the gap between food gel structure

7/29/2019 The Gap Between Food Gel Structure

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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 interactions. 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 observation. Interchange of disulfide bonds, leading to intermolecular 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 polysaccharide gel than a globular protein gels, as

it is a cold-setting thermoreversible gelation process. In comparison with carrageenan 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 rearrangements 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 rearrangement 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 equilibrium

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 comparable 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.

the gap between food gel structure

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