The role of Bacillus cereus in the sweet-curdling defect of fluid milk

The recent increase in number of isolations of psychrotrophic bacilli and the sweet curd formation in fluid milk have created the need for more information as to the role of Bacillus cereus. The purpose of this study was to determine the influence of the current trends in the dairy industry towards higher pasteurization temperatures and extended storage times on activation and germination of psychrotrophic Bacillus cereus, as well as the determination of stimulatory influence of the growth of the common Pseudomonas organisms in raw milk on these activated spores. Twenty-eight percent of commercially pasteurized milk samples procured from various plants throughout Tennessee exhibited the sweet-curdling defect on refrigerated storage within 10 days. This defect was observed many times at the bottom but sometimes throughout the carton of milk. Psychrotrophic Bacillus cereus was isolated as a causative organism from these sweet-curdled milk samples. The isolates differed from the type culture organism only with regard to their ability to grow and bring about sweet curd formation in skimmilk at refrigeration temperature. These isolates of psychrotrophic bacilli even though possessing the same cultural and biochemical characteristics, exhibited marked difference in their response to various heat-activation temperatures as well as the initial excessive growth of Pseudomonas species in raw skimmilk. Spores of the three isolates exhibited greater activity after activation at 80 C for 15 seconds, which happens to be the higher pasteurization temperature currently used by many dairy plants, than the standard pasteurization at 71.5 C for 15 seconds. The sweet curd formation was invariably noticed within 10 hours of incubation at 35 C and was always associated with exponential growth of the bacilli with counts ranging from 40 million to more than 60 million. The two common Pseudomonas species, Pseudomonas fluorescens and Pseudomonas fragi, were isolated from the stored samples of fluid milk for this study. Excessive growth of these Pseudomonas species in the raw skimmilk prior to processing had a stimulatory effect on psychrotrophic Bacillus cereus isolates in combination with heat-activation. Two of three isolates of Bacillus cereus provided statistical evidence of the stimulatory effect due to growth of these Pseudomonas organisms especially when the spores of psychrotrophic bacilli were activated at 80 C for 15 seconds.