Behaviour of Bacillus cereus spores during heat processing of whey and in Mizithra cheese

The behaviour of Bacillus cereus spores, during whey processing for production of Mizithra whey cheese, as well as during the storage of Mizithra at 22 °C for 10 days and at 7 °C for 20 days, was studied. The whey was inoculated with different populations of B. cereus spores when the temperature of whey reached 40 °C. Subsequently the whey was heated to reach the 90 °C for protein coagulation and production of Mizithra cheese. Samples, for counting of B. cereus, were taken when the temperature reached at 50°, 60°, 70°, 80°, 85°, 90°, and 90°C 20 min., as well as also during the storage of Mizithra at 22°C and at 7°C. For each temperature and for all trials used, the best prediction regression equation was estimated using semi-logarithm time depended models in polynomial form, with the depended variable considered in loges. Comparative analysis of the results revealed that no significant differences were observed in B. cereus populations between 40°C, 50°C and 60°C. On the contrary, the bacterial population showed a significant decreased at 80°C, compared with the respective populations observed in lower temperatures. Similarly, after the curd was formed (85°C), the counted B. cereus spores observed at 90°C were significantly decreased, compared with the respective population at 85°C. The decrease of spore's number continued during the cooking of the curd for 20 minutes at 90°C. After the heating process over the storage time of Mizithra cheese at 22°C and at 7°C. This behaviour observed almost in all trials referred to different initial populations of B. cereus spores. It was observed that, at 22°C, B. cereus spores present in cheese, after the heating process (zero time), germinated and the B. cereus population increased almost up to 2 log at the 2 nd day, followed by a decrease after the 3rd day of storage. Significant fluctuations of B. cereus populations over the 20 days of storage, also occurred at 7°C, for the majority of the trials, which are not microbiologically evaluated. Further statistical comparisons revealed that, the temperature of 7°C does not support the development of B. cereus spores in Mizithra cheese. The results support the hypothesis that Mizithra cheese is favorable medium for the germination of spores and multiplication of B. Cereus, if the temperature of storage is favourable. Even small numbers of survived spores can give rise to populations capable of causing food poisoning, if the product is not kept under refrigeration.