Renneting properties of milk from individual ewes: influence of genetic and non-genetic variables, and relationship with physicochemical characteristics

Abstract

The physicochemical characteristics and the renneting properties of milks from Lacaune ewes are reported. The ewes belonged to divergent genetic lines termed ‘low’ or ‘high’ and were selected on a combination of fat and protein yield. The 179 second to fifth parity ewes were sampled three times during lactation at about 50, 135 and 175 d in milk. Physicochemical and renneting variables were principally influenced by days in milk and somatic cell count, whereas β-lactoglobulin genotype, parity and divergent lines had relatively few effects. Fat and protein content increased throughout lactation, whereas rennet gel firming rate decreased. Salt distribution and micellar characteristics were also significantly affected by days in milk. Milks with high somatic cell count were characterized by high values of pH and soluble nitrogen content and rennet coagulation properties were significantly affected at >3×10⁵−5×10⁵ cells/ml milk. Divergent lines had a strong effect on milk yield, but no effect on milk composition consistent with the selection criteria. Gel firming rate was slightly higher in milks from high yield ewes. Protein content, Ca concentration, pH and micelle characteristics were the main factors related to variations in the renneting properties of the milks.