Effect of somatic cell count and lactation stage on sheep milk quality

Abstract

In order to evaluate the effects of mammary health status and lactation phase on the qualitative parameters of ovine milk, 213 individual milk samples were repeatedly collected from 40 primiparous Sarda ewes on a monthly basis. Yield, physico-chemical characteristics, casein fractions quantitative distribution, somatic cell count (SCC), cheese making properties and plasmin-plasminogen activity were determined on each sample. Repeated individual milk SCC were used as a marker of udder health status, allowing the definition of three classes: "Healthy" (H), "Infected" (I) or "Doubtful" (D). Samples were grouped into 4 classes of days in milk (DIM). To evaluate the influence of mammary health status and phase of lactation, a mixed model was performed using the ewe as random effect. Milk physico-chemical parameters were influenced both by the udder health status and by lactation phase. In particular, the udder health status adversely affected αs1 and β1-casein fractions (P<0.01) and curd firmness (P<0.05). Samples reactive to rennet were 92.73%, 70.65% and 64.60% in "H", "D" and "I," respectively. Lactation phase influenced the overall milk composition and technological characteristics. Plasmin activity was higher in the "I" group than in the others (16.1 vs 11.8 and 11.2 U/ml; P<0.01) and it significantly (P<0.01) increased during lactation. In conclusion, both mastitis and mammary involution are proven to exert a detrimental effect on milk quality since they enhance its endogenous proteolytic activity.