Changes in Refrigerated Milk Caused by Enterobacteriaceae

Abstract

Laboratory-pasteurized samples of whole raw milk were inoculated with psychrotrophic strains of Citrobacter freundii, Enterobacter agglomerans, E. cloacae, Escherichia coli, Klebsiella ozaenae and Serratia liquefaciens and incubated for 7 days at 7.degree. C. All test organisms caused a pH reduction that ranged from 0.12 (C. freundii) to 0.69 U (K. ozaenae), but none reduced pH to lower than 6.0. Following growth of E. agglomerans, K. ozaenae, S. liquefaciens and E. cloacae, the milk coagulated upon heating or upon addition of alcohol; with C. freundii and E. coli, the milk did not coagulate. All organisms caused reductions in the redox potential (Eh) of milk, ranging from 20 mV (E. coli) to 152 mV (S. liquefaciens). All organisms caused an increase in free tyrosine plus tryptophan in milk ranging from 3.5 (E. agglomerans) to 12.5 .mu.g/ml (E. cloacae) as well as an increase in free fatty acids in milk which ranged from 0.29 (C. freundii) to 0.89 .mu.eq/ml (K. ozaenae). In spite of the significant proteolytic and lipolytic activities exhibited in milk by some of the organisms, none showed proteolytic activity in skim milk-nutrient agar or lipolytic activity on Tween 80 agar.