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

The behaviour of Bacillus cereus spores, during whey processing for product ion of "Mizithra" whey cheese, as well as during the storage of Mizithra at 22 degrees C for 10 days and at 7 degrees C for 20 days, was studied. The whey was inoculated with different populations of B. cereus spores when the temperature of whey reached 40 degrees C. Subsequently the whey was heated to reach the 90 degrees C for protein coagulation and production of Mizith ra cheese. Samples, for counting of B. cereus, were taken when the temperat ure reached at 50 degrees, 60 degrees, 70 degrees, 80 degrees, 85 degrees, 90 degrees, and 90 degrees C 20 min., as well as also during the storage of Mizithra at 22 degrees C and at 7 degrees C. For each temperature and for all trials used, the best prediction regression equation was estimated usin g semi-logarithm time depended models in polynomial form, with the depended variable considered in leges. Comparative analysis of the results revealed that no significant differences were observed in B. cereus populations bet ween 40 degrees C, 50 degrees C and 60 degrees C. On the contrary, the bact erial population showed a significant decreased at 80 degrees C, compared w ith the respective populations observed in lower temperatures. Similarly, a fter the curd was formed (85 degrees C), the counted B, cereus spores obser ved at 90 degrees C were significantly decreased, compared with the respect ive population at 85 degrees C. The decrease of spore's number continued du ring the cooking of the curd for 20 minutes at 90 degrees C. After the heat ing process, the survived B. cereus spores showed a significant progress ov er the storage time of Mizithra cheese at 22 degrees C and at 7 degrees C. This behaviour observed almost in all trials referred to different initial populations of B. cereus spores. II was observed that, at 22 degrees C, B. cereus spores present in cheese, after the heating process (zero time), ger minated and the B. cereus population increased almost up to 2 log at the 2( nd) day, followed by a decrease after the 3(rd) day of storage. Significant fluctuations of B. cereus populations over the 20 days of storage, also oc curred at 7 degrees C, for the majority of the trials, which are not microb iologically evaluated. Further statistical comparisons revealed that, the t emperature of 7 degrees C does not support the development of B, cereus spo res in Mizithra cheese. The results support the hypothesis that Mizithra ch eese is favorable medium for the germination of spores and multiplication o f B. cereus, if the temperature of storage is favorable. Even small numbers of survived spores can give rise to populations capable of causing food po isoning, if the product is not kept under refrigeration.