CHARACTERIZATION AND CONTROL OF PSYCHROTOLERANT BACTERIAL SPOILAGE ORGANISMS ASSOCIATED WITH PASTEURIZED FLUID MILK

Abstract

Millions of gallons of pasteurized fluid milk meant for consumption in the U.S. are discarded. Bacterial spoilage is the largest contributing factor for product loss of pasteurized fluid milk. Microbes can be present in pasteurized milk via two main routes: (i) survival of pasteurization by bacteria present in raw milk (generally Gram-positive sporeformers), and (ii) post-pasteurization contamination (PPC) of the product. This product spoilage is further complicated when spoilage microbes can survive refrigeration temperatures and even grow in the cold storage climate. The studies presented here focused on ways to achieve higher quality, longer lasting pasteurized fluid milk by exploring (i) the application of molecular technologies to better understand and potentially track psychrotolerant coliforms responsible for post-pasteurization contamination of fluid milk and (ii) the associations between psychrotolerant sporeforming spoilage organism presence in fluid milk and dairy farm management practices. Our data revealed that psychrotolerant coliforms introduced as PPC in fluid milk have considerable taxonomic and phenotypic diversity, indicating that hygienic issues within a fluid milk processing plant may lead to introduction of a diverse group of coliform contaminants capable of having a direct impact on pasteurized milk quality and the consumer’s sensory experience. Our cross-sectional study identified dairy farm management practices related to milking time hygiene may simultaneously lower bulk tank somatic cell count on dairy farms as well as psychrotolerant sporeformer levels in bulk tank milk, suggesting that on-farm adjustments in management specifically focused on udder cleanliness may directly impact the shelf-life of pasteurized fluid milk. Finally, we showed through our longitudinal study that dairy farm management practices related to overall farm cleanliness were associated with a decrease in the presence of psychrotolerant Bacillales spores in bulk tank milk after 21 days at 6°C post-heat treatment. The combined results from the cross-sectional and longitudinal studies indicate that on-farm adjustments in management focused on both general farm and cow cleanliness may have a direct impact on psychrotolerant Bacillales spore presence and therefore impact the shelf-life of pasteurized fluid milk. The studies presented here contribute to our understanding of psychrotolerant spoilage organisms isolated from fluid milk and provide insights into potential control strategies aimed at achieving high quality, longer lasting pasteurized fluid milk.