Rennet coagulation properties of retentates obtained by ultrafiltration of skim milks heated to different temperatures

Skim milks were heated at temperatures ranging from 72 degrees C x 15 s to 100 degrees C x 120 s to give levels of whey protein denaturation (WPD) varying from similar to 1 to 70%. The different milks were then ultrafiltered to yield retentates with protein levels ranging from similar to 3 to 19%. Rennet was added on a volume basis and the coagulation properties of renneted retentates measured by low amplitude oscillation rheometry. For all heat treatments, the shear storage modulus, G', increased more than proportionally with protein concentration (P), i.e. G' proportional to P-n, where n similar to 2, at times > > 3600 s. A similar trend was noted for curd firming rates, with the power law index being similar to 2.4. On heating under normal pasteurization conditions, i.e. 72 degrees C x 15 s, gelation time increased slightly with protein concentration. At the higher heat treatments, gelation time decreased markedly with protein concentration (especially at levels of WPD similar to 70%), and approached those of the normal pasteurized milk at protein levels of 12-18%, depending upon the heat treatment and gel time definition. (C) 1996 Elsevier Science Limited