Rationale The originality of this experiment was to study simultaneously several native properties of milk (lipolysis, heat stability and freezing point) that determine milk consumption or processing in the context of climate change and efforts to decrease environmental impacts. Aim We assessed effects of interactions among ambient temperature, dietary nitrogen content and milking shift (morning and evening) on the composition, structure and native properties of milk. Methods The experiment followed a double 4 × 4 Latin square design, using eight dairy cows for four periods of 3 weeks each. We compared two levels of dietary nitrogen content (13.5% crude protein (N−) or 17.5% crude protein (N+)) and two levels of ambient temperature (18°C for the entire day (T−) or a diurnal peak of 28°C for 7 h (T+)). Major Findings Dietary nitrogen content had no effect on milk fat or protein content or fatty acid composition. N+ significantly decreased calcium content, tended to increase the diameter of milk fat globules, but had no effect on that of casein micelles. N+ decreased lipolysis and the freezing point of milk but had no effect on its heat stability. T+ increased the crude protein, total phosphorus and colloidal phosphorus contents, and decreased the diameter of casein micelles and the Na + /K + ratio. T+ increased the saturated fatty acid concentration and decreased the mono‐unsaturated fatty acid concentration. T+ also increased pH and heat stability. Milking shift had no effect on milk protein content, but evening milking increased fat content, unsaturated fatty acid concentration and Δ9‐desaturase activity. Evening milking increased the freezing point and lipolysis and decreased heat stability. The effect of temperature must be retested under better‐controlled temperatures. Scientific or Industrial Implications Knowledge of the technological behaviour of milk in the face of variations in feed and climate will allow better adaptation of dairy processing.
Hurtaud et al. (Thu,) studied this question.