Influence of preliminary treatments on structural properties of casein micelles affecting the rennetability

Abstract

Different reconstituted skimmed milk powders, as well as defatted pasteurized and raw milk, were used to probe the effects of preheating conditions on rennetability and molecular availability of casein to coagulate and to form a curd. The experiments for following coagulation and gel formation were carried out using the formagraph at pH 6.4, 35 °C, and 3.8% total protein. Two new approaches to estimate the surface protein hydrophobicity (SPH) and to quantify the glycosylated and non-glycosylated caseinomacropeptide (CMP) were used to study the molecular state of casein micelles during the primary and secondary phases of renneting. The latter method was supported by ion-exchange fast-protein liquid chromatography (FPLC) using Mono-Q and Mono-S columns. Results with raw milk 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 severity 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 β-lactoglobulin (β-Lg) on the micellar surface, especially with the outer part of non-glycosylated κ-casein (κ-Cn). The extent of association is related to the severity of preliminary milk treatment, which may be measured by a reduced liberation of non-glycosylated CMP.