Reactivation of alkaline phosphatase in ultra high-temperature, short-time processed liquid milk products

Alkaline phosphatase (enzyme) reactivation was studied in liquid milk products heated to 87.8 to 121.1 C for less than 1 s in a continuous, two-phase, slug-flow heat exchanger. The effects of magnesium concentration, pH, incubation temperature, homogenization pressure, processing temperature, fat content, and initial enzyme concentration were investigated. Jersey milk from one farm showed seasonal variations in enzyme concentration and its reactivation behavior. Increased reactivation in products with high fat content was due to high initial enzyme concentration in the product. Homogenization of products before heating decreased reactivation. Maximum reactivation occurred in products heated to 104.4 C, incubated at 34 C, and adjusted to pH 6.5. Maximum velocity of reactivation and reactivation constant varied with milk samples. Activation energy for the control and samples with magnesium was 22.646 +/- .118 and 24.100 +/- .210 kJ mole-1, respectively. The enzyme from raw and reactivated cream contained two major isozymes, and the reactivated isozyme differed from the control. The official method for differentiating residual and reactivated enzymes was modified in terms of magnesium concentration and extent of reactivation of the enzyme in the reactivated product.