OVINE MILK .4. SEASONAL-CHANGES IN MICROBIOLOGICAL QUALITY OF RAW-MILK AND YOGURT

Bulk milk was collected at regular intervals during lactation from a c ommercial flock of dairy sheep. The microbiological quality of the mil k and of yogurts made from the milk was studied. The microbiological q uality of the bulk milk was very high early in the season, but fell to a level comparable with mixed bulk milk from cows during the latter p art of lactation. Adventitious contamination was minimal except in one month where modest levels of coliform contamination were detected. St aph. aureus was also detected at low levels. After heat treatment neit her Staph. aureus nor coliforms were found in the milk or in the yogur ts. The microbiological quality of the yogurts made from sheep milk wa s high throughout the season and both organisms used in the starter cu lture survived at a high concentration. However, it was noted that the lactobacilli used in the starter were sensitive to changes in milk co mposition, to storage and to processing conditions. Both starters were recovered at higher numbers from stirred than from set product. Irres pective of these differences, the concentration of starter bacteria in yogurts was substantially higher (streptococci 10(8)-10(9) CFU/g and lactobacilli 10(5)-10(6) CFU/g) than that required to ensure microbiol ogical safety.