotherm, for example, sample composition, crystalline or amorphous sample structure, sample

preparation, temperature, and method of determination.

17

Figure 8. Classification of van der Waals adsorption isotherms (Brunauer et al. 1940)

Figure 9. Sorption isotherm (Okos et al. 2007)

According to Figure 9, type II sorption isotherms may be divided into three main regions

(Al-Muhtaseb et al. 2002):

- Region A: strongly bound and least mobile water (unfreezable at -40°C, not available for

chemical reactions, no plasticizing effect);

- Region B: less tightly bound water (also unfreezable at -40°C);

- Region C: exhibits properties of bulk water (available as solvent for reactions, supports the

growth of microorganisms).

18

The boundary of zones A and B represents the monolayer moisture content, which can

be thought as single layer of water molecules bound only to highly polar groups of the solid

matrix. Most dried foods show moisture content comparable to the monolayer value. At that

moisture content dry products exhibit their highest stability (Sablani et al. 2007a).

Mathematical models are usually used to describe sorption isotherms. The most

commonly ones are Brunauer-Emmett-Teller (BET) and Guggenhein-Anderson-de Boer (GAB)

models. BET is only linear at lower aw and fails above 0.5. The BET is a two-parameter equation

and is expressed as:

(2)

where M is the equilibrium moisture content (dry basis, db), M0 is the monolayer moisture

content (db), and C is a constant related to the heat of sorption of monolayer.

The GAB model is an extension of the Langmuir and BET theory, having three

parameters. The model is successful up to high water activities, but underestimates moisture

content values at aw > 0.93. The GAB equation is expressed as:

(3)

where M is the equilibrium moisture content (db), M0 is the monolayer moisture content (db), C

is a constant related to the heat of sorption of monolayer, and K is a constant related to the heat

of sorption of multilayer region (Basu et al. 2006; Brunauer et al. 1938)

Several methods are available for determining sorption isotherms. These methods can

be classified into the following categories: gravimetric (involves the measurement of weight

changes); manometric (measures the vapor pressure of water in the vapor space surrounding

the food); and hygrometric (measures the equilibrium relative humidity of air in contact with a

food product, at a specific moisture content) (Gal 1981).

Research about sorption isotherms of fruit powders is vast. Syamaladevi et al. (2009)

studied the water adsorption isotherm of freeze-dried raspberry at room temperature. The

isotherm exhibited a sigmoidal shape and was modeled using BET and GAB models. The

19

monolayer water content was 0.059 kg water/kg solids for the BET model and 0.074 kg water/

kg solids for the GAB model.

Sorption isotherms of vacuum dried mulberry at different temperatures (10, 20 and 30°C)

were analyzed by Maskan et al. (1998). The isotherms obtained for the three temperatures were

very similar and showed a sigmoidal shape. Although isotherms followed a typical type II

behavior, a sharp increase in moisture content at high water activities was observed, probably

due to the elevated sugar content of the product.

A J-shape isotherm was characteristic of freeze-dried acerola, a high sugar fruit. The

samples sorbed relatively small amounts of water at low aw, but an increase of sorption capacity

was observed at high water activities. Sorption isotherms were practically not affected by the

different freezing techniques analyzed (cryogenic freezing with liquid nitrogen, with vapor of

nitrogen, and by putting the samples in a freezer) (Marques et al. 2007).

Information about moisture sorption characteristics of dried açaí is important for

selection of appropriate packaging materials and prediction of its stability during storage.

Previous work of Tonon et al. (2009a) with spray dried açaí at 25°C showed that the product

presented a type III isotherm, which is probably due to the presence of carrier agents

(crystalline molecules). Da Silva et al. (2008) also studied the water sorption of spray dried açaí

and reported a type III isotherm for the product, at all temperatures used (15, 25 and 35°C).

Up to now, there is no information about the water sorption characteristics of freeze

dried, RW-dried or hot-air dried açaí.

Glass Transition Temperature

The moisture sorption isotherm of a food product is influenced, among others, by the

structure of the solid. Solids can exist as a crystal (ordered molecular arrangement) or as an

amorphous product (random molecular arrangement) (Bell et al. 2000).

The amorphous state can be further divided into glassy and rubbery states. Products in

the glassy state show low internal mobility and high internal viscosity, and are described as

solids. Rubbery materials show viscoelastic behavior and are described as supercooled liquids

(Le Meste et al. 2002).

The conversion between the glassy and the rubbery states is known as glass transition

and it occurs over a temperature range, which is frequently referred to as single temperature,

called glass transition temperature (Tg). Glass transition is a second-order transition and it is

characterized by a change in physical properties of the material, mainly heat capacity, free

20

volume, thermal expansion coefficient, and viscoelastic properties. The main factors affecting

glass transition are moisture content and temperature (Slade et al. 1995b; Roudaut et al. 2004;

Kasapis 2008).

The concept of glass transition has been used to predict food stability, especially of low

moisture products. Changes from the glassy to the rubbery state have shown to affect food

structure and have been suggested to influence molecular mobility and, consequently, the rates

of chemical reactions. Products in the glassy state (lower molecular mobility) have a higher

stability compared to products in the rubbery state (Roos 2003). At temperatures above glass

transition, the solid state is converted to a rubbery material and undesirable changes will occur,

like caking, stickiness, crystallization, collapse and loss of crispiness. As a result, the final

product might be unattractive and unfit for consumption (Schmidt 2004).

According to studies in the field of polymer science, the glass transition temperature of a

mixture is a non-linear function of the glass transition temperature of the individual components.

Due to the complexity of food systems and the occurrence of several thermal changes and

chemical reactions in the same temperature range, it is usually difficult to determine Tg of food

products (Bhandari et al. 2000).

Glass transition temperature is highly dependent on the molecular weight of the material.

In foods, Tg decreases with increase in low molecular weight compounds, mainly water. It is well

known that water acts as a plasticizer, increasing mobility due to increased free volume and

decreased local viscosity (Slade et al. 1995a; Slade et al. 1991). Also, due to the non-

equilibrium nature of the amorphous phase, glass transition is time-dependent. Therefore,

different glass transition temperatures may be found for the same product depending on the

time scale of the method used (Roos 2003). In addition, sample preparation, sample history and

the analytical procedure used will also influence glass transition temperature.

Many experimental methods can be used to measure glass transition temperature.

Among them, differential scanning calorimetry (DSC) is one of the most commonly used. DSC

measures changes in heat flow during heating or cooling of the test material. The glass

transition is manifested as a step change in the heat flow, due to different heat capacities of the

glassy (lower specific heat capacity) and rubbery states (higher specific heat capacity) (Schmidt

2004; Liu et al. 2006).

Methods used to produce materials in the amorphous state involve mainly two events:

rapid decrease in water content or rapid cooling (avoiding the formation of the crystalline state

due to lack of time). Dehydration processes result in foods in the amorphous state. For higher

stability, it is important to assure that dried products are in the glassy state. Any change in that

21

state may result in alteration of physico-sensorial attributes, structural damage and increase in

the rate of biochemical reactions, affecting the quality of the product (Bhandari et al. 1999).

Knowledge about glass transition temperature of açaí is still lacking. Tonon et al.

(2009a) analyzed the glass transition temperature of spray-dried açaí juice produced with

different carrier agents (maltodextrin with 10DE, maltodextrin with 20DE, gum Arabic and

tapioca starch). Temperatures varying from, approximately, -62 to 74°C were reported,

depending on the water activity at which the product was stored and the carrier agent used. The

researchers stated that spray dried pure açaí juice might have a much lower glass transition

temperature than the values obtained for the materials produced with additives. No information

about glass transition temperature of freeze dried, RW-dried or hot-air dried açaí is currently

available.

Anthocyanins

Anthocyanins are pigments that belong to the flavonoid group. They are responsible for

a wide variety of colors in plants, including purple, blue, red and orange. Anthocyanins are

water-soluble and have been widely used as food colorants (Castañeda-Ovando et al. 2009).

Anthocyanidins are the basic structure of anthocyanins. When anthocyanidins are

bonded to a sugar moiety (glycoside form), they are known as anthocyanins (Mazza et al.

1987). Figure 10 shows the general structure, also called flavilium cation, of anthocyanins.

Figure 10. General anthocyanin structure (the flavilium cation) (Mazza et al. 1987) R1 and R2 are H, OH or OCH3, R3 is a glycosyl or H and R4 is OH or a glycosyl

Differences in anthocyanins are due to: 1) number of hydroxyl/methoxyl groups present;

2) nature, number and sites of sugar attachment; and 3) types and number of aliphatic or

aromatic acids attached to the sugars. The most commonly encountered sugars are glucose,

galactose, rhamnose and arabinose. Caffeic, p-coumaric, ferulic and sinapic acids are the most

22

common aromatic acids, while acetic, malonic, malic, oxalic and succinic acids are examples of

aliphatic acids involved in acylation of sugars (Clifford 2000).

Kong et al. (2003) estimated that more than 400 different anthocyanins exist in nature.

The researchers also report the existence of 17 anthocyanidins, with pelargonidin, peonidin,

cyanidin, malvidin, petunidin and delphinidin being the most common in plants.

Anthocyanins are soluble in polar solvents and are generally extracted by using acidified

methanol. Identification and quantification of anthocyanins are done, primarily, by HPLC and

HPLC-MS. Several studies about anthocyanins in açaí juice have been published, but with

contradictory results. Lichtenthaler et al. (2005), Gallori et al. (2004) and Vera de Rosso et al.

(2008) detected cyaniding-3-rutinoside and cyaniding-3-glucoside as the main anthocyanins in

açaí whereas Del Pozo-Insfran et al. (2004) reported cyaniding-3-glucoside and pelargonidin-3-

glucoside as the main compounds. Schauss et al. (2006b) analyzed anthocyanins in freeze-

dried açaí and concluded that the cyaniding-3-rutinoside and cyaniding-3-glucoside were the

most abundant ones.

One of the most significant properties of anthocyanins is their antioxidant capacity. By

donating phenolic hydrogen atoms, these compounds are able to capture free radicals and

prevent oxidative damage. It is being suggested that anthocyanins contribute to the protective

effect of fruits and vegetables against degenerative and chronic diseases. These compounds

also exhibit therapeutic benefits, including cardioprotective, antiinflammatory and

anticarcinogenic properties (Heinonen et al. 1998; Stoner et al. 2007; Zafra-Stone et al. 2007).

Many assays have been used to measure the antioxidant capacity of anthocyanins and

other phenolic compounds. Among them, Oxygen Radical Absorbance Capacity (ORAC) has

been extensively used. ORAC provides a measure of antioxidant capacity against peroxyl

radicals. The test uses the following components: (a) an azo radical initiator, usually 2,2’-

azobis(2-methylpropionamidine) dihydrochloride (AAPH); (b) a molecular probe, normally

fluorescein (FL), to monitor the reaction progress; (c) the antioxidant compound of interest; and

(d) reaction kinetic parameter to quantify the antioxidant capacity. The method, basically,

involves the mixture of samples, controls (phosphate buffer) or standard (6-hydroxy-2,5,7,8-

tetramethylchroman-2-carboxylic acid - Trolox) with fluorescein and incubation at 37°C, followed

by addition of AAPH to initiate the reaction. The fluorescence intensity is measured periodically

and, as the reaction progresses, fluorescein is consumed and its intensity decreases.

Fluorescein decay is inhibited / limited in the presence of antioxidants (Huang et al. 2005).

Anthocyanins are fairly unstable and quite susceptible to degradation. Their greatest

stability occurs under acidic conditions. The main factors affecting anthocyanins stability are pH,

23

chemical structure, temperature, and oxygen concentration. Secondary factors include light,

metal ions, and presence of degradative enzymes (Markakis 1982).

In an aqueous medium, depending on the pH of the solution, anthocyanins can exist in

different chemical forms (Figure 11): red flavylium cation (AH+), blue quinonoidal base (A),

colorless carbinol pseudobase (B), and colorless chalcone (C). Usually, as the pH increases

anthocyanins are converted into colorless forms (Brouillard 1982).

Figure 11. Anthocyanin structures in aqueous solutions (Brouillard 1982)

Anthocyanins are susceptible to molecular oxygen due to their unsaturated nature and

reaction with oxygen causes degradation of their color. Anthocyanins are also readily destroyed

by heat during processing and storage of foods (Delgado-Vargas et al. 2000).

Mejia-Meza et al. (2010) studied the effect of drying on the anthocyanin content and

antioxidant capacity of raspberries. Dehydration resulted in a significant decrease in

anthocyanin content. Losses ranged from, approximately, 30-80% for aglycones and 75-95% for

glycosides, depending on the drying method used. Considerable loss of antioxidant capacity

was also observed.

The effect of different drying methods – convective drying, vacuum-microwave drying,

vacuum drying and freeze drying - on bioactive compounds of strawberry fruits was studied by

Wojdyło et al. (2009). The researchers found that drying destroyed anthocyanins and flavanols.

Also, all dried products showed a significantly lower antioxidant capacity.

24

Lohachoompol et al. (2004), by analyzing anthocyanin degradation during drying of

blueberries, reported losses of 41% for samples dried in a cabinet drier and 49% for fruits dried

using osmotic pretreatment followed by cabinet drying.

Meschter (1953) examined the effect of temperature on the stability of anthocyanins in

strawberry preserves. At a storage temperature of 20°C, the half-life of the pigment was 54 days

whereas at 38°C the half-life was 10 days. Processing strawberry preserve at 100°C would

result in a half-life of 1 hour.

Flavor Analysis

Flavor is a sensory impression of a food and is established by the chemical senses of

taste, smell and “trigeminal” (d'Acampora Zellner et al. 2008). Among those senses, smell is,

probably, the most important flavor determinant. Smell can be divided into aroma - smell of a

food before it is put in the mouth - and odor - retronasal smell of a food in the mouth (Acree

1993).

The study of flavor is of great importance for food, since, together with color, texture and

other parameters, it determines food quality (Acree 1993). Flavor is also used to define

characteristics and identity of products, such as in wines (Flamini 2005)

The knowledge and identification of odor active compounds, also called flavor

compounds, has improved with the advent of gas chromatography (GC) and mass spectrometry

(MS) (d'Acampora Zellner et al. 2008). Gas chromatography is a separation technique, not an

identification technique. However, when coupled with a mass spectrometer the instruments

provide a great identification method (Mussinan 1993). Currently, approximately 7000 flavor

compounds have been identified (Rowe 2005).

It has been reported that just a small portion of the large number of volatiles occurring in

foods contributes to their perceived odor (Grosch 1994). Also, human perception of volatile

compounds depends on the extent of release from the matrix and their odor properties (Van

Ruth 2001). Further, the contribution of those molecules to the flavor profile of a product is not

equal and the peak area generated by a “chemical” detector is not directly translated into

intensity (d'Acampora Zellner et al. 2008).

Therefore, the grouping of olfactometry and GC (GC-O) results in another useful tool for

discrimination of odor active compounds. As the human nose is more sensitive than “chemical”

detectors for many flavor compounds, GC-O uses the human olfactory system as one of the

25

detectors. In this method, the human subject sniffs the gas chromatographic effluent of a food

extract, determining which volatiles are odor active and providing descriptors. With the use of

GC-O it is possible to list and order the odorants present in a food (Van Ruth 2001).

The detection of flavor compounds by GC-O depends on several factors, such as odor

threshold (main factor), amount of food sampled, dilution of the volatile fraction, and sample size

analyzed. Therefore, a screening method is necessary to identify the most important odorants

(Grosch 1993). Aroma extract dilution analysis (AEDA) is probably the most frequently used

method for that purpose. In this technique, the flavor extract is serially diluted (usually 1:2, 1:3,

1:5 or 1:10) and each dilution is analyzed by GC-O. The result is expressed as a flavor dilution

(FD) factor, which corresponds to the highest dilution at which the odorant can be perceived

(Ferreira et al. 2002).

The flavor profile of a food product is closely related to the extraction procedure used,

which should yield an extract representative of the sample. For that reason, the choice of an

adequate isolation procedure is also very important. Two very popular techniques used for

sample preparation are solvent-assisted flavor evaporation (SAFE) and solid-phase

microextraction (SPME) (d'Acampora Zellner et al. 2008).

SAFE is a technique that extracts volatiles under high vacuum and low temperature

conditions. The method involves adding, into the apparatus, small droplets of samples, which

will be immediately transformed into a vapor spray. As a result, volatiles and solvent are

transferred into a flask, which is cryogenically cooled with liquid nitrogen. The method is suitable

for a solvent extract or a food matrix (aqueous foods or matrices with high oil content) (Engel et

al. 1999).

SPME uses a fused silica fiber coated with a specific stationary phase to adsorb the

flavor compounds. The fiber is first submerged in a liquid phase or exposed to a gaseous

phase, allowing the analytes to be directly extracted. Next, absorbed analytes are thermally

desorbed in the GC injection port. The method involves short preparation time and saves

solvent and disposal costs (Kataoka et al. 2000; Deibler et al. 1999).

Many factors can affect the flavor of a food product and processing is, probably, one of

the most important. Thermal processing of foods usually results in loss of volatile compounds,

or reduction of their concentration, and development of new odors due to a series of chemical

reactions, such as Maillard and lipid oxidation. These factors will change the flavor profile of the

product, which can affect its acceptability (Ruiz Perez-Cacho et al. 2008).

The characterization of the flavor compounds of açaí juice is of great interest to food

companies, since a large number of new products containing the fruit are being launched. It is

26

also important to evaluate how drying affects the flavor of açaí. However, up to date, information

about the flavor profile of açaí juice or dried açaí is still not available.

The effect of heat treatment on the flavor profile of fruits has been widely studied. Feng

et al. (1999) analyzed the effect of different drying methods on flavor retention of blueberries.

Heating caused some odor compounds to disappear while new volatiles were created or had

their concentration increased, which changed the flavor profile of the fruit.

Changes in volatile compounds during dehydration of d’Agen prunes was evaluated by

Sabarez et al. (2000). Four major components were identified in fresh plums. After drying, three

of the four main compounds completely disappeared and three new volatiles were generated.

Among the new ones, the researchers highlighted the presence of a furan derivative, which was

probably derived from caramelization or Maillard reactions.

Wang et al. (2007) compared the volatiles of banana powder dehydrated by air drying

(AD), vacuum belt drying (VBD), and freeze drying (FD). The researchers found that the

characteristic aroma of fresh banana was present in the dried samples, but the level of flavor in

those products was different depending on which drying technique had been used. As the

drying temperatures used in AD and VBD were higher than that of FD, some of the original

esters disappeared in the AD and VBD samples while some new compounds were formed.

Luning et al. (1995) evaluated the effect of hot air drying on flavor compounds of bell

peppers and found that many “fresh” odor notes decreased or disappeared after drying,

whereas several compounds derived from lipid oxidation were formed or had their

concentrations increased during the drying process.

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Chapter 3: Effects of freeze drying, Refractance Window drying, and hot-air drying on the quality

parameters of açaí

Materials and Methods

Chemicals

- Moisture Working Isotherm: NaCl, KI, Mg(NO3)2·6H2O, K2CO3, MgCl2, and LiCl salts were

obtained from Fisher (Fair Lawn, NJ), and KC2H3O2 was obtained from Sigma-Aldrich (St. Louis,

MO).

- HPLC: Cyanidin-3-glucoside and cyaniding-3-rutinoside were purchased from Polyphenols

Laboratories (Sandnes, Norway).

- ORAC assay: 2,2’-Azobis-(2-methylpropionamidine) dihydrochloride (AAPH) and 6-Hydroxy-

2,5,7,8- tetramethylchroman-2-carboxylic acid (Trolox) were obtained from Sigma-Aldrich (St.

Louis, MO). Fluorescein was purchased from Fisher (Hanover, IL). Potassium phosphate

monobasic and potassium phosphate dibasic were purchased from Fisher (Fair Lawn, NJ).

- Flavor Standards: All standard compounds were purchased from Sigma-Aldrich (St. Louis,

MO), except for (Z) 3-hexen-1-ol and α-ionone, which were obtained from Bedoukian Research

(Danbury, CT); isoeugenol and 1-octen-3-one were purchased from Alfa-Aesar (Ward Hill, MA);

(E,E,Z) 2,4,6 nonatrienal was synthesized following the method described by Schuh et al.

(2005); (Z) 2-nonenal was synthesized from (Z) 2-nonen-1-ol (Bedoukian Research, Danbury,

CT) by oxidation with Dess-Martin periodinane (0.3M in dichloromethane) based on the

procedure described by Meyer et al. (1994); β-damascenone was provided by Firmenich

(Princeton, NJ), and (Z) 1,5-octadien-3-one was synthesized as previously described by Ullrich

et al. (1988a).

Açaí Juice

Açaí juice (36lb) was purchased from Earthfruits (South Jordan, UT) and shipped frozen

overnight to the Department of Food Science and Human Nutrition at University of Illinois at

Urbana-Champaign. The juice was produced with fruits of Euterpe oleracea Martius species and

contained 11-13% total solids, 2 – 5 °Brix, pH < 4.6 and 0.4 – 0.45% acidity (data from

specification sheet). Juice production involved a pasteurization step.

36

Upon arrival, the juice was thawed overnight and then divided into 3 buckets containing

14 lb (to be used for hot-air drying), 14 lb (to be used for Refractance Window drying), and 8 lb

(to be used for analysis of the fresh juice).

Drying Methods

Freeze drying

Freeze-dried açaí was kindly donated by Liotécnica (Embu, São Paulo, Brazil). The

product was vacuum packed and was kept in the original package at -20°C until beginning of

storage test. The fruits used to produce the freeze-dried product were from the same species

and were harvested at the same location as the fruits used to produce the açaí juice from

Earthfruits.

Although freeze-dried açaí was from a different lot of fruit, this product was used for

comparison with RW-dried and hot-air dried samples in order to evaluate the effect of each

drying method.

Hot-air drying

For hot-air drying, açaí juice was thawed overnight and then placed in metal trays, 1-2

cm deep. Product was dried at 65°C for approximately 20 hours. Drying was performed on a

convection cabinet dryer V-33 (Despatch Oven Co, Minneapolis, MN). The procedure was

based on the work of Krokida et al. (2003).

Refractance Window drying

Frozen açaí juice was shipped overnight to Tacoma, WA. MCD Technologies performed

the RW drying of the product. The dwell time of the juice on the dryer’s heated surface varied

between 1min 15 sec to 1min 29 sec. The circulating water temperature was approximately

94°C and the product temperature was around 62 to 71°C.

Storage Test

After drying, products were placed in amber glass bottles, flushed with nitrogen for 4 min

and stored at room temperature (25°C) for up to 3 months. Samples of dried açaí were taken

from storage every month and placed at -20°C until analysis (Piga et al. 2003). Açaí juice was

analyzed only on day zero and the results were used as a reference for comparison with dried

samples.

37

Moisture Content and Water Activity

The moisture content of the açaí powders (2 g) was determined using a vacuum oven

(Equatherm, Curtin Matheson Scientific, Inc., Houston, TX) at 60°C and 30 inches Hg for 24

hours (Yu et al. 2008). The moisture content of the açaí juice was determined in two steps: first,

16 g of juice were dried in a convection oven (Precision Scientific, Inc.) at 80°C for 2.5 hours;

then, samples were dried in a vacuum oven (Equatherm, Curtin Matheson Scientific, Inc.,

Houston, TX) at 60°C and 30 inches Hg for 24 hours.

The water activity of açaí powders was measured using the AquaLab aw meter

(Decagon Devices, Inc., Pullman, WA) at 25°C.

Color

Instrumental color was determined using a HunterLab spectrocolorimeter (LabScan II,

Hunter Associates Laboratory Inc., Reston, VA), calibrated with white and black standards. The

spectral curve was determined over the 400 to 700 nm range. Color values were expressed as

L (whitness/darkness), a (redness/greenness) and b (yellowness/blueness). Also the hue angle

(Eq. (4)) and chroma (Eq. (5)) were used to characterize the products.

(4)

(5)

Moisture Sorption Working Isotherm

To obtain the sorption isotherms, duplicate samples from each drying method (1.5 g)

were placed in previously weighed plastic cups. Samples were then placed in “Lock & Lock”

(Heritage Mint, Ltd., Scottsdale, AZ) containers and equilibrated to seven saturated salt slurries

at 25°C. The relative humidity values of NaCl (75%), KI (69%), K2CO3 (43%), MgCl2 (33%),

KC2H3O2 (23%), and LiCl (11%) were obtained from Greenspan (1977), and Mg(NO3)2·6H2O

(55%) was obtained from Hirose et al. (2006).

During the equilibration period, samples were weighed every week until the observed

weight change was within criteria (2 mg/g). After that, equilibrium moisture content (db) of the

dried açaí was calculated based on the weight change and initial moisture content. The water

activity of samples and salt slurries were also determined using the AquaLab. The moisture

content of the samples and the relative humidity values from the references were used to

38

generate the working isotherms, which were then fit to GAB and BET models. BET model was

applied to water activity values lower than 0.5.

The root mean square error (RMSE) was calculated and used to represent the fitting

ability of the models in relation to the number of data points.

Glass Transition Temperature

Glass transition temperature was determined by differential scanning calorimetry in a

DSC Q2000 (TA Instruments, Newcastle, DE) using Tzero hermetically sealed aluminum pans

and lids. DSC was equipped with a refrigerated cooling system.

Açaí powders (~10 mg) were placed into the DSC pans and analyzed. Samples were

first cooled to -65°C, at 50°C/min, then equilibrated to -65°C, and finally heated to 250°C, at

10°C/min. An empty pan was used as a reference. Dry nitrogen, 50 mL/min, was used as a

purge gas. Analysis of data was done using the software Universal Analysis 2000.

Preliminary studies suggested that the high fat content of açaí was interfering with the

DSC results. Therefore, it was decided to also perform an oil extraction and run DSC with both

the oil and solid phases in order to try to identify the glass transition temperature of the product

and to estimate the influence of the fat on the thermogram. For oil extraction 1g of dried sample

plus 5 mL of pentane:ether (50:50, v/v) were placed in test tubes. Tubes were shaken 30min

and centrifuged. Supernatant containing the oil phase was collected and the above extraction

procedure was repeated 2 more times. Solvent was removed under a stream of nitrogen in

order to recover the oil and solids.

Anthocyanin Analysis

Extraction Procedure

Extraction of anthocyanins was based on the method of Wu et al. (2004b) with

modifications. For açaí juice, 5 g of samples were placed in 125 mL screw cap erlenmeyers.

Next, 25 mL of methanol and 150 μL of acetic acid (resulting in 85:15:0.5 methanol/water/acetic

acid; v/v) were added and samples were extracted on an orbital shaker (250 rpm) at room

temperature for 2 h. Product was then centrifuged at 4550 g for 10 min, at 4°C. The supernatant

was filtered using a 0.2 μm Teflon syringe filter prior to HPLC analysis.

For açaí powders, 1 g of freeze-dried, 1g of RW-dried or 3 g of hot-air dried samples

were placed in 125 mL screw cap erlenmeyers. After that, 25 mL of methanol/water/acetic acid

(85:15:0.5; v/v) were added and samples were extracted on an orbital shaker (250rpm) at room

39

temperature for 2 h. Product was then centrifuged at 4550 g for 10 min, at 4°C. The supernatant

was filtered using a 0.2 μm Teflon syringe filter prior to HPLC analysis.

Analysis by HPLC and HPLC-MS

Anthocyanin separation was based on the method of Grace et al. (2009) with

modifications.

Analysis was performed on a Waters Alliance 2695 HPLC (Waters, Milford, MA)

equipped with a photodiode array detector (HP Series 1050). Separation was carried out on a

reversed phase Phenomenex Prodigy 5u ODS (3) 100A column, 250 x 4.6mm x 5 micron, with

a Phenomenex Prodigy 5u ODS (3) 100A guard column, 30 x 4.6mm x 5 micron. Mobile phases

consisted of 5% formic acid aqueous solution (A) and 100% methanol (B). The flow rate was

1ml/min and detection was at 520 nm. The gradient system used was 10%, 15%, 20%, 25%,

30%, 45%, 10%, and 10% of solvent B at 0, 5, 15, 20, 25, 35, 37, and 50 min, respectively. The

volume of samples injected was 15 μL.

The HPLC-MS analysis was made with an LCQ Deca XP mass spectrometer (Thermo

Finnigan Corp., San Jose, CA), electrospray ionization (ESI) in the positive ion mode (m/z 200-

2000) and a photodiode array (PDA) detector (200-600 nm). Separation was carried out on a

150 x 2.1mm x 5 micron C18 reversed-phase column (Vydac, Western Analytical, Murrieta, CA).

Mobile phases consisted of 0.1% formic acid in water (A) and 100% methanol (B). A step

gradient of 10%, 15%, 20%, 25%, 30%, 60%, 10%, and 10% of solvent B at 0, 5, 15, 20, 25, 45,

47, and 60 min, respectively, a flow rate of 200 μL/min, and an injection volume of 15 μL were

employed.

For quantification and identification purposes cyaniding-3-glucoside (C-3-G) and

cyaniding-3-rutinoside (C-3-R) were used as external standards. Results were expressed as mg

anthocyanin (C-3-G + C-3-R) / g dry weight (DW).

Oxygen Radical Absorbance Capacity (ORAC)

Extraction Procedure

Extracts were prepared according to the method of Ou et al. (2001) with modifications.

For açaí juice, 4.17 g of sample were placed in 125 mL screw cap erlenmeyers. Then,

6.3 mL of water and 10 mL of acetone (resulting in 50:50 acetone/water; v/v) were added and

samples were extracted on an orbital shaker (250 rpm) at room temperature for 2 h. Product

40

was centrifuged at 16300 g for 15 min, at 4°C. Supernatant was diluted (1:200) with phosphate

buffer before analysis.

For dried açaí, 0.5 g of powders were placed in 125 mL screw cap erlenmeyers.

Samples were extracted with 20 mL of acetone/water (50:50; v/v) on an orbital shaker (250 rpm)

at room temperature for 2 h. Product was then centrifuged at 16300 g for 15min, at 4°C.

Supernatant was diluted (1:200) with phosphate buffer before analysis.

Solutions

- Phosphate buffer: 0.075 M phosphate buffer, pH = 7.4, was used for all dilutions and as a

blank in the assay.

- Fluorescein (FL): A stock fluorescein solution (Stock #1) was prepared by dissolving 0.0188 g

FL in 50 mL of phosphate buffer. A second stock solution (Stock #2) was prepared by diluting

516 μL of Stock #1 in 25 mL of phosphate buffer. Then, 141 μL of Stock #2 were diluted in 25

mL of phosphate buffer and 120 μL of this solution was added to each well.

- AAPH: The AAPH solution was prepared by dissolving 0.1085 g AAPH in 10 mL of phosphate

buffer. Sixty microliters of that solution were added to each well.

- Trolox standards: A Trolox stock solution (1mM) was prepared by adding 0.0129 g of Trolox in

50 mL of phosphate buffer. The stock solution was further diluted with phosphate buffer to give

the following concentrations: 150, 100, 80, 60, 40 and 20 μM. The Trolox solutions were used

as standards to generate the calibration curve.

Microplate Assay

For ORAC assay, the method of Dávalos et al. (2004) was used. The assay was carried

out in black-walled 96-well plates (Fisher Scientific, Hanover Park, IL). Aliquots of 20 μL of

phosphate buffer (blank); either 20 μL of Trolox standards or 20 μL of samples; were added into

appropriate wells. Then, 120 μL of fluorescein was added to each well and the plate was

incubated at 37°C for 15 min. After incubation, 60 μL of AAPH were added. The plate was read

in a fluorescent plate reader, FLx800tbi (Bio-Tek, Winooski, VT) at 37°C, sensitivity 60,

excitation 485 nm and emission 582 nm. Readings were done every 2 min for 120 min.

The final ORAC values were calculated by using a regression equation between the

Trolox concentrations and the net area under the FL decay curve. Data are expressed as

micromoles of Trolox equivalents (TE) per gram dry weight (μmol TE/g DW).

41

Flavor Analysis

Açaí juice

Internal Standards

An internal standard solution, containing 2-methyl-3-heptanone and 2-ethylbutyric acid,

was prepared at concentration of 1 mg/mL each.

Sample Preparation

Açaí juice, 500 g, was first thawed and then placed in a beaker. Under stirring, 5 μL of

internal standard solution was added to the product. Juice was then mixed for 2 min and sieved

prior to extraction of volatiles.

Isolation of Volatiles

Solvent-assisted flavor evaporation (SAFE) was used to isolate the volatile compounds

and was based on the procedure of Lozano et al. (2007b), with slight modifications. Over a

period of 1h the sample was fed in dropping aliquots into the SAFE system (distillation head at

40°C, sample flask at 40°C and vacuum at less than 1 x 10-4 Torr). SAFE unit was then kept at

10-5 Torr for 2 h prior to removal of the product from the system. Product was kept outside for 30

min and then 50 mL of ether was added. Mixture was thawed overnight before extraction.

Extraction was done according to the procedure of Lozano et al. (2007a), with some

modifications. First, 2 M NaOH was added to the SAFE extract until pH=9 (approximately 0.5

mL). Product was then transferred to a separatory funnel and 40 g of NaCl were added. The

mixture was shaken for 3 min and let stand for 3 min. The ether layer was placed in a 500 mL

round-bottom flask and the aqueous layer was extracted two more times with 30 mL of ether,

following the same procedure. After that, the aqueous layer was acidified with 4N HCl to a pH=2

(around 0.7 mL). Aqueous layer was extracted three times with 30 mL of ether.

The combined ether layers (in 500 mL round-bottom flask) were concentrated to 30 mL

by distillation at 42°C using a Vigreaux column. The concentrate was fractionated into

neutral/basic (NB) and acidic (AC) fractions, according to the procedure of Karagül-Yüceer et al.

(2003) with modifications. Extract was first washed with sodium carbonate (3 x 20 mL), followed

by separation of the ether (NB fraction) and aqueous (AC fraction) layers. The ether layer was

washed twice with 10 mL of saturated NaCl and the upper ether phase was collected and

concentrated to 10 mL by distillation at 42°C using a Vigreaux column. It was next dried over

42

anhydrous Na2SO4 and concentrated to 1 mL, again by distillation. Extract containing NB

volatiles was transferred to 2 mL vial and concentrated to 200 μL under N2. The aqueous layer

was washed twice with 15 mL of ether (which was discarded) and then acidified with 4N HCl

saturated with NaCl to pH ~2. Acidic volatiles were extracted with ether (3 x 20 mL). Ether layer

was concentrated to 10 mL by distillation at 42°C using a Vigreaux column, dried over

anhydrous Na2SO4 and concentrated to 1 mL again by distillation. AC extract was transferred to

2 mL vial and concentrated to 200 μL under N2. Both fractions were kept at -70°C until analyses.

In order to determine the predominant odorants in açaí juice, each of the fractions was diluted

sequentially with diethyl ether in a ratio of 1:3 according to the aroma extract dilution analysis

technique. Each dilution (1 μL) was later injected in GC-O.

Identification of volatiles

GC-MS and GC-O were used to identify the main volatile compounds of açaí juice. GC-

MS was performed on an Agilent 6890N GC/5973N mass selective detector (MSD) system.

Each extract (1 μL) was injected in the cold on-column injection mode. Two columns with the

same dimensions (30 m length x 0.25 mm i.d x 0.25 µm film thickness) but different polarities

(Stabilwax and SAC-5) (Restek, Bellefont, PA and Supelco, Bellefont, PA, respectively) were

used. The oven was programmed from 35 to 225°C at a rate of 4°C/min with initial and final hold

times of 5 and 25 min, respectively. Helium was used as carrier gas at a constant rate of 1.0

mL/min. MSD conditions were as follows: MS transfer line heater, 280 °C; ionization energy, 70

eV; mass range, 33–350 amu; and scan rate, 5 scans/s. For better sensitivity the electron

multiplier voltage was 200 V above the autotune setting.

GC-O was conducted on an Agilent 6890 GC (Palo Alto, CA) equipped with an FID and

a sniffing port (DATU, Geneva, NY). Two different polarity columns (Stabilwax and RTX-

5SILMS) (Restek, Bellefont, PA) with the same dimensions (15 m length x 0.32 mm i.d x 0.5 µm

film thickness) were used to obtain odor description and retention indices. Each extract (1 μL)

was injected in the cold on-column injection mode. The oven was programmed from 35 to

225°C at a rate of 10°C/min for NB fraction and 6°C/min for AC fraction in Stabilwax column and

6°C/min for both fractions in RTX-5SILMS column. Initial and final hold times were 5 and 25

min, respectively. Helium was used as carrier gas at a constant rate of 1.0 mL/min.

Temperature of FID and transfer line were held at 250°C. The end of the capillary column was

split 1:5 between the FID and sniffing port.

Compounds were, first, tentatively identified based on comparison with published

retention indices and odor quality (Rychlik et al. 1998; Acree et al. 2004). Positive identification

43

of odorants was based on matching retention indices, odor description (both on two different

polarity columns), and mass spectra of unknown compounds with those of authentic standards.

An n-alkane series was used for the determination of retention indices.

Dried Açaí

Headspace volatiles were extracted and concentrated using SPME headspace

sampling. Açaí juice (used as control) and dried samples from day 0 and after 3 months of

storage were used for the analysis.

Sample Preparation

For açaí juice, 5 mL of samples, 1 g of NaCl and a micro stirring bar were placed into 20

mL screw cap glass vials with PTFE/Silicone-coated septa (SPME vials). For açaí powders,

samples were first rehydrated with deodorized water to a final moisture content of 87.86%

(same moisture content of juice). Then, 5 mL of samples were added to SPME vials as

previously described. An internal standard (IS) solution containing 2-methyl-3-heptanone, was

prepared at concentration of 0.0232 mg/mL and 2 μL of IS were added to each vial.

Headspace sampling

Headspace sampling was based on the procedure of Mahattanatawee et al. (2007), with

modifications. SPME vials were, first, incubated for 15 min at 40°C in a single magnetic mixer,

with agitation set to 250 rpm. After that, a SPME fiber (50/30μm DVB/Carboxen/PDMS on a

1cm StableFlex fiber, Supelco, Bellefont, PA) was inserted into the headspace of the sample

vial and exposed for 45 min at 40°C. Fiber was, then, thermally desorbed in the GC-MS injector

port for 14 min at 260°C (vent after 4 min). The fiber was preconditioned prior to analysis at

270°C for 1 h.

CG-MS

GC-MS was performed on an Agilent 6890N GC/5973N mass selective detector (MSD)

system equipped with an RTX-Wax (30 m length x 0.25 mm i.d x 0.25 µm film thickness) column

(Restek, Bellefont, PA). The oven was programmed from 35 to 225°C at a rate of 4°C/min with

initial and final hold times of 5 and 15 min, respectively. Helium was used as carrier gas at a

constant flow rate of 1.0 mL/min. MSD conditions were as follows: MS transfer line heater, 280

44

°C; ionization energy, 70 eV; mass range, 33–350 amu; and scan rate, 5 scans/s. For better

sensitivity the electron multiplier voltage was 200 V above the autotune setting.

Compound Quantification

For quantification purposes, standard curves of selected flavor compounds were used.

Known amounts of authentic standards plus internal standard were added into a deodorized

açaí juice matrix and SPME was performed in the same way as described above. The ratios of

authentic standards and IS concentrations were: 1:1, 1:10, 1:50 and 1:100. Deodorized juice

matrix was achieved by running the product through SAFE twice and collecting the solids left

after water evaporation.

Statistical Analysis

Significant differences between treatments were determined using the one-way ANOVA

followed by Tukey’s test at α = 0.05.

Results and Discussion Moisture Content

The moisture content of the juice was 87.86 ± 0.04%, confirming that the product used

was a Type B açaí (11-14% solids) (Table 2). The moisture content of the dried products over

time is presented in Figure 12. The RW-dried samples presented the highest moisture content

(2.19% wb), followed by freeze-dried (1.55% wb) and hot-air dried samples (1.36% wb)

(reported moisture content values on day 0). Similar results were obtained during the entire

study.

Drying time and air relative humidity are very important in drying operations since they

affect the final moisture content of the product (Kaya et al. 2008; Krokida et al. 1998). During

RW drying, ambient air is in contact with the product and, therefore, air relative humidity is not

45

controlled. The higher moisture content of RW-dried açaí may have been caused by insuficient

drying time or air with high relative humidity.

Figure 13 shows the effect of storage on the moisture content of açaí powders. The

moisture content of all dried products on day 0 was significantly lower than the values obtained

for samples after one month of storage. Changes in moisture might have been caused by

changes in the material, resulting in more volatiles being produced during vacuum oven

measurement. It could also be due to a measurement error on day 0. Water could also have

been produced during the first month of storage, resulting in higher moisture content, although

this fact is not compatible with the water activity results. Further research is necessary in order

to explain why moisture content was significantly lower on day 0.

Figure 12. Effect of different drying methods on moisture content * Within each segment, bars with different letters are significantly different

46

Figure 13. Effect of storage on moisture content * Within each segment, bars with different letters are significantly different

Water Activity

All dried samples exhibited low water activity values, suggesting good product stability.

When comparing different treatments (Figure 14) at day 0, the RW-dried açaí showed a

significantly higher water activity (~0.240), followed by the freeze-dried (~0.196) and hot-air

dried samples (~0.119), respectively. For samples having a relatively high moisture content, the

water activitiy was also higher. The water activity for samples from the three drying methods is

thus in the sequence of RW > FD > HAD.

The effect of storage time on water activity is presented in Figure 15. Water activity

values, within each individual treatment, did not change over the studied period of time,

suggesting that glass is a good packaging material to avoid moisture uptake from the

surrondings during storage. Similar results were reported by Mahayothee et al. (2009), working

with dried lychees stored in polyethylene film bags, and Yongsawatdigul et al. (1996), working

with dehydrated cranberries (packaging material was not described).

47

Figure 14. Effect of different drying methods on water activity * Within each segment, bars with different letters are significantly different

Figure 15. Effect of storage on water activity * Within each segment, bars with different letters are significantly different

Color

For evaluation of color, L, a and b values were measured using a Hunter colorimeter.

The values of a and b were used to calculate hue angle and chroma, measurements that are

most significantly related to visual color scores than the individual a and b values (Shishehgarha

et al. 2002). A comparison of the color readings of açaí juice with those in samples dried by

48

different methods at day 0 is given in Table 3. The L values increased significantly in all dried

samples compared with the juice, with the L for the RW-dried being significantly higher or

"lighter" than the hot-air and freeze-dried. The changes in a and b values after drying seemed to

follow similar trend. A significant decrease in a and b values can be observed in all the samples.

Obviously, the dried açaí had a color totally different from that of the juice.

In the analysis of color, plotting a and b values results in a color wheel, which subtends

360°, with red-purple placed at an angle 0°, yellow at 90°, bluish-green at 180° and blue at 270°,

counterclockwise. The hue angle determines in which quadrant the sample will be placed and

gives information about the actual hue of the product (McGuire 1992). Drying caused a

significant decrease in hue angles for all dried samples (Table 3). However, all the samples

were still in the same quadrant (Figure 16) and the changes in the hue angle, and consequently

the hue, although significant, were relatively small. The higher hue angle of the juice represents

a less redish and, consequently, higher yellowish color, which is probably due to its oily

appearance. On the other hand, the lower hue angles of the dried samples stand for a more

redish color. A red color reinforcement in dried samples has been attributed to pigment

concentration due to water reduction (Hammami et al. 1997). In dried samples, the chroma

values, which indicate color saturation or intensity, decreased compared to the açaí juice. A less

vivid and less intense color may be expected in the dried products. Overall changes in color due

to drying were most likely caused by anthocyanin degradation and formation of brown pigments

by enzymatic browning and Maillard reaction.

Table 3. Color measurements of açaí juice and powders on day 0

* Within each column, bars with different letters are significantly different

L a b Hue angle ChromaJuice 12.42c 16.61a 4.94a 0.29a 17.33aFreeze-dried 14.56b 4.94c 0.35b 0.075b 4.97cHot-air dried 15.26b 3.66d 0.48b 0.13ab 3.69dRW-dried 19.05a 6.44b 0.59b 0.092b 6.47b

49

Figure 16. Representation of sample’s hue

The color changes as affected by different drying methods are presented in Figures 17-

19, using lightness, hue angle and chroma. Figure 17 shows the effect of the three drying

methods on lightness. No significant difference was observed between freeze and hot-air dried,

samples. However, the RW-dried samples presented consistently higher L values compared to

the other dried products. This higher lightness might have been caused by the açaí oil, which

may have coated the surface of the thin layer of product used in this technology, giving it a shiny

appearance and the impression of a lighter color. Figure 18 presents the effects of the drying

techniques on hue angle. No significant difference in this parameter among the treatments was

observed for up to two months of storage. However, on the third month, the hot-air dried

samples showed a significantly higher hue angle, which represents a decrease in its red color.

When considering chroma (Figure 19), the RW-dried açaí showed the highest chroma value,

followed by the freeze and hot-air dried samples. A more saturated color in RW-dried samples

might have been influenced by the oil coating, which gives it a more vivid appearance.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 2 4 6 8 10 12 14 16 18

a

b

Freeze-dried Hot-air dried RW-dried Juice

Red-Purple

Yellow

50

Figure 17. Effect of different drying methods on Lightness * Within each segment, bars with different letters are significantly different

Figure 18. Effect of different drying methods on hue angle * Within each segment, bars with different letters are significantly different

51

Figure 19. Effect of different drying methods on chroma * Within each segment, bars with different letters are significantly different

When considering the color differences within each drying method as a function of

storage time, values of the lightness, hue angle, and chroma are plotted against each drying

method and are shown in Figures 20-22. For all the drying technologies, lightness decreased

during storage, which means that the products were turning darker. This was most likely due to

the formation of brown pigments as a result of Maillard reaction and/or polymerization of

oxidized phenolic compounds (Perera 2005). At the water activity values of the dried samples,

occurrence of chemical reactions was not expected. However, it has been reported that even at

low water activities and below glass transition temperature, Maillard reaction can occur (Karmas

1992; Telis et al. 2009). All dried samples presented an increase in hue angle over storage. A

significant increase in hue angle indicates a reduction in red color. Storage also caused a

significant increase in chroma for all dried samples. The occurrence of non-enzymatic browning

reactions may account for part of the changes observed in hue angle and chroma during

storage.

52

Figure 20. Effect of storage on Lightness * Within each segment, bars with different letters are significantly different

Figure 21. Effect of storage on hue angle * Within each segment, bars with different letters are significantly different

53

Figure 22. Effect of storage on chroma * Within each segment, bars with different letters are significantly different

Moisture Sorption Working Isotherm

Moisture working isotherms of freeze-dried, RW-dried and hot-air dried açaí are

presented in Figure 23. All the isotherms were sigmoidal in shape, as characteristic of most

foods, and presented similar water sorption behavior. The hot-air dried sample exhibited a

slightly higher water adsorption capacity than the RW and freeze-dried materials at all aw

values. A type II isotherm was also observed by Maskan et al. (1998) for vacuum dried

mulberries and Syamaladevi et al. (2009) for freeze-dried raspberry.

The water sorption data of the dried samples were fitted to the Guggenheim-Anderson-

de Boer (GAB) and Brunauer-Emmett-Teller (BET) models. The estimated GAB and BET

parameters are presented in Table 4. Both the BET and GAB models showed a good fit to the

experimental data, as shown by low root mean square errors (RMSE). The GAB model is an

extension of the BET model by introducing the constant K. The GAB model can be used to

predict the isotherm parameters at a water activity of up to 0.9, whereas BET allows isotherm

modeling for aw up to 0.5. The constants in the GAB equation are temperature dependent. In

this experiment the values of C fell between 1 and 20 and K between 0.7 and 1.0, which is in

the range of most food products reported in the literature (Rahman 1995).

Both the BET and GAB models are based on the monolayer moisture concept and

provide the monolayer moisture content of the product, an important parameter for assuring

food stability. As it can be seen from Table 3, the monolayer moisture content values of the

54

three dried açaí samples calculated using the BET model were lower than those obtained from

the GAB model, a phenomenon that has been documented in published papers. Timmermann

et al. (2001) stated that GAB monolayer moisture content is always higher than BET monolayer.

The moisture content of all dried samples was below their monolayer moisture content

value, indicating good product stability over time. Bell et al. (2000) reported that, for most dried

foods, the monolayer value is usually at water activity of 0.2-0.4 and, in general, the results of

the current project fall between that range (except for hot-air dried samples, which had an

average aw of 0.119). The same authors showed that, for most chemical reactions in aqueous

phase, the rate of quality loss begins to increase above aw 0.2-0.3. Beyond that range, water

adsorbed is enough to behave as a solvent, increasing the mobility of the reactants and the rate

of degradative reactions. An exception of that is lipid oxidation, for which the rate of reaction

increases as the water activity decreases below the monolayer. The occurrence of lipid

oxidation will be discussed later.

As previously mentioned, freeze-dried, RW-dried and hot-air dried samples presented a

type II isotherm. The monolayer moisture content values obtained for those samples varied from

2.43% to 2.53% according to the BET model and from 2.5% to 2.62% according to the GAB

model, depending on the drying method used (Table 4). The monolayer values obtained in this

research were lower than the ones previously reported. Tonon et al. (2009) reported a type III

isotherm for spray dried açaí at 25°C, probably due to the presence of carrier agents (crystalline

molecules). The monolayer moisture content reported by Tonon et al. (2009) varied from 3.1%

to 5.8% according to the BET model and from 3.2% to 6.3% when obtained from the GAB

model, depending on the carrier agent used. Da Silva et al. (2008) also worked with spray dried

açaí and reported a type III isotherm for the product, at all temperatures used (15, 25 and 35°C).

The reported monolayer moisture content, according to the BET model, was 7.7% (25°C).

55

Figure 23. Moisture sorption working isotherms of the freeze-dried, RW-dried and hot-air dried açaí

Table 4. Estimated GAB and BET parameters

Anthoycanins

Cyaniding-3-glucoside (peak 1) and cyaniding-3-rutinoside (peak 2) were found to be the

major anthocyanins (ACNs) in all açaí samples, which is in agreement with previous reports of

Lichtenthaler et al. (2005), Schauss et al. (2006b), and Vera de Rosso et al. (2008). Of them,

cyaniding-3-rutinoside was found to be the predominant ACN, having a concentration of 5 to

30% above the concentration of cyaniding-3-glucoside depending on the drying method. The

MS spectral data of these compounds are presented in Table 5 and a typical HPLC

0

1

2

3

4

5

6

7

8

9

0 0.2 0.4 0.6 0.8

Water activity

Moi

stur

e C

onte

nt (%

db)

Hot-air dried Freeze-dried RW-dried

Model Parameters Freeze-dried RW-dried Hot-air driedC 7.13 6.82 11.82M0 2.5 2.56 2.62K 0.92 0.93 0.92RMSE 0.08 0.07 0.08

C 6.38 6.23 10.91M0 2.43 2.49 2.53RMSE 0.02 0.03 0.03

GAB

BET

56

chromatogram for açaí juice is shown in Figure 24. HPLC chromatograms for freeze, RW and

hot-air dried samples are presented in Figures 32-34 in Appendix A.

Table 5. Identification of anthocyanins

Figure 24. HPLC chromatogram for açaí juice on day 0

The effect of different drying methods on anthocyanin content (reported as sum of

cyaniding-3-glucoside and cyaniding-3-rutinoside) is shown in Figure 25. On day 0, the freeze-

dried samples presented an anthocyanin content (5.46 mg/g DW) comparable to that from the

açaí juice (5.74 mg/g DW), followed by the RW-dried (4.87 mg/g DW) and hot-air dried (2.79

mg/g DW). During storage, the anthocyanin content of freeze dried samples was always

significantly higher than that of the RW dried açaí, which was significantly higher than that of

hot-air dried samples. The anthocyanin concentration of hot-air dried açaí was approximately

40-50% lower than that of other açaí samples. Freeze drying resulted in the greatest retention of

anthocyanins compared to other drying methods, which confirms the superiority of this

technique. A better retention of anthocyanins by freeze drying compared to other drying

methods was also reported by Nsonzi et al. (1998) for blueberries and Michalczyk et al. (2009)

Peak no. tR (min) MS (m/z) Compound

1 24.4 449 cyanidin-3-glucoside

2 26.3 595 cyanidin-3-rutinoside

57

for raspberry, strawberry and bilberry. Although significantly different, the RW-dried samples

showed an anthocyanin content similar to the freeze-dried, indicating that this technique is

superior to hot-air drying, with respect to preserving ACNs. Anthocyanins are very unstable and

can be easily degraded during hot-air drying due to the use of high temperatures for long time

and presence of oxygen (Lohachoompol et al. 2004; Piga et al. 2003).

Figure 25. Effect of different drying methods on anthocyanin content * Within each segment, bars with different letters are significantly different

The contents of anthocyanins found in this study, 5.7 mg/g DW for açaí juice and 2.8 to

5.5 mg/g DW for dried samples, are different from previously reported data. For açaí juice Vera

de Rosso et al. (2008) reported around 3.0 mg/g DW, whereas Pacheco-Palencia et al. (2009)

reported approximately 13 mg/g DW. Anthocyanin contents of 0.5 mg/g DW and 3.19 mg/g DW

have been reported by Gallori et al. (2004) and Schauss et al. (2006b), respectively, for freeze

dried açaí. Differences in pigment content are to be expected since the level of anthocyanins is

affected by factors like climatic conditions, harvesting period, maturity stage and processing

techniques, which, in case of açaí juice, normally include a thermal pasteurization. Lichtenthaler

et al. (2005) reported anthocyanin contents varying between 13 and 463mg/L for different açaí

juice samples. In fact, the authors also analyzed the juice of fruits collected from the same trees

in different years and noticed that anthocyanin levels ranged from 88 to 211mg/L.

Cyaniding-3-glucoside and cyaniding-3-rutinoside account for more than 95% of the total

anthocyanins in açaí (Pacheco-Palencia et al. 2009; Schauss et al. 2006b). Interestingly, the

content of anthocyanins in this fruit is much lower than that of other berries. For instance, wild

58

blueberries were reported to have around 487 mg/100g FW (~45 mg/g DW), while blackberries

and black raspberries were said to have approximately 245 and 687 mg/100g FW (~19 and 46

mg/g DW, respectively) (Wu et al. 2006).

The effect of storage on anthocyanin content of dried samples is presented in Figure 26.

The content of ACNs, within each drying method, did not change over the storage period,

indicating good stability of those compounds under the experimental conditions of the project.

It has been reported that water activity plays an important role on anthocyanin

degradation (Amr et al. 2007; Garzón et al. 2001). High water contents result in high molecular

mobility, which facilitates chemical reactions. According to Erlandson et al. (1972) high water

activity values results in either hydrolysis of the glycosidic bond to form aglycones, which are

unstable, or opening of the pyrilium ring to form a substituted chalcone. The low water activity

values, as well as the low moisture contents of below monolayer values, of the açaí powders

used in this study may contribute to a relative good retention of ACNs during storage.

Brønnum-Hansen et al. (1985) evaluated the effect of different water activity values and

storage on stability of freeze-dried elderberry anthocyanins. They reported that samples stored

at or below the monolayer moisture content showed the highest stability, with essentially no loss

of anthocyanins after 1 year of storage.

Tonon et al. (2010) studied the effects of water activity (0.328 and 0.529) and storage

temperature (25 and 35°C) on anthocyanin stability of spray dried açaí. A higher temperature

and water activity led to faster ACN degradation. Since all samples were stored at aw values

below the critical water activity, degradation was not expected. According to the authors,

diffusion of oxygen may have caused anthocyanin degradation.

Figure 26. Effect of storage on anthocyanin content * Within each segment, bars with different letters are significantly different

59

Antioxidant Capacity

The antioxidant capacity of açaí samples against peroxyl radicals was assessed by

ORAC assay. The effect of different drying methods on the antioxidant capacity of açaí is shown

in Figure 27. On day 0, the freeze-dried samples presented an ORAC value (993.12 μmol TE/g

DW) comparable to the açaí juice (1125.33 μmol TE/g DW), suggesting that this drying method

retained most of the bioactive compounds responsible for the antioxidant properties of açaí. The

RW-dried samples showed an intermediate antioxidant capacity (710.39 μmol TE/g DW) while

hot-air dried samples exhibited the lowest ORAC value (528.35 μmol TE/g DW). The ranking for

the ORAC among the three drying methods remained the same over the 3-month storage, with

the freeze-dried having the highest while the hot-air dried being with the lowest ORAC values.

Schauss et al. (2006a) studied the antioxidant capacity of freeze-dried açaí and reported

an ORAC value of 996.9 μmol TE/g DW. In the present research, a similar ORAC value, 993.12

μmol TE/g DW, was obtained for the freeze-dried product on day 0. Different results for the

antioxidant capacity of açaí juice have been reported in the literature. Pacheco-Palencia et al.

(2007) recorded an ORAC value of 54.4 μmol TE/mL juice (~453 μmol TE/g DW), while Del

Pozo-Insfran et al. (2004) and Pacheco-Palencia et al. (2009) reported 48.6 μmol TE/mL juice

(~405 μmol TE/g DW) and 87.4 μmol TE/g juice (~728 μmol TE/g DW), respectively. In this

research, the açaí juice was found to have an ORAC value of 1125.3 μmol TE/g DW. The

differences in antioxidant capacity of açaí might be due to differences in polyphenolic

composition, which is affected by geographical and seasonal differences as well as by post-

harvest practices and the processing method used.

It has been stated that açaí has the highest antioxidant capacity, of any fruit reported to

date, against peroxyl radicals (Schauss et al. 2006a). In this research, the açaí juice (1125.3

μmol TE/g DW) and freeze-dried açaí powders (~838 - 993 μmol TE/g DW) were found to have

very high antioxidant capacity, being superior to other berries with high ORAC values, such as

fresh lowbush blueberries (92.09 μmol TE/g fruit or ~837 μmol TE/g DW) and cranberries (92.56

μmol TE/g fruit or ~718 μmol TE/g DW) (Wu et al. 2004a). Despite of being lower than the

freeze-dried product, RW-dried and hot-air dried açaí showed ORAC values higher than other

fruits, such as fresh blackberries (52.45 μmol TE/g fruit or ~400 μmol TE/g DW), raspberries

(47.65 μmol TE/g fruit or ~336 μmol TE/g DW), and strawberries (35.41 μmol TE/g fruit or ~398

μmol TE/g DW) (Wu et al. 2004a).

The effect of storage on the antioxidant capacity of dried products is presented in Figure

28. For all the three dried samples, no discernable changes in antioxidant capacity were

60

observed over time. These results agree with our previous discussion that anthocyanin content

did not change during storage. Anthocyanins have been identified as important antioxidants in

açaí, together with other compounds, some of which are yet to be identified.

Figure 27. Effect of different drying methods on antioxidant capacity * Within each segment, bars with different letters are significantly different

Figure 28. Effect of storage on antioxidant capacity * Within each segment, bars with different letters are significantly different

61

Flavor Analysis

Açaí Juice

Predominant odorants of açaí juice were identified by means of AEDA and are

presented in Table 6. Twenty-six neutral/basic and 4 acidic compounds were identified, with

flavor dilution (FD) factors ranging from 9 to 2187. All compounds were positively identified by

RI, MS and odor.

Esters are important flavor compounds of many fruits and fruit juices. Interestingly, only

one ester, ethyl butyrate, was identified in açaí juice. Ethyl butyrate is an important odor-active

compound found in many foods, being significant in fruit juices such as orange, apple and

strawberry. It is actually described as the most important odorant in orange juice, being

responsible for fruity, pineapple notes (Nisperos-Carriedo et al. 1990; Perez-Cacho et al. 2008).

Similar to açaí juice, watermelon is another fruit that do not have many esters in its volatile

composition (Beaulieu et al. 2006).

The most important flavor compounds identified in açaí juice were (E,Z) 2,6-nonadienal,

α-ionone, and 2-phenylethanol and/or β-ionone, with FD factors of 2187. (E,Z) 2,6-nonadienal

was most likely derived from lipoxygenase activity on linolenic acid (Tressl et al. 1981). It could

also have been produced by autoxidation of methyl linolenate (Ullrich et al. 1988b). This

compound is the main volatile in cucumber, being also important in melon and watermelon

(Buescher et al. 2001; Perry et al. 2009; Pino et al. 2003). Alfa-ionone, a norisoprenoid, is a

product of degradation of carotenoids. Mahattanatawee et al. (2005), working with orange juice,

reported that this compound is formed by oxidative cleavage of double bonds in α-carotene and

α-cryptoxanthin. This odorant is responsible for a floral note and is an important constituent in

other fruits, such as raspberries (Hampel et al. 2007) and blackberries (Qian et al. 2005). 2-

phenylethanol and β-ionone were probably co-eluted and it was not possible to identify which

compound was the major responsible for the additional floral notes found in açaí juice. Since β -

ionone has a much lower threshold compared to 2-phenylethanol, this volatile may be the most

important one. 2-phenylethanol is derived from phenylalanine and the pathway for its

biosynthesis in plants is not well understood yet (Tieman et al. 2007). The compound is the

major aroma volatile in roses (Sakai et al. 2007) and it is also of great importance for the flavor

of lychee and apple cider (Ong et al. 1998; Xu et al. 2007). Beta-ionone, like α-ionone, is a

norisoprenoid and has been reported as an important volatile in other fruits, such as

blackberries and raspberries (Du et al. 2010; Klesk et al. 2004).

62

Many aldehydes derived from lipid oxidation were identified in the açaí juice. It is well

known that the disruption of plant tissues induces lipoxygenase activity, leading to the formation

of hydroperoxydes. By action of hydroperoxyde lyases, hydroperoxydes are then converted into

volatile aldehydes. Thermal oxidation, autoxidation and photo-oxidation are also responsible for

the formation of fatty acid derived aldehydes (Tressl et al. 1981). As previously mentioned, açaí

has a high lipid content and, probably, both pathways were responsible for the formation of

those compounds in açaí juice. Volatile production most likely occurred during post-harvest,

processing (mainly) and storage. Among the aldehydes identified, (E,Z) 2,6-nonadienal (already

discussed above), (E,E,Z) 2,4,6-nonatrienal, (E) 2-nonenal, (E,E) 2,4-nonadienal, and (E,E) 2,4-

decadienal showed the highest FD factors. (E,E,Z) 2,4,6-nonatrienal can be formed by

enzymatic degradation and autoxidation of linolenic acid (Schuh et al. 2005); (E) 2-nonenal has

been reported as a product of thermal decomposition of hydroperoxides from linoleic and

linolenic acids (Lozano et al. 2007a); and (E,E) 2,4-nonadienal and (E,E) 2,4-decadienal can be

formed by oxidation of linoleic acid (Frankel 1983).

Lipid oxidation-derived aldehydes may be further reduced to alcohols by alcohol

dehydrogenases. This may be the mechanism behind the formation of (Z) 3-hexen-1-ol. This

volatile has been reported as a product of the metabolism of linolenic acid hydroperoxydes by

hydroperoxyde lyases and alcohol dehydrogenases (Angerosa et al. 1999). (Z) 3-hexen-1-ol is

responsible for fresh green and grassy odors. The compound is one of the main contributors to

the characteristic odor of green leaves. It has also been detected in elderberry juice and grapes

(Fan et al. 2010; Hatanaka 1993; Jensen et al. 2001).

The ketones 1-octen-3-one and (Z) 1,5-octadien-3-one may also have been derived from

lipid oxidation, probably of linoleic acid (Tressl et al. 1981). (Z) 1,5-octadien-3-one imparted a

plastic, styrene flavor, whereas 1-octen-3-one was characterized as mushroom-like or metallic.

These compounds constitute important odorants in mushrooms. 1-octen-3-one has been also

identified in lychees and elderberry juice (Mahattanatawee et al. 2007; Jensen et al. 2001).

Some thermally generated volatiles were as well identified. Methional is formed by

Strecker degradation reaction involving methionine. The compound is responsible for a potato-

like note (Di et al. 2003). Phenylacetaldehyde is a Strecker aldehyde of phenylalanine, with a

characteristic floral note (Hofmann et al. 2000). Vanillin (4-hydroxy-3-methoxybenzaldehyde)

has been reported to be a thermally induced decomposition product of ferulic acid and

contributes with a pleasant vanilla, sweet note (Peleg et al. 1992). Those compounds might

have been generated, or had their concentration increased, during pasteurization of the juice.

63

In addition, two phenolic compounds, guaiacol and isoeugenol, were present in the açaí

juice, showing relatively high FD factors. Guaiacol is responsible for smoky, medicinal and/or

phenolic notes, and is usually characterized as an off-flavor in fruit juices. The presence of this

compound in juices has been related to product spoilage, mainly by Alicyclobacillus

acidoterrestris. The spores of this microorganism are resistant to pasteurization and can grow

over a wide range of temperatures, producing off-flavors (Eisele et al. 2005; Perez-Cacho et al.

2007). In the case of açaí, spoilage might have occurred in the juice or in the fruit, before

processing, since the elevated temperatures and relative humidity of the harvesting/production

area may favor microbial growth. On the other hand, isoeugenol provides a pleasant floral

scent. The compound is one of the main volatiles of the petunia flower and was also detected in

blueberries (Dexter et al. 2007; Su et al. 2010). Moreover, an unknown compound (no. 4), with

high FD factor (729) was detected in açaí juice. This odorant was responsible for green, apple

notes.

Several acids were also identified. Acetic acid contributed to vinegar, sour notes, while

butanoic and 3-methyl butanoic acids were responsible for cheesy, sweaty notes. Butanoic and

3-methyl butanoic acids are volatile short-chain fatty acids and provide important notes for the

characteristic flavor of cheeses (Innocente et al. 2000).

64

Table 6. Odor-active compounds of açaí juice identified by AEDA

All compounds identified by RI, MS and odor * Compounds were co-eluted Dried Açaí

To evaluate the effect of drying and storage on the flavor of açaí the SPME technique

was employed and three groups of odor-active compounds were selected for analysis. 2-methyl

butanal, 3-methyl butanal, benzaldehyde and phenylacetaldehyde were chosen as indicators of

Maillard reaction; pentanal, hexanal, heptanal, (E) 2-hexenal, (E,E) 2,4-heptadienal and (E,E)

2,4-decadienal were used as indicators of lipid oxidation; and 2-phenylethanol, (Z) 3-hexen-1-ol

and octanal were selected as characteristic flavor compounds identified in açaí juice by AEDA

(named as “target compounds”).

Table 7 presents the concentration of the analyzed volatile compounds in each product

on day 0. It can be seen that dehydration changed the flavor of the açaí juice. In general, the

no. compound aroma Stabilwax RTX-5 FD factor fraction1 2,3-butanedione buttery 969 <700 9 NB2 ethyl butyrate fruity, bubble gum 1036 801 27 NB3 hexanal green, cut-leaf 1078 796 27 NB4 unknown green, apple 1135 — 729 NB5 octanal orange, pungent, citrus 1287 1003 9 NB6 1-octen-3-one mushroom, metallic 1295 976 243 NB7 unknown green, floral 1348 — 9 NB8 (Z )1,5-octadien-3-one plastic, styrene 1367 981 27 NB9 (Z ) 3-hexen-1-ol green leaf 1381 859 729 NB10 acetic acid vinegar 1429 <700 27 AC11 methional potato 1443 906 27 NB12 (Z ) 2-nonenal hay, stale 1495 1147 27 NB13 (E ) 2-nonenal hay, stale 1524 1160 729 NB14 (E,Z ) 2,6-nonadienal fatty, cucumber 1574 1152 2187 NB15 butanoic acid cheesy, fecal 1615 820 27 AC16 phenylacetaldehyde rosy, floral 1626 1045 27 NB17 3-methyl butanoic acid cheesy, sweaty 1652 862 27 AC18 (E,E ) 2,4-nonadienal fatty, fried 1687 1217 243 NB19 unknown stale, hay, fatty 1715 — 27 NB20 (E,Z ) 2,4-decadienal cucumber, fatty 1756 1291 27 NB21 (E,E ) 2,4-decadienal fried, fatty 1796 1320 243 NB22 β-damascenone apple sauce, honey 1807 1380 9 NB23 α-ionone floral, wine-like 1835 — 2187 NB24 2-methoxy phenol (guaiacol) smoky, hospital 1841 1087 243 NB/AC25 (E,E,Z ) 2,4,6-nonatrienal oats, fatty 1858 1238 729 NB26 2-phenylethanol rosy, wine-like 1892 1115 NB27 β-ionone rosy 1901 — NB28 eugenol cloves, cinnamon 2146 1359 81 NB29 isoeugenol cloves 2319 1455 729 NB30 vanillin vanilla 2531 1421 27 AC

RI

2187 *

65

concentration of Strecker aldehydes was higher in dried samples, indicating the occurrence of

Maillard reaction during drying. The majority of lipid oxidation compounds also had their

concentration increased with drying. This was expected since açaí juice is rich in lipids and the

application of heat during processing would most likely induce lipid oxidation and the release of

volatile aldehydes. All the Maillard reaction indicators and three indicators of lipid oxidation were

detected in açaí juice. These compounds might be either originally present in açaí, being

characteristic of the fruit flavor, or might have been produced during juice pasteurization and

storage. Lower concentrations of (Z) 3-hexen-1-ol were found in dried products, suggesting that

this volatile was lost during drying. It has been reported that the small size of the molecule and

its polarity are factors that cause C6 alcohols to evaporate together with water (De Torres et al.

2010). The concentration of octanal did not change due to processing whereas 2-phenylethanol

presented contradictory results (significantly higher in freeze-dried samples, lower in RW-dried

samples and equal to açaí juice for hot-air dried samples).

Table 7. Concentration of volatile compounds in açaí juice and powders on day 0 (expressed in μg/L)

* Within each row, bars with different letters are significantly different

Comparison among the three dried products on day 0 revealed that, in general, RW and

hot-air dried samples had a lower volatile concentration compared to freeze-dried samples. This

may not mean that RW and hot-air drying resulted in less Maillard reaction and/or lipid

Juice FD RW HADStrecker Aldehydes2-methyl butanal 85 ± 18 b 212 ± 66 a 91 ± 27 b 175 ± 36 ab3-methyl butanal 147 ± 32 b 278 ± 65 ab 234 ± 66 ab 404 ± 94 abenzaldehyde 23 ± 4 bc 133 ± 18 a 14 ± 3.4 c 43 ± 11 bphenylacetaldehyde 235 ± 123 bc 456 ± 77 ab 119 ± 35 c 669 ± 155 a

Lipid Oxidationpentanal 0 b 126 ± 8.3 ab 204 ± 99 a 151 ± 105 abhexanal 233 ± 93 a 200 ± 67 ab 49 ± 5 c 55 ± 12 bcheptanal 3.4 ± 1.4 b 7.3 ± 0.5 a 2.8 ± 0.8 b 9.8 ± 2.2 a(E) 2-hexenal 118 ± 48 a 95 ± 14 a 52 ± 5.5 ab 24 ± 11 b(E,E ) 2,4-heptadienal 0 b 50 ± 7.3 a 4.7 ± 0.4 b 6.4 ± 1 b(E,E ) 2,4-decadienal 0 b 161 ± 29 a 0 b 0 b

Target compoundsoctanal 3.9 ± 1.1 a 5.6 ± 1.4 a 4.5 ± 0.6 a 5.4 ± 2.8 a(Z ) 3-hexen-1-ol 1172 ± 373 a 106 ± 18 b 22 ± 5.8 b 43 ± 15 b2-phenylethanol 276 ± 81 b 613 ± 145 a 41 ± 8.7 c 158 ± 35 bc

66

oxidation. As previously described, both techniques expose the product to oxygen and relatively

high temperatures. In case of hot-air drying, the exposure occurs during a long time. Therefore,

the lower concentration of odorants might have been caused by evaporation / loss of volatiles.

The volatile composition of each dried product was monitored during storage and results

are presented in Table 8. All dried samples showed a significant increase in the concentration of

lipid oxidation-derived compounds after 3 months of storage. These results indicate that lipid

oxidation was, probably, the most important reaction that took place in açaí powders. It is well

known that oxidation rate at low water activity, which is the case of dried açaí samples, is high,

because no enough water is available to reduce free radical concentration (Angelo 1992).

Storage of dried samples also resulted in an increase in the concentration of all Strecker

aldehydes, suggesting the occurrence of Maillard reaction. However, this increase in volatile

concentration was significant only for RW-dried products. RW-dried açaí showed the highest

water activity values. This may help explain the higher extent of Maillard reaction in those

samples. With respect to the target compounds, in general, the concentration of (Z) 3-hexen-1-

ol and 2-phenylethanol did not change during storage. However, the concentration of octanal

was significantly higher after 3 months. Octanal is also a lipid oxidation derivative, which might

explain the increase in its concentration during storage.

Table 8. Concentration of volatile compounds in freeze, RW and hot-air dried açaí on day 0 and after 3 months of storage (expressed in μg/L)

* Within each segment, bars with different letters are significantly different

day 0 3M day 0 3M day 0 3MStrecker Aldehydes2-methyl butanal 212 ± 66 a 247 ± 95 a 91 ± 27 b 221 ± 47 a 175 ± 36a 241 ± 56 a3-methyl butanal 278 ± 65 a 468 ± 206 a 234 ± 66 b 561 ± 103 a 404 ± 94 a 552 ± 108 abenzaldehyde 133 ± 18 a 206 ± 78 a 14 ± 3.4 b 82 ± 25 a 43 ± 11 a 73 ± 15 aphenylacetaldehyde 456 ± 77 a 592 ± 236 a 119 ± 35 b 461 ± 105 a 669 ± 155 a 719 ± 77 a

Lipid Oxidationpentanal 126 ± 8.3 b 466 ± 170 a 204 ± 99 a 335 ± 39 a 151 ± 105 b 510 ± 130 ahexanal 200 ± 67 b 1363 ± 516 a 49 ± 5 b 522 ± 82 a 55 ± 12 b 912 ± 194 aheptanal 7.3 ± 0.5 b 37 ± 11 a 2.8 ± 0.8 b 12 ± 1.4 a 9.8 ± 2.2 b 17 ± 4 a(E) 2-hexenal 95 ± 14 a 108 ± 40 a 52 ± 5.5 b 110 ± 18 a 24 ± 11 b 60 ± 10 a(E,E ) 2,4-heptadienal 50 ± 7.3 b 135 ± 41 a 4.7 ± 0.4 b 83 ± 17 a 6.4 ± 1 b 96 ± 20 a(E,E ) 2,4-decadienal 161 ± 29 b 212 ± 14 a 0 0 0 0

Target compoundsoctanal 5.6 ± 1.4 b 52 ± 21 a 4.5 ± 0.6 b 11 ± 1.4 a 5.4 ± 2.8 b 16 ± 4.2 a(Z ) 3-hexen-1-ol 106 ± 18 a 112 ± 32 a 22 ± 5.8 a 29 ± 4.7 a 43 ± 15 a 51 ± 5.1 a2-phenylethanol 613 ± 145 a 771 ± 285 a 41 ± 8.7 b 76 ± 11 a 158 ± 35 a 145 ± 7.5 a

RW HADFD

67

At the end of storage, the concentrations of the selected flavor compounds in all dried

samples were compared (Table 9). In general, all açaí powders showed similar concentration of

Strecker aldehydes. Freeze-dried samples showed significantly higher concentration of three

lipid oxidation indicators (hexanal, heptanal and (E,E) 2,4-decadienal) and all target

compounds. Additional research is necessary in order to better understand the occurrence of

Maillard and lipid oxidation during storage. Also a consumer test should be performed to find out

consumer preferences. Some of the results reported for flavor could as well be due to matrix

effects.

Table 9. Concentration of volatile compounds in açaí powders after a 3-month storage (expressed in μg/L).

* Within each row, bars with different letters are significantly different

Glass Transition Temperature (Tg)

Knowledge of glass transition temperature is important because this parameter has been

related to food stability. Glass transition of amorphous food systems has been found to cause

structural and textural changes, which increases molecular mobility and decreases product

stability. In general, amorphous foods are more stable in the glassy state, in which mobility and,

therefore, degradative reactions are limited (Slade et al. 1991).

FD RW HADStrecker Aldehydes2-methyl butanal 247 ± 95 a 221 ± 47 a 241 ± 56 a3-methyl butanal 468 ± 206 a 561 ± 103 a 552 ± 108 abenzaldehyde 206 ± 78 a 82 ± 25 b 73 ± 15 bphenylacetaldehyde 592 ± 236 a 461 ± 105 a 719 ± 77 a

Lipid Oxidationpentanal 466 ± 170 a 335 ± 39 a 510 ± 130 ahexanal 1363 ± 516 a 522 ± 82 b 912 ± 194 abheptanal 37 ± 11 a 12 ± 1.4 b 17 ± 4 b(E) 2-hexenal 108 ± 40 a 110 ± 18 a 60 ± 10 a(E,E ) 2,4-heptadienal 135 ± 41 a 83 ± 17 a 96 ± 20 a(E,E ) 2,4-decadienal 212 ± 14 a 0 b 0 b

Target compoundsoctanal 52 ± 21 a 11 ± 1.4 b 16 ± 4.2 b(Z ) 3-hexen-1-ol 112 ± 32 a 29 ± 4.7 b 51 ± 5.1 b2-phenylethanol 771 ± 285 a 76 ± 11 b 145 ± 7.5 b

68

In this project, the glass transition temperature was studied using differencial scanning

calorimetry (DSC). A typical DSC thermogram is presented in Figure 29 for the freeze-dried

sample. Thermograms for RW-dried and hot-air dried samples can be found in Figures 35 and

38 (Appendix B). Similar curves were obtained for all dried samples. All DSC thermograms

showed three major endothermic peaks: the first one between -5 and -3°C, the second between

10 and 12°C and the third between 150 and 200°C. The last endothermic peak was probably

due to sample decomposition. We hypothesized that the first two endothermic peaks were

caused by melting of lipids. There seemed to be glass transition between 50 and 60°C.

Figure 29. DSC thermogram for freeze-dried açaí

In order to confirm our hypothesis and due to the uncertainty of finding the product’s

glass transition temperature, samples were subjected to an oil extraction and then both the oil

and solids fractions were used for a second DSC analysis. Figure 30 shows the thermogram for

the oil of freeze-dried açaí. Thermograms for the oil of RW-dried and hot-air dried açaí are

shown in Figures 36 and 39, respectively (Appendix B). Once again, the curves for the oil of all

dried samples were similar. Interestingly, the thermograms for the oil fractions were

69

characterized by the same first two endothermic peaks found in the dried samples, validating

our hypothesis.

Figure 30. DSC thermogram for freeze-dried açaí oil

Since the lipids present in dried açaí melt into two endothermic peaks at relatively low

temperatures, it is possible to conclude that they are liquid at room temperature. Being liquid

gives them higher mobility to participate in degradative reactions, such as lipid oxidation. This

agrees with the results discussed previously for flavor, which indicated the occurrence of lipid

oxidation during storage of açaí powders. For higher stability against this reaction, addition of

carrier agents, such as maltodextrin, prior to drying would be very important. Those compounds

would form a “more glassy” matrix, encapsulating the oil and protecting it from degradation.

Schauss et al. (2006b) studied the fatty acid composition of freeze-dried açaí and found

that the predominant fatty acid was oleic acid (56.2%), followed by palmitic acid (24.1%) and

linoleic acid (12.5%). The melting points of those individual fatty acids are around 13°C, 63°C

and -5°C, respectively. The same authors reported that ~74% of the total fatty acids present in

freeze-dried açaí is composed by unsaturated fatty acids. This helps explain the low melting

temperature of the endotherms.

70

Fatty acids are, probably, present in açaí powders in the form of triacylglycerol (TAG).

TAG polymorphism may have influenced the shape and position of the melting peaks. In most

thermograms, for both the whole sample (Figures 29, 35 and 38) and the oil (Figures 30, 36 and

39), it is possible to notice that if a line is drawn between the baseline of the first melting peak

and the baseline of the second melting peak, an exothermic peak appears between the two

endothermic peaks. This might be due to TAG conversion to a more stable stable polymorph via

recrystallization (exothermic peak). These more stable polymorphs will later melt together with

other forms to give the second endothermic peak. Different TAG composition, due to different

drying methods (Gutiérrez et al. 2008), and TAG polymorphs may result in slightly different

melting temperatures and bigger or smaller exothermic peaks. It would be interesting to further

investigate the lipid fraction of açaí powders in order to better describe their melting behavior.

In preliminary studies, crimped pans were used in DSC analysis. Comparing the

thermograms obtained when using crimped pans with those obtained when using hermetically

sealed pans revealed the same first two endothermic peaks. However, since crimped pans

permit moisture evaporation, an evaporation peak was found and no decomposition peak was

observed up to 250°C. When using hermetically sealed pans, the decomposition peak was

present between 150 and 200°C. This indicates that the presence of water allows

decomposition to begin at lower temperatures.

Figure 31 shows the thermogram for freeze-dried açaí solids. Thermograms for RW and

hot-air dried solids are presented in Figures 37 and 40 (Appendix B). Interestingly, the

thermograms for the solid fractions showed a very subtle step change in the heat flow between

50 and 60°C, which may indicate the occurrence of a glass transition around that temperature.

This step change can be more easily visualized in the thermogram of RW-dried solids. The

magnitude of change in heat capacity is proportional to the amount of amorphous material

undergoing glass transition. The subtle step change observed in the thermograms might be due

to the small amount of amorphous material in the samples undergoing glass transition. In

addition, due to the complexity of food products, it is usually difficult to determine their Tg. Glass

transition usually occurs over a range of temperatures. Further studies are required in order to

better identify the Tg of the product.

71

Figure 31. DSC thermogram for freeze-dried açaí solids

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Chapter 4: Conclusions

This project evaluated the impact of three different drying methods (freeze drying, RW

drying, and hot-air drying) and subsequent storage on the quality of açaí. To accomplish the

objective, anthocyanin content, antioxidant capacity, moisture content, water activity, moisture

sorption isotherm, glass transition temperature, flavor and color were analyzed.

All three methods produced dried samples with low moisture contents, in the range of

monolayer values, thus having a low water activity and relatively good product stability during

storage. The dried powders showed a sigmoidal shape isotherm, with hot-air dried açaí sorbing

slightly more water than the other samples at all aw values. The GAB and BET models both

produced a good fit to the isotherm data.

DSC analysis showed a very subtle step change in heat flow for the solid fraction of all

dried samples, suggesting the occurrence of a glass transition for the oil-free solid between 50

and 60°C. Thermograms for the oil fractions revealed that the lipids present in açaí powders

are liquid at room temperature, which will increase their mobility, making them more prone to

degradative reations, such as lipid oxidation.

Drying definitely affected the color of açaí. All dried samples exhibited higher lightness

and lower hue angle and chroma compared to açaí juice. Comparison among the treatments

revealed that RW-dried açaí had the highest lightness and chroma values. In general, no

significant difference was observed for hue angle. The color of the RW-dried samples was

probably influenced by the açaí oil, which made comparison with other dried samples difficult.

Color changes during storage were marked by a decrease in lightness and an increase in hue

angle and chroma for all dried samples. The color changes during storage might have been

caused by the formation of brown pigments due to Maillard reaction and/or polymerization of

oxidized phenolic compounds. A consumer test would have to be performed in order to

determine which dried sample has the most representative color of açaí.

Analysis of the flavor of açaí juice identified (E,Z) 2,6-nonadienal, α-ionone and β-ionone

and/or 2-phenylethanol as the most important odorants. A number of thermally generated

compounds and aldehydes derived from lipid oxidation were also identified. These compounds

may either be characteristic of the fruit or might have been formed during thermal

pasteurization, drying, and storage. The effects of drying and storage on the flavor of acai were

also remarkable. The concentrations of Strecker aldehydes and lipid oxidation compounds in

dried samples were higher in dried powders compared to that in açaí juice, indicating the

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occurrence of Maillard reaction and lipid oxidation during drying. In general, RW and hot-air

dried samples showed a lower concentration of Strecker aldehydes and lipid oxidation

compounds compared to freeze-dried açaí. It is possible that loss of volatiles occurred during

RW and hot-air drying, resulting in lower concentrations of those odorants. During storage, all

dried samples showed an increase in lipid oxidation compounds, indicating the occurrence of

that reaction. Significant increase in Mailard reaction products was observed only in RW-dried

samples. At the end of the storage period, all three dried powders showed similar volatile

concentration. It would be important to perform a consumer test in order to verify which dried

sample has the most representative flavor of açaí juice and to find out if consumers can

perceive the changes that occurred during storage.

The anthocyanin concentrations in the freeze-dried açaí were comparable to those in the

açaí juice, suggesting that this drying method preserved most of the pigments. The RW-dried

açaí showed intermediate anthocyanin content whereas hot-air dried samples recorded the

lowest pigment concentration. During storage, anthocyanin content did not change within each

treatment.

For antioxidant capacity, the highest ORAC value (993.12 μmol TE/g DW) was obtained

in the freeze-dried samples, being again comparable to açaí juice. The RW-dried açaí had a

medium ORAC value while the hot-air dried samples presented the lowest antioxidant capacity.

The ranking for antioxidant capacity is in the same sequence as that for the anthocyanin content

(FD>RW>HAD), indicating that freeze drying preserved the majority of the bioactive compounds

present in açaí. All dried samples maintained their antioxidant capacity during storage.

In general, the results presented in this work indicate that the freeze-dried açaí retained

the antioxidant properties of the raw material to a higher degree compared to other dried

powders. Therefore, lyophilisates can probably satisfy the need for a more shelf stable product

with a better retention of bioactive compounds. The RW-dried samples showed superior quality

compared to the hot-air dried samples. Moreover, an economic analysis taking into

consideration of capital investement or operation costs of each drying method is needed when

doing drying method selection for açaí preservation.

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Appendix A: HPLC Chromatograms

Figure 32. HPLC chromatogram for freeze dried açaí on day 0

81

Figure 33. HPLC chromatogram for RW-dried açaí on day 0

1

2

82

Figure 34. HPLC chromatogram for hot-air dried açaí on day 0

1

2

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Appendix B: DSC Thermograms

Figure 35. DSC thermogram for RW-dried açaí

84

Figure 36. DSC thermogram for RW-dried açaí oil

85

Figure 37. DSC thermogram for RW-dried açaí solids

86

Figure 38. DSC thermogram for hot-air dried açaí

87

Figure 39. DSC thermogram for hot-air dried açaí oil

88

Figure 40. DSC thermogram for hot-air dried açaí solids