Activities of alkaline phosphatase, γ-glutamyltransferase and lactoperoxidase in cow, sheep and goat's milk in relation to heat treatment

This study compared the effects of isochrone heating with different temperature–time combinations on the residual activities of alkaline phosphatase (ALP), γ-glutamyltransferase (GGT) and lactoperoxidase (LPO) in bovine (Holstein Friesian Cow), ovine (East Friesian Dairy Sheep) and caprine (German Improved Fawn) milk. Averages of 774, 1413 and 67 U/l ALP were determined in raw milk from cows, sheep and goats, respectively. The GGT values averaged 4143, 1871 and 603 U/l and the LPO activities averaged 2015, 2796 and 5190 U/l. After a holder pasteurisation (62 °C for 30 min to 65 °C for 32 min), the ALP activity in milk from the three mammals was <0.6 U/l, whereas the residual activity of GGT remained between 70% and 80% relative to raw milk after heating to 62 °C for 30 min, and 10–40% after heating to 65 °C for 32 min. The LPO activity of bovine milk remained unchanged, while that of ovine and caprine milk was reduced by about 5%. Milk heated with high-temperature-short-time (HTST, 75 °C for 28 s) conditions showed ALP activities of <0.1 U/l, whereas the residual activities of GGT were 6%, 40% and 11%, and the activities of LPO were 39%, 53% and 43% relative to raw milk from cows, sheep and goats, respectively. Residual ALP activities in commercial bovine and caprine HTST and high-heated milk samples from the German market were <0.07 U/l. In UHT-heated milk samples of bovine origin, ALP activities of 0.170–1.035 U/l were found due to reactivation of the enzyme. Studies evaluating the inactivation of ALP, GGT and LPO activities by isochrone heating of milk (35–85 °C for 90 s) have shown that the heat stability of the indigenous enzymes increased in the following order: ALP < GGT< LPO. Furthermore, it has been shown that the residual enzyme activities in heated milk of non-bovine origin were considerably higher than bovine milk. This must be considered if the limiting values for indigenous enzyme activities in milk are fixed.