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Micellar Changes in Skimmilk Treated with Alkali or Acid

Department of Food Science and Technology, University of California, Davis

ABSTRACT

Samples of skimmilk were adjusted to various pH levels with N-KOH or N-HCl, and after 3 hr at 20 C were restored to the original pH. The rennet coagulation times of milks transiently above pH 10 increased logarithmically with increasing pH. Milks transiently at acid pH's coagulated more rapidly.

Since soluble calcium is an important variable influencing coagulation time, it was measured by a dialysis procedure to determine its role. Colloidal calcium content was nearly constant in the milks of the higher pH range, but gradually decreased in milks of the lower pH range.

Turbidity decreased when the samples were adjusted to above pH 8.7 and below pH 5.4. In the alkali range the micelles were swollen and altered by pH change, whereas colloidal calcium phosphate was not disturbed.

Accordingly, the rapid decrease of rennet coagulation time in milks transiently at lower pH was attributed to the increase of soluble calcium. The logarithmic increase in coagulation time in milks treated above pH 10 was attributed to protein changes that altered micellar interaction and H₂S liberation.