Controlling the viscosity of milk concentrates through tailored casein-whey protein interactions
Heat-induced interactions between caseins and whey proteins form the basis for their functionality in many applications. We have hypothesized that the pH of the milk before preheat-treatment and adjusting preheating temperatures will influence the distribution of casein-whey protein complexes between the micellar and the soluble (serum or continuous) phase of milk and this will affect the viscosity of the continuous phase in concentrated milk. Therefore, the objective of the present study was to investigate the effect of pH and preheating temperatures on the viscosity of skim milk concentrates.
Reconstituted milk, 10%w/w total solids (TS), adjusted to pH 6.5, 6.7 (control) and 6.9 was preheated either at 80 or 90°C for 5 min. Following these treatments, the milk was concentrated to final TS of 45 and 50 % (w/w) under vacuum at 60°C using a rotary evaporator. Dynamic viscosity of the resulting concentrates was measured at 55°C at a constant shear rate of 100 s-1using a Stresstech Rheometer. Particle size was determined using a Malvern Zeta-sizer-Nano-ZS. The heated and pH adjusted samples were centrifuged at 25000g for 1h to obtain soluble (serum) and micellar phases. The protein interactions in these milk samples were characterized in detail using non-reduced and reduced sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The experiments were repeated at least two times and results were tested by ANOVA and statistical significance at P<0.05 was determined, using the statistical software SAS.
Significant differences in the viscosity of milk concentrates were observed (P<0.05) when the milk was pre-adjusted to different pH and pre-heated at different temperatures. The results showed that the concentrates obtained from milk preheated at pH 6.5 and 80°C for 5 min had significantly (P<0.05) lower viscosity compared to that pre-heated at pH 6.7 and 6.9, at both TS levels studied (45 or 50%w/w). The samples pre-heated at pH 6.5 also showed increase in particle size in contrast to the samples preheated at pH 6.7 and 6.9. Such differences can be explained by differences in the interactions of casein and whey proteins and distribution of casein-whey protein complexes distributed at continuous phase and micellar phase as shown by SDS-PAGE.
It can be concluded that adjusting the pH and preheating temperature of milks can be used as levers for controlling viscosity of milk concentrate for powder manufacturing, which ultimately has an influence on the efficiency of drying.
Keywords: milk concentrate, viscosity, casein-whey proteins, pH.