Use of Milk Enzymes as Indices of Heat Treatment

Methods for the assay of naturally occurring enzymes in milk were investigated and suitable techniques obtained. The effect of laboratory pasteurization at 65, 70, 75 and 80°C for 15 s on the activity of nine enzymes present in milk was studied. The activity of these enzymes remaining after heating at temperatures between 65 and 80°C with a holding time of 15 s using a plate heat exchanger was generally less than expected from the laboratory data. Four enzymes, acid phosphatase, amylase, lactoperoxidase and xanthine oxidase, were chosen for further study and holding times were varied at temperatures between 65 and 80°C in order to calculate D- and z-values. Data are also presented on the variation in the activity of the enzymes studied in raw milk. The results obtained in the present study were compared with those in the literature. Of the enzymes studied, lactoperoxidase offers the most promising method for detecting heat treatments of the order 76°C for 15 s. Commercially available strips for the detection of enzyme activity are available (API ZYM), but they are of little use in assessing the heat treatment which has been applied to milk.