Evaluation procedures to predict the keeping quality of ultra-high temperature (UHT) processed skim milk

This study evaluated the ability of an improved carbon-14 protease assay as a rapid procedure to screen raw milk and predict the keeping quality of UHT processed skim milk. Raw milk composition and microbiological quality were evaluated as possible parameters which could determine the keeping quality of UHT processed skim milk. Protein, total solids, fat, free amino groups, standard plate count, psychrotrophic bacteria count, proteolytic bacteria count, somatic cell count, and initial protease activity (measured by the radioisotope procedure) were determined on 51 raw milk samples pooled from area farms. University farm and University creamery storage tank, which were then skimmed and UHT processed and held for 60 days at 25 C. The samples were analyzed for change in free amino groups upon coagulation or at 60 days whichever occured first. Viscosity was measured on a weekly basis except on the first two weeks. The samples which lasted 60 days were analyzed for bitterness by a panel of experienced judges.

Shelf-life was determined as days to coagulation or 60 days, whichever was sooner. Shelf-life was weakly correlated to initial protease assay (r=.26: p= .06). Initial protease activity was inversely and weakly correlated to bitterness flavor score (r=-.27: p=.08). Shelf-life was also weakly and inversely correlated to somatic cell count(-.25: p= .08). However, initial protease activity was positively correlated to psychrotrophic bacteria count but not with other bacteria counts. Bacterial counts were correlated to each other and to somatic cell count. Change in free amino groups were inversely correlated to shelf-life (r=- .30; p= .03). Viscosity was weakly correlated with bitterness flavor (r=-.48; p=.05). The relationship between initial protease activity and shelf-life was not a dramatic one, thus a longer evaluating period Is recommended. However, the positive correlation between Initial protease activity and psychrotrophic bacteria count Is an Important finding and suggests the potential of radiolabeled protease assay for rapid enumeration of psychrotrophic bacteria.