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.

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(Reçu le 8 avril 1994; accepté le 22 août 1994) Résumé -Les caractéristiques physico-chimiques et l'aptitude à la coagulation par la présure du lait de brebis collecté dans la région de Roquefort ont été suivies au cours d'une période de lactation. Les résultats obtenus mettent en évidence une évolution nette de la composition globale du lait au cours de la période de production. L'étude de la répartition du calcium et du phosphore inorganique entre les phases soluble et colloïdale fait apparaître une augmentation de la fraction colloïdale. Concernant les caractéristiques micellaires, on note un accroissement du diamètre des micelles et une diminution de la minéralisation calcique. Le degré d'hydratation des micelles et les proportions relatives des caséines évoluent peu. Le suivi des paramètres de coagulation montre une augmentation nette du temps de prise, s'accompagnant d'une réduction de la vitesse de raffermissement. La fermeté du gel croît légè-rement et l'aptitude à l'exsudation du lactosérum diminue.

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Summary -A fast procedure for the detection of bovine milk in Roquefort cheese is described. It is based on the separation by rapid isoelectric focusing on Phastêysternë apparatus of 12-caseinsfrom the milk of the 2 species. The presence of bovine milk is also confirmed by the detection in the electrophoretic pattern of a~-casein derived peptide from bovine milk, during cheese ripening, identified by immunoblotting. By using Ihis procedure, levels of bovine milk as low as 5% were easily delecled in Roquefort cheese ripened for a period varying from 10 days to 5 months.