Effect of udder health status and lactation phase on the characteristics of Sardinian ewe milk.

Mammary involution and inflammation are known to negatively affect milk quality. A trial was carried out to elucidate the mechanism by which udder health status and lactational phase determine compositional modifications in ovine milk. A total of 60 individual milk samples was collected from a group of 20 pluriparous Sardinian ewes from mid to late lactation. Each sample was assessed for its chemical characteristics, quantitative distribution of casein fractions, lactodynamographic characteristics, and enzymatic activity. Udders were classed as healthy, doubtful, or infected on the basis of repeated somatic cell counts, and samples were grouped in 3 classes of days in milk. Results indicated that both udder inflammation and mammary involution can increase plasmin (PL) activity (15.6 vs. 18.4 U/mL in healthy vs. infected udders; 14.0 vs. 20.2 U/mL in phase 1 vs. 3), which is responsible for an evident protein breakdown in milk. Significant differences between groups were observed for several characteristics. With regard to udder heath status, casein index was lower in the infected vs. healthy udders (74.8 vs. 76.6%), and beta(tot)-casein showed a similar trend (43.9 vs. 46.6%). As a consequence of protein degradation, gamma-casein (5.78 vs. 2.82%) and proteolysis index (7.60 vs. 3.82) increased in the infected group with respect to the healthy group. Udder health status also affected milk technological traits. Udder inflammation resulted in longer clotting time (20.7 vs. 16.5 min for infected vs. healthy, respectively) and in poorer curd firmness (35.6 vs. 47.6 mm for infected vs. healthy, respectively). Frequency of samples reactive to rennet was 100, 93, and 67%, respectively, for healthy, doubtful, and infected groups. With regard to lactational phase, a decrease in alpha(s1)-casein (39.13 vs. 29.36%) and beta(1)-casein (23.41 vs. 19.36%) occurred during phase 1 vs. 3, whereas kappa + alpha(s2)-casein increased (12.30 vs. 21.56%, phase 1 vs. 3). Correlation coefficients confirmed the role of PL in protein degradation. It was concluded that PL activity was strongly affected by both lactational phase and udder health status and, in turn, could be an important agent enhancing milk quality detriment.