Milk Lipase Activity: A Method for its Determination, and its Relationship to the Estrual Cycle

Abstract

Samples of milk were dried and defatted by extracting one vol. of milk at room temp., twice with acetone, once with equal parts of acetone and ether and twice with ether each time using 5 vol. of sol- vent. A very soft, fine precipitate was obtained which kept indefinitely. The dried milk and substrate were incubated without preservative and extracted with ether. Alcohol was then added to the extract which was titrated with aqueous NaOH. Tributyrin, diacetin, triacetin, tricaproin, ethyl oleate and sterilized butter oil were used as substrates. Tributyrin appeared to be a more sensitive indicator of milk which developed rancidity than did tricaproin, triacetin, tri-palmitin or sterilized butter oil. There appeared to be no relationship between the lipase action which causes the development of hydrolytic rancidity and that which attacks the substrates diacetin and ethyl oleate. Milk lipase appeared to show some selectivity in its hydrolysis, acting more rapidly on the volatile than on the non-volatile substrates. When milk was kept an additional 24 hrs. before drying, there was a marked increase in tributyrinase activity. Cream obtained by gravity separation had much greater lipase activity than did skim milk from the same sample. Data on milk collected at frequent intervals from open cows showed a definite relationship between the estrus cycle and tributyrinase activity. Greatest activity occurred several days before and the least activity during or just after estrus. This may explain why some cows produce milk which develops rancidity for short periods early in lactation.