University of Ghent Faculty of Bioscience Engineering Lodz … · 2016-07-28 · in my life....

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University of Ghent

Faculty of Bioscience Engineering

Lodz University of Technology

Faculty of Biotechnology and Food Sciences

Academic year 2015 – 2016

Fabrication and characterization of arrested

non- aqueous foam

Kinga Karp

Promotor: Prof. dr. Ashok Patel

Promotor: Dr. ir. Anna Podsędek (Lodz University of Technology)

Tutor: MSc. Mohd Dona Bin Sintang

Master’s dissertation submitted in partial fulfillment of the requirements for the degree of

Master of Science in Biotechnology (Food Biotechnology) at Lodz University of Technology

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ACKNOWLEDGEMENTS

After few months of intense learning for me, not only in scientific area, but also on personal

level, it is time when I would like to thank all those people who have supported me throughout

this period.

First of all, I would like to express my deepest gratitude to my promoter - Prof. Ashok Patel

for the possibility to join his group and for proving me valuable and constructive suggestion

during development of this thesis. It was a great pleasure for me to be a part of his research

team and worked in Department of Food Safety and Food Quality.

My special thanks go to Dona Sintang for his supervision, knowledge, suggestions and

patience.

I am also grateful to Associate Dean from Lodz University of Technology –

doc. Dr. ir. Stanisław Brzeziński for his indulgence, help in solving my problems and

confidence.

I thank my fellow labmates: Zulema Perez Valdivielso, Jorgen Goemaere and Kobe for

motivation and enjoyable working atmosphere.

Last but not at least, I would like extend my heartfelt gratitude to the most important persons

in my life. Przemek, I dedicate you this dissertation. Words cannot express how grateful I am

for you support whenever I needed it. Thank you for your devotion and willingness to help me

as best you can. Dear parents, without your love and support, none of this would have been

possible. Marlena, I also would like to thank you for your encouraging me in throughout this

endeavor. Dziękuję Wam za wszystko, Kocham Was.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS ....................................................................................................................... I

TABLE OF CONTENTS.......................................................................................................................... II

LIST OF ABBREVIATIONS ..................................................................................................................... V

ABSTRACT ......................................................................................................................................... VI

POLISH ABSTRACT (STRESZCZENIE) ................................................................................................... VII

INTRODUCTION .......................................................................................................................... VIII

1. LITERATURE REVIEW ........................................................................................................... 1

2.1. Foams ................................................................................................................................. 1

2.1.1. Mechanism of foam decay ................................................................................................ 2

2.1.1.1. Drainage ............................................................................................................. 2

2.1.1.2. Coarsening .......................................................................................................... 3

2.1.1.3. Coalescence ........................................................................................................ 4

2.2. Non-aqueous foam .............................................................................................................. 4

2.2.1. Stabilization mechanism of non-aqueous foam ............................................................ 4

2.2.2. Stabilizing agents ........................................................................................................ 5

3. MATERIALS AND METHODS ................................................................................................ 7

3.1. Materials ............................................................................................................................ 7

3.2. Preparation of the foam ....................................................................................................... 7

3.2.1. Rapeseed oil + sucrose ester emulsifier ....................................................................... 7

3.2.2. Rapeseed oil + combination of sucrose ester and sunflower lecithin............................. 8

3.3. Preparation of the oleogel ................................................................................................... 8

3.4. Characterization of resulting foams and oleogels ................................................................. 8

3.4.1. Microstructure of foams .............................................................................................. 8

3.4.1.1. Optical microscopy ............................................................................................. 8

3.4.1.2. Scanning electron microscopy (SEM) .................................................................. 8

3.4.2. Calculation of the air bubble in the foam stabilized by SE and combination of SE and

SFLE .......................................................................................................................... 9

3.4.3. Rheological measurements .......................................................................................... 9

3.4.3.1. Oscillatory measurement ..................................................................................... 9

3.4.3.2. Flow measurement .............................................................................................. 9

3.4.4. Termal analysis ..........................................................................................................10

3.4.4.1. Differential Scanning Calorimetry ......................................................................10

3.4.4.2. Hot stage microscopy .........................................................................................10

III

3.4.5. Air fraction analysis ...................................................................................................10

4. RESULTS .................................................................................................................................13

4.1. Macroscopy observation of the resulting foam ...................................................................14

4.1.1. The overrun of foams ................................................................................................14

4.1.2. Foam stabilized by sucrose ester ................................................................................14

4.1.3. Foam stabilized by SFLE and combination with SE....................................................15

4.2. Influence of temperature on crystallization of oleogel and melting of foam and oleogels .......16

4.2.1. Foam and oleogel stabilized by SE .............................................................................17

4.2.2. Foam and oleogel stabilized by combination of SE and SFLE ....................................19

4.3. Properties of foam and oleogels .........................................................................................21

4.3.1. Oscillatory measurements of foam and oleogel stabilized by SE .................................21

4.3.2. Oscillatory measurements of foam and oleogel stabilized by combination of SE and

SFLE .........................................................................................................................24

4.3.3. Flow measurements of foam and oleogels stablilized by SE........................................26

4.3.3.1. Flow viscosity ....................................................................................................26

4.3.3.2. Shear ramp .........................................................................................................28

4.3.3.3. Thixotropy .........................................................................................................29

4.3.4. Flow measurements of foam and oleogels stablilized by combination of SE and SFL .30

4.3.4.1. Flow viscosity ....................................................................................................30

4.3.4.2. Shear ramp .........................................................................................................32

4.3.4.3. Thixotropy .........................................................................................................33

4.4. Microscopy observation of resulting foam..........................................................................34

4.4.1. Foam stabilized by SE ................................................................................................34

4.4.2. Foam stabilized by combination of SE and SFLE .......................................................35

4.4.3. Air bubble appearance after amplitude stress measurement .........................................36

4.4.3.1. Foam stabilized by SE ........................................................................................36

4.4.3.2. Foam stabilized by combination of SE and SFLE ...............................................37

4.5. Influence of temperature on structure of air bubbles ...........................................................38

4.5.1. 5% foam ....................................................................................................................39

4.5.2. 7,5% foam .................................................................................................................40

4.5.3. 10% foam ..................................................................................................................41

4.5.4. Combination SE-SFLE (9:1) ......................................................................................42



IV


4.5.8. 10% SFLE .................................................................................................................46

4.6. Size of air bubbles .............................................................................................................47

4.6.1. Foam stabilized by SE ................................................................................................47

4.6.2. Foam stabilized by combination of SE and SFLE .......................................................48

4.7. Air fraction analazys ..........................................................................................................49

5. DISCUSSION ...........................................................................................................................52

5.1. Fabrication of foam ...........................................................................................................52

5.2. Foam stability as a function of temperature ........................................................................52

6. CONCLUSIONS .......................................................................................................................54

7. REFERENCES .........................................................................................................................55

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LIST OF ABBREVIATIONS

Cryo- SEM cryo-scanning electron microscopy

DSC differential scanning calorimetry

FS frequency sweep

G’, G’’, G* elastic, viscous and complex modulus, respectively

HBL hydrophilic lipophilic balance

HSM hot-stage microscopy

LVR linear viscoelastic region

PBs Plateau borders

PNMR pulsed nuclear magnetic resonance

RPO rapeseed oil

SE sucrose ester

SFLE sunflower lecithin

SR stress ramp

VI

ABSTRACT

Most food products are complex colloidal systems which comprise of multiple phases and

interfaces that are created by close contact of dispersed and continuous phases. Foams are one

such systems where dispersed air bubbles are incorporated and stabilized in a continuous

phase. Recently, there has been an increased interest in edible foams because of their

enormous potential in reformulating food products with reduced calories. In addition, due to

their unique texture and mouth feel, foams have also gained popularity in the discipline of

gastronomy. Unfortunately, the research in the area of food foams have not received the same

level of attention as edible emulsions or gels. Moreover, most work done in this field till date,

is mainly restricted to aqueous foams with focus on the stabilization of air-water interfaces.

The aim of this work is to fundamentally investigate the stabilization of air-oil interfaces in

order to create non-aqueous foams using edible vegetable oil and food-approved emulsifiers.

The challenge of dispersing a large volume of air bubbles in a hydrophobic continuous phase

(liquid oil) is explored to create foam oleogels. In order to make them suitable for various

applications in edible products, we optimized the responsiveness of these systems to

temperature. The air bubbles help to improve the mechanical properties of foams compare to

its oleogels, and prove with the rheological measurements. Furthermore, we also investigated

the parameters such as temperature of crystallization and melting, to correlate between the

crystallization and fabrication process.

VII

POLISH ABSTRACT (STRESZCZENIE)

Większość produktów spożywczych to złożone kompleksy koloidalne, zawierające wiele faz

i połączeń, tworzonych przez bliski kontakt fazy ciągłej i rozproszonej. Piany są jednym

z takich systemów, gdzie rozproszone pęcherzyki powietrza są wprowadzane do fazy ciągłej

i w niej stabilizowane. Na przestrzeni ostatnich lat znacznie wzrosło zainteresowanie

jadalnymi pianami, ze względu na ich niezwykły potencjał w zamianie tradycyjnych

produktów spożywczych na te o obniżonej liczbie kalorii. Co więcej, ze względu na ich

unikalną strukturę i odczucie w jamie ustnej, piany zyskały również popularność w

gastronomii molekularnej. Niestety, jak dotąd, badania w dziedzinie pian spożywczych nie

osiągnęły podobnego poziomu zainteresowanie, jak jadalne emulsje czy żele. Ponadto,

większość badań wykonanych w tym zakresie ogranicza się do wodnych pian, z naciskiem na

stabilizacje połączeń wody i powietrza.

Celem tej pracy jest natomiast fundamentalne zbadanie stabilizacji połączeń powietrza i oleju.

Niewodne piany zostały wytworzone przy użyciu jadalnego oleju roślinnego i emulgatorów

spożywczych. W celu stworzenia oleożeli w formie piany, wyzwanie rozproszenia dużej ilość

powietrza w hydrofobowej fazie ciągłej, jaką jest olej, zostało podjęte. Ponadto, mając na

uwadze szerokie zastosowanie wytworzonej piany w produktach spożywczych, reakcje tych

systemów na działanie temperatury zostały określone. Podczas pomiarów reologicznych

zaobserwowano, że pęcherzyki powietrza przyczyniają się do polepszenia właściwości

mechanicznych piany. Co więcej, parametry takie jak temperatura krystalizacji czy topnienia

zostały zbadane, aby skorelować ze sobą proces krystalizacji i wytwarzania piany.

VIII

INTRODUCTION

Over the past decades, the area of foam science has emerged as a rapidly growing research

domain because of the potential of foams for applications in a wide range of fields. However,

most of the work done in this area is related to aqueous foam systems. Stability of oil foams

have surprisingly received a very little attention. One of the main reasons for this is the

significant challenge encountered in stabilizing air-oil interfaces as both air and oil are

hydrophobic in nature. Some limited studies in the past have explored the possibility of

stabilizing such non-aqueous systems using solid particles (surfactant crystals, colloidal

inorganic particles etc.). Oil foams are attractive colloidal systems which could have potential

applications in developing novel food products with reduced calories and unique textures (for

molecular gastronomy).

The main aim of this research is to investigate the use of specialty surfactants and

combination of surfactants in stabilization of non-aqueous foams of edible oil. The aim will

be achieved using these specific objectives

fabrication of non-aqueous foams stabilized by food-grade surfactants such as sucrose

esters and lecithin (and their combinations)

characterize the properties of resulting foams using advanced microscopy, rheological

measurements, DSC and diffusive NMR.

This master dissertation consists of 7 chapters. Chapter 1 presents Introduction of thesis and

mentions the aspects on which the work is focused. Chapter 2, Literature overview, in

general familiarizes with both water and oil based foams, their applications, mechanism of

destructions and also stabilizing agents. The subsequent chapter 3 provides Materials and

Methods used in this work. In chapter 4, Results, composed of 7 subsections, there are

presented the final results of all conducted experiments. The first subsections 4.1 is focused

on external appearance of resulting foams. Subsection 4.2 presents the crystallization and

melting peaks for oleogels and melting results for foams. The following subsection 4.3

investigates the properties of foam and oleogels using oscillatory and flow tests. The last four

subsections are dedicated to internal appearance of foams. The following chapter 5,

Discussion, comprehensively discuss obtained results. Chapter 6, Conclusions and future

perspectives provides summary of this work’s results and also concentrates the attention for

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future developments of non-aqueous foam science as a promising field in food technology. In

the last chapter 7, there are collected all References, on which the literature review is based.

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1. LITERATURE REVIEW

2.1. Foams

Foams are non-equilibrium, two-phase media, in which there are dispersed gas bubbles of

many sizes in the liquid- or solid-continuous phase. The volume of gas-phase fraction

provides the unique properties of foams. Generally two mechanical behavior of foams can be

identified: solid-like and liquid-like. They are depending on packing fraction as well as

applied stress. Furthermore, foams demonstrate a couple of physical properties such as low

density, high specific surface area, low interphase slip velocity, large expansion ratio and a

finite yield stress. This features determine the tendency to evolve over time and gives rise to

the multifarious applications of foams ranging from food manufacturing, pharmaceutical

products, well known detergents and also industrial process like oil recovery [51, 54, 55].

Moreover, the foam applications are constantly on the increase. From this reason, customers

expect the highest quality and texture of foam products they use. Thus an understanding of

foams and their unique features is of high importance to improve innovative, everyday

products and to have a control over the industrial processes [8, 51].

So far, the most common and well understood types of foam have been aqueous-based media

containing the water as a continuous phase [13, 20, 29, 20, 37, 38, 43, 51, 54, 55]. Their

essential component responsible for propensity of a liquid to foam is stabilizing agents. These

ingredients also bear a responsibility for prolonging stability of the resulting foam. Stabilizing

agents which can be surfactants of low molecular weight, proteins, amphiphilic polymers and

nanoparticles, reside at the gas-liquid interfaces [11, 28, 53]. In general, classic foams which

are stabilized by surfactant molecules (Fig.1.) reveal short existence from minutes to hours

such as champagne or beer foam. Afterwards they give in kinetic evolution [34, 37].

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Fig. 1. Aqueous foam with polyhedral bubbles stabilized by surfactants. Liquid film between

bubbles (top right) is covered by surfactant monolayers and Plateau border junction (bottom

right). Surfactant molecules are represented by a circle (polar head), in contact with water,

and a hydrophobic chain, in contact with air. The surfactant is also solubilized in water and

present in the bulk liquid. Figure is reproduced form [39].

2.1.1. Mechanism of foam decay

Foams are thermodynamically metastable systems and they are liable to irreversible decay

due to three main mechanisms of foam destruction which are strongly linked to each other. It

is liquid (gravitational) drainage between adjacent bubbles, Ostwald ripening (coarsening) and

coalescence of neighboring bubbles (film rupture) [14, 55]. This mechanisms usually act

simultaneously and enhance mutually. However, recent studies prove that is it possible to

oppose the sign of foam decay and obtain the foam which is stable even several months [2, 7,

19, 52].

2.1.1.1. Drainage

Foam drainage is a process that depends on gravity and capillary action. It describes the flow

of liquid, which initially is spread between the bubbles. The liquid flows through arbitrary

directed interstitial channels which consist of thin films, Plateau borders (PBs) and nodes

(Fig. 1.). PBs are made through joining of three films while the nodes are the result of four

PBs merger. The phenomenon of liquid drainage is that the gas bubbles, with sizes greater

than a few microns rise rapidly to the surface under the force of gravity while the liquid is

accumulated at the bottom (Fig.2.). This process continues till the films separating bubbles are

so thinned that other mechanisms of foam decay come into play [22, 25, 42, 55].

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Fig. 2. Draining foam. The top of the foam is dry and consist of polyhedral bubbles. The

bottom of the foam, which is connected to the liquid, is wet and composed of spherical

bubbles. Figure is reproduced form [4].

2.1.1.2. Coarsening

Coarsening is a well understood process and often is referred to as Ostwald ripening. The

fundamental mechanism of this phenomenon relates to different Laplace pressures of the gas

in the adjacent foam bubbles which are separated by thin films. The molecules of gas are

transferred from smaller bubbles with a greater internal pressure to larger ones characterized

by smaller Laplace pressure (Fig.3.). This diffusion leads to an increase in the average bubble

size after some time, and in effect, to bubbles rupture [11, 37, 55].

Fig. 3. Coarsening of two-dimensional foam. Figure is reproduced form [1].

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2.1.1.3. Coalescence

Film rupture is perceived as a continuation of gravitational drainage. When the bubbles come

close enough together, the separating film between them become extremely thin which can

lead to bubble rupture and coalescence (Fig.4.) [36]. The main mechanism of this process has

not been clearly understood, so far. Foam coalescence can occur while the air bubbles reaches

the critical size [20] otherwise when the critical value of liquid medium is reached [12] or else

when the critical value of capillary pressure is reached [26].

Fig. 4. Coalescence of foam after 10 hours. Figure is reproduced form [1].

2.2. Non-aqueous foam

Non-aqueous foam are widespread and occur in many industries such as cosmetics (e.g. make

up removers and shaving cream) and food technology. In foods, the purpose of their

applications is creating low calories food products and also creating unique texture for

gastronomy. The characteristic chew, mouth-feel and lower fat content, for instance in

bubble-containing chocolate, is of crucial importance for customers [21]. However, there are

also cases when foaming can be unwanted and detrimental. This is concerned with petroleum

as well as crude-oil gas recovery and requires the use of appropriate antifoams [8, 10, 35].

That is why the comprehension of non-aqueous foam’s features is essential to control

industrial processes and also to improve innovative goods. Nevertheless they have not been

investigate so often contrary to liquid-base foams and there have been only few papers

devoted to oil-continuous foams, thus far.

2.2.1. Stabilization mechanism of non-aqueous foam

There is no difficulty in finding literature which is focused on aqueous foam stabilization

mechanism [23, 29, 32, 33, 44, 51, 55]. It can be useful in understanding oil-based foams

stability mechanism, because this one is basically the same as in case of water-based foams.

5

Nevertheless it is still not easy to obtain stable non-aqueous foams. The significant difference

between the systems containing various continuous phases is their surface tension. In case of

hydrocarbon-based fluid foams the tension at the liquid – gas interface is in range of 20-25

mN·m-1

and is significantly lower with regard to the tension of water, which is 72 mN·m−1

at

25 °C. From this reason, the tendency of oil-soluble surfactant to adsorb at the oil-air interface

is disadvantageous from an energy point of view and makes the stabilizing foam process

troublsome [9, 16].

Moreover, due to the low dielectric constants of oil systems, the electrostatic double layer

repulsion is also minimal in contrast to aqueous foams. Consequently, the dissociation of ions

is limited and significant electrostatic stabilization is prevented.

Although the manufacture of oil - continuous foams is quite challenging, there are defined

different strategies to stabilize the films and to resist bubble coalescence in air-oil mixtures.

This effect is possible to achieve by using the appropriate surfactants, colloidal particles and

also multi-phase condensed media.

2.2.2. Stabilizing agents

Notwithstanding the fact that the surface tension of oil-air mixture is relatively low, some of

surface-active molecules are able to adsorb to the surface and cause modification of surface

rheology. Early work by Sanders [41] was devoted to understand the relationship between

foam stability and surfactant solubility as a main factor of stabilization oil systems. The

example of two types of non-aqueous foam systems – glycol and mineral oil stabilized by

ethoxylated fatty alcohol and polyethylene glycol proves that oil-soluble surfactants are not

capable of fabricate stable foam contrary to solid stabilizers with appropriate wettability

properties. Rose and co-workers [40] investigated the lubricant oil and also indicated that the

foaming capacity depends on solubility parameters of surfactant. The conditions, in which the

surfactant remains insoluble and hence is characterized by good surface activity, have a

positive effect on both foamability and stability. Based on p-xylene/ triethanolammonium

oleate system, Friberg et al. [18] demonstrated that it is possible to obtain foam with high

stability only if it contains lamellar liquid crystalline phase. In case when the system was

isotropic liquid, no foam was achieved successfully. Moreover, later Friberg’s study confirms

the assumption of hydrocarbon foam stability dependence on liquid crystal amount [17].

There are also more recent work confirming conclusions drawn by Rose. Binks and co-

workers [6] in their research with mixtures of hydrocarbon oil solvent with a range of low

6

molar mass and polymeric surfactants showed that foam formation increase close to solubility

phase boundary. The low solvent affinity to solute enhance the adsorption at the oil-air

surface and also the higher concentration of surfactants is correlated with increasing viscosity

which better affects foaming properties. Shrestha et al. [27, 45-50] introduce exhaustive

papers devoted long-term stable foams stabilized by monoglycerol and diglycerol fatty acid

esters. The studies have shown that the mentioned stabilizing agents form the crystals in

liquid which coated air bubbles and change the foams rheological properties by arresting

diffusion. What is more, the stability of foam depends on hydrocarbon chain length [27,

45].The great importance is also the high melting point of monoglycerides which allows

forming crystalline particles at 25°C. In the case of monoglycerides, the self life at room

temperature was observed form minutes (for 11 carbons) to hours (for 13 carbons) and was

associated with the shape, concentration and stabilizer size, as well [45, 48-50]. They have

shown that the smaller surfactants produce foam with higher stability. Foam fabricated from

olive oil, stablilized by diglycerolmonipalmipate exhibited outstanding stability, even longer

than one month at room temperature when contained 10 wt % of surfactant [50]. Also

Kunieda and co-workers [27] using organic solvents such as squalane, squalene or liquid

paraffin successfully elaborated super-stable nonaqueous foams in diglycerol fatty acid esters.

Inspired by Shrestha et al. [45-50], recently Brun with co-workers [9] obtained a stable foam

in a vegetable oil (rapeseed oil) using edible surfactant with a long chain. They proved that

the air bubbles are stabilized by dense layer of surfactant crystals. Another strategy was

demonstrated by Bergeron et al. [3]. They used two different types of fluorocarbons which are

able to reduce dodecane-air surface tension until 5 mN·m−1

. In this case the foam was

stabilized by overlapping of surfactant coatings on adjoining bubbles.

7

3. MATERIALS AND METHODS

3.1. Materials

The rapeseed oil (RPO) and sunflower lecithin (SFLE) were supplied by the Vandemoortele

Lipids N.V., Belgium. The sucrose ester with hydrophilic lipophilic balance (HBL) value of 2

was purchased from SISTERNA, Netherlands.

3.2. Preparation of the foam

3.2.1. Rapeseed oil + sucrose ester emulsifier

The foam was obtained by the following procedure: accurately weighed, particular quantity of

rapeseed oil and sucrose ester, depending of the final solution concentration (5%, 7,5%, 10%)

were heated in glass vessels on heater plate at 90°C for about 10 minutes. After achieving the

same temperatures of RPO and emulsifier (to avoid formation of lumps), the oil was added to

the beaker with sucrose and stirred with magnetic stirrer 300 rpm. In next step the mixture

was poured to the plastic tube to the volume 15 ml and mixed using kitchen mixer for 2

minutes at constant rotation rate at room temperature (Fig.5.). In this step, the air bubbles are

being introduced into the oil medium. Immediately , the tubes with foam were cooled in a

freezer approximately 5 min at -18°C and then transferred to a fridge at 4°C. After 1h the

overrun of samples, as a function of SE concentration, were determined by the following

formula [36]:

(1) overrun [%] =

x 100%

where VI is a initial volume of oil phase in the tube and VII is a final volume of foam after

mixing.

Fig. 5. Aeration of rapeseed oil in which the sucrose ester was dispread.

Dispersion of SE

in rapeseed oil

(120°C, 5 min)

Aeration (high

temperature)

8

3.2.2. Rapeseed oil + combination of sucrose ester and sunflower lecithin

In order to prepare 10% foam stabilized by mixture of SE and SFLE, 18g of RPO and 2g in

different combinations of SE and SFLE were heated in glass beaker on heating plate at 90°C

for about 10 minutes. After achieving the same temperatures of oil and emulsifier (to avoid

formation of lumps), the RPO was added to the beaker with mixture and stirred with magnetic

stirrer 300 rpm. The next step was conducted as described in 3.2.1. and shown at Fig.5. After

1h the overrun of samples, as a function of SE:SFLE concentration, were determined by the

formula (1).

3.3. Preparation of the oleogel

The oleogels were obtained by mixing an appropriate amount of oil and sucrose ester or

combination of sucrose ester and sunflower lecithin on a heating plate at 90°C (preheated to

the same temperature) under mild agitation (300 rpm) using magnetic stirrer. Cooling down to

room temperature resulted in the formation of oleogels, which were stored in a fridge at 4°C.

3.4. Characterization of resulting foams and oleogels

3.4.1. Microstructure of foams

3.4.1.1.Optical microscopy

In order to obtain information about the internal structure of foam and to visualize the size of

air bubbles optical microscope with digital camera Leica DM2500 (Leica Microsystems,

Belgium) was used. A droplet of previously prepared foam (3.2.1.; 3.2.2.) was carefully

placed on microscope glass slide and covered by thin cover slip. The samples of foam were

viewed under normal and polarized light. The structure of foams was also observed after

amplitude sweep measurements, to take notice of influence of stress applied during the

measurement on the appearance of the air bubbles.

3.4.1.2.Scanning electron microscopy (SEM)

The SEM micrographs were obtained as follows: the samples were placed in the slots of a

stub, plunge-frozen in liquid nitrogen, and transferred into the cryo-preparation chamber

(PP3010T Cryo-SEM Preparation System, Quorum Technologies, UK). There were freeze-

fractured and then sputter-coated with Pt. The examination was performed using JEOL JSM

7100F SEM (JEOL Ltd., Tokyo, Japan).

9

3.4.2. Calculation of the air bubble in the foam stabilized by SE and

combination of SE and SFLE

In order to calculate the air bubbles, there was used an optical microscope Leica DM2500

(Leica Microsystems, Belgium) with digital camera. The images were recorded in 4 different

field of view using 10× objective lens magnification. In each field of view the dimensions of

about 130 air bubbles were measured.

3.4.3. Rheological measurements

A range of rheological measurements included oscillatory experiments (like amplitude stress

and frequency sweep) and flow tests (like thixotropy, shear ramp and flow viscosity) were

done. All rheological behavior measurements of foams and oleogels were carried out at

constant temperature of 5°C (except flow viscosity test) with an AR 2000ex (TA Instruments,

USA) equipped with a Paltier temperature control system. A parallel plate cross-hatched

geometry of diameter 40 mm with rough surface, to prevent slip was used. The geometry gap

was set at 1 mm. In order to obtain reproducible data, all samples were analyzed at a similar

time after preparation, on the same instrument, and a special carefulness during the loading

process were preserved.

3.4.3.1.Oscillatory measurement

Amplitude stress sweeps were conducted at a frequency of 1 Hz for the SE:SFLE

combinations foam and gels. The samples were subjected to an increasing oscillatory stress

from 0.1 Pa up to 1000 Pa to obtain the linear viscoelastic region (LVR). Then, the samples

were subjected to frequency sweep at varying frequency ranging from 0.01 to 100 Hz.

3.4.3.2.Flow measurement

At thixotropy tests, the samples were undergone the cycles of low and high shear rates at 4

intervals (0,1 and 10 s-1

, respectively). Yield stress was measured with increasing shear stress

form 0,1 to 500 Pa. At flow viscosity measurement, the samples firstly were heating from 5 to

90°C and then cooling back to 5°C at constant ramp rate 10°C/min and constant shear rate 0,1

s-1

.

10

3.4.4. Termal analysis

3.4.4.1.Differential Scanning Calorimetry

In order to precisely determine a temperature and enthalpy of melting of foams and oleogels,

and crystallization of oleogels, there was used a Q1000 differential scanning calorimeter (TA

Instruments, USA). The weighted samples were hermetically sealed in aluminum pans. The

melting profile of foams and oleogel stored at 5°C for at least 2 days was measured by heating

the pans from 5 to 100°C, with heating rate of 5°C/min. The crystallization and melting

(fresh) profile of the respective oleogels were obtained by heating the pans to 100°C to

eliminate any crystals history. The pans were then cooled to 5°C with cooling rate of

10°C/min and reheated to 100°C with heating rate of 5°C/min.

3.4.4.2.Hot stage microscopy

The foam was analyzed as a function of temperature by using DM2500 (Leica Microsystems,

Belgum) equipped with stage compartment (Linkam hot stage system). The droplet of sample

was placed on a microscope slide, then covered by a cover slip, placed on metal hot stage and

subsequently subjected to heating in a range of temperature from 5°C to 90°C at constant rate

5°C/min. By increasing the temperature, the melting point of foam was determined and the

physic changes in structure of air bubbles were observed. The images were acquired.

3.4.5. Air fraction analysis

Pulsed field gradient (pfg) NMR measurements were performed at 5 °C on a benchtop Maran

Ultra spectrometer (Oxford Instruments, UK) operating at a frequency of 23.4 MHz. The

samples were filled in 18 mm diameter glass NMR-tubes (Oxford Instruments, UK) for a

height of about 40 mm. A Teflon spacer of 27 mm was used so that the detection volume was

identical for all analyzed samples, i.e. from 0 mm (sample tube bottom) to 20 mm height. The

oil diffusion signal (I0) was measured using a stimulated echo pulse sequence. The gradient

strength (G) was 0 T/m, the diffusion time (Δ) was set to 200 ms and the gradient duration (δ)

was fixed at 3 ms. The NMR receiver gain was set at 4% and the number of scans was 32.

In the presence of air, the diffusion signal of an aerated sample is expected to decrease

proportionally to the air (volume) fraction as compared to the signal of an equi-volume non-

aerated sample. Therefore, the air fraction can be determined as follows:

(2) Air fraction (vol.%) =

11

Dobstr/D0

Fig. 6. The variation of the obstruction factor Dobstr/D0 defined as the reduced self-diffusion

coefficient of small solvent molecules in solution as a function of the volume fraction of

obstructing particles of different geometries. Figure is reproduced form [24].

The shape of the obstructing particle can be evaluated based on the obstruction factor Dobstr/D0

(Fig 6). To that end, the diffusion coefficient of the aerated (Dobstr) and non-aerated sample

(D0) was calculated by measuring the echo intensity of the NMR-signal as a function of the

gradient strength G (in 17 steps from 0 to 2.2 T/m). In the presence of air particles, the oil

molecules will experience hindered diffusion, which results in a decrease of the diffusion

coefficient.

A formula exists to derive the volume fraction ( ) for spherical obstructing particles

[24]. However, quantitative analysis might be hampered if obstructed diffusion is not

completely obtained. In the latter case, the value of Dobstr is highly ∆-dependent; Dobstr

decreases with increasing ∆, whereby an increase in ∆ will increase the signal loss due to (T1)

relaxation. Using the resulting (overestimated) diffusion coefficient will result in an

underestimation of

In case the oil molecules experience obstructed diffusion, the diffusion coefficient is expected

to remain constant with ∆.

The T2-relaksometry distribution measurements of the relaxing oil protons were conducted

using the Carr Purcell Meiboom Gill (CPMG) sequence. The NMR receiver gain was set at

4% and the number of scans was 32. T2 distributions were obtained by Contin analysis using

12

the WinDXP 1.8.1.0 software (Oxford Instruments, UK), from which the signal intensity (A)

can be integrated.

In the presence of air, the integrated signal intensity of an aerated sample is expected to

decrease proportionally to the air (volume) fraction as compared to the signal of an equi-

volume non-aerated sample. Therefore, the air fraction can be determined as follows:

(3) Air fraction (vol.%) =

13

4. RESULTS

In this section there are presented the final results with brief outlines. All results are precisely

discussed in the following Chapter 5 (Discussion).

The first subsections 4.1 is focused on external appearance of resulting foams stabilized by

two different food-approved emulsifiers as sucrose ester, sunflower lecithin and their

combinations, as well. A particular attention is dedicated to overrun of resulting foam and to

evolution of foam volume and structure under the influence of time depending on the

concentration of using surfactant.

The following subsections 4.2 presents the crystallization and melting peaks for oleogels and

melting results for foams. All temperatures and enthalpies are determined both for SE and

SE:SFLE foams.

In subsection 4.3, data on properties of foam and oleogels are studied. All results are

presented based on type of measurement. The results of oscillatory tests as LVR and FS

define a strength and yielding of resulting foams. Flow test provide information about

deflection point (flow viscosity), yield stress (shear ramp) and viscosity recovery on removal

of shear. The results of all tests are presented in comparison with oleogels properties.

The last four subsections are dedicated to internal appearance of foams. The first of them (4.4)

demonstrates microstructure of foams depending on the type and concentration of using

surfactants. The attention is also dedicated to modifications in bubbles structure after stress

applied during the amplitude sweep measurements. The following subsection (4.5)

investigates the influence of temperature on appearance of air bubbles and determine the

temperature of melting crystals, which are responsible for stabilizing the structure of foam.

The impact of concentration of using surfactant on size of air bubbles is revealed in the next

subsection (4.6). In last subsection (4.7), the changes in air fraction with increasing

concentration of SE are presented by Pfg-NMR diffusometry and relaxometry distribution

measurements.

14

4.1. Macroscopy observation of the resulting foam

4.1.1. The overrun of foams

Table 1. The overrun depended on a type of used stabilizing agents and their concentrations.

Stabilizing agent Concentration VI VII Overrun [%]

SE

5% 15 27,5 83

7,5% 15 30 100

10% 15 32,5 117

Combination of

SE:SFLE

9:1 15 30 100

8:2 15 30 100

7:3 15 27,5 83

6:4 15 27,5 83

5:5 15 27,5 83

4:6 15 27,5 83

3:7 15 27,5 83

2:8 15 25 67

1:9 15 22,5 50

10% 15 15 0

4.1.2. Foam stabilized by sucrose ester

The overrun as a function of SE concentration was measured (Fig. 7) and presented in Table

1. Foam was apparently homogenous and there was no signs of drainage observed after 2

weeks. What is more, foam reminded in gelled state and did not exhibit the tendency to flow,

even when turned upside down. The oil leakage at the bottom was not observed during

prolonged storage.

15

5% 7,5% 10%

Fig. 7. Photography of plastic tubes filled with foam taken after 24h of storage in the fridge at

4°C.

4.1.3. Foam stabilized by SFLE and combination with SE

The overrun and differences in macroscopic appearance of foam after 1 day, 1 week and 2

weeks are presented in Fig. 8. No foam was observed after aeration of SFLE in oil after 1 day.

There were observed distinct changes in structure of foam in 9:1-5:5 SFLE:SE concentration.

The foam underwent phase separation and a fraction of oil began to be collected at the bottom

of the vessel (Fig 8 b-c). It was caused by insufficient firmness of crystal network to keep the

initial structure of resulting foams.

a)

10% 9:1 8:2 7:3 6:4 5:5 4:6 3:7 2:8 1:9 10%

SFLE SE

16

b)

10% 9:1 8:2 7:3 6:4 5:5 4:6 3:7 2:8 1:9 10%

SFLE SE

c)

10% 9:1 8:2 7:3 6:4 5:5 4:6 3:7 2:8 1:9 10%

SFLE SE

Fig. 8. Photographs of evolution of foam volume and structure under the influence of a time:

a) 24h after preparation b) 1 week after preparation c) 2 weeks after preparation. Foams were

stored in the fridge at 4°C.

4.2. Influence of temperature on crystallization of oleogel and melting

of foam and oleogels

Both the temperatures and enthalpies of melting and crystallization were determined using

differential scanning calorimetry (DSC). All thermograms as a function of SE and SE:SFLE

concentration are presented in sub-section 6.5.1. and 6.5.2., respectively in Fig.9 and Fig. 10.

Moreover the numerical data can be found in Table 2 and Table 3. As a reference, the

thermograms of only SE are presented in Fig. 9(a-c). The SE shows an one sharp exothermic

peak upon cooling (Fig. 9a) and one endothermic peak upon heating (Fig. 9c), which

correspond to crystallization and melting process respectively.

17

4.2.1. Foam and oleogel stabilized by SE

With increasing concentration of SE there was observed slight increase in melting

temperature of oleogels and foams (max 0,5°C). The temperature of crystallization increased

linearly with SE concentration, as well. For each concentration, oleogels exhibit lower

temperature of melting and crystallization than foams. There was observed that the increase of

enthalpy also depends on the concentration of SE and is lower for oleogels than for foams.

The thermograms for oleogels and foams exhibit similar shape.

In comparison with only SE, the crystallization temperature of oleogels in different

concentration was lower about 35% while the difference in temperature of melting oscillated

around 2%.

0

0,5

1

1,5

2

2,5

3

3,5

4

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

SE

a)

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Gel 10%

Gel 7,5%

Gel 5%

b)

18

Fig. 9. Heat flow curves as a function of SE concentration for pure SE (a, c), oleogel (b, d)

and foam (e), upon cooling (a, b) and upon heating (c-e).

-1,5

-1

-0,5

0

0,5

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

SE

c)

-0,9

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Gel 10%

Gel 7,5%

Gel 5%

d)

-1

-0,9

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

5 15 25 35 45 55 65 75 85

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Foam 10%

Foam 7,5%

Foam 5%

e)

19

Table 2. Temperature and enthalpy of foam and oleogels stabilized by SE at different

concentration.

Concentration Sample

Melting Crystalization

Temp.[°C] Enthalpy

[J/g] Temp.[°C]

Enthalpy

[J/g]

100% SE 67,36 81,74 54,59 70,96

5 wt% SE Foam 65,94 3,167

Oleogel 65,89 0,6242 33,67 0,8213

7,5 wt% SE Foam 66,38 4,814

Oleogel 65,80 2,171 36,60 1,537

10 wt% SE Foam 66,63 6,489

Oleogel 65,84 3,507 37,53 4,385

4.2.2. Foam and oleogel stabilized by combination of SE and SFLE

There was observed that with increasing concentration of SE in SE:SFLE mixture the

temperature of melting and crystallization increased linearly. For each concentration, oleogels

exhibit lower temperature of melting and crystallization than foams. The enthalpy for both

process also increased with increasing SE concentration (except 9:1) and hhe higher value

was observed in concentration 8:2.

In comparison with only SE, the crystallization temperature of oleogels in different

concentration was lower about 30% while the difference in temperature of melting oscillated

around 4% (for 9:1) and 20% (for 6:4).

20

Fig. 10. Heat flow curves as a function of SE:SFLE concentration for oleogel (a, b) and foam

(c), upon cooling (a) and upon heating (b, e).

0,2

0,6

1

1,4

1,8

2,2

2,6

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Gel 9:1

Gel 8:2

Gel 7:3

Gel 6:4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

5 15 25 35 45 55 65 75 85 95

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Gel 9:1 Gel 8:2

Gel 7:3 Gel 6:4

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

5 15 25 35 45 55 65 75 85

Hea

t Fl

ow

[W

/g]

Temperature [°C]

Foam 9:1 Foam 8:2

Foam 7:3 Foam 6:4

c)

a)

b)

21

Table 3. Temperature and enthalpy of foam and oleogels stabilized by combination of

SE:SFLE at different concentration.

Concentration

SE:SFLE Sample

Melting Crystalization

Temp.[°C] Enthalpy

[J/g] Temp.[°C]

Enthalpy

[J/g]

6:4 Foam 53,97 2,082

Oleogel 55,12 9,035 36,18 10,91

7:3 Foam 56,80 3,562

Oleogel 57,95 5,547 36,82 10,91

8:2 Foam 62,41 4,168

Oleogel 59,30 8,695 37,22 12,50

9:1 Foam 64,66 4,162

Oleogel 63,93 1,756 37,51 4,893

4.3. Properties of foam and oleogels

4.3.1. Oscillatory measurements of foam and oleogel stabilized by SE

Plots from oscillatory amplitude and frequency sweeps for foams and oleogels are presented

in Fig. 12 and prove the information about the impact of SE concentration on foam and

oleogel strength and yielding. For all concentrations of foam, the elastic modulus (G’) was

higher than viscous modulus (G’’) throughout entire range of strain. For oleogels, similar

situation was observed only for 10 wt%. In case of 7,5 wt% and 5 wt%, viscous modulus was

higher than elastic modulus, what indicates on weak properties of oleogels at these

concentrations. At low values of oscillatory stress, the elastic modulus values were of the

order of 103-10

5 Pa for foams and 10

2 Pa for 10 wt% oleogel. As seen on the graphs, the

foams properties strongly depend on SE concentration. Furthermore, all samples of foam

showed yielding behavior. The oscillatory yield stess (defined by point where G” values

becomes larger than G’) was shifted towards higher stress values by increasing concentration

of SE. In order to investigate the response of foams to applied rate of deformation or

frequency, frequency sweep measurement at a fixed amplitude was also carried out (see Fig.

12 c). The frequency sweep curve for oleogel (Fig. 12 d) was plotted only for 10 wt% due to

the fact that G’ < G’’ in oleogels with low SE concentration. As suggested by G* and n*

values at the low frequencies, the rheological behavior of foams and oleogel is shifted from an

22

elastic to a viscous. A slightly positive slop of G* curves for foams suggest that samples had a

strength with a slight dependence on the applied frequency, whereas the oleogel’s strength is

strongly related to the applied frequency.

0,05 0,5 5 50 500

G''

[Pa]

G' [

Pa]

osc. stress [Pa]

5% G' 7,5% G' 10% G' 5% G'' 7,5% G'' 10% G''

0,001

0,01

0,1

1

10

100

0,001

0,01

0,1

1

10

100

0,05 0,5 5 50 500

osc. stress [Pa]

G''

[Pa]

G' [

Pa]

5% G' 7,5% G' 10% G' 5% G'' 7,5% G'' 10% G''

b)

a)

23

Fig. 11. (a and b) Amplitude sweep curves for foams and oleogels, respectively; (c and d)

Frequency sweep curves for foams and oleogel, respectively.

0,05 0,5 5 50

n*

[Pa·

s]

G*

[P

a]

ang. frequency [rad/s]

5% G* 7,5% G* 10% G* 5% n* 7,5% n* 10% n*

0,5 5 50 500

n*

[Pa·

s]

G*

[Pa]


10% G*

10% n*

d)

c)

24

4.3.2. Oscillatory measurements of foam and oleogel stabilized by combination

of SE and SFLE

The results of oscillatory amplitude and frequency sweeps for foams and oleogels are

presented in Fig. 13 and prove the information about the impact of SE:SFLE concentration on

foam and oleogel strength and yielding. For all concentrations of foam and oleogels, the

elastic modulus (G’) was higher than viscous modulus (G’’) throughout entire range of stress.

As Fig. 13a suggests, the foam at 8:2 SE:SFLE exhibits the strongest properties while the

weakest is 6:4, respectively. The same dependence was indicated by curves plotted for

oleogels. At low values of oscillatory stress, the elastic modulus values were of the order of

103-10

5 Pa for foams and 10

1-10

3 for oleogels. As seen on the graphs, all samples of foam

showed yielding behavior. The highest oscillatory yield stress (defined by point where G”

values becomes larger than G’) was observed for 8:2 SE:SFLE foam. In order to investigate

the response of foams to applied rate of deformation or frequency, frequency sweep

measurement at a fixed amplitude was also carried out (see Fig. 13 c and d). As suggested by

G* and n* values at the low frequencies, the rheological behavior of foams and oleogel is

shifted from an elastic to a viscous. A slightly positive slop of G* curves for foams and

oleogels suggest that samples had a strength with a slight dependence on the applied

frequency.

0,1 1 10 100

G''

[Pa]

G' [

Pa]

osc. stress [Pa]

6:4 G' 7:3 G' 8:2 G' 9:1 G' 6:4 G'' 7:3 G'' 8:2 G'' 9:1 G''

a)

25

0,001

0,01

0,1

1

10

100

1000

10000

0,001

0,01

0,1

1

10

100

1000

10000

0,01 0,1 1 10 100 1000

G''

[Pa]

G' [

Pa]

osc. stress [Pa]

6:4 G' 7:3 G' 8:2 G' 9:1 G'

6:4 G'' 7:3 G'' 8:2 G'' 9:1 G''

0,5 5 50 500

n*

[Pa

·s]

G*

[Pa]


6G* 7G* 8G* 6n* 7n* 8n* 9G* 9n*

c)

b)

26

Fig. 12. (a and b) Amplitude sweeps for foams and oleogels, respectively; (c and d)

Frequency sweeps for foams and oleogel, respectively.

4.3.3. Flow measurements of foam and oleogels stablilized by SE

4.3.3.1. Flow viscosity

The viscosity curves as a function of temperature were determined for oleogels at different SE

concentration and are presented in Fig. 13. The highest value of viscosity were observed for

oleogel stabilized by 10 wt% SE. Based on the data obtained during measurement, deflection

points (point when viscosity begins to grow) were determined (see Table 5). Under the

influence of decreasing temperature the oleogels become more viscous. What is more, the

temperature drop after foam aeration was investigated and it was 40°C (Tinitial = 90°C, Tfinal =

50°C). The values of viscosity in initial temperature (5°C) and in temperature after aeration

process (50°C) are given in Table 4.

0,5 5 50 500

n*

[Pa·

s]

G*

[Pa]


6:4 G* 7:3 G* 8:2 G* 9:1 G* 6:4 n* 7:3 n* 8:4 n* 9:1 n*

d)

27

Fig. 13. Viscosity curves as a function of temperature for different SE concentration.

Table 4. Viscosity values as at different temperatures for oleogels at different SE

concentration.

Sample Viscosity at 50°C

[Pa·s]

Viscosity at 5°C

[Pa·s] Deflection point [°C]

5 wt% 2,468 8,745 60,2

7,5 wt% 2,245 49,05 56,8

10 wt% 4,774 118,5 53,5

Table 5. Deflection points of oleogels stabilized by SE.

Sample Temperature [°C] Viscosity [Pa·s]

5 wt% 60,2 0,4588

7,5 wt% 56,8 0,5058

10 wt% 53,5 2,708

0

20

40

60

80

100

120

140

5 25 45 65 85

Vis

cosi

ty [

Pa·

s]

Temperature °C

5%

7,50%

10%

28

4.3.3.2. Shear ramp

The results corresponding to yield stress (the stress that is needed to initiate flow of a sample)

are presented in Fig. 14 (a-b) for foams and Fig. 14 (c) for oleogels, respectively. It was

observed that 10 wt% foam has almost 10 fold higher resistance threshold to applied stress

than 5 wt% foam. The lowest value of yield stress exhibits 7,5 wt% foam. In comparison with

oleogels (see Table 6), foams are the systems which are more difficult to make them flowed

(have a greater shear stress resistance).

Fig. 14. Comparison of yield stress for foams (a-b) and oleogels stabilized by different

concentration of SE. Curves plotted as shear rate versus shear stress.

-0,5

0

0,5

1

1,5

2

2,5

3

3,5

0,01 0,1 1 10 100

She

ar r

ate

[1

/s]

Shear stress [Pa]

Foam 5%

Foam 7,5%

a)

-100

0

100

200

300

400

500

600

700

800

0,01 0,1 1 10 100 1000

She

ar r

ate

[1

/s]

Shear stress [Pa]

Foam 10%

b)

0

100

200

300

0,1 1 10 100

She

ar

rate

[1

/s]

Shear stress [Pa]

GEL 5%

GEL 7,5%

GEL 10%

c)

29

Table 6. Average values of yield stress for foams and oleogels at different SE concentration.

Sample Average yield stress [Pa]

Foam Oleogel

5 wt% 37,07 ± 4,74 4,93 ± 0,0005

7,5 wt% 22,03 ± 5,35 10,76 ± 1,4942

10 wt% 313,9 ± 0,85 7,88 ± 0,0007

4.3.3.3. Thixotropy

The structure recovery properties were measured at 4 intervals of high and low shear rates and

thixotropy plots for foams and oleogels stabilized by different concentrations of SE are

presented in Fig (a-b) respectively. The viscosity was changed as a function of time. The

thixotropic recovery for foams were as follow: 5 wt% (16,92%), 7,5 wt% (42,4%), 10 wt%

(22%). Oleogels exhibited much higher percentage of recovery than foams. The thixotropic

recovery for oleogels were as follow: 5 wt% (87,52%), 7,5 wt% (93,24%), 10 wt% (80,02%).

It was observed that 7,5% SE concentration exhibit the highest percentage of recovery both

for foam and oleogel. The oleogels had significantly lower viscosity than foams.

Fig. 15. Viscosity changes followed in time against low and high shear rates (0,1 – 10 s-1

respectively) for foams (a) and oleogels (b).

5 15 25 35

Vis

cosi

ty [

Pa·

s]

Time global [min]

Foam 5%

Foam 7,5%

Foam 10%

0,1

1

10

5 15 25 35

Vis

cosi

ty [

Pa·

s]

Time global [min]

Gel 5%

Gel 7,5%

Gel 10%

b) a)

30

4.3.4. Flow measurements of foam and oleogels stablilized by combination of SE

and SFLE

4.3.4.1. Flow viscosity

The viscosity curves as a function of temperature were determined for oleogels at different

SE:SFLE concentration and are presented in Fig. 16 (a-c). The highest value of viscosity at

5°C were observed for oleogel stabilized by SE:SFLE at concentration 8:2 (see Table 7) and

compared with SFLE viscosity the value increased almost 1000 fold form 1,761 Pa·s to 1234

Pa·s. Based on the data obtained during measurement, deflection points of samples (point

when viscosity begins to grow) were determined (see Table 8). Under the influence of

decreasing temperature, the oleogels become more viscous. The most similar values of

viscosity to SFLE in oil at temperature after oleogels aeration (50°C) exhibits sample at

concentration 6:4. The highest viscosity at this temperature shows oleogel at concentration 9:1

(see Table 7). The 10% SFLE and 6:4 (SE:SFLE) have almost similar in viscosity at 50°C.

But the 6:4 SE:SFLE capable to retain the air bubbles inside the system better than 10%

SFLE. This shows that SE imparts additional advantages in the fabrication of oil foam. On

one hand, the SE can stabilize the air bubbles even at 5% concentration without any sign of

drainage. On the other hand, the SFLE forms air bubbles that appear more similar to foam

produced by detergents. Unfortunately, the SFLE foams can only stand for very short period

of time. Therefore, combining the SFLE and SE together can retain the characteristic of the

individual component in oil foam.

0

5

10

15

20

25

30

35

40

45

50

5 25 45 65 85

Vis

cosi

ty [

Pa·

s]

Temperature °C

SE:SFLE 6:4

SE:SFLE 7:3

0

200

400

600

800

1000

1200

1400

5 25 45 65 85

Vis

cosi

ty [

Pa·

s]

Temperature °C

SE:SFLE 8:2

SE:SFLE 9:1

b) a)

31

Fig. 16. Viscosity curves as a function of temperature for SFLE (c) and for their different

combination with SE (a-b).

Table 7. Viscosity values as at different temperatures for oleogels at different SE:SFLE

concentration.

Sample Viscosity at 50°C

[Pa·s]

Viscosity at 5°C

[Pa·s]

Deflection point

temperature [°C]

10 wt% SFLE 0,8135 1,761 38,4

6:4 0,8847 16,57 56,9

7:3 0,4607 49,74 41,7

8:2 0,3556 1234 41,8

9:1 3,064 253,3 46,8

Table 8. Deflection points of oleogels stabilized by combination of SE:SFLE.

Sample Temperature [°C] Viscosity [Pa·s]

10 wt% SFLE 38,4 0,9459

6:4 56,9 0,7298

7:3 41,7 1,507

8:2 41,8 5,164

9:1 46,8 6,218

0,5

0,7

0,9

1,1

1,3

1,5

1,7

1,9

2,1

2,3

2,5

5 25 45 65 85

Vis

cosi

ty [

Pa·

s]

Temperature °C

SFLE 10%

c)

32

4.3.4.2. Shear ramp

The results corresponding to yield stress (the stress that is needed to initiate flow of a sample)

are presented in Fig. 17 (a-b) for foams and Fig. 17 (c) for oleogels, respectively. The highest

resistance threshold to applied stress exhibits foam at SE:SFLE concentration 9:1 (128 Pa)

and the lowest at concentration 8:2 (21,51 Pa). In case of oleogels, the highest value of yield

stress was observed at concentration 7:3 (249,6 Pa). Direct relationship between concentration

of using surfactant and yield stress values for foams and oleogels was not found (see Table 9).

Fig. 17. Comparison of yield stress for foams (a-b) and oleogels stabilized by different

concentration of SE:SFLE. Curves plotted as shear rate versus shear stress.

-500

0

500

1000

1500

2000

2500

0,01 1 100

She

ar

rate

[1

/s]

Shear stress [Pa]

Foam 6:4

Foam 9:1

a)

-100

0

100

200

300

400

500

600

700

800

900

0,01 1 100

She

ar r

ate

[1

/s]

Shear stress [Pa]

Foam 7:3

Foam 8:2

b)

0

500

1000

1500

2000

0,01 0,10 1,00 10,00 100,00 1 000,00

She

ar r

ate

[1/s

]

Shear stress [Pa]

Gel 6:4 Gel 7:3

Gel 8:2 Gel 9:1

c)

33

Table 9. Average values of yield stress for foams and oleogels at different SE:SFLE

concentration.

Sample Average yield stress [Pa]

Foam Oleogel

9:1 128 ± 0 47,50 ± 13,45

8:2 21,51 ± 4,49 171,63 ± 23,53

7:3 24,42 ± 17,14 249,6 ± 0

6:4 92,93 ± 11,91 79,17 ± 0,08

4.3.4.3. Thixotropy

The structure recovery properties were measured at 4 intervals of high and low shear rates and

thixotropy plots for foams and oleogels stabilized by different concentrations of SE:SFLE are

presented in Fig (a-b) respectively. The viscosity was changed as a function of time. The

thixotropic recovery for foams were as follow: 6:4 (76,78%), 7:3 (36,24%), 8:2 (30,21%), 9:1

(27,52%). It was observed that for foam percentage of thixotropic recovery increase with

increasing concentration of SFLE. The thixotropic recovery for oleogels were as follow: 6:4

(76,69%), 7:3 (29,77%), 8:2 (25,19%), 9:1 (27,37%). Both foam and oleogel exhibited the

highest percentage of recovery. The recover percentage shows that the presence of SFLE

helps to reinforce the network which contribute for better recovery of SE:SFLE foams.

Fig. 18. Viscosity changes followed in time against low and high shear rates (0,1 – 10 s-1

,

respectively) for foams (a) and oleogels (b).

5 15 25 35

Vis

cosi

ty [

Pa·

s]

Time global [min]

Foam 6:4 Foam 7:3

Foam 8:2 Foam 9:1

5 15 25 35

Vis

cosi

ty [

Pa·

s]

Time global [min]

Gel 6:4 Gel 7:3

Gel 8:2 Gel 9:1

a) b)

34

4.4. Microscopy observation of resulting foam

4.4.1. Foam stabilized by SE

Optical microscopy images of oil foam in three SE concentrations are presented in Fig. 9 (a-c)

and subsequently cryo-SEM images are given in Fig 9 (d-f). In each concentration the vast

majority of bubbles have a diameter lower than 100 µm. It was observed that with increasing

concentration of SE the average of bubbles decreased and the structure of foam was more

condensed.

d) a)

e) b)

c) f)

35

Fig. 19. (a)-(c) Optical microscopy images of foams stabilized by crystals of SE at

concentration of 5 wt%, 7,5 wt% and 10 wt%, respectively. (d)-(f) Cryo-SEM images of foam

stabilized by crystals of SE at concentration of 5 wt%, 7,5 wt% and 10 wt%, respectively.

4.4.2. Foam stabilized by combination of SE and SFLE

Optical microscopy images of oil foam stabilized by different concentration of SE:SFLE are

given in Fig. 10 (a, c, e, g) and a image of foam stabilized by only SFLE is presented in Fig.

10 (h). Subsequently cryo-SEM images for SE:SFLE 9:1, 8:2, 7:3 are presented to the right in

Fig 10 (b, d, f). There was observed that the volume of air bubbles in the foam increases with

increasing amount of SE in mixture (see cryo-SEM images). Furthermore optical microscopy

revealed a textured surface of air bubbles and presence of clear crystals between bubbles

which stabilized the system. The size of air bubbles is more diverse than in foam stabilized by

only SE and an average of bubbles increases with increasing concentration of SFLE.

c) d)

a) b)

36

Fig. 20. (a,c,e,g) Optical microscopy images of foams stabilized by crystals of combination of

SE and SFLE at concentration of 9:1, 8:2, 7:3 and 6:4, respectively. (h) Optical microscopy

image of foams stabilized by crystals of SFLE at concentration 10 wt%. (b,d,f) Cryo-SEM

images of foams stabilized by crystals of combination of SE and SFLE at concentration of

9:1, 8:2 and 7:3, respectively.

4.4.3. Air bubble appearance after amplitude stress measurement

4.4.3.1. Foam stabilized by SE

Optical microscopy images of oil foam after amplitude stress measurement are presented in

Fig. 21 (a-c). With increasing concentration of SE less destruction signs in structure of air

bubbles were observed. This relationship indicates an increase in strength of foam according

to SE concentration. In addition, the SE supplies crystalline particles to the system which in

turn improve the structure of air bubbles. This can be clearly seen in Fig. 21 (a-c), wherein the

volume of air bubbles retained after the amplitude sweep is a function of concentration.

f) e)

g) h)

37

Fig. 21. (a-c) Optical microscopy images of air bubbles stabilized by crystals of SE at

concentration of 5 wt%, 7,5 wt% and 10 wt%, respectively, taken after amplitude stress

measurements.

4.4.3.2. Foam stabilized by combination of SE and SFLE

Optical microscopy images of oil foam after amplitude stress measurement are presented in

Fig. 22 (a-c). With increasing concentration of SE insignificantly less destruction signs in

structure of air bubbles were observed.

a) b)

c)

a) b)

38

Fig. 22. (a-d) Optical microscopy images of foams stabilized by crystals of combination of SE

and SFLE at concentration of 9:1, 8:2, 7:3 and 6:4, respectively, taken after amplitude stress

measurements.

4.5. Influence of temperature on structure of air bubbles

The influence of stability as a function of temperature was studied by using optical

microscope equipped with heating stage (hot stage). The images of thermal transition in

structure of air bubbles are demonstrated in Fig. 23-25 for foam stabilized by SE and in Fig

26-30 for foam stabilized by combination of SFLE with SE. It was observed that increasing

concentration of SE affects positively on the resistance of air bubbles to coalesce and

consequently affects the delay destruction of the system as a function of temperature.

However, above 65°C, the films which separate the air bubbles become extremely thin and

the vast majority of bubbles rupture and coalescence, regardless of SE concentration. This

was associated with melting point of SE crystals. In case of foam stabilized by combination of

SE:SFLE, with increasing concentration of SFLE, the coalescence of air bubbles was

observed earlier (in lower temperature) and air bubbles became angular (see Fig. 29).

c)

d)

39

4.5.1. 5% foam

Fig. 23. Optical images of collapsing air bubbles with increasing temperature in 5 wt% foam

stabilized by SE.

40

4.5.2. 7,5% foam

Fig. 24. Optical images of collapsing air bubbles with increasing temperature in 7,5 wt%

foam stabilized by SE.

41

4.5.3. 10% foam

Fig. 25. Optical images of collapsing air bubbles with increasing temperature in 10 wt% foam

stabilized by SE.

42

4.5.4. Combination SE-SFLE (9:1)

Fig. 26. Optical images of collapsing air bubbles with increasing temperature in foam

stabilized by SE:SFLE at concentration 9:1.

43




44




45




46

4.5.8. 10% SFLE


stabilized by 10 wt% SFLE.

47

4.6. Size of air bubbles

The size of air bubbles in foam stabilized by SE and SE:SFLE combination are presented in

sub-section 6.3.1. and 6.3.2., respectively in Fig.11 and Fig.12. Based on the results, the size

of air bubbles depends on the concentration of emulsifier used during preparation. With

increasing concentration of SE, the size of air bubbles shifts towards lower values with

smaller diameters.

4.6.1. Foam stabilized by SE

The biggest air bubbles were observed at 5 wt% foam and their diameter oscillated in the

range of 8,55 to 143 µm, with a predominance of bubbles with size about 25 µm. Foam

oleogel at 7,5 wt% contains air bubbles correspondingly smaller with the greatest frequency

of 15 µm. The smallest air bubbles were observed at 10 wt% foam.

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140

No

of

bu

bb

les

bubble diameter [µm]

Frequency a)

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70

No

of

bu

bb

les


Frequency b)

48

Fig. 31. Size of air bubbles as a function of SE concentration (a-c) 5 wt%, 7,5 wt% and 10

wt%, respectively.

4.6.2. Foam stabilized by combination of SE and SFLE

The air bubbles with the greater size were observed at concentration of 6:4 SE:SFLE. Their

diameter oscillated in the range of 18,4 to 241 µm, with a predominance of bubbles with size

about 55 µm. The greatest frequency at 7:3 and 8:2 had the air bubbles in size of 20 and 15

µm respectively. In foam at 9:1 concentration also prevailed the bubbles in size about 15 µm.

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

No

of

bu

bb

les


Frequency c)

0

5

10

15

20

25

30

35

40

45

50

0 30 60 90 120 150 180 210 240

No

of

bu

bb

les


Frequency a)

0

20

40

60

80

100

120

140

0 25 50 75 100 125 150 175

No

of

bu

bb

les


Frequency b)

49

Fig. 32. Size of air bubbles as a function of SE:SFLE concentration (a-d) 6:4, 7:3, 8:2, 9:1,

respectively.

4.7. Air fraction analazys

The diffusion signals and integrated signal intensity of the samples are given in Tables 10 and

11.

Table 12 shows that the air fractions as obtained from the diffusometry method (Eq. (2)) are

higher as compared to the T2-relaxometry method (Eq. (3)).

Both methods indicate that an increase of the sucrose ester concentration from 5 to 10%

increases the air fraction by a factor of 1.43.

Based on Figure 6 and the obtained obstruction factor (Dobstr/D0) in Table 12, the geometry of

the obstructing (air) particles is spherical or long prolate. The small values of

for the oleogel containing 5% sucrose esters indicate that the oil molecules did not

experience completely restricted diffusion and hence, Dobstr is overestimated using ∆=0.2 s

(Table 13). Due to the higher air fraction of the 10% sucrose ester containing sample, the oil

molecules experience more restricted diffusion ∆=0.2 s, which results in -values closer to

the values obtained from Eq. (2) and (3).

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105

No

of

bu

bb

les


Frequency c)

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

No

of

bu

bb

les


Frequency d)

50

Table 10. Diffusion signal (I0) of oleogel samples containing 5% or 10% sucrose ester.

I0 (-)

Rep. 1 2 3 Average

5% SE- Non-aerated 2330 2295 - 2312 25

5% SE- Aerated 1241 1319 1319

10% SE- Non-aerated 2088 2082 - 2085 4

10% SE- Aerated 777 801 747

Table 11. Integrated signal intensity (A) of oleogel samples containing 5% or 10% sucrose

ester.

A (-)

Rep. 1 2 3 Average

5% SE- Non-aerated 53821 53938 - 53880 82

5% SE- Aerated 34780 36923 36136

10% SE- Non-aerated 50146 50199 - 50173 37

10% SE- Aerated 26683 26389 26151

Table 12. Air fraction of oleogel samples containing 5% or 10% sucrose ester as determined

using Eq.(2) and Eq. (3).

Air fraction (vol.%)

Rep. 1 2 3 Average

5% SE

Eq. (1) 46.3 43.0 42.9 44.1 1.9

Eq. (2) 35.4 31.5 32.9 33.3 2.0

Dobstr/D0 0.93 0.96 0.91 0.93 0.03

14.7 8.0 19.4 14.0 5.7

10% SE

Eq. (1) 62.7 61.6 64.2 62.8 1.3

Eq. (2) 46.8 47.4 47.9 47.4 0.5

Dobstr/D0 0.80 0.82 0.81 0.81 0.01

51.4 44.9 46.2 47.5 3.5

51

Table 13. Obstructed and free diffusion coefficient values.

D (m2/s)

Rep. 1 2 3 Average (∙1E12 m2/s)

5% SE- Non-aerated (D0) 3.54E-12 3.57E-12 - 3.56 0.02

5% SE- Aerated (Dobstr) 3.31E-12 3.42E-12 3.24E-12 3.33 0.09

10% SE- Non-aerated (D0) 3.53E-12 3.52E-12 - 3.53 0.01

10% SE- Aerated (Dobstr) 2.81E-12 2.88E-12 2.87E-12 2.85 0.04

52

5. DISCUSSION

Over the last years, edible foams gained a substantial increase of the interest because of their

enormous potential in reformulating food products with reduced calories. In addition, due to

their characteristic chew, mouth-feel, lower fat content and unique texture, they have become

highly desirable by the customers. However, most of work done in area of food foams was

restricted to aqueous foam systems. The main goal of this study was to fabricate the oil

foam stabilized by food-grade surfactant as sucrose ester. Keeping in mind the future

application of the product, therefore the manufacturing costs, lecithin is combined with

sucrose ester and creating mixed combination systems. Lecithin is known to stabilize air-

water interface but in hydrophobic solvent, the air bubbles are not stable. In addition, lecithin

helps to reduce dependency of foam oleogel towards sucrose ester as stabilizing agent.

5.1. Fabrication of foam

The first aim of this work was to fabricate the oil foams using two different food-approved

stabilizing agents at different concentrations. For this purpose, various concentrations of

sucrose ester and mixture of sucrose ester and sunflower lecithin were used. After one day, it

was observed that changing the concentration of SE significantly influences the overrun of

resulting foams. In case of SE, the overrun increased with increasing concentration of used

emulsifier. The maximum overrun was observed for 10 wt% foam and it was equal to 117%.

In case of using only SFLE and the combinations, the result was similar, with increasing

concentration of SFLE, the overrun decreased. A foam stabilized by only SFLE was only

stayed for one hour at 5°C and slowly coalesce afterwards. The SE improves the overrun of

foams due to its contribution to more crystalline particles, which adsorb on the interface and

in the interstitial spaces between the air bubbles. These crystalline particles form a connection

to the other particles by means of hydrophobic interaction. This in turn creates three-

dimensional network that immobilized the air bubbles, thus prevent them from coalesce.

5.2. Foam stability as a function of temperature

The next challenge of this study was to investigate the impact of temperature on foam

stability. To understand and explain this dependence, the temperature of crystallization and

melting was investigating. Foam stabilized by different concentration of SE, stored at

temperature below melting point did not exhibit any signs of drainage for a long time. Rise of

temperature above melting point, resulted in the flow of the foam. By hot stage microscopy,

53

we observed the thermal transformations in structure of foam upon heating. Due to the

progressive melting of crystals, the layer separating the air bubbles become thinner and as a

consequence, the bubble size increase.

54

6. CONCLUSIONS

In summary, the oil foams were successfully fabricated using two food-grade surfactants of

sucrose ester and sunflower lecithin. The combinations of these emulsifiers were also

investigated in order to compare their properties, keeping in mind their future application.

Rheological characterization revealed that concentration of sucrose ester had strong influence

on rheological properties of resulting foams. With increasing concentration of SE the highest

viscosity and the structure recovery of foams are observed. Moreover, the microscopy

observation showed that 10% foam stabilized by SE is the most condensed with smaller air

bubbles size. The relationship between increasing concentration of SE and increasing the air

fraction in foam was proved using pulsed field gradient (pfg) NMR measurements, as

well. From this study, it is shown that the foam oleogels have better rheological properties

than its oleogels counterpart, without air bubbles. The storage modulus (Gʹ) values in foam

are almost double than the values observed in oleogels. Therefore, it’s can be concluded here

that, incorporation of air bubbles is capable to tune the mechanical properties of the oleogels.

In addition, a new form of oleogel, foam oleogel, can be fabricated by altering the process of

making conventional oleogels. This study is different from the existing techniques in which

the foaming process is carried out at high temperature in order to force the accumulation of

surface active agents at the air-oil interfaces in their molten state. In contrast, the existing

techniques carry out incorporation of air bubbles in presence of pre-formed crystals (cooled

dispersion). We believe that the possibility of incorporating air bubbles at high temperature

provides improvement in the foaming functionality of food emulsifiers by firstly allowing

incorporation of a large number of air bubbles in low viscosity medium and secondly by

forcing accumulation of a higher concentration of emulsifiers at the interfaces where they can

crystallize on cooling.

55

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University of Ghent Faculty of Bioscience Engineering Lodz … · 2016-07-28 · in my life....

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