Abstract
Considering the cellular injury of microorganisms caused by the processing operation, the detection of coliforms in frozen dairy products was examined in a model system. Viable counts of test organisms (Escherichia-Aerobacter group) frozen and stored at -20°C in sterilized skim milk were determined by selective and nonselective media. Ten to 73% of viable population was unable to reproduce on desoxycholate agar and violet red bile agar. However, the frozen injured cells displayed no sensitivity to brilliant green lactose bile broth. The percentage of injured survivors became greater when the cells were frozen and maintained in skim milk of relatively low concentration of solids-not-fat. Sucrose exerted a considerable protective action against the injury due to freezing. The present results suggest that the sublethal injury should be considered carefully in the microbiological evaluation of frozen dairy products.
