Effect of physiological stage and season on dairy milk composition and technological characteristics (coagulation properties, lipolysis)

Abstract

The effect of physiological stage (especially the end of gestation in milk producing cows) and of season on the composition and technological characteristics of milk have been studied on the basis of data reported in the literature. At a given lactation stage, the milk fat and protein contents are lower in summer than in winter in an opposite pattern to that noted with milk production. The maximum differences between monthly averages of milk fat and protein contents and milk production amounted to 3 g/kg, 2 g/kg and 2.5 kg/d respectively. Within the first 2 to 3 months of lactation the coagulation time increased by 30 % to 40 %, the rate of firming doubled and the curd firmness at 30 min decreased by half. Even higher changes in these parameters were noted within the last weeks of gestation : the coagulation time and the rate of firming were reduced by 75 % between the 5^th and the last week before calving. These modifications might result from changes in milk pH. Indeed, spontaneous lipolysis in milk depends primarily on gestation stage and level of production. When cows are not dry, the degree of lipolysis is at maximum 2 weeks pre-calving (a level 40 times higher than normal) and decreases back to the normal level on the first day of lactation. Similar changes in lipolysis are induced by treatment with reproductive hormones in non-gestating cows.