Proteolysis and partial dephosphorylation of casein are affected by high somatic cell counts in sheep milk

Mastitis, an inflammatory affection of the mammary gland, causes milk somatic cells (SC) to increase. The individual milk composition changes while that of bulk milk is remarkably constant, particularly in the lactose content. A study has been conducted on non-infected ewes as a control group and diagnosed unilateral subclinical or clinical mastitis. Milk samples were taken from 240 half udders, with low (<500 x 103/ml), medium (500-1000 x 103/ml) and high somatic cell count (SCC) (1000-2000 x 103/ml) of a flock of 120 primiparous Sarda ewes. In milk with high SCC, the majority of the peptides measured by ESI Q-TOF MS arose from αs1-casein (CN), αs2-CN, β-CN and κ-CN. The focus of the present work was on phosphopeptides (CPP) released by milk enzymes. In unhealthy milk, seventeen CPP and twenty partially dephosphorylated CPP were identified through the selective enrichment of CPP on hydroxyapatite (HA). This enrichment was coupled to MALDI-TOF. Although MALDI-TOF analysis faces quantitative limitations in the detection of highly phosphorylated peptides, it was, however, useful for identifying partially or fully dephosphorylated peptides. These results are well correlated with the alkaline phosphatase (ALP) activity of milk. The endogenous overall proteolysis of CN afforded the increase of the hydrolytic products of β-CN. Therefore, in addition to ALP, plasmin (PL) activity is indicated as valid predictor of endogenous proteolysis in sheep‟s milk.