MICROORGANISMS IN PASTEURIZED MILK AFTER REFRIGERATED STORAGE .3. EFFECT OF MILK PROCESSOR

The variation between manufacturers with respect to the types of microorganisms responsible for the spoilage of pasteurised milk was determined by examining milk from three processors every fortnight for 15 months. Gram-negative bacteria, particularly species of Pseudomonas were the predominant spoilage microflora in milk from all manufacturers after storage at 4 and 7-degrees-C. A greater number and variety of strains of Pseudomonas, particularly P fragi, were found in milk with poorer keeping quality but differences in shelflife for the three manufacturers could not be completely related to species. Some extremely lipolytic or proteolytic strains of Pseudomonas were associated with specific milk processors, but the overall activity of isolates from each manufacturer was similar. The results suggest that the ability of pseudomonads to degrade milk fat and protein may contribute in part to survival in the milk processing environment. It was concluded that standard of hygiene (initial number of contaminants) and the lipolytic and proteolytic activity of the strains that grow to spoilage levels are the most important factors which determine the potential keeping quality of individual packages of milk.