Heat stability of micellar casein concentrates as affected by temperature and pH

The increased interest in using micellar casein concentrates (MCC) obtained by microfiltration in the manufacture of shelf-stable high-protein beverages creates a need to understand the effect of sterilization treatments on the stability of this ingredient. The goals of this work were to (1) elucidate the effects of pH and heat treatment temperatures in the sterilization range on the stability of MCC, and (2) use the generated knowledge to develop solutions for stabilizing the MCC during sterilization treatments. Micellar casein concentrate powders were reconstituted, and the resulting casein dispersions were adjusted to pH values of 6.5 to 7.3. Subsequently, the MCC samples were heated in an oil bath to 110 to 150°C. The treated samples were evaluated for particle size, soluble minerals, and casein dissociation. At pH <6.7, all heat-treated samples were visibly aggregated or coagulated. At pH 6.9, higher temperatures led to increased particle size, whereas at pH >6.9, few or no changes were observed after heat treatment. Casein dissociation increased with increasing pH for all caseins, at all temperatures, with dissociation of κ-casein and β-casein being the most pronounced. At higher pH, the levels of dissociated αs-casein decreased after heat treatment, suggesting aggregation of αs-casein in the presence of Ca and protection lost by κ-casein. It was concluded that increased stability of MCC requires increasing the pH or lowering the processing temperature. After applying these modifications, MCC was submitted to both retorting and UHT sterilization, at equivalent lethality. A significant reduction in particle size was obtained and no coagulation or aggregation occurred after retorting or UHT under the modified conditions as compared with the controls. The knowledge generated in this study will allow the effective stabilization of MCC in practical applications, such as the production of high-protein, shelf-stable beverages.