The structure of casein in heated suspensions affects the acid gelation of milk.

In the purpose of improving texture properties of yogurt, milk composition was modified first by substitution of 12% of micellar casein by Na- or Ca-caseinate and by a native phosphocaseinate as control (Pcas) and second by fractionation into a small and a large micelle fractions using centrifugation. Analyses of suspensions include total (TN), non casein (NCN) and non-protein nitrogen (NPN), before and after heat treatment at 90degreesC for 10 min, mineral partition between colloidal and soluble phase and particle diameter. Bacterial acidification at 42degreesC was followed by the dynamic viscoelastic method. Whey protein (WP) amount was the same in the 3 substituted suspensions, but the Na-caseinate suspension contained less calcium than the others. The higher moduli and tan 6 were obtained for Na-, Ca-caseinate and Pcas in decreasing order, meaning a more viscous character in a firmer gel with a lower level of structuration. The smaller micelles led to a lower modulus, though it had the higher concentration of WP and though the smaller micellar fraction contains more k-casein, which could enhance the formation of heat-induced complexes between casein and WP. Gelation of the larger micelles was observed at a higher pH value (pH 5.5) than for the smaller micelles (pH 5.3).