Shelf life and microbial content of fluid milk from nine processing plants in Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5d86p223t

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  • High-temperature-short-time pasteurized fluid milk commonly has a shelf life around 16-19 days. The aim of this study was to determine the shelf life of fluid milk in Oregon and characterize the bacterial growth over 23 days. Samples of pasteurized 2% milk were collected at six time points from all nine fluid milk-processing plants within Oregon. Milk was collected at 23, 21, 19, 16, and 8 days pre-testing, allowing for aging to occur. Fresh samples were collected 24 hours pre-testing. Raw milk samples were collected twice at 23 days and within 24 hours of testing for enumeration of endospores. Samples from each time point were cultured for Total Plate Count (TPC), and gram-negative counts (GNC). Concurrently, shelf lives of the milk were determined by employing triangle tests (N=25) where milk-drinking consumers were asked to distinguish between fresh and aged milk samples. Shelf lives varied greatly among the nine processing plants ranging from less than 8 to 21 days. Two accelerated milk shelf-life tests, the Pre-Incubation Count (PI) and the Moseley Keeping Quality (MKQ) test were also conducted. Both accelerated shelf life tests failed to accurately predict the shelf lives among plants. TPC's at 23 days of age were within one log value of 10⁶cfu/ml among all nine dairy plants. Endospore counts (ESC) ranged from 10³-10⁷ at 23 days. ESC counts trailed the TPC and gram-negative counts, as high numbers of spores were not seen until after 16 days. GNC counts followed the TPC closely. The four dairies with the shortest shelf lives utilized coated paperboard carton packaging. Microbial content and packaging material showed no correlation. Pasteurization temperature did not correlate to the bacterial growth, although the five plants with the shortest shelf lives utilized the longest holding times. Bacterial species in raw and pasteurized milk samples were identified using Terminal Restriction Fragment Length Polymorphism (t-RFLP). Paenibacillus spp. were the most commonly found organism in the pasteurized samples. Pseudomonas spp. were only found in one pasteurized sample, however they were found in 56% of the raw milk samples. Streptococcus spp. were found in the pasteurized milk of the four plants with the shortest shelf lives and are a possible cause of reduced shelf life. This study demonstrates that although pasteurized milk processing is a well-established process, there is still significant variability in the milk quality and shelf life between dairy plants.
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