B. Lieske, INFLUENCE OF PRELIMINARY TREATMENTS ON STRUCTURAL-PROPERTIES OF CASEIN MICELLES AFFECTING THE RENNETABILITY, Le Lait, 77(1), 1997, pp. 201-209
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.