[Utilization of pure proteins and peptides for the study of chemical and nutritional changes suffered by milk during heating].

Milk proteins, and in particular the caseins, undergo during heat treatments (120 degrees C, 20-30 min) physico chemical and nutritional modifications. Using pure proteins (alphas and beta caseins) and peptides, it has been possible to dissociate the effects of heat treatments which, in milk can hide each other. Physico chemical properties of caseins (the ability to bind Ca++ and anionic dyes, acid-basic titration curves, electrophoretic behavior) are strongly altered, while the constitutive aminoacids are less modified. The accessibility of the aminogroups of lysine to the fluorodinitrobenzen and to the O-méthylisourea is lowered with a small rate. Lysinoalanyle interactions between serine and lysine are weak at a neutral pH while the isopeptide bonds epsilonN (gamma glutamyl)lysyle are only produced with more severe conditions as those of sterilization. The digestibility to the proteases is enhanced with low heat treatments then decreases with more severe treatments (120 degrees C-80 min.). Many peptides, which are released from the caseins during heating are issued from low specific split peptidic bonds; these peptides have often stimulating properties on the growth of lactic bacteria.