Journal Pre-proof

Innovative pre-treatmentS to enhance food DRYING: a current review

B Llavata, JV Garcıa-Perez, S Simal, JA Carcel

PII: S2214-7993(19)30130-4

DOI: https://doi.org/10.1016/j.cofs.2019.12.001

Reference: COFS 531

To appear in: Current Opinion in Food Science

Please cite this article as: Llavata B, Garcıa-Perez J, Simal S, Carcel J, Innovativepre-treatmentS to enhance food DRYING: a current review, Current Opinion in Food Science(2019), doi: https://doi.org/10.1016/j.cofs.2019.12.001

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© 2019 Published by Elsevier.

INNOVATIVE PRE-TREATMENTS TO ENHANCE FOOD DRYING: A CURRENT REVIEW

1Llavata, B., 1García-Pérez, J.V., 2Simal, S., 1Cárcel, J.A.

1Analysis and Simulation of Agro-food Processes Group. Food Technology Department, Universitat Politècnica de València, Camino de Vera s/n, E46022, Valencia, Spain

2Department of Chemistry, University of the Balearic Islands, Ctra. Valldemossa, km 7.5, E07122, Palma de Mallorca, Spain

Corresponding author:

Juan A. Cárcel

E-mail:jcarcel@tal.upv.es

Tf: +34 96 387 93 65

Graphical abstract

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Highlights

The influence of pre-treatments on drying kinetic was updated

Ultrasound, pulsed electric field, high pressure and ethanol were reviewed

Pre-treatments significantly reduce energy consumption

Pre-treatments can affect bioactive compounds retention, colour or texture

This influence depends on pre-treatment, process variables and products

Abstract

The application of pre-treatments before drying represents an alternative to better

preserve fresh food properties and reduce the energy needs. The aim of this review

was to analyse the influence of different pre-treatments (ultrasound, pulsed electric

fields, high pressure processing or ethanol) on drying. For this purpose, the effect on

food matrices, drying kinetics and different quality parameters has been addressed

through the review of the most recent studies. The results can differ greatly depending

on the type of pre-treatment and the product considered but, in some cases, an

increase in the drying rate and a better retention of quality can be observed. Even so, it

is necessary to continue studying these pre-treatments to better understand the effect caused.

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Keywords: ultrasound, pulsed electric fields, high pressure, ethanol, kinetics, quality

1. Introduction

Drying is one of the oldest, widely-applied food preservation operations. It consists of

the reduction in the water content slowing down microbial or enzymatic degradation

reactions. The quality of the product obtained depends largely on the methodology

used. Generally, processes that involve low temperatures (such as freeze-drying)

require long drying times, so high temperature drying techniques are more extensive,

although this may compromise the quality. So, the food industry faces the challenge of

achieving shorter drying times under moderate conditions and maximizing product

quality.

Conventionally, the use of different pre-treatments has been one of the most

commonly-followed strategies. In this sense, the blanching of vegetable tissue, one of

the most widely-used pre-treatments, implies enzymatic inactivation, intracellular air

expulsion, a reduction in colour and taste loss, as well as an improvement in the drying

rate [1]. Another conventional pre-treatment is osmotic dehydration. It consists of the

introduction of the food matrix into a hypertonic solution which entails a partial loss of

water, shortening the subsequent drying time, and reducing the gain of solids, which

results in products with better organoleptic or functional properties [2].

In recent years, the introduction of emerging technologies as drying pre-treatments has

come under consideration. The application of ultrasound, pulsed electric fields, high

pressures or ethanol pre-treatments does not involve high temperatures but can

shorten the drying time, improve the final quality of products, and is more

environmentally-friendly because of the lower energy consumption.

The aim of this paper is to review the current findings in some of these alternative pre-

treatments addressing their effectiveness at drying enhancement as well as the

influence on quality parameters, such as the retention of bioactive compounds, the

colour or the texture of the final product.

1. Pre-treatments

1.1 Ultrasound

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Ultrasound (US) consists of mechanical waves with a frequency greater than 20 kHz.

When applied with enough energy, it produces effects, such as cavitation, the

successive compressions and expansions of the treated material or an intense

microstirring at interfaces, which facilitate mass transfer (Figure 1A). However, the

large impedance difference between the generator systems and gas media makes the

propagation of the waves quite difficult [3]. This is the reason why the offer of

commercial transducers for liquid systems, where it is easy to propagate the acoustic

waves, is greater than for gas systems. In this sense, many studies can be found that

deal with the application of ultrasound in hyper or hypotonic solutions as pre-treatments

before drying.

The pre-treatments in hypertonic media (osmotic dehydration, OD) result in water loss

(WL) and solid gain (SG) in the matrix treated. US application during the OD process

can contribute to enhance the mass flows. The stress produced by US generates

microchannels that make the water movement inside the solid matrix easier. Moreover,

the weakening of the cellular structure and the creation of microcracks facilitate the

penetration of soluble solids into the structure. In this sense, US has been observed to

be effective in mass transfer during OD in blueberries [4] and carrots [5]. Generally, the

longer the pre-treatment time [6–8], the higher the frequencies [9,10] and the more

ultrasonic power applied [11], the more significant the effects of US.

In the case of US pre-treatment in hypotonic media, the sample can also undergo a

series of microstructural changes such as the deformation of cell walls that results in

the appearance of microchannels and pores. Some authors have studied the

microstructure of pre-treated materials as a means of better understanding the US

effect [12,13]. Thus, Wang et al. [14], studying the case of carrots treated from 360 to

1080 W (30 min, 20 kHz), or Wu et al. [11] analysing Pakchoi stems treated from 300

to 900 W (30 min, 20 kHz), found that the higher the ultrasonic powers is, the greater

the effect on cell structures will be. The immersion time needed to achieve the desired

effect also depends on the food matrix to be treated. Thus, Wang et al. [15] observed

that the kiwifruit treated at 20 kHz and 400 W needed at least 20 minutes to achieve

pore formation, while Miano et al. [16] found that at least 60 minutes of immersion were

required in the pre-treatment of potato at 25 kHz and 364 W.

All these microstructural changes lead to a clear improvement in the mass transfer

during subsequent drying. In this way, it has been reported that US pre-treatment

significantly shortens the drying time in processes, such as freeze-drying [17], hot air

drying [18], microwave [19] or infrared [12]. Generally, the longer the pre-treatment

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time is, the greater the US effect will be, increasing the effective diffusivity of water

during drying [16,20]. Thus, a drying time reduction of 26% was reported in persimmon

fruit [8] when a 20 minute pre-treatment was applied, but this reduction was 39% when

a 30 minute pre-treatment was considered. The US power applied also has a

significant influence on the drying rate. Thus, in the case of Pakchoi stems [11], a

drying time reduction of 25% was obtained when applying 300 W, but this was of 32%

when applying 600 W or 42% when it was 900 W; or in bananas [21] with a 14%

shorter drying time when using 500 W and 22% shorter when applying 1000 W. The

drying time is correlated with energy consumption as has been observed during the

drying of different products, such as almond [20], walnut [19], or potato [18].

However, other authors did not find a significant influence on the drying rate of the US

pre-treatment. Thus, Corrêa et al. [22] attributed the non-effect of the US pre-treatment

to the non-porous product structure and the high fibre content. In addition, the use of a

highly concentrated sugar solution can make the US transmission difficult as a result of

the energy losses due to viscosity. Mierzwa et al. [5] also attributed the non-effect of

the US pre-treatment to the absorption of solids from the osmotic dehydration that

could block the pores of the food matrix and thereby worsen the water transfer.

On the other hand, the improvement of mass transfer in liquid pre-treatments brought

about by US can lead to an enhanced loss in bioactive compounds. Thus, vitamin C

can be diluted in the solution due to its soluble character and facilitated by the

microchannels formed in the food. This has been observed in strawberry [9], kiwifruit

[15] or Pakchoi stems [11]. Losses in phenolic compounds in the osmotic solution have

also been reported in persimmon fruit [8], strawberries [9] and pomegranate arils [10].

Nevertheless, the drying time reduction produced by US pre-treatment involves less

exposure to the high temperatures during drying. In this sense, a greater vitamin C final

content has been found in quince slices [23] and honeyberry fruits [24] and a greater

phenolic compound content in quince [23], banana [21] and sweet potatoes [25].

Colour plays a crucial role in the final consumer acceptance of products. In this sense,

the shorter time exposure to drying produced by the US pre-treatment can reduce the

colour changes with regards to the fresh product [15,21,26]. Moreover, when the US

propagation is carried out in hypertonic media there is an uptake of solids and a

release of gas from the food, which permit a better retention of pigments, as has been

observed in plums [27] or strawberry [28]. On the contrary, in other materials, such as

carrots [5] or tilapia fillets [29], the uptake of solids during OD affects the colour negatively.

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Texture is another important parameter in terms of product quality. Exposure to US and

the subsequent drying results in changes in the tissues. In this sense, the formation of

cracks and microchannels caused by sonication softens the structure, producing food

with a lower degree of hardness and elasticity, as observed by Wang et al. [14] working

on carrots or Zhao et al. [12] studying shiitake mushrooms. On the contrary, although

the US pre-treatment in hypertonic media can reduce hardness in the early stages of

pre-treatment, there is a subsequent increase due to the progressive uptake of solids,

as was observed in the cases of strawberry [9] and pomegranate [10].

1.2 Pulsed electric field

Pulsed electric field (PEF) technology consists of the application of very short, high

voltage, electric pulses to the food matrix. Its main effect is electroporation, which is the

formation of pores (permanent or reversible) in the cell membrane (Figure 1B),

facilitating the mass transport through it. This technology has been studied for different

applications, such as the intensification of compound extraction [30], non-thermal

pasteurization [31] or the improvement of drying. The cell disintegration index (Z-index)

has been widely used to quantify the level of electroporation induced by PEF and

depends on variables, such as matrix structure [32], treatment intensity [33,34] or

number of pulses [35].

PEF has been used as a pre-treatment in different drying processes, such as osmotic

dehydration, vacuum drying, convective drying or freeze-drying. Thus, Yu et al. [36]

observed that in the osmotic dehydration of blueberries, the PEF pre-treatment (3

kV/cm) can significantly accelerate the WL and SG rates. Some studies [37,38] point to

a more pronounced effect on water diffusivity than solute, probably due to membrane

selectivity as well as the greater molecular weight of solids. The PEF pre-treatment

also increases the kinetics of vacuum drying. The level of sample damage can depend

on both the product and the conditions. Thus, when working on blueberries, Yu et al.

[39] observed that the PEF can increase the proportion of damaged samples due to the

pressure gradient generated. However, when drying basil leaves, Telfser and Gómez-

Galindo [35] found no increase in the amount of damage done. As regards convective

air drying, the PEF pre-treatment of samples can increase the drying velocity

[34,40,41]; the greatest effects are found when drying is carried out at moderate

temperatures. For example, 45 °C for onion (1.07 kV/cm) [33] and blueberries (3

kV/cm) [39], 50 °C for potatoes (0.6 kV/cm) [42] or 60 °C for carrots (0.9 kV/cm) [32]. In

the case of other products, such as parsnips (0.9 kV/cm) [32], PEF effects can be more

marked when drying at higher temperatures (70 °C). These apparently contradictory

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effects can be explained by the different interaction of PEF with the initial structure

(porosity, cellular shape and size…) and composition of samples. The

electropermeabilization can also significantly enhance freeze-drying processes, as

reported by Lammerskitten et al. [43] when studying apple. However, the effectiveness

does not reach the levels achieved in other drying processes. Thus, when studying the

drying of basil, Telfser and Galindo reported drying time reductions of 57%, 33% and

25 % for convective, vacuum and freeze-drying respectively [35].

As concerns the effect of the PEF pre-treatment on the retention of the bioactive

compound, no clear trend has been found. In some cases, it has been reported that the

use of PEF has not brought about any significant losses in antioxidant capacity [36,39].

Moreover, PEF treatments combined with sulphites [40] or multicomponent solutions

[37] can allow the uptake of components which improve the antioxidant activity of

products. On the contrary, other studies reported that the electropermeabilization

produced a greater migration of water and other compounds, facilitating the

degradation activity of enzymes, which results in a decrease in antioxidant capacity

[36,39]. In this sense, Lammerskitten et al. [44] observed both an antioxidant

degradation effect and the formation of new phenolic compounds during the drying of

apple samples pre-treated with PEF. On the other hand, the pre-treatment can affect

the antioxidant compounds, such as carotenoids, in different ways. Thus, Huang et al.

[40] observed that the PEF pre-treatment led to an increase in the β-carotene in dried

apricots (45 °C) due to a better extraction as a consequence of electropermeabilization.

This was not observed by Fratianni et al. [45] when studying dried carrots (50 to 70 °C),

which can be attributed to the fact that PEF made these compounds more sensitive to

high temperatures.

Samples pre-treated with PEF generally preserve their colour better due to the release

of intercellular content that affects the activity of some enzymes [32,37]. However, this

depends on the food structure and process conditions. In the case of basil [35] or

apricot [40], for instance, convective drying after PEF pre-treatment can lead to a

decrease in luminosity compared to untreated samples. Nevertheless, when freeze-

dying is applied, the luminosity can increase, this is probably linked to an increase in

product porosity [44].

As for the texture, temperature can induce a greater effect on texture than PEF in the

case of convective drying. In this sense, when working on carrots and parsnips, Lyng et

al. [32] found a higher shear stress in samples dried at high temperatures (70 °C) than

at moderate (50/60 °C), while no differences were found between those samples that

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had been PEF pre-treated (0.9 kV/cm) and those non-pre-treated at the same

temperature. On the contrary, Tylewicz et al. reported that the PEF pre-treatment

produced a dramatic reduction in the hardness of osmotically dehydrated strawberries,

this decrease being proportional to the applied electric field (0.1 – 0.2 – 0.4 kV/cm).

This was attributed to cellular rupture and the creation of pores that led to a softening

of the samples, although in the later stages of the process, a slight increase in

hardness could be evidenced as a result of the uptake of solids. This softening has

also been reported when studying freeze-dried apples (1.07 kV/cm) [44]. In any case, it

must be mentioned the extremely difficult to compare results due not only to the

characteristics of the PEF applied, but also to the subsequent drying process and to

highly variable nature of the products.

Recently, some authors have studied the combination of PEF and US. Using both

technologies for the pre-treatment has obtained a significant shortening in the drying

time of foods, such as carrots [46] or cranberries [47]. The order of application of these

technologies could be an important factor. Thus, some authors have found that the

application of PEF followed by US significantly accelerated the drying process when

compared to the samples treated with US followed by PEF [48]. In addition, the

PEF+US pre-treated samples retained not only the original colour better, but also

polyphenols, anthocyanins or flavonoids. There are, however, still very few studies in

this field.

1.3 High Pressure Processing

High-Pressure Processing (HPP) consists of maintaining food under hydrostatic

pressures (100-800 MPa) during a certain period of time (Figure 1C). The liquids may

be treated directly, but solids have to be previously packaged. This technique has been

widely studied for enzymatic inactivation [49], non-thermal sterilization [50], high

pressure assisted thermal processes [51] or bioactive compound extraction [52]. Its

application as a pre-treatment in drying processes has also been reported.

It is known that HPP causes the modification of the cellular structure, reducing the

turgor pressure and then affecting the permeability and mass transfer. In this sense,

Belmiro et al. [53] found a higher drying rate in beans pre-treated with HPP, especially

at high pressures (600 MPa) and for longer times (10 min). A higher drying rate due to

better effective water diffusivity was also found by George et al. [54], who reported a

significant effect on ginger treated at 400 MPa for 10 min, similarly to Swami Hulle and

Rao [55] when working on Aloe vera samples. These authors found there was a

greater effect of the pressure at moderate temperatures (50/60 °C), than at higher ones

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(70 °C). This has been demonstrated by means of the microstructural analysis of HPP

samples, showing a more irregular structure in ginger [54], with the evident separation

of cells and thinner walls, and without the presence of organelles in Aloe vera samples

[55]. Some of these changes have already been reported by other authors when

studying carrots, sweet potatoes and cocoyams [56].

Changes in the cellular membrane produced by HPP, led to an improvement in the

phenolic compound content, due to their better extraction. Thus, George et al. [54]

found an increase in the 6-gingerol content of samples pre-treated with 400 MPa for 10

minutes. Previous studies of Aloe vera already obtained similar results, observing that

the antioxidant capacity remained stable or improved during the HPP pre-treatment

[55].

The influence of HPP pre-treatments on the colour of dried products is quite limited

[55], as has been observed in studies carried out, for example, on potato [57]. On the

other hand, some of these authors reported a significant decrease in hardness and

compressive strength, explained by changes produced in the cell wall [53,54].

However, other authors, such as Swami Hulle and Rao [55], found an increase in

hardness due to the possible inactivation of pectimethylesterase in treatments of Aloe

vera done at pressures above 500 MPa.

1.4 Ethanol

The application of ethanol as a pre-treatment is a simple but efficient technique that

has also been studied for the purposes of drying intensification. The effect of an

ethanol pre-treatment is based on its ability to dissolve components of the cell wall,

causing changes in the structure and increasing its permeability [58]. Moreover, the

Marangoni effect (Figure 1D), which is based on the formation of a surface tension

gradient between the ethanol and the food’s water content, can significantly enhance

the water transport [59].

Microstructural analyses have shown that the ethanol pre-treated samples have more

compact, thin-walled cells as well as an intracellular air loss [60,61]. As these structural

modifications in the food matrix permitted a greater extraction of water, significant

reductions in the drying time were reported for scallions (25%) [58], potatoes (10%)

[59] and pumpkins (49.5%) [61]. Similar results had already been obtained by means of

the injection of ethanol into balls of mixed rice and soybean protein [62], and in

modified atmospheres during the drying of bananas [63]. The increase in the drying

rate in samples pre-treated with ethanol can be explained by the interaction of ethanol

with water, resulting in a mixture with higher vapour pressure. Otherwise, it is worth

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noting that the combination of ethanol with other techniques, such as vacuum drying

[58], US [60,64] or perforation [59], leads to their intensification.

On the other hand, a greater retention of bioactive compounds in dried samples that

have been pre-treated with ethanol has been reported. Thus, the absorption of ethanol

could provide greater protection against the oxidation of different components, such as

vitamin C [58]. In addition, the shorter drying time means less exposure to high

temperatures and, therefore, greater bioactive retention [65]. A higher volatile

compound content for pineapples treated with ethanol has also been postulated, due to

the condensation of ethanol on the sample surface [66,67].

Minor changes in colour differences have been observed when this pre-treatment is

applied due to a possible inhibition of enzymatic activity, avoiding browning reactions.

This has been found in scallion samples [58] and garlic [64]. With regard to the

mechanical properties of the dehydrated samples, not too much information has been

reported, so it would be interesting to carry out further research in this field.

2. Conclusion

This paper reviews the effectiveness of the application of different drying pre-

treatments. The reduction in the drying time achieved by pre-treating the samples can

result in a better preservation of food quality, as well as significant energy savings.

However, the extent of the effects depends on the pre-treatment, the food matrix

considered and the objective followed. Thus, it has also been shown that each pre-

treatment has some limitations and can sometimes lead to a loss in the quality of

treated foods. In addition, further studies are needed to evaluate and quantify the effect

of pre-treatments, especially in the case of HPP and ethanol. Very interesting results

have been obtained through both the combination of these two pre-treatments or via

the combination of either of them with other techniques; further research into them,

therefore, could be of special interest.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

The authors would like to acknowledge the financial support of the National Institute of

Research and Agro-Food Technology (INIA), co-financed with ERDF funds (RTA2015-

00060-C04-02 and RTA2015-00060-C04-03).

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Figure caption

Figure 1. Main effects induced by the different pre-treatments

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