INFLUENCE OF PRELIMINARY TREATMENTS ON STRUCTURAL-PROPERTIES OF CASEIN MICELLES AFFECTING THE RENNETABILITY

Different reconstituted skimmed milk powders, as well as defatted past eurized and raw milk, were used to probe the effects of preheating con ditions on rennetability and molecular availability of casein to coagu late and to form a curd. The experiments for following coagulation and gel formation were carried out using the formagraph at pH 6.4, 35 deg rees C, and 3.8% total protein. Two new approaches to estimate the sur face protein hydrophobicity (SPH) and to quantify the glycosylated and non-glycosylated caseinomacropeptide (CMP) were used to study the mol ecular state of casein micelles during the primary and secondary phase s of renneting. The latter method was supported by ion-exchange fast-p rotein liquid chromatography (FPLC) using Mono-Q and Mono-S columns Re sults with raw milli showed that the SPH of native micellar casein is about eight times higher than that of the individual casein components . In processed milks, the range of maximum SPH decreased as the severi ty of preheating increased. It was found that a high SPH is correlated with an optimum micellar structure. The latter is progressively lost by association of denatured beta-lactoglobulin (beta-Lg) on the micell ar surface, especially with the outer part of non-glycosylated kappa-c asein (kappa-Cn). The extent of association is related to the severity of preliminary milk treatment, which may be measured by a reduced lib eration of non-glycosylated CMP.