Bacterial Ecology Of High Temperature Short Time Pasteurized Fluid Milk
The Grade "A" Pasteurized Milk Ordinance (PMO) specifies minimum processing conditions of 72 degrees C for at least 15 s for high-temperature, short-time (HTST) pasteurized milk products. Currently, many US milk processing plants exceed these minimum requirements for fluid milk products. To test the effect of pasteurization temperatures on bacterial numbers in HTST pasteurized milk, 2% milkfat raw milk was heated to 60 degrees C, homogenized, and treated for 25 s at one of four different temperatures (72.9, 77.2, 79.9 or 85.2 degrees C) and then held at 6 degrees C for 21 d. Aerobic plate counts were monitored in pasteurized milk samples at d 1, 7, 14, and 21 post-processing. Bacterial numbers in milk processed at 72.9 degrees C were lower (P less than 0.05) than in milk processed at the other three temperatures on each sampling day, indicating that HTST fluid milk processing temperatures significantly affect bacterial numbers in fluid milk. To assess the microbial ecology of the different milk samples during refrigerated storage, a total of 490 psychrotolerant endospore-forming bacteria were identified using DNA sequence-based subtyping methods. Regardless of processing temperature, greater than 85% of the isolates characterized at d 0, 1 and 7 postprocessing were of the genus Bacillus, while over 92% of isolates characterized at d 14 and 21 post-processing were of the genus Paenibacillus, indicating that the predominant genera present in HTST-processed milk shifts from Bacillus spp. to Paenibacillus spp. during refrigerated storage. To determine the microbial ecology of pasteurized milk from different geographical regions within the United States, 2% fat pasteurized fluid milk samples were obtained from 18 dairy plants from five geographical areas representing the Northeast, Southeast, South, Midwest, and West. Of the 589 bacterial isolates identified using DNA sequence-based subtyping methods, 348 belonged to genera characterized as gram-positive endospore forming bacteria (i.e., Bacillus and Paenibacillus). 244 of the 348 gram-positive endospore forming bacteria isolated in the present study classified into 46 allelic types identical to those previously identified from samples obtained in New York State, indicating the widespread presence of these microbes in fluid milk production and processing systems in the United States. greater than 84% of the gram-positive sporeforming isolates characterized at d 1, 7, and 10 were of the genus Bacillus, while over 92% of isolates characterized at d 17 of shelf life were of the genus Paenibacillus, indicating that the predominant gram-positive spoilage genera shifts from Bacillus spp. to Paenibacillus spp. during refrigerated storage. Gram-positive endospore forming bacteria with the same allelic types were obtained from milk samples originating from all 5 geographical regions in the US. In summary, (i) HTST processing temperatures affected bacterial numbers in refrigerated milk, with higher bacterial numbers in milk processed at higher temperatures; (ii) no significant association was observed between genus isolated and pasteurization temperature, suggesting that the genera were not differentially affected by the different processing temperatures; (iii) although typically present at low numbers in raw milk, Paenibacillus spp. are capable of growing to numbers that limit HTST pasteurized refrigerated milk shelf-life; and (iv) gram-positive endospore forming bacteria with the same allelic types were obtained from milk samples originating from all 5 geographical regions in the US.
Dissertation or Thesis