Abstract
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.
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