Thermal inactivation and variability of γ-glutamyltransferase and α-l-fucosidase enzymatic activity in sheep milk

•Thermal inactivation of EC 2.3.2.2 and EC 3.2.1.51 is presented in sheep milk.•No differences in both enzymatic activities were ascribable to the animal diet.•Enzymatic activity of EC 2.3.2.2 is roughly constant over the whole lactation cycle.•Enzymatic activity of EC 3.2.1.51 increases linearly during lactation cycle.•Enzymes can be used as reliable markers of low thermal treatments in sheep milk.

We present a study of the thermal inactivation and variation of activity of two enzymes in sheep milk: γ-glutamyltransferase and α-l-fucosidase. We find that the thermal treatment of milk causes inactivation of α-l-fucosidase between 57 °C and 68 °C, and γ-glutamyltransferase between 68 °C and 80 °C. In untreated sheep milk, the highest level of enzymatic activity (EA) for these two enzymes is in the pH range 5.5-6.0 and 8.0-8.5, respectively. Skimming treatment causes the loss of 35% of the activity of γ-glutamyltransferase. No differences in the EA were ascribable to the diets evaluated in this study. We observed EAs of α-l-fucosidase and γ-glutamyltransferase over the entire lactation cycle, noting that the EA of the former increased from 81 to 207 U mL−1 (average 159 ± 30 U mL−1) while the EA of the latter from 3.12 to 4.89 U mL−1 (average 3.83 ± 0.34 U mL−1). Relationships between both enzymes and selected milk parameters (lipids, proteins, lactose, urea and somatic cells) are highlighted by Principal Component Analysis.