Almost immediately after the commercialization of pasteurization for fluid milk over 100 years ago, post-pasteurization contaminants (PPC) were identified as spoilage organisms. Better cleaning and sanitation practices have greatly reduced the frequency of PPC, but PPC is still an issue today. Later, it was found that spores, produced by gram-positive organism were capable of surviving high temperature, short time (HTST) pasteurization and can subsequently germinate into vegetative cells capable of growing at refrigeration temperatures to levels that induce spoilage defects. These spore forming organisms are more difficult to address than PPC because these organisms usually contaminate raw milk at the farm level. While there are strategies to reduce spores at the farm-level without the need for new equipment, removing spores at the processor level requires investment in technologies that reduce spores (e.g., bactofugation, microfiltration). It is now known microfiltration represents a technology that can reduce spores to low levels; however, even when paired with HTST pasteurization, spoilage can still occur during refrigerated storage. Microfiltration is a promising technology for extending shelf-life, but the spoilage potential of organisms that can pass through filtration membranes needs to be further understood (i.e., ability to produce flavor defects). The three studies in this dissertation are investigations of (i) PPC in single-serve HTST fluid milk (also referred to as school milk), (ii) the effect of different HTST temperatures and different refrigeration temperatures on microbial survival and growth in fluid milk, (iii) the effect of different microfiltration pore sizes and milk types (i.e., skim or whole) combined with the effects investigated in (ii) on microbial survival and growth in fluid milk. Results from (i) revealed a high frequency of gram-negative PPC in single-serve milk, while a cleaning intervention aimed at a single filler component was ineffective at reducing PPC. The results from (ii), revealed that the effect of storage temperature on shelf-life was greater than the effect of HTST pasteurization temperature. Further, we found evidence that supports previous findings that increasing HTST pasteurization temperature likely shortens shelf-life. Lastly, (iii) revealed that regardless of pore size and milk type, microfiltration can prolong shelf-life; however, Microbacterium represents an emerging organism of concern in microfiltered fluid milk. These findings further contribute to the understanding of fluid milk microbial challenges and also identify opportunities for future research.