[Development and survival of Yersinia enterocolitica in pasteurized milk and ice cream]

Studied were the laboratory development and survival of Yersinia enterocolitica organisms in pasteurized milk and ice-cream that had been contaminated with varying amounts of microbial cells. The milk was kept at 4 degrees, 9 degrees, and 21 degrees C for 120 days, and the ice-cream--at--18 degrees and --23 degrees C for 8 months. Yersinia development and survival were found to be dependent on temperature as well as on the numbers and activity of microflora. When milk was kept at temperatures below 10 degrees C the organisms retained viability for more than 120 days, while at room temperature (20 degrees--22 degrees C) they remained active for up to 30-60 days. The presence of coliform bacteria inhibited the development of Yersinia, its replication and survival rates being reduced. These were dependent in ice-cream on the amount of viable cells in the initial raw material. When contamination was at 10-15 cells per cu. cm Yersinia organisms remained viable up to the 45th day, and at 100 cells per cu. cm--up to the 90th day. At 1000 cells/cu. cm and more viable cells could be found up to the eighth month, their count over a 6-month period being reduced from 84.2 up to 98.5 per cent.