Preventative maintenance has limited effect on decreasing incidence of postpasteurization contamination in single-serve milk cartons.

The microbial quality of fluid milk is improved via processing steps such as pasteurization, which reduces the overall microbial population of milk and is particularly effective against gram-negative, heat-sensitive bacteria. Therefore, the presence of gram-negative bacteria such as Pseudomonas in properly pasteurized fluid milk is indicative of postpasteurization contamination (PPC). Although several proposed routes of contamination have been suggested, one likely source is the filling equipment. Preventative maintenance (PM) is the routine upkeep of equipment, such as fillers, which includes the replacement of wearable rubber components (e.g., gaskets, seal rings). If not regularly replaced, these components may form cracks and allow establishment of bacterial niches that may not be addressed by regular cleaning and sanitation procedures. To understand the role of PM in reducing rates of PPC, we conducted 3 total visits to 2 dairy processing facilities. At the facilities, we collected all wearable rubber components replaced during PM as well as milk cartons produced pre- and post-PM. Milk cartons were stored at 6°C for 14 d to simulate mild temperature abuse, and components were enriched with brain heart infusion broth at 32°C for 24 h. All samples were streaked on standard methods agar and crystal violet tetrazolium agar for the assessment of PPC. Our results indicate that even when milk was free of PPC before entering the filler, PPC was still present in 26% to 64% of tested finished cartons. Additionally, we found that gram-negative bacteria were present in 0% to 99% of wearable components from each of the 3 visits. Bacterial isolates taken from milk were most frequently identified as Pseudomonas and Microbacterium, whereas those taken from components were primarily Staphylococcus. Overall, we observed that PM did not reduce the rates of PPC, and the effectiveness of PM may have been influenced by employee behavior and standard practices at the processing facilities. Future studies should further investigate areas within filling equipment that may serve as bacterial niches for gram-negative spoilage microorganisms.