Relationship of Protease Activity to Shelf-Life of Skim and Whole Milk

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Abstract
This study was to investigate causes of a possible difference in shelf-life of pasteurized skim milk and whole milk. Samples of skim and whole milk were obtained the day of processing, in 235 ml containers, from commercial dairies throughout South Carolina [USA]. They were stored at 4.5.degree. C for 0, 4, 8, 10, 12, 14 and 16 days; 7.degree. C for 0, 4, 6, 8, 10 and 12 days; and 10.degree. C for 0, 1, 2, 3, 4, 5 and 6 days. On each sampling day milks were tested for coliform count, psychrotrophic count, flavor score and relative protease activity. The shelf-life of skim milk was significantly less than that of whole milk when both were stored at 4.5.degree. and 7.degree. C, but not at 10.degree. C. Bacteria counts were not significantly different; they were of no predictive value as anticipated changes in flavor score. Relationship between flavor score and relative protease activity of skim and whole milk was linear. Relative protease activity was significantly higher in skim milk as compared to whole milk stored at 4.5.degree. and 7.degree. C. A higher protease activity in skim milk may account partially for its decreased shelf-life.