Acid gelation in heated and unheated milks: Interactions between serum protein complexes and the surfaces of casein micelles

The acid-induced interactions between different protein particles in milk (casein micelles and serum protein/kappa-casein complexes) were studied in a series of different mixtures of heated and unheated proteins using diffusing wave spectroscopy (DWS) and small deformation rheology. The measurements were made as functions of pH during acidification by addition of glucono-delta-lactone (GDL). Heat treatment (85 degrees C, 10 min) affected the composition of the serum and the reactivity of casein micellar surface based on the pH at which the casein micelles aggregated during acidification. It was observed that the gel points as defined by DWS and rheology did not always coincide. The experiments showed that all systems containing heated serum proteins gelled at a higher pH than those containing unheated serum proteins. For systems containing heated micelles, an intermediate network can be formed between heat-induced aggregates of serum proteins and kappa-casein formed at the surfaces of the micelles and dispersed as soluble complexes in the serum. This can explain the observation that DWS measurements detected aggregation of casein micelles at an earlier stage than did rheology. For systems containing unheated micelles and soluble complexes from heated milk, the results appear to be explained only by a direct interaction between soluble serum protein complexes and the casein micelles themselves, once the pH has decreased to below about 5.5. Comparison of the different systems studied gives a more complete description of the possible mechanism of interaction of the different protein materials during the acid-induced coagulation of milk-based systems.