VISCOELASTIC PROPERTIES OF HEAT-SET WHEY PROTEIN EMULSION GELS

VISCOELASTIC PROPERTIES OF HEAT-SET WHEY PROTEIN EMULSION GELS

JIANSHE CHEN and ERIC DICKINSON’

Procter Department of Food Science University of Leeds

Leeds LS2 9JT, UK

(Manuscript received December 4, 1997; in final form February 17, 1998)

ABSTRACT

The viscoelastic properties of heat-set whey protein gels and whey protein-stabilized emulsion gels have been studied using the dynamic oscillatory rheometry technique. The storage modulus was monirored and analysed for pure protein gels and emulsion gels over a wide range of protein concentrations. The dependence of storage modulus on protein concentration is different for gels of low and high modulus. At low protein concentrations, the increase of storage modulus is much more sensitive to the increase of protein concentration. The protein-coated oil droplets behave as active jiller particles and dramatically enhance the gel strength. The effect of the oil volume fraction on the rheology has been investigated for emulsion gels containing 11 vol. %, 20 vol. % and 45 vol. % Trisun oil. The formula of van der Poel fails to describe the experimental results. l l i s is attributed to the strongly flocculated state of the emulsion system.

INTRODUCTION

Many food gels can be considered as complex mixtures of various components, in which particulate inclusions (fillers) are embedded in a polymeric gel matrix. The textural properties of these composite food gels are in principle determined by the combined structural properties of the gel matrix and the filler particles (Tolstoguzov and Braudo 1983). The filler particles can be classified either as ‘active’ or ‘inactive’, according to their effects on the rheological properties. An inactive filler has little chemical affinity for the polymer of the matrix and hence does not strengthen the gel. In contrast, an active filler normally has a strong interaction with the gel matrix and hence reinforces the gel strength (Ring and Stainsby 1982). This behaviour has been confirmed by microscopy for globular protein emulsion gels. Yost and Kinsella

I Author to whom correspondence should be addressed

Journal of Texture Studies 29 (1998) 285-304. All Righrs Reserved. “Copyrighi 1998 by Food & Nuiriiion Press, Inc., Trumbull, Conneciicui 285

286 1. CHEN and E. DICKINSON

(1992) observed that emulsified butterfat droplets in whey protein isolate gels were intimately associated with the gel protein matrix. Langley and coworkers (1989, 1991) investigated the mechanical and structural properties of model food composites and found that particles with a hydrophilic surface were an integral part of the composite microstructure.

The small shear deformation moduli G’ and G” are commonly used to express the viscoelasticity of biopolymer gels and emulsion gels. The storage modulus G’ is a measure of the energy stored elastically during an oscillation cycle, and the loss modulus G” is a measure of the energy dissipated as viscous flow. An extensive literature over last half century has shown that the magnitude of the small deformation storage modulus of a composite viscoelastic material is directly related to the rheology of its matrix (Kerner 1956; Lipatov 1977; Chow 1980; Shih et al. 1990; Clark 1991; Stainsby 1991). Investigation on soy 11s globulin protein gels and emulsion gels has shown straight lines of 1nG’ (storage modulus) versus lnc, (protein concentration). A slope of n = 4 was observed for pure protein gels in the range of 4-10 wt % protein concentration, whereas a slope of n = 2 was obtained for emulsion gels in the range 1.8-6 wt % protein concentration (Matsumura et al. 1993). Nishinari and Watase (1983) observed similar straight lines for agar gels in the range of 1-4 wt % concentration.

Many theoretical efforts have also been made to estimate the shear modulus of a composite gel. Kerner (1956) calculated shear moduli of a composite material consisting of a suspension of grains. A well-known formula to estimate the shear modulus of a composite gel was proposed by van der Poel (1958) and then simplified by Smith (1975). Van Vliet extended this formula to predict the shear modulus of filled gels containing liquid emulsion droplets (van Vliet 1988; Luyten and van Vliet 1989). A general conclusion was that the extent of reinforcement depends primarily on the volume fraction of the filler, though the size distribution and the shape of the particles can also be important. Walstra and co-workers developed a model to describe gel formation and structure based on the concept of fractal geometry (Bremer et al. 1989; Walstra er al. 1991). A scaling theory was developed by considering the gel network as a collection of flocs (Shih et al. 1990). The fractal dimensionality D, of the underlying fractal clusters is an essential parameter to describe a particle-based colloidal gel, and this parameter can be calculated in principle from the experimental rheological data. The Brownian dynamics simulation of aggregated particle networks provides another powerful approach for investigating the relationship between interparticle interactions and the rheology of colloidal gel systems (Dickinson 1994; Whittle and Dickinson 1997; Wijmans and Dickinson 1998).

A series of experimental investigations on model protein-stabilized emulsion gels containing various surfactants has been carried out in last few years in this laboratory. Having established previously that the protein-covered oil droplets

HEAT-SET WHEY PROTEIN EMULSION GELS 287

function as active filler and enhance the strength of emulsion gels (Dickinson ef al. 1996; Dickinson and Hong 1996, 1997; Dickinson and Yamamoto 1996a-c, Chen and Dickinson 1998), we here investigate particularly the effects of protein concentration and the volume fraction of oil phase on the shear modulus of the heat-set emulsion gels. Whey protein isolate has been chosen as the sole emulsifier and gelling agent in this work.

MATERIALS AND METHODS

Materials

Whey protein isolate PSDI-2400 (WPI) was obtained from MD Foods Ingredients (Videbaek, Denmark). According to the supplier’s specification, the protein content consisted of > 95% P-lactoglobulin, the fat content was below 1 % , and the mineral contents were as follow: Ca, 0.02%; Mg, < 0.10%; P, 0.35%; Na, 1.15%. The analysis of the WPI sample by fast protein liquid chromatography (FPLC) showed a sharp double peak characteristic of a mixture of the P-lactoglobulin A and B variants. Commercial triolein oil (Trisun 80) was supplied by Danisco (Brabrand, Denmark). The triglyceride fatty acid composition of the oil was as follows: 80% oleic, 9% linoleic, 4% stearic, and 4% palmitic. To remove surface-active impurities, the oil was pretreated by passage through a Florisil column (Gaonkar 1989).

Emulsion Preparation

The whey protein solution was made by dissolving the protein in 0.05 M TrisHCl buffer (pH 7 .O) through gentle stirring. Oil-in-water emulsions were prepared at room temperature using a laboratory-scale jet homogenizer operating at 300 bar. Three different volume fractions of oil phase were considered: 11 vol. %, 20 vol. % and 45 vol. % . The particle-size distribution of the diluted fresh emulsion samples was determined using a Malvern Mastersizer S2.01. The mean particle size of the emulsion droplets was characterized by the average diameters,

d,, =Cinid:/Cinidz,

d,, =Cinidf/Cinidf,

where n, is number of droplets of diameter d,.

288 J. CHEN and E. DICKINSON

Rheological Measurements

Small-deformation viscoelasticity was investigated by dynamic oscillatory rheometry using a controlled stress Bohlin CS-50 rheometer. Protein solution or protein-stabilized emulsion (2 mL) was carefully poured into a Couette-type cylindrical cell (2.5 cm i.d., 2.75 cm 0.d.) and covered with a thin layer of low-viscosity silicone oil to prevent evaporation. Heat-set gelation was induced in situ by (1) heating the sample at constant rate of 3C m i d from 30 to 85C, (2) holding at 85C for 35 min, (3) cooling at a rate of 1C m i d from 85 to 30C, and (4) holding at 30C for 20 min. Shear rheological properties of the gels produced at the end of this cycle were shown to be in the linear viscoelastic region at I 0.5% strain). The storage and loss moduli were measured at a constant frequency of 1 Hz. Figure 1 shows a typical graph of storage shear modulus during the whole treatment as a function of time and temperature.

RESULTS AND DISCUSSION

The viscoelastic properties of heat-set whey protein gels (0 vol. % oil) and whey protein-stabilized emulsion gels (11, 20 or 45 vol.% oil) have been investigated over a wide range of protein concentration. The major protein component of WPI is 0-lactoglobulin. At room temperature /3-lactoglobulin exists as a dimer in aqueous solution at a pH between 5.5 and 7.5. On heating a P-lactoglobulin solution intramolecular and intermolecular changes take place. Raising the temperature to 50C induces the dissociation of the dimer into its monomeric units, and above approximately 60C the monomers start to unfold. The denaturation is then followed by an aggregation reaction so that the whole process becomes irreversible (Verheul et al. 1995). The denaturing temperature of the whey protein solution on heating is around 80C depending on its bulk concentration. The higher the protein concentration, the lower is the denaturing temperature (Wong et al. 1996). After being heated at 85C for 35 min, most of the globular 0-lactoglobulin molecules are fully denatured. It can be seen from Fig. 1 that the elastic modulus of the protein-stabilized emulsion shows a rapid increase initially at 85C and then almost levels off before increasing further on cooling down to 30C. The increase in storage modulus at 85C is mainly due to the aggregation of the denatured protein molecules through hydrophobic interactions and disulphide bonds. The relative rates of change of G’ during the heating and cooling stages (a) and (b) in Fig. 1 are due to the different rates of temperature change and the different balance of contributions from the various types of molecular interactions during the sequential stages of the thermal processing. The main reason for the increase in G’ during cooling from 85C to 30C is the development of enhanced hydrogen bonding (Beveridge et al. 1984).


The steady value of G' at 30C during stage (d) shows that the gel formed at the end of the thermal processing cycle has a stable structure, and it also provides confirmation that the determined gel strength is unaffected by the rheology measurement itself (i.e. that the measurements are indeed in the linear region).

20000

16000

12000

2 G

8000

4000

0

0 G'fPa - Ternp/C

0 0 0

100

80

60

'0 a 40 c)

20

0 0 30 60 90 120

Urnin

FIG. 1 . DEVELOPMENT OF VISCOELASTIC PROPERTIES OF WHEY

20 VOL. % TRISUN OIL, 50 MM TRISHCL, PH 7) The storage modulus G' is recorded over whole experimental time-scale, where (a), (b),

(c ) , and (d) are the four stages of heat treatment (see text)

PROTEIN-STABILIZED EMULSION GEL (10 WT % PROTEIN,

Figure 2 shows the set of elastic modulus values for the final whey protein gels and emulsion gels. The natural logarithm of G' has been plotted against the protein concentration cp in the various systems. The storage moduli of both protein gels and emulsion gels show a strong protein concentration dependence. This is much more notable for the systems of low protein content. The storage modulus increases more strongly with cp at the low protein concentrations than at the high protein concentrations. Figure 2 shows that the emulsion gels show a stronger increase in strength with protein content as compared to the pure protein gels. The 1nG' versus cp curves move leftwards with increasing oil

290 J . CHEN and E. DICKINSON

volume fraction. Generally speaking, the set of pure protein gels shows a smooth monotonic curve over the whole protein concentration range, while the functional behaviour for the emulsion gels appears slightly more complicated.

9

$ 6 -

3

0

0% oil *11%oil 20% oil

A 45 Yo oil

. * I * * * *

0 4 8 12 16 20

C d w t Yo

FIG. 2. VISCOELASTIC PROPERTIES OF HEAT-SET PROTEIN AND EMULSION GELS MEASURED AT 30C. 1 HZ AND AT 0.5 % STRAIN

The points are experimental data, the solid lines are fitted curves using Eq. (3). Logarithm of G' is plotted against the protein concentration cp in the system.

The strong increase in emulsion gel strength means that the dispersed oil droplets not only function as space fillers but also help to build up the gel matrix structure through interaction between protein molecules at the droplet surface and those in the gel matrix. The system of whey protein-covered oil droplets can thus be called an active filler or a hydrophilic filler (Langley and Green 1989; Langley et af. 1991). The storage modulus of the heat-set emulsion gels containing 20 vol.% or 45 vol.% oil shows a very steep increase at very low protein concentrations which is then followed by a linear increase. An interesting phenomenon is the sharp increase/decrease/increase of storage modulus of

HEAT-SET WHEY PROTEIN EMULSION GELS 29 1

emulsion gels containing 11 vol. % oil: a peak storage modulus is observed in emulsion gels containing about 2 wt % protein. This may be due to a sudden change in gel network structure or gelation mechanism (see below).

A complication in these systems is the state of aggregation of the fresh emulsions before heating. Figures 3 and 4 show the apparent average particle sizes of fresh emulsions containing different protein concentrations and oil volume fractions. The large changes in apparent average particle size of the emulsions are due to droplet flocculation. A simple way to confirm the flocculation of protein-coated droplets is to add excess surfactant (e.g. Tween 20) to the emulsion which completely displaces protein from the oil droplet surface without destabilizing the emulsion (Chen and Dickinson 1998), and the intrinsic average size of dispersed primary droplets can then be established. The intrinsic average particle size after adding excess Tween 20 is much smaller than the apparent particle size, and varies only slightly as protein content changes, i.e., d,, = 0.6 pm and d,, S 2 pm.

60 20

16

40

E 5 30 P

20

10

A

A

0 0 3 6 9 12

C$wt”!

12

8

4

0

n

3 5

FIG. 3. MEAN APPARENT PARTICLE SIZE OF FRESH PROTEIN-STABILIZED EMULSIONS (TRIANGLE SYMBOLS: EMULSIONS CONTAINING 20 VOL. %;

SQUARE SYMBOLS: EMULSIONS CONTAINING 45 VOL. % OIL; SOLID SYMBOLS: d4,; OPEN SYMBOLS: dJ

The average particle size from light scattering is plotted against the protein concentration cp in the system.


20

16

12

8

4

0

8

6

P

4 5 3

2

0 0 2 4 6 8 10 12 14 16

c$wt Yo

FIG. 4. MEAN APPARENT PARTICLE SIZE OF FRESH PROTEIN-STABILIZED EMULSIONS CONTAINING 1 1 VOL. % OIL (SOLID SYMBOL: d03; OPEN SYMBOL: dJ

The average particle size from light scattering is plotted against the protein concentration cp in the system.

The reason for the flocculation is open to speculation. It has been reported that emulsions made with whey protein isolates become flocculated as the pH is lowered towards the isoelectric point and the ionic strength increases (Hunt and Dalgleish 1994). A likely explanation for flocculation at pH 7 is the greater extent of protein aggregation caused by the ionic species in the protein sample (Hongsprabhas and Barbut 1997) or by the partially denatured state of the protein induced during whey protein separation and purification. Steric protection by protein molecules is overwhelmed by chain-chain contacts between protein molecules on different droplets (Hunter 1993) and, therefore, the emulsion becomes flocculated. For emulsions containing 11 vol. % oil, restabilization was observed for protein concentrations > 4 wt % . There are two possible reasons. One is that the average distance between oil droplets is greater in these systems than in emulsions containing 20 vol.% or 45 vol.% oil. Therefore, chain-chain contacts between droplets are less likely. Another possible reason is that, at higher protein concentrations, there is more than enough protein fully to cover the oil droplets, and hence there is no possibility for any bridging flocculation.

Obviously, any preaggregation of the emulsion droplets prior to heat treatment will affect the build-up of the gel network during the heat treatment.


On the other hand, the effective volume fraction of a system of aggregated droplets is higher than just the actual volume fraction of the added oil. A consequence of this is that the final gel will tend to have higher modulus because of the higher density of interdroplet connections. For emulsions containing 20 vol.% oil, there is large increase in average apparent particle size at protein concentrations between 6 wt % and 10 wt %. However, no large increase in storage modulus was detected for these emulsion gels. A plausible explanation is that, at these high protein concentrations, the gel matrix is so strong that it overwhelms the effect of the filler. This means that the reinforcement by filler particles becomes less significant for gels that already have a very strong matrix. The increase/decrease/increase of 1nG’ against c, for emulsion gels containing 11 vol. % Trisun oil at low protein concentrations is also probably related to the state of flocculation of the fresh emulsions. Figure 4 shows the average apparent particle size in fresh emulsions containing 11 vol. % oil and various protein concentrations. We infer that the emulsions are extensively flocculated when the protein concentration is lower than 2 wt % . The sharp decrease in 1nG’ between 2 wt % and 3 wt % protein is probably due to the greater degree of stabilization of the fresh emulsions. When the protein concentration is higher than 4 wt % , the fresh emulsion is little flocculated. Therefore, the storage modulus of the heat-set emulsion gels increases again with increase of protein concentration in a similar manner to that for the other emulsion gels and protein gels. It has been suggested (van Vliet 1988) that the higher than expected elastic modulus of filled acid milk gels is also due to aggregation of the milk fat globules. Again, the argument is that aggregates can be considered to represent dispersed particles of total effective volume much higher than the sum of volumes of the individual globules.

To summarize the effect of protein concentration on the viscoelastic properties of the heat-set protein gels and heat-set emulsion gels, the data of 1nG‘ against c, have been fitted using the equation

where c, is the protein concentration, and Pi (i = 1-4) are fitted parameters. The quantity P, is an adjustable parameter that can be considered to represent the critical protein concentration at which In (G’/Pa) equals zero (i.e. G’ = I Pa). The least-squares fitted curves in Fig. 2 represent very well the experimental results (the sharp peak near c, = 2 wt % for the emulsion gels containing 11 vol. % oil has not been fitted). The x2 values for the 0, 11, 20 and 45 vol. % oil systems are 0.024, 0.230, 0.046 and 0.043, respectively. At very high protein concentrations (i.e. cp % P4 and cp %- PJ, the value of 1nG‘ is mainly determined by the second term on right-hand-side of equation (3); under these


conditions, it will therefore increase linearly with increase in cp. When cp is very small (i.e. c,+P,), however, 1nG’ is mainly determined by the first term.

Of the four fitted parameters in Eq. (3), of most interest and physical significance is P,, the critical protein concentration. (The physical meaning of P,, P, and P, remains unclear; we treat them simply as empirical fitting parameters.) The changing value of P4 shows how the oil volume fraction affects the strength of the emulsion gels (see Fig. 5 ) . The term lnP, decreases almost linearly with oil volume fraction. The critical protein concentration for pure whey protein gels is 5.2 f 0.2 wt %. This value is quite small compared to values for other whey protein gels studied previously in our laboratory (Dickinson and Yamamoto 1996a-c). For example, it was found that the storage modulus of a heat-treated 10 wt % solution of a commercial ‘pure’ P-lactoglobulin was only 1.7 Pa (Dickinson and Yamamoto 1996b). The reason for the difference from the present results is presumably due to the different sources of the protein, different concentrations of ionic components, and most particularly to partial denaturation of the present whey protein during the production process. For the emulsion gel containing 45 vol. % Trisun oil phase, the critical protein concentration is only ca. 0.13 wt %, which is so low that it seems almost below that needed to emulsify the oil.

1.5

0.5

0” E - -0.5

-1.5

-2.5 0 10 20 30 40 50

Vh

FIG. 5 . EFFECT OF OIL CONTENT ON THE CRITICAL PROTEIN CONCENTRATION OF

Logarithm of cp is plotted against the oil volume fraction 4. HEAT-SET EMULSION GELS


Oil volume fraction

n1

“2

Dfl

Dn

In the literature, the concentration dependence of the small deformation moduli of biopolymer gels and biopolymer-stabilized emulsion gels is often approximated by a power law (Nishinari and Watase 1983; Matsumura et al. 1993):

G’=kc ”. (4)

0 ?h 11 % 20 YQ 45 Yo

9.2 5.6 I .7 3.4

4.3 3.6 3.0 1.1

2.8 2.6 I .8 2.4

2.5 2.4 2.3 1.2

Here, k and n are constants, and c represents the concentration of biopolymer in the system. Figure 6 gives a set of plots of 1nG’ versus In cp for our heat-set whey protein gels (0 % oil) and emulsion gels (1 1, 20 and 45 vol. % oil). For each of the systems, the plots of 1nG’ vs lnc, show two different slopes, i.e., n, at low protein concentrations and n, at high protein concentrations. Calculated values of the slope are listed in Table 1. For all four sets of systems (except that with 20 vol. % oil), the value of n, is much higher than that of n,, which suggests that gel strength is more sensitive to protein concentration change in weak gels than in strong ones. This trend resembles results of Hirai (1955) who found two different slopes for the very different system of a gelatin hydrogel: n, = 3.7-4.0 and n, = 1.7-2.0. The reason for the rather different behaviour for the set of emulsion gels containing 20 vol. % oil (n, < nJ is not clear. A likely explanation is the very different state of flocculation of the fresh emulsions containing different amounts of protein (see Fig. 3).

TABLE 1 . THE FITTED SLOPES AND APPARENT FRACTAL DIMENSIONALITIES OF PROTEIN

AND EMULSION GELS CALCULATED USING EQ. (4) AND (5 ) . THE ESTIMATED ERROR IS WITHIN f 5 % .

Figure 6 shows that the break points in the 1nG’ vs In cp lines for the pure protein gels and emulsion gels occur at a similar value of 1nG’. This fact may suggest that there are two different mechanisms involved in the gelling. At a certain value of gel strength, the cross-linking of the P-lactoglobulin molecules


or the formation of gel matrix may change over from one mechanism to another. It has been reported (Verheul el al. 1995) that there are two consecutive reactions in the gelling of heat-treated P-lactoglobulin solutions. The first denaturation step is in principle an equilibrium reaction between the native state and the partly unfolded state of the protein. Upon unfolding, the thiol group is exposed and becomes reactive, which is then followed by the second irreversible aggregation reaction. According to this interpretation, the gelling dynamics will be determined by the aggregation reaction. Therefore, it seems reasonable to speculate that the denatured 0-lactoglobulin molecules have different aggregation kinetics at low and high protein concentrations (where, respectively, the weak and strong gels are formed).

10

0

i!J E -

6

4

2

11 % oil 8 20 % oil A 45 % oil

L

- 1 0 1 2 3

Inc,

FIG. 6. DEPENDENCE OF GEL STRENGTH ON THE PROTEIN CONCENTRATION FOR

AND 45 V0L.X OIL Logarithm of G' is plotted against logarithm of c,.

HEAT-SET PROTEIN AND EMULSION GELS CONTAINING 1 1 VOL. X , 20 VOL. %

Considering the structure of a particle gel network as a collection of fractal flocs closely packed throughout the sample, Walstra and coworkers developed a scaling theory to describe the viscoelasticity of colloidal gels (Bremer er al. 1989; Walstra et al. 1991):

G /=K42/(3-Df) ( 5 )

IIEAT-SET WIEY PROTEIN EMULSION GELS 297

In Eq. (9, D, is the fractal dimensionality of the aggregates; K is an elastic constant that is independent of the volume fraction, but is dependent on the size of the primary particles and on the interactions between these particles. If D, is a constant, then Eq. ( 5 ) is another version of Eq. (4). Assuming that the fractal scaling approach is suitable for these systems over the whole range of concentrations (which is rather questionable), we can calculate the effective fractal dimensionalities of the protein gels and emulsion gels. The results are shown in Table 1 . The effective value of the low-modulus protein gels D, is close to 3 (i.e. close to being nonfractal), but it is reduced to D, = 2.5 for strong protein gels. The effective fractal dimensionality seems to decrease with increase of oil volume fraction. For emulsion gels containing 45 vol. '31 Trisun oil, the fitted fractal dimensionalities are only 2.4 (Dn) and 1.2 (Dn). The latter value seems unrealistically low. Figure 7 shows the graph of slope n2 and fractal dimension D, of protein and emulsion gels against the oil volume fraction 4. The slope n, shows linear decrease with the increase of the oil volume fraction. D, also shows a monotonic decrease with the increase of oil volume fraction. It seems difficult at this stage to relate these fractal dimensionalities in a meaningful way to the nature of emulsion gel network structure.

5

4

3

E" 2

1

0 0 10 20 30 40 50

@PA

3

2

U is

1

0

FIG. 7 . EFFECT OF OIL VOLUME FRACTION 6 ON THE SLOPE n, AND EFFECTIVE FRACTAL DIMENSION D,2

The slope n? is obtained from Eq. (4) and Fig. 6, and D,2 is calculated from the fractal scaling theory using Eq. (5).


1

Figure 8 shows another way to represent the rheological data - as contour lines of emulsion gels having the same storage modulus values. This graph confirms the active role of oil droplets in strengthening the gel. An emulsion gel containing 45 vol. % oil needs only 3 wt % protein to produce a gel that has same strength as a pure 14 wt % protein gel. In Fig. 8, the contour lines for concentrated emulsion gels are generally closer together than for pure protein gels. That is, the higher the oil volume fraction, the denser the contour lines. The densest contour lines are observed for the emulsion gels containing either 20 vol. % and 45 vol. !Z oil and low protein concentrations (low storage modulus). This is understandable from the results in Fig. 2 , where the increase of G’ for these same systems is much steeper at low protein concentrations.

t l 0 0 t 500 t 1000

3000 -x- 5000

-

\+ 1 14

12

10 8 3 a 2

6

4

2

0 0 10 20 30 40 50

@hJol%

FIG. 8. CONTOUR LINES OF STORAGE MODULUS FOR HEAT-SET PROTEIN GELS AND EMULSION GELS

The protein concentration in the system. c,, is plotted against the oil volume fraction #. The inset numbers are the values of storage modulus G’ (units: pascals).

In order to examine the effect of the concentration of emulsion droplets on the elasticity of emulsion gels, it seems appropriate to compare storage moduli of emulsion gels having the same strength of gel matrix but different volume fractions of oil. Figure 9 compares data for three systems containing 10 wt %,


11 wt % and 12 wt % whey protein (here, unlike previously, the protein concentrations are based oiily on [he aqueous phase). The ratio of the storage modulus of the emulsion gel (G’) to that of the pure protein gel (G,,,’) increases with increasing volume fraction of oil phase. The smaller the protein concentration in the aqueous phase, the steeper is the increase. That is, the dispersed particulate material is much more efficient in enhancing the strength of a protein gel that has a weak matrix. For example, for the emulsion gel with 20 vol. % oil and 10 wt % protein in the aqueous phase, the ratio G’/G,‘ is ca. 6.5. However, for an emulsion gel with same volume fraction of oil, but with 12 wt % protein in the aqueous phase, the value of G’/G,’ is less than 2.5.

9 -

- E ’ - 0 c!J >

5 -

3 -

+low% -A- 1 1 wt% t 12 wt%

M=G’dG’,

0 10 20 30 40 50 $/voJ.

FIG. 9. INFLUENCE OF OIL VOLUME FRACTION 6 ON THE RATIO OF THE MODULUS OF COMPOSITE TO THE MODULUS OF MATRIX (G‘Q‘) FOR

EMULSION GELS CONTAINING 10 WT %, 11 W % AND 12 WT % PROTEIN IN THE AQUEOUS PHASE

The parameter M stands for the ratio of the modulus of filler to the modulus of matrix (Gf’/G,,,’). The points and solid lines are experimental results and the dashed lines are calculated

results from van der Poel’s formula (Eq. (6) to @)I.

A formula to estimate the shear modulus of a particulate composite has been developed by van der Poel (1958) and simplified by Smith (1975). It has been shown that an active filler can only result in reinforcement of the composite


when the filler particles are stiffer than the gel matrix. The extent of the increase in gel modulus depends on the modulus of the filler material (Gf). A parameter M was introduced by van der Poel (1958) to represent the ratio of the moduli of filler and gel matrix (M=G,’ / G,,,‘), so that the reinforcement of gel strength can be expressed as a function of M (and other factors). Smith (1975) gives the second-order solution as

where Y, is the Poisson ratio of the matrix and Q is defined by

Q=(8 -10~, )M+7-5~, (7)

The modulus (Gf’) of a liquid filler particle has been estimated by van Vliet using the following simple equation (van Vliet 1988; Luyten and van Vliet 1989):

Gi=2y/r. (8)

Here y is the interfacial tension between the particle and medium, and r is the radius of the droplet. Calculated results from van der Poel’s theory are also shown in Fig. 9 for systems with G’,/G,,,‘ = 12 and 70. There is a large qualitative difference between the experimental and theoretical results. The predicted moduli do not fit the experimental data. A likely reason for this is that the fresh emulsion is already flocculated before the heat-treatment, whereas in the theory the droplets are considered as evenly distributed dispersed particles (van Vliet 1988). Because the volume of an aggregate is greater than the sum of the volumes of the contributing oil droplets, in practice the effective volume fraction of entities responsible for the increase in G’ is much higher than the volume fraction of added oil. As an approximation, we may estimate an effective volume fraction for the flocculated emulsions. The experimental data in Fig. 9 can be roughly fitted with van der Poel’s model if we use &ffec,ive/4 = 2.5 . In addition, van der Poel’s model is only generally valid for systems containing nondeformable particles with no interaction between the filler particles. In our practical emulsion gels, the filler is composed of deformable oil droplets. This could be another reason for the large difference between theory and experiment.

HEAT-SET WHEY PROTEIN EMULSION GELS 30 1

CONCLUDING REMARKS

The viscoelastic properties of heat-set pure whey protein gels and emulsion gels have been investigated as function of protein concentration and oil volume fraction. Protein concentration is the main factor affecting gel strength. Higher protein concentrations produce stronger gels. The experimental data have been well fitted by an empirical equation and the critical protein concentrations have been obtained for emulsion gels containing different oil volume fractions. The higher the oil volume fraction the lower is the critical protein concentration. The protein-covered emulsion droplets can be classified as active filler particles that enhance the gel strength. The plots of In G’ versus In c, show two lines with different slopes. Two different fractal dimensionalities can be fitted using the fractal scaling theory. The values of slope n2 and fractal dimensionality D, decrease with increase in oil volume fraction, which implies that gel strength becomes less sensitive to increase in protein concentration for emulsion gels with large oil volume fraction and at high protein concentrations. Although the oil droplets behave as active fillers, the widely used van der Poel formula fails to match the experimental results. The state of flocculation of the fresh emulsions is believed to be the main reason for the difference between theory and experiment. It is possible that some of the rheological changes observed at certain protein or oil concentrations could be attributable to shear-induced protein interactions occurring during the homogenization process.

ACKNOWLEDGMENT

This research was supported by Contract FAIR-CT96-1216 of the European Commission Framework IV Programme.

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BREMER, L.G.B., VAN VLIET, T. and WALSTRA, P. 1989. Theoretical and experimental study of the fractal nature of the structure of casein gels. J. Chem. SOC., Faraday Trans. 1, 85, 3359-3372.

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VISCOELASTIC PROPERTIES OF HEAT-SET WHEY PROTEIN EMULSION GELS

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