Flow behaviour of WP samples

Figure 5.7 –A, -B and –C and Table 5.3 present the rheological parameters of WP dispersions, including apparent viscosity, consistency coefficient, k (Pa.sn), and flow behaviour index, n (dimensionless), obtained by the model fitting, which used to characterise the flow behaviour of the WP samples (10, 17.5 and 25%, respectively) at pH 4, 5 and ~ 6. The k and n values were obtained with high correlation coefficients by fitted to the Power Law model. As Figure 5.7 –A, -B and –C demonstrate the apparent viscosity of all WP dispersions has decreased with increasing shear rate, thus they could be referred to as non-Newtonian fluids with shear thinning behaviour (Rao, 1999). The statistics also showed that pH alone has a significant (P<0.05) effect on k values where as the interaction between pH and concentration has significantly (P<0.05) affected both the k and

n values of WP dispersions. The molecular properties of WP always influence their flow behaviour as described in Chapter 4. Whey proteins, especially β-Lg, show a Newtonian behaviour up to 5% w/w protein, but shear thinning at higher concentrations (Fox and McSweeney, 2003). The pseudo-plastic (shear thinning) character of these dispersions may have resulted from the disruption of weak inter-particle linkages at sufficiently high enough hydrodynamic forces which were generated when shearing (Rao, 1999) and also the affinity of protein molecules to align with the direction of flow (Damodaran et al., 2008). According to Figure 5.7 -A, pH did not show considerable effect in changing the shear thinning behaviour of WP at comparatively diluted 10% concentration as shown by almost mirrored flow curves.

Table 5.3 Rheological parameters of whey protein dispersions, containing 10, 17.5 or 25% (w/w) proteins, obtained by fitting the experimental data to Power Law model

k– consistency coefficient; n – flow behavior index. R2_{– correlation coefficient;} *_{SEM - Pooled}

standard error of the mean, P < 0.05. Means present the average of at least 4 independent observations (n=4). The different small letter superscripts in a column indicate significant difference (P < 0.05). Concentration, % w/w pH k, mPa.s n _{n R}2 10 4 19.5 a 0.76b 0.98 5 16.5a 0.73a 0.97 ~ 6 27.8a 0.64a 0.97 17.5 4 59.8a 0.87bc 0.99 5 31.0a 0.82b 0.99 ~ 6 27.8a 0.78b 0.99 25 4 911.3c 0.75b 0.98 5 237.0b 0.77b 0.94 ~ 6 55.3a 0.94c 0.99 *_{SEM 45.0 0.03 0.02}

Figure 5.7 Apparent viscosity of 10% (A), 17.5% (B) and ~25% (C) (%w/w protein) WP dispersions at pH 4, 5 or 6 during a controlled shear rate sweep (0.1 - 100 s-1) at 20°C.

In diluted solutions, the polymer molecules behave independently, and the viscosity depends on the dimensions of the polymer such as hydrodynamic volume and the radius of gyration (Rao, 1999). As results indicate, the WP, particularly as compact globular proteins, may not have any substantial difference in their dimensions in the presence of vast amount of solvent although the pH is changed.

However, when the concentration was raised up to 17.5% protein, the sample at pH 4 showed the highest initial viscosity (yield stress) relative to the samples at other two pH values. It also exhibited a comparatively higher viscosity values through out the shear rate ramp. The WP dispersion at pH 5 possessed an intermediate effect on apparent viscosity and the sample ~ pH 6 showed the lowest viscosity during the shear rate sweep. The greater viscosity of pH 4 sample indicates its higher resistance to induced flow relative to other two samples. Due to WP charged nature, their rheological behaviour may also be influenced by pH of the medium. At acidic pH, the degree of hydration of proteins may become greater due to greater net positive charge on protein molecules leading to a greater affinity of water molecules towards proteins. In addition, the intra-molecular repulsion present in WP under an acidic pH may facilitate partial unfolding of proteins in conjunction with hydrodynamic forces which may finally end up in increased viscosity of the dispersion. Solutions of randomly coiled molecules frequently display greater viscosity than solutions of compactly folded macromolecules of same molecular weight (Damodaran et al., 2008). Similarly the lowest apparent viscosity of pH 6 dispersion may have resulted from enhanced repulsive forces predominating at lower shear rates and likely weak interactions among protein aggregates easily disrupted under increased shear.

The change of apparent viscosity during shearing of WP dispersions containing ~ 25% protein (Figure 5.7 -C) was fairly different from the other two concentrations since the highest initial yield point was observed for pH 5 sample. However, with the application of shear force, the apparent viscosity declined sharply indicating brittleness of this system. Such a high initial viscosity of this sample may be

attributed to elevated protein-protein attractive interactions through hydrophobic and ionic surface patches present in WP under a minimum electrostatic repulsion. In addition, the protein-protein attraction may have been facilitated via increased volume fraction of dispersed particles at a lower water content minimizing the inter particle distances (Patocka et al., 2006). Moreover, the pH 4 sample maintained the overall maximum viscosity similar to what was observed for the sample with 17.5% proteins.

Several important observations can be made from Table 5.3. The comparatively larger k values were obtained at lower pH at higher protein concentrations. The k

value or consistency index is directly proportional to the resistance of a material to flow (Rao, 1999). On the other hand, the flow behaviour indexes (n) had a similar pattern for samples containing 10 or 17.5% proteins, deviating more from Newtonian flow with elevation of pH. This was however reversed for the concentrated sample (~25%) with the highest n value obtained at pH 6 which could indicate enhanced flow due to greater intra-molecular repulsions.

5.3.7 Viscoelastic properties of whey protein dispersions during heat-induced