METHODICAL OPTIMIZATION OF THE DETERMINATION OF RAW-MILK PROPORTIONS IN PASTEURIZED MILK AND RECONSTITUTED MILK VIA ALKALINE-PHOSPHATASE ACTIVITY

In the present paper investigations on the optimization of the determination of raw milk proportions in pasteurized and in reconstituted milk via measurement of alkaline phosphatase activity are described. Alkaline phosphatase belongs to the group of minor proteins in milk and has been used for more than 50 years for furnishing proof of correct pasteurization of milk. So far, the IDF-standard methods as well as the European reference method as static measurement are applied for detecting alkaline phosphatase in milk. In the present study the optimized method for the determination of the volume activity of alkaline phosphatase in milk (l.c. 16) was used for the detection of this enzyme in pasteurized and in reconstituted pasteurized milk with and without raw milk proportions. Here, a partial regeneration of the thermally inactivated phosphatase was observed after clarification of milk with a detergent (Genapol X 80) also in properly pasteurized milk. In the reconstituted pasteurized milks studied this proportion of phosphatase able to reactivate was also determined. However, measurement of a blank value with a scalded milk sample from the milk analysed has allowed all other factors from the milk matrix which might influence the measurement to be compensated. The determination of raw milk proportions of 0.5 and 2% using the optimized method (l.c. 16) is proportional to the recovered proportion of alkaline phosphatase activity. The order of enzyme activity established for the raw milk proportions is in agreement with the results of a longitudinal study of the activity of alkaline phosphatase in untreated bulk milk (mean activity :2400 u/l). A further validation of the method for the determination of raw milk proportions in pasteurized and reconstituted milk via measurement of the activity of alkaline phosphatase is to be done by the competent committees of the BGA (federal association of the German wholesale and foreign trade) according to 35 LMBG (food law) in cooperation with DIN (German industry standards).