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Effect of Certain Salts on the Stability of Skimmilk as Determined by Rennet Coagulation Time and Alcohol Test¹

Department of Dairy Science, University of Alberta, Edmonton, Alberta, Canada

ABSTRACT

The addition of calcium ions to skimmilk greatly decreased the stability as measured by rennet coagulation time and alcohol test. Addition of as little as 1 mg calcium ions per 100 ml of milk shortened the rennet coagulation time. Measurement of the soluble and ionic calcium before and after addition of calcium ions indicated that only about one-fourth of the added calcium remained in the ionic form. Adding up to 60 mg/100 ml of calcium ions shifted the ratio of soluble to ionic calcium from 0.33 to 0.42 and the ratio of ionic to soluble calcium from 0.56 to 0.64. Addition of citrate increased rennet coagulation time and alcohol test value, whereas phosphate did not have a similar stabilizing effect. Addition of citrate counteracted the effect of added calcium, presumably by forming complex (Ca Citr)^– ions. Phosphate apparently cannot form complex ions with calcium.

Measurement of the rennet coagulation time with an automatic blood clot-timer was sensitive and reproducible for measuring milk stability. It gave results similar to the alcohol test, but had definite advantages over the latter.

¹ Financial support for this work was received from the National Research Council of Canada.

² Present address: Department of Dairy and Food Industries, University of Wisconsin, Madison.