Effects of Somatic Cell Count on Quality and Shelf-Life of Pasteurized Fluid Milki

摘要

Milk was collected from eight Holstein cows four times before and four times after intramammary infec- tion with Streptococcus agalactiae. Postinfection milk had significantly higher somatic cell count (SCC) (849,000 cells/ml) than preinfection milk (45,000 cells/ ml). High SCC raw milk had more lipolysis and proteol- ysis than low SCC raw milk. Pasteurized, homogenized, 2/100 fat milks from pre- and postinfection periods were stored at 5 deg. C and analyzed for lipolysis, proteolysis, microbial quality, and sensory attributes at 1, 7, 14, and 21 d post processing. During refrigerated storage, the average rates of free fatty acid increase (i.e., lipoly- sis) and casein hydrolysis in high SCC milk were, re- spectively, three and two times faster than those in low SCC milk. In general, standard plate counts, coliform counts, and psychrotrophic bacterial counts of both the high and low SCC milks remained low (<100,000 cfu/ ml) during 5 deg. C storage. Low SCC milk maintained high organoleptic quality for the entire 21-d shelf life period. However, for high SCC milk, between 14 and 21 d, sensory defects were detected, which resulted in low overall quality ratings. The sensory defects mainly in- cluded rancidity and bitterness and were consistent with higher levels of lipolysis and proteolysis. Hence, mastitis adversely affected the quality of pasteurized fluid milk. It is recommended that the fluid milk indus- try consider implementation of premium quality pay- ment programs for low SCC milks.