Prevalence, subtyping, tracking, and select spoilage manifestations of Gram-negative bacterial fluid milk postpasteurization contaminants, particularly Pseudomonas spp.

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Human consumption of domesticated ruminant milk began over 10,000 years ago during the Neolithic Revolution. The extremely perishable nature of fresh milk has remained a persistent challenge from these prehistoric times to the present day. Bacteria present in and on the udder, in the farm environment, and on the hands and implements of milkers contaminate the milk and rapidly proliferate in its nutrient-rich conditions. For this reason, untreated fresh milk spoils within hours at ambient temperatures. This changed with the dual advents of mechanical refrigeration and pasteurization in the 19th century. Although the primary purpose of pasteurization is to reduce the foodborne pathogen population of milk to an acceptable risk level, pasteurization also drastically reduces the population of nonpathogenic vegetative bacterial cells responsible for spoilage. For this reason, refrigerated contemporary high-temperature, short-time pasteurized milk has an optimum shelf life of approximately 21 days. The largest obstacle to achieving this optimum is recontamination of milk with heat-labile bacteria during processing following pasteurization, a phenomenon called postpasteurization contamination (PPC).The 4 studies presented in this dissertation advance our knowledge of PPC in several relevant areas: (1) The prevalence and identity of postpasteurization contaminant Gram negative bacteria in large Northeastern United States fluid milk processing plants; (2) an assessment of plant sanitation and employee training interventions for the reduction of PPC rates in fluid milk; (3) a single gene sequencing-based subtyping technique for Pseudomonas spp., currently the most common postpasteurization contaminant; and (4) genotypic and phenotypic characterization of 2 Pseudomonas spp. responsible for blue and gray color defects in dairy products. The results of these studies emphasized several high-level conclusions regarding the present state of PPC and our knowledge thereof. PPC remains concerningly prevalent in the fluid milk produced by many large dairy plants, averaging approximately 50% of packages sampled and reaching 100% of packages in certain instances. Despite continuous improvements in processing equipment sanitary design and sanitation chemistry, aging plant infrastructure and numerous additional pragmatic factors likely prevent the reduction or elimination of PPC from fluid milk through simple interventions for many processors. A large diversity of Pseudomonas spp. subtypes, mostly of species within the P. fluorescens and P. putida groups, exists within dairy plants and contaminates dairy products singly or multiply in isolated incidents or with evidence of persistent contamination over periods of up to 2 years. Finally, genetically and temporally distant isolates of 2 Pseudomonas spp. share a common genetic element associated with the production of blue and gray pigments in milk and fresh uncultured cheese. Despite the breadth of the studies reported herein, many additional questions about the nature of PPC and how it can most effectively be addressed have been raised or emphasized by this work. Suggestions for further research directions are included in the discussion sections of each chapter of this dissertation. It is clear at present that the underlying causes and mechanisms of postpasteurization contamination will need to be better understood to provide effective, evidence-based guidance for the dairy industry. Until a time when this occurs, untimely spoilage of fresh milk will remain with us, just as it always has.

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273 pages


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bacteria; contamination; dairy; milk; pasteurization; pseudomonas


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Union Local


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Wiedmann, Martin

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Moraru, Carmen I.
Worobo, Randy W.

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Food Science and Technology

Degree Name

Ph. D., Food Science and Technology

Degree Level

Doctor of Philosophy

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Government Document




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Attribution 4.0 International


dissertation or thesis

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