Graduate Thesis Or Dissertation
 

Rework Practices in the Dairy Industry and Their Potential Contribution to Microbial Spoilage

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zk51vq42z

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  • Rework is a common practice in the dairy industry as a way for processors to minimize waste while recovering costs for dairy products that are unsaleable. The regulations available for reworking fluid dairy products only outline product eligibility requirements with a focus on repasteurization of the products containing rework as a way to control the safety of the product by eradicating pathogenic bacteria. However, the pasteurization of milk is not a sterilization process. The quality and shelf life of pasteurized milk is often limited by the presence of psychrotrophic bacterial spores that can survive and/or germinate following the HTST-pasteurization (72°C, 15 seconds) process and subsequently grow in refrigeration temperatures (≤7°C). Paenibacillus odorifer, a non-pathogenic psychrotrophic spore-former ubiquitous to the farm environment, is the dominant bacterial species in pasteurized fluid milk at the end of shelf life (21 days), often at elevated levels (≥6 log CFU/ml) that can cause noticeable product defects. Because of the reported spoilage and sporulation potential of P. odorifer, it is possible that this organism could cause premature spoilage events and degrade quality in fluid milk products, regardless of the repasteurization step. The first objective of this study was to understand rework in the fluid dairy processing industry including how processors define rework as well as their motivations, handling practices, and decision-making processes. The second objective of this study was to evaluate the spoilage potential of P. odorifer in reworked fluid and chocolate flavored milk and how the growth behavior of this organism would contribute to the microbial burden of finished product containing rework. Seven commercial fluid milk processors from the Pacific Northwest were surveyed regarding their rework handling practices. Processors reported nine typical reworking motivations with reclaim and packaging problems being the two most common. The age of the product allowed to be reworked ranged from 3-21 days after initial pasteurization with typical dilution rates reported to be <20% rework commingled with ≥80% fresh product. Some processors had performed in-house studies to assist in making rework processing decisions while others followed the guidelines outlined Pasteurized Milk Ordinance (PMO) (United States Public Health Service/Food and Drug, 2017). Quality defects were anecdotally attributed to products containing rework more often than fresh product. Defects included organoleptic changes, processing issues, and elevated microbial counts. To evaluate the spoilage potential of P. odorifer, commercial UHT-pasteurized fluid milk and chocolate milk were inoculated with four strains of P. odorifer previously isolated from pasteurized fluid milk (n=2) and chocolate milk (n=2) to achieve an initial cell density of 1-2 log CFU/ml. Samples were stored at 4°C and 7°C and microbial populations and organoleptic observations (visual, aroma) were monitored throughout a 31-day shelf life. Maximum growth rates that occurred over the first 7-10 days at both temperatures were similar between products for all strains (4°C = 0.39-0.53 log CFU/ml/day; 7°C = 0.83-0.98 log CFU/ml/day). All strains demonstrated a slower growth rate and lower cell density after 21 days of storage at 4°C in fluid milk (6.83-7.45 log CFU/ml) compared to chocolate milk (7.74-7.83 log CFU/ml). Thermally-resistant subpopulations of P. odorifer were undetectable for the first 21 days of storage at 4°C in fluid milk and pasteurization conditions were confirmed to be effective at inactivating P. odorifer vegetative cells. P. odorifer is capable of developing thermally-resistant cell types including spores; however, it was confirmed that these subpopulations do not develop in fluid milk unless P. odorifer reaches high cell density which will not occur within 21 days of processing when milk is stored at 4°C. These results indicate P. odorifer is unlikely to be the cause for negative quality implications of fluid milk products containing rework. Collectively, these efforts have characterized rework practices used in the dairy industry and have clarified that P. odorifer is unlikely to contribute to increased quality deterioration in products containing rework.
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