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The gap between food gel structure, texture and perception
Abstract
This paper summarizes the current status of research into the structure, texture and perception of
food gels based on proteins and polysaccharides. The first part of this paper deals with
mechanisms of biopolymer gelation. In the second part the effects of post-gelation phenomena
are covered, demonstrating that it is of importance for the quality of food to realise that gels are
metastable systems that continue to evolve after the initial gelation process has taken place.
Phase separated networks are presented in the context of the huge interest from the industrial
perspective and a short overview of physical mechanisms is given in a third part. In the final part
examples from the dairy industry and flavour perception are presented.
2005 Elsevier Ltd. All rights reserved.
General introduction
For companies wishing to produce attractive gelled food products it is of increasing importance to
understand the relationships between the perception of food gel texture and its structure. The
replacement of different ingredients usually leads to changes in food structure that are often
perceived by consumers as giving a less attractive texture or mouth feel. New products in
functional foods, low-fat products, vegetarian products and gelatine replacements were not
always successful as these products had inferior texture and were rejected by the consumer. The
key to prevent these debacles is to control and adjust the sensoric perception of the product. The
prerequisite to do so is to well understand the relationship between food structure and its sensory
properties.
The product-transformation chain is the base of this relationship. Nowadays, the product-
transformation chain needs to be considered in a more integrated approach than in the past. In
contrast with the logic of the conventional upstream approach (do your best with the existing
ingredients) the chain needs re-engineering via a downstream analysis. Down-stream analysis
starts from the sensations that consumers expect or desire of food products, down to texture and
microstructure to end at the ingredient level. This approach should make it possible to improve
the industrial processes and adapt to consumer requirements regarding the sustainability of food
products, the development of environmentally friendly processes as well as the safety and
diversity of end products.
This paper will give a first step to this integrated approach in an overview of the relationshipbetween food structure and sensory perception. The first part of this paper describes the
microstructure of biopolymer gels and the mechanism of biopolymer gelation with a focus on
polysaccharide and protein gels. Post-gelation phenomena may lead to undesirable effects like
syneresis and are the focus of the second section. A huge interest for phase-separated networks
has given rise to the third part of this paper concerning biopolymer mixtures. In addition, to finish,
we will delve into the relationship between gel structure, texture and perception.
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2. Biopolymer gelation
Before starting an overview on biopolymer gelation a short intermezzo on the definition of a gel isneeded. The definition of a gelled material as given by Ferry (1980) is a gel is a substantially
diluted system which exhibits no steady state flow. This covers a range of substances which
exhibit solid- like properties while a vast excess of solvent is present. Comparable definitions are
given by Atkins (1990) and in the Encyclopedia of Polymer Science and Engineering (Tanaka, 1987).
Gels consist either from space filling networks of interacting particles such as fat crystals in the
case of butter or from cross-linked polymers that form a space-filling network such as in the case
of the white of a boiled egg. The interaction between polymers or particles can by way of covalent
reactions or by physical interactions between different types of polymers such as depletion forces
and van der waals forces.
The former category (referred to as chemical gels) will not be discussed in this paper, since most
food gels belong to the second type (referred to as physical gels). Food gels can be further
distinguished on the basis of different criteria. In this context, we prefer to distinguish protein gels
from fine stranded polysaccharide gels. Examples of the former category are heat-set protein gels
from ovalbumin, whey proteins and soy proteins (Ikeda & Morris, 2002; Renkema, Gruppen, & van
Vliet, 2002; Weijers, Visschers, & Nicolai, 2002). The latter category includes polysaccharides such
as carrageenan (van de Velde & de Ruiter, 2002), alginate (Draget, Smidsrd & Skjak-Brk, 2002)
and pectin (Ralet, Bonnin, & Thibault, 2002).
A comparison between these two major classes of biopolymer gels, protein and polysaccharide,
gives rise to the following items; the critical gelation concentration is generally five to ten fold
higher in the case of proteins while the stress at failure is three fold lower; protein gels are often
turbid except in the case of gels formed at low ionic strength and gelatin gels; syneresis and
precipitation phenomena are commonly encountered in protein gels (Renard, Robert, Garnier,
Dufour, & Lefebvre, 2000). The network forming capacity of proteins is considerably reduced when
proteins are denatured during the purification stage (Renard, Lavenant, Sanchez, Hemar, & Horne,
2002; Renard, Robert, Faucheron, & Sanchez, 1999; Sanchez, Pouliot, Renard & Paquin, 1999). The
ability to gel is also reduced in the presence of high sugar concentration (Bryant & McClements,
2000). Moreover, except for the case of gelatine gels there are no crystalline phases observed in
protein gels contrary to what is observed in certain polysaccharide gels. Finally, and probably themost relevant characteristic for technological applications is that protein gelation is not easy to
control.
Does this mean that proteins are bad candidates to create structure and texture in food products?
Certainly not. Proteins are an important part of our diet since they have very good nutritional
qualities. They are rich in essential amino acids and sometimes in organic minerals (calcium and
phosphate for milk proteins). Moreover, proteins also can give color and taste to foods. Apart
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from these nutritional qualities, protein gels may be produced in several ways: increasing
temperature, pressure or exposure to microwaves, acidification or changing the solvent quality.
Several enzymatic treatments can also be used such as chymosin (rennet) or trans-glutaminase for
instance. All these processes allow the formation of different microstructures in the protein
networks which give rise to important classes of foods such as cheese, yoghurt and tofu.
2.1. Globular protein gels
Globular proteins, such as b-lactoglobulin, ovalbumin and plant storage proteins (soy, pea etc) are
well known for their gel forming capacities (Cayot & Lorient, 1997; Doi & Kitabatake, 1997;
Utsumi, Matsumara & Mori, 1997). These globular protein gels are generally referred to as heat
set gels (Clark & Lee-Tuffnell, 1986), as the gelation is usually induced by the thermal unfolding of
the globular proteins. Such heat-set mechanisms are generally irreversible and gels formed during
heating retain their gel structure upon cooling and repeatedly heating. Examples of globular
protein gels are depicted in Figs. 1 and 2 for the particular case of b-lactoglobulin (Aymard, 1995).
Fig. 1 corresponds to the events occurring during the early stages of aggregation of the protein in
the conditions where repulsive interactions are predominant. At very low ionic strength,
aggregates formed have a rodlike shape and contain very few branching points. The stability of
these structures is reached by long range, weak attractive inter- actions. At low ionic strength
these fine-stranded gels may resemble to some aspects to polysaccharide gels. With increasing
ionic strength both the flexibility of the aggregates and the number of branching points increase
giving rise to coarser and less translucent gels.
Fig. 2 depicts the protein gels obtained when electrostatic repulsions are screened. The
aggregation occurs in two steps in these conditions. First small globular aggregates are formed
which subsequently aggregate to form fractal structures. Fractal means self-similarity at many
lengthscales of obser- vation. Interchange of disulfide bonds, leading to intermole- cular bonding is
probably involved in the formation of the elementary subunit (Aymard, Gimel, Nicolai, & Durand,
1996). These gels are often called particulate or particle gels. In the case of fine-stranded gels, the
reactivity of the sulphydryl groups is very small which inhibits the formation of the elementary
subunit. However, in both cases, the step leading to the formation of fractal aggregates involves
mainly an end to end aggregation, with occasional branching. The amount of branching and the
persistence length of the aggregates are determined by the concentration of added salt, which
screens the repulsive electrostatic interactions (Aymard, Nicolai, Durand, & Clark, 1999).
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2.2. Polysaccharide gels
In contrast to globular protein gels, most polysaccharide gels are reversible cold setting gels. Gel
formation proceeds via a disorder-to-order transition induced by cooling (Morris, 1998). This
process is reversible and gel melting is therefore possible by re-heating. The way in which
crosslinks between individual chains are formed is different from polysaccharide to
polysaccharide. Grant and co-workers (Grant, Morris, Rees, Smith, & Thom, 1973) proposed the
egg-box model for gelation of alginate or pectin (Fig. 3). Affinity of some parts of the chain for
calcium gives rise to junction zones formation, thus promoting intermolecular associations.
Alginates consists of (1/4) linked b-D-mannuronic acid and a-L-guluronic acid residues of widely
varying composition and sequence, in which
the calcium binding sites are formed by homopolymeric regions of guluronic acid residues (Draget
et al., 2002). In pectin, particularly in the case of low-methoxy pectin, the Ca- binding sites are
formed by the so-called smooth regions of homogalacturonic acid regions (Ralet et al., 2002).
Besides these smooth regions, pectic polysaccharides consist of hairy rhamnogalacturonan
regions to which complex neutral sugars side chains are attached. Initially the egg boxes were
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considered to be multimolecular aggregates (Fig. 3). More recent studies (Morris, Gidley, Murray,
Powell, & Rees, 1980) support a simpler dimerisation model for the junction zones (Fig. 3b).
A second example concerns the gelation of carrageenans, a group of sulfated galactans extracted
from certain species of red seaweeds (van de Velde & de Ruiter, 2002). In general terms, k-
carrageenan gels are hard, strong and brittle gels that are freeze/thaw instable, whereas i-carrageenan forms soft and weak gels that are freeze/thaw stable. The first step in the gelation of
carrageenan is the transition from a disordered (random coil) to the ordered (helical) state
(Viebke, Piculell & Nilsson, 1994). The helical conformation is promoted by the addition of salts or
by lowering the temperature. Monovalent cations (KC, RbC, CsC and NHC) promote the
aggregation of 4 k-carrageenan double helices to form so-called aggregated domains (Fig. 4)
(Morris, 1998). Gel formation in i-carra-geenan gels is assumed to take place at the helical level by
branching and association through incomplete formation of double helices (Viebke et al., 1994).
The number of branching points plays an important role in the gel strength of i-carrageenan gels
(van de Velde et al., 2002).
2.3. Gelatine gels
In some respects, gelatine behaves more like a polysacchar- ide gel than a globular protein gels, as
it is a cold-setting thermoreversible gelation process. In comparison with carra- geenan and other
gel forming polysaccharides, such as gellan gum and agarose, gelatine exhibit a coil-to-helix or
disordered- to-ordered transition upon cooling. Fig. 5 illustrates the mechanism of the formation
of a gelatine gel. Gelation is governed by the partial reformation of triple helices found in collagen
during cooling. In a first step, a polypeptidic chain takes an orientation to induce a reactive site
then, condensation of two others chains near the reactive site occurs giving rise to triple helix
formation (Godard, Biebuyck, Barriat, Naveau, & Mercier, 1980; Normand, 1995).
Gelatine gels are nearly purely elastic but Eldridge & Ferry (1954) observed that gelatine gels
flowed under stress at verylong timescales. This flow behaviour in gelatine gels is explained by thereversibility of the junction zones. An equilibrium may exist between creation and destruction of
junction zones in the network under stress, allowing thus the flow of the material without return
to the initial state. This rheological behaviour is also encountered in globular protein gels or in
particle gels in general.
3. Post gelation phenomena
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3.1. Structural rearrangements in protein gels
Gels are materials with characteristics from both liquids and solids. Many particle or emulsion gels
can be described as networks made up of particles connected by viscoelastic bonds with long
relaxation times (Mellema, Heesakkers, van Opheusden, & van Vliet, 2000). They are thus in a
metastable state from a thermodynamic point of view. Bonds between particles in such colloidalgels are not always static. They may change during the observation time, either spontaneously or
due to external forces. The dynamic character of the bonds may also lead to gradual changes in
the structure that, in turn, change the rheological properties. Such ageing phenomena are
spontaneous in the sense that they do not result from mechanical disturbance such as shear-
induced bond breaking (Mellema, Walstra, van Opheusden, & van Vliet, 2002). The ageing
commonly involves a gradual coarsening of the structure and a change in firmness. An ultimate
effect of structural rearrangement is syneresis or expulsion of liquid determined both by the stress
acting on the gel and the permeability of the network. Syneresis can be either desired, like in
manufacture of cheese or undesired like in manufacture of yoghurt. The question is at which
spatial and time scales these structural rearrangements inside the gel occur.
Syneresis and spontaneous rupture are macroscopic phenomena can be caused by three types of
microscopic processes that are linked to the basic building blocks of protein gels. As described
before, most protein gels are fractal structures of small (10500 nm) aggregates of denatured
protein units. Interparticle rearrangements or reorganization within these particles occur mainly
during aggregation. These
particles further interact with each other to form a spacefilling network. Intraparticle
rearrangements or particle fusion on this lengthscale occurring mainly during gel ageing may be
probed by electron microscopy or by the measurement of tangent delta at low frequency by
means of small amplitude oscillatory shear measurements. Tangent delta (the loss modulus over
the storage modulus ratio) is a measure of the proportion of bonds with a relaxation time about
equal to the reciprocal of the applied frequency. In other words, it is a measure of the dynamics of
the system at length scales from molecules to particles. The lengthscale of these types of changes
is in the micrometer range and can be probed by computer simulations,light scattering or confocal
microscopy. Intercluster rearrange- ments also called coarsening or micro-syneresis and typically
involve the formation of water-channels in the structure. They can be probed by confocal or
classical microscopy, permea- metry or by large deformation rheological measurements.
3.2. Intraparticle rearrangements
Intraparticle rearrangements were clearly observed in the cold-gelation experiments carried out
by Alting, Hamer, de Kruif, Paques, and Visschers (2003). In these experiments, a pre-heated
solution ofwhey proteins with a well defined size(usually around 50 nm) was gradually acidified
with glucono- d-lactone. By varying the amount of glucono-d-lactone it was possible to control the
time during which the system was in the gelled state. The protein particle size grew significantly
(up to 500 nm) during the time the system was in the gel-state. Furthermore, this particle-fusion
type of process depended strongly on the availability of free thiols at the surface of the
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aggregates. When these groups were chemically blocked, no particle-fusion was observed. The
time-scale of this process was rather fast ranging from minutes to about one hour at room
temperature.
3.3. Interparticle and intercluster rearrangements
Interparticle and intercluster rearrangements are illustrated for the case of rennet-induced casein
gels in Fig. 6. Mellema et al. (2002) observed that the storage modulus G continues to increase as
a function of time after rennet addition. The increase of G in fresh gels can be due to an increased
contact area between the micelles by intraparticle rearrangements (particle fusion). The total
increase of G depends strongly on pH (lower pH yields lower G) and temperature (higher
temperatures yield lower G).
Interparticle rearrangements can occur when there is sufficient freedom for the micelles to move
around. This may be the case in the early stages of gel formation. The occurrence of intercluster
rearrangements is supported by confocal microscopy observations since thinning and fracturing of
strands are observed as a function of time during gel ageing (Mellema et al., 2000). Also gel ageing
is accompanied by a gradual formation of larger pores. The authors summarize the effect of pH
and temperature on structural rearrangements and rheological properties by concluding that
lowering pH or increasing temperature speeds up the gel ageing or rearrange- ment process.
Syneresis can be seen as the macroscopic consequence of the transient character of the gel
structure. This conclusion supports the fact that a gel from a thermodynamic point of view is in a
metastable state and a significant amount of bonds within the network have a reversible
character. The driving force of these structural rearrangements comes from the non equili- brium
conditions of the gel material. As a consequence, the permanent reorganisation within the
material aims to minimize the free energy of the system. Gels in that sense can be considered as
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soft glassy materials where the non crystalline and random organisation within the system induces
a metastable state.
4. Phase separated networks
4.1. Structures of phase-separated systems
Proteinpolysaccharide mixtures that are subject to phase- separation are interesting for the
creation of new structures simply because the addition of polysaccharides at low concentrations
can create major differences in structure and rheological properties of mixed systems as a result of
intertwining gelation and phase-separation processes. In addition, they can also induce major
differences in consistency or texture with only minor effect on other organoleptic properties.
Three examples of such systems are briefly presented here. A short impression of what is known
about the physical background of these phenomena and how this helps to control the functionality
in terms of structure and texture for different mixtures will be given thereafter.
When phase separation in proteinpolysaccharide mixtures occurs the included phase may formspherical domains. Their size can vary but is commonly in the range of 220 mm. The observed
formation of spherical inclusions suggests that a significant liquidliquid interface energy is acting
within these mixtures. For instance, in gelatinmaltodextrin mixtures, phase separation quite
often leads to the entrapment of droplets within droplets (Norton & Frith, 2001). Gelation of one
or both phases is a good way to arrest the process of ripening of the phase structure. Gelation of
one phase has been observed by several authors and is usually explained by an additional
separation which occurs within the droplets themselves rather than by diffusion of the polymers
to the already formed droplets. The second example depicted on Fig. 7 is when conditions lead to
favourable electrostatic interactions between b-lactoglobulin and arabic gum, associative phase
separation occurs and droplets called coacervates are formed that grow markedly as a function oftime (Schmitt, 2000). Ultimately, this process leads to a structured phase that contains both types
of biopolymers.
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Mixed solutions of chemically or conformationally different biopolymers are unstable and
separate into two liquid phases. The chemical analysis of the phases is usually presented in
theform of a phase diagram (Doublier, Garnier, Renard, & Sanchez, 2000). Phase diagrams
establishment allows the distinction between the one phase region of miscibility and the two
phase region of incompatibility where two liquid phases are formed when the system reaches the
thermodynamic equilibrium. The binodal line delimits the fronteer between the miscibility and the
incompatibility areas. The points located on the binodal give the composition of each phase at
equilibrium. These points are connected by the tie-lines. The spinodal line separates the
metastable and unstable regions of the two-phase system. This overall behaviour is a common
rule for biopolymer mixtures in solution.
Phase separation in mixed biopolymer solutions is always accompanied by rheological changes.
The rheological beha-viour of mixed systems differ noticeably from pure biopolymer solutions.
Micellar caseinguar gum mixtures are a typical example. The viscoelastic behaviour of pure guar
gum is that of a regular macromolecular solution with G lower than G at low frequency and a
crossover at high frequency (Fig. 8). Response from micellar casein is typical of a suspension withalso a strong variation of G with frequency. Both one-component systems do not evidence any
organisation of the medium. In contrast, caseinguar gum mixtures exhibit viscoelastic moduli less
dependent upon frequency and their value is much higher than for guar gum alone (Bourriot,
Garnier & Doublier, 1999). In addition, the G curve tends to level off towards the low frequency
range and to cross the G 00 curve (Fig. 8). These results suggest a solid-like behavior resulting from
a structuration of the medium. The storage modulus of such a gel-like system appears however
very low. The corresponding microstructures revealed by confocal scanning laser microscopy of
the mixtures located in the one- phase region or two-phase region are also shown. Fluorescence of
the mixtures in the one-phase region is regularly distributed indicating that casein is spread all
over the medium (Fig. 8). In the case of the mixtures located in the two-phase region, distributionof the fluorescence is not regular. Casein is localized in white areas which have a broad size
distribution, these large zones originate from a concentration of casein micelles. Two phases
coexist obviously in the system, one containing mainly casein micelles whereas the other one is
enriched with guar gum.
To understand the behaviour of these systems we need to realise that hydrocolloids can form
mixed gels when the concentration of each of them in the mixed solution exceeds the minimum
concentration for gelation. Normally, the minimum concentration for heat-induced gelation of
biopolymers varies from 0.1 to 15% weight/weight. The minimum concentration for gelation
usually decreases when another incompatible biopolymer is added. This is due to an excludedvolume effect in the single phase mixed solution. In the phase separated biopolymer systems, the
same effect may be due to water redistribution between phases during gelation. The range of
minimum concentrations for hydrocolloid gelation is usually lower than that of the phase
separation threshold. This means that a mixture of biopolymer gelling agents can form gels in
concentration areas lying both above and below the bimodal curve. Accordingly, gelation of the
mixed solution leads to filled gels above the binodal and to single-phase mixed gels below the
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binodal (Zasypkin, Braudo & Tolstoguzov, 1997). Filled gels correspond to gel rich in one polymer
and filled with liquid particles rich in the other polymer or to gels filled with gel particles, each
phase being a filled gel. Single-phase mixed gels correspond to single-phase gel of polymer 1 for
example filled with polymer 2 or conversely.
In all these systems, the final structure and morphology ofthe composite system is determined by
the interface between the phases. Much progress has been made to be able to measure the
interfacial tension in mixed systems (Scholten, Tuinier, Tromp, & Lekkerkerker, 2002). In order to
design the microstructure of mixed systems, one clue for food industries will be to understand therole and importance of the interface between the phases in particular at the molecular level. In
addition, the development of predictive models for material properties of composites based on
their microstructure will be necessary (Norton & Frith, 2001).
5. Relation between gel structure, texture and flavour perception
5.1. Texture design in dairy industry
From a perception point of view, firmness and creaminess are the major sensory attributes
important for consumer preference of fermented dairy products such as yoghurt, cheese,
fermented cream and milk based desserts. These are extremely complex attributes that can not becaptured with a single physical property. From the rheological point of view, two important
characteristics of dairy products can be distinguished: viscosity and elasticity. Both properties are
important for the organoleptic quality of a product and for its appealing appearance and pleasant
mouth feel. Viscosity is the property of a material to resist deformation. In the context of
fermented dairy products viscosity relates directly to attributes such as as slimy, thin and fluid.
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Elasticity represents the property of a material to recover after a deformation occurred. This
property corresponds to simple attributes such as firm body and gum-like.
Exopolysaccharides (EPSs) are synthesised by lactic acid bacteria during fermentation and act both
on firmness and creaminess of these products. EPSs increase the viscosity of the serum phase,
bind hydration water and, thus, reduce the water flow in the matrix space. Moreover,exopolysaccharides decrease syneresis and improve product stability. Dairy industries have
rationalised the relationship between these sensory attributes and the applications of
exopolysaccharides in these products (Duboc & Mollet, 2001) in the following way.
Different lactic acid bacteria are known to produce neutral and charged EPSs (de Vuyst & Degeest,
1999; Ruas-Madiedo, Hugenholtz & Zoon, 2002). These two classes of EPSs have quite distinct
functional properties (Duboc & Mollet, 2001). The viscosity of the product with neutral
exopolysaccharide increased with time and was about ten times higher than the viscosity of a
control product obtained with a non- polysaccharide producing strain. Surprisingly, the viscosity of
the product with the charged exopolysaccharide was compar- able to the control product. In
contrast, the elasticity (as measured the storage modulus G 0 ) was significantly higher in the
product containing a negatively charged polysaccharide. These experiments show that neutral
exopolysaccharide contributes to the viscosity but not to the elasticity. On the other hand,
negatively charged polysaccharide contributes to the elasticity, but not to the viscosity, since they
interact with the positively charged casein particles by electrostatic interactions, reinforcing the
strength of the network and consequently increasing G 0 . As a consequence different strains of
lactic acid bacteria that produce either charged or neutral EPSs can be selected to tailor
mouthfeel.
5.2. Flavourmatrix interactions
Besides texture, flavour release is the second important factor determining the perception of
gelled food products. Addition ofhydrocolloids to foods causes a change in the flavour perceived.
The mechanism for this change is not well understood. In the case of proteins flavour molecules
can bind to the protein, causing a decrease in effective concentration and therefore a decrease in
flavour perception. Since flavours are present in food at low levels anyway, a relatively small
amount of binding can exert a significant effect in terms of perceived flavour. However, binding of
flavours to most other hydrocolloids (gums) is minimal in high water containing food products.
If the binding of flavours to hydrocolloids is minimal, then alternative explanations need to be
invoked to explain the decrease in flavour perception. One suggestion is that the increasedviscosity of products containing hydrocolloids causes slower mass transfer, therefore slower
flavour release and decreased flavour perception from these products during eating. MS-nose
technology has been developed to follow the concentration of volatile flavours in the noses of
people eating food (Fig. 9) (Taylor, Besnard, Puaud, & Linforth, 2001). By obtaining sensory
information from the subjects in conjunction with the concentration measurements on the same
samples, it is possible to study the link between the perceived intensity offlavour chemicals to
their actual concentration and release from the food. These techniques have been applied to
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study how hydrocolloids cause a change in flavour perception and direct effects of texture on the
perceived intensity (so-called cross-modal interactions) could be demonstrated (Weel et al., 2002).
Taylor and collaborators at the Samworth Flavour Labora- tory in UK (Taylor et al., 2001) studied
the effect of gelatin concentration on the intensity of furfuryl acetate perception. There is a strong
trend of decreasing perception with increasing gelatine concentration (Fig. 10A) (Taylor, 2000).
Measuring the in-nose concentration shows a delay in the timing of the maximum intensity of the
flavour signal reaching the nose but no difference in maximum intensity of release (Fig. 10B)
(Taylor, 2000). The texture of the gelled food containing the flavour has an effect on the
perception of the flavour, whereas the amount of flavour released is unaffected. In addition,
Taylor concluded that the rate of release is best correlated with the observed change in sensory
perception from the different gel samples.
In another study Boelrijk and co-workers at NIZO food research demonstrated that the perception
diacetyl and ethylbutyrate was directly influenced by the hardness of gels (Personal data). In their
experiments it was clear that although similar concentrations of the flavours were released into
the nose, gels with increased hardness were perceived as less intense flavoured.
6. Outlook and concluding remarks
We would like to conclude with the following French quote: Ce nest pas assez de savoir les
principes, il faut savoir manipuler that translates into Its not enough to know the principles, we
have to know how to manipulate. This sentence was found in the first edition of the chemical
manipulations written by Michael Faraday. This quote is simply to say that the chemical and
physical principles applied in food science are not always at our convenience from a desired
sensations point of view. Anyone who cooks daily knows that it is quite easy to fail in the
preparation of a mayonnaise or a souffle. From this simple consideration, we may imagine how
difficult it can be in the scale up applications encountered in food processes.
Acknowledgements
First of all, DR would like to thank the organisers and more particularly professor Hamer for the
invitation to be a keynote speaker in the WCFS Food Summit. The authors gratefully acknowledge
Dr Alexandra Boelrijk (NIZO food research), Dr Els de Hoog (Wageningen Centre for Food Sciences
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& NIZO food research), Ir. Marja Kanning (NIZO food research), Dr Ir. Ton van Vliet (Wageningen
Centre for Food Sciences & Wageningen University), Ir. Mireille Weijers (Wageningen Centre for
Food Sciences & Wageningen University) for critically reading (parts of) the manuscript.