Changes in milk composition as affected by subclinical mastitis in sheep and goats

The present study focused on gaining a better insight into how subclinical mastitis at the gland level in dairy sheep and goats affects milk yield and compositional changes in relation to curd yield. Animals chosen for the study had one udder half infected with identified coagulase-negative staphyloccocci (CNS) species and the contra-lateral gland was free of bacteria. Udder halves were tested for udder bacterial condition, California mastitis test, somatic cell count (SCC), milk yield, milk composition (fat, protein, lactose, casein (CN), and whey protein), curd yield (Yc) and milk clotting time (Tc). Data were statistically analyzed with a nested block design. Curd yield from the infected halves was lower than that from the uninfected ones for both sheep and goats, although CN concentrations were almost equal in the two glands. The data suggest that knowledge of the gross CN content in the milk is insufficient for predicting Yc, probably because of modifications in the CN micelles or in the various casein micelle components that are more detrimental to curd formation than they are to the CN concentration itself. The primary enzymatic coagulation is based on the action of rennin on κ-casein, which thereafter exposes hydrophobic sites on the casein micelle, thus making it available for the secondary aggregation reaction. The complications in the coagulation process are also supported by the longer Tc of the milk from the infected glands. However, the effect of rennet on the caseins and thereafter the coagulation process may be impeded by only partial hydrolysis of κ-casein and more pronounced hydrolysis of the other caseins by enzymes such as PL and cathepsin.