Gamma-glutamyl transpeptidase in milk and butter as an indicator of heat treatment

Naturally present γ-glutamyl transpeptidase (GGTP) in whole and skim milk was inactivated by heat treatment at >79°C for 16 s. Of the total activity in whole milk, 72% was found in the skim milk fraction. Little seasonal variation was noted in either whole or skim raw milk over a period of 300 days. Using a commercially available test kit for GGTP, as little as 0·1% raw milk or cream could be detected in pasteurized skim milk and butter. An alternative GGTP method examined was less sensitive than the commercial method. However, it was necessary for cream products with low GGTP activity since cream interfered with the commercial assay. No reactivation of GGTP was found in whole milk or butter under a variety of conditions. Commercial milk and cream samples were negative for GGTP activity. The results suggest that GGTP analysis could be useful for monitoring the heat-treatment give to fluid milk products.