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foam stability

Table 4.2 indicates that the heat treatment significantly (P<0.05) decreased the solubility of WP (H1 and H5) compared to control. Native WP are globular with higher numbers of surface hydrophilic residues and buried hydrophobic and cysteine groups resulting high aqueous solubility (Fox and McSweeney, 2003; Zhu and Damodaran, 1994). β-Lg, α-La and BSA have 5, 8, 35 cysteine residues respectively. Naturally around neutral pH the intramolecular disulfide bonds between cysteine residues stabilize the tertiary structure of these globular proteins. β-Lg exists as dimers having 2 disulfide bridges and a buried free thiol group, α-La is a monomer with 4 disulfide bridges but no thiol groups and BSA is also a monomer having 17 disulfide bridges and 1 thiol group (Monahan et al., 1993; Paulsson and Dejmek, 1990).

Table 4.2 Colloidal and interfacial properties of different whey protein preparations produced using heat and high pressure shearing

Treatments: N & H - non heat treated and heat treated whey proteins, respectively. HCT – heat coagulation time; EAI – emulsion activity index; ESI – emulsion stability index. *SEM: pooled standard error of the mean, P < 0.05, (n=8). The different small letter superscripts in a column indicate significant difference (P < 0.05).

Treatment # of passes Solubility, % Heat stability, % HCT, s EAI, m 2_g-1 ESI, h Adsorbed protein, mg/mL Overrun, % Foam stability, s Control 0 96.5 b _100.5b _14.5a ₅₁₈₈a _23.3abc _32.1a ₂₇₅b ₀a N 1 101.0c 99.1b 18.9b 4654a 23.2bc 31.8a 1187d 23.7a 5 94.7b _100.0b _17.4b ₄₆₆₆a _22.3c _33.4b ₈₅₀c _301.8b H 1 28.0a 93.4a 87.8c 8545b 24.0ab 41.2c 0a 0a 5 31.2 a 89.1a 102.5d 9257c 24.4a 41.2c 0a 0a *_{SEM 1.3 1.9 1.13 189.07 0.4 0.14 34 15.0}

During the application of dynamic high-pressure, the forced induced phenomena of cavitation, shear, turbulence and temperature rise occur simultaneously (Bouaouina et al., 2006). This may cause conformational rearrangements in quaternary and tertiary structures of proteins (Bouaouina et al., 2006) with consequent changes of some functional properties such as significantly (P<0.05) increased solubility (sample N1 compared to C) and the significant (P<0.05) decrease in solubility of sample N5 compared to N1 as observed in this experiment.

As shown in Table 4.2 heat treatment had a significant (P<0.05) negative effect on heat stability of WP. This comparatively low heat stability in heat treated samples (H1, H5 compared to C, N1 and N5) may be due to a possible further coagulation and precipitation of protein molecules that have already been denatured. However the combined effect of heat and number of passes significantly (P<0.05) increased the heat coagulation time (HCT) observed at 140°C (Table 4.2, H1 and H5). As described earlier native WP are readily denatured upon heating. But heat treated samples may withstand heat since they have been already denatured and mostly contain no active sites such as free thiol groups to initiate aggregation, which in turn retards heat coagulation. On the other hand, dynamic high pressure has a disruptive effect on intramolecular hydrophobic and electrostatic interactions which finally leads to the subsequent reformation of intra and inter molecular bonds within or between protein molecules (Bouaouina et al., 2006). Therefore microfluidization may have increased the interactions of protein molecules, reducing further availability of reactive sites, consequently increasing the heat stability.

The emulsifying activity index has been significantly (P<0.05) increased (Table 4.2) by heat treatment (H1, H5 compared to C, N1 and N5). The number of passes further increased it for denatured samples (H5 compared to H1). In addition, the combined effect of heat and number of passes significantly (P<0.05) increased the concentration of adsorbed protein on the surface of oil droplets (Table 4.2). The emulsifying activity index is a function of oil volume fraction, protein concentration and the type of equipment used to produce the emulsion (Pearce and Kinsella, 1978). Generally heat may reduce the emulsifying characteristics of proteins due to irreversible protein denaturation; however, partial protein unfolding would improve emulsifying ability (Phillips et al., 1990b). Both surface hydrophobicity which affects the affinity of the

protein for the oil-water interphase and molecular flexibility which influences the ability to unfold and interact with other proteins are important in determining emulsifying activity (Monahan et al., 1993). However the results revealed an increase in the emulsifying activity in heat denatured samples. Therefore the affinity between protein and dispersed phase might be greater than that of protein-protein and thus imparting a thermodynamically more favourable condition to form stable emulsions. This situation would have been assisted by the method used for emulsion preparation during homogenisation of emulsions using dynamic high pressure at 140 MPa. The capacity of protein to stabilize emulsions is related to the interfacial area that can be coated by proteins (Pearce and Kinsella, 1978). The exposure of buried hydrophobic groups upon heat treatment may have caused this enhanced emulsification. In addition, microfluidization may have further increased the emulsion properties of heated samples by dispersing micro aggregates and changing surface properties by unmasking hidden hydrophobic residues and repositioning them towards oil phase. All emulsions produced in this study had high emulsion stability regardless of treatments. Emulsion stability depends on the consistency of the interphase, which does not change with time. Emulsions with an appropriate pH and increased net negative charge present a barrier to the close approach of droplets thus retarding the rate of coalescence, resulting in more stable emulsions (Klemaszewski and Kinsella, 1991). In our study, the experiments were carried out at neutral pH. At this pH, WP have a net negative charge which in turn imparts greater emulsion stability by retarding coalescence through repulsion.

Results also showed (Table 4.2) that the foaming properties of WP were detrimentally affected by heating (H1, H5 compared to C, N1 and N5) while the number of passes significantly (P<0.05) increased the foam overrun (N1 compared to C) and stability (N5 compared to C and N1). The extensive aggregation of WP caused by heat denaturation may have reduced the ability of proteins to produce stable films. However all non-heat treated WP samples (N1 and N5) along with controls also did not show good foaming ability. Lipids in WPC can seriously impair the foaming ability since surface active, polar lipids interfere with protein films by situating themselves at air/water interface (Fennama, 1996). Proximate analysis (Table 4.1) revealed of considerable fat content in our samples. In addition to that fact, these

the internal pressure of air bubbles compared to WP. As a result bubbles expand and finally collapse rapidly resulting in poor foaming. According to our results, dynamic high pressure shearing has positively affected both foam overrun and stability. The foaming properties of WP concentrates are significantly correlated with the amount of

β-Lg, presenting approximately 50% of the total WP (Fitzsimons et al., 2007), and the extent of WP denaturation (Phillips et al., 1990).