Preheating milk at 140 °C for 1 min at pH 6-6, 6-8, 7-0 or 7-2 shifted the heat coagulation time (HCT)/pH profile to acidic values without significantly affecting the maximum stability. Whey proteins (both /?-lactoglobulin and a-lactalbumin) co-sedimented with the casein micelles after heating milk at pH < 6-9 and the whey protein-coated micelles, dispersed in milk ultrafiltrate, showed characteristic maximaminima in their HCT/pH profile. Heating milk at higher pH values (> 6-9) resulted in the dissociation of whey proteins and K-casein-rich protein from the micelles and the residual micelles were unstable, without a maximum-minimum in the HCT/pH profile. Preformed whey protein-casein micelle complexes formed by preheating (140 C C for 1 min) milk at pH 6-7 dissociated from the micelles on reheating (140 °C for 1 min) at pH > 6-9. The dissociation of micellar-/c-casein, perhaps complexed with whey proteins, may reduce the micellar zeta potential at pH ~ 6-9 sufficiently to cause a minimum in the HCT/pH profile of milk.

No abstract

The dissociation of casein micelles when heated to approximately 65 degrees C in the presence of ethanol [1:1 mixture (v/v) of milk and 65% (w/w) aqueous ethanol] was investigated using L* values and transmission measurements. Mixtures of milk and ethanol became transparent on heating, which suggests dissociation of casein micelles. Results of experiments using confocal laser scanning microscopy, light scattering (static and dynamic), and dialysis to examine the changes of milk during heating in the presence of ethanol supported the assertion that such treatments result in dissociation of casein micelles, as did studies of model beta-casein micellar systems.

Skim milk powders were prepared from control and transglutaminase-treated skimmed milk. The heat stability of reconstituted transglutaminase-treated skimmed milk (9.0% total solids) was markedly increased in the pH region of minimum stability (pH 6.8 to 7.1) compared with control milk, while the heat stability of reconstituted concentrated transglutaminase-treated skimmed milk (22.5% total solids) increased progressively as a function of pH relative to control milk. The effect of transglutaminase treatment on the heat stability of skimmed milk may have commercial applications, but extensive research is necessary to gain a better understanding of the mechanism by which transglutaminase improves heat stability.

A method that uses Gerber fat testing equipment to measure free oil in melted Mozzarella cheese was developed and evaluated. Recovery of butteroil added to Mozzarella cheese samples was virtually complete. Coefficients of variation (six replicates) for three cheese samples were 3.26, 3.56, and 3.89%. The test was used to measure free oil formation in Mozzarella cheese during 3 wk refrigerated storage.