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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
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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
4.5.5. Combination SE-SFLE (8:2) ......................................................................................43
4.5.6. Combination SE-SFLE (7:3) ......................................................................................44
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4.5.7. Combination SE-SFLE (6:4) ......................................................................................45
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
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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.
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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.
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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.
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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
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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.
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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)
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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).
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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
.
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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]
ang. frequency [rad/s]
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]
ang. frequency [rad/s]
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]
ang. frequency [rad/s]
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
4.5.5. Combination SE-SFLE (8:2)
Fig. 27. Optical images of collapsing air bubbles with increasing temperature in foam
stabilized by SE:SFLE at concentration 8:2.
-
44
4.5.6. Combination SE-SFLE (7:3)
Fig. 28. Optical images of collapsing air bubbles with increasing temperature in foam
stabilized by SE:SFLE at concentration 7:3.
-
45
4.5.7. Combination SE-SFLE (6:4)
Fig. 29. Optical images of collapsing air bubbles with increasing temperature in foam
stabilized by SE:SFLE at concentration 6:4.
-
46
4.5.8. 10% SFLE
Fig. 30. Optical images of collapsing air bubbles with increasing temperature in foam
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
bubble diameter [µm]
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
bubble diameter [µm]
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
bubble diameter [µm]
Frequency a)
0
20
40
60
80
100
120
140
0 25 50 75 100 125 150 175
No
of
bu
bb
les
bubble diameter [µm]
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
bubble diameter [µm]
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
bubble diameter [µm]
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
7. REFERENCES
[1] Ageing of foams [Online] https://www.equipes.lps.u-psud.fr/sil/spip.php?rubrique31,
[Access date: 5.04.2016].
[2] Alargova R.G., Warhadpande D.
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