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

The microbial quality of milk is improved via processing steps such as pasteurization, which reduces the overall microbial population in milk and which is particularly effective against gram-negative, heat-sensitive bacteria. Thus, the presence of gram-negative bacteria such as Pseudomonas in properly pasteurized milk is indicative of post-pasteurization contamination (PPC). Although several proposed routes of contamination have been suggested, one important probable 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 and seal rings). If not regularly replaced, these components may form cracks and may result in establishment of contaminating bacteria that may not be addressed by regular cleaning and sanitation procedures. To understand the causes of PPC better, the goal in the study cited was to assess the effects of PM on the incidence of PPC in milk. The hypothesis was that the wearable components had the potential to serve as sources of PPC in fluid milk, and their replacement during routine PM should reduce contamination of milk cartons by gram-negative bacteria.

Two dairy processing facilities were visited three times. At the facilities, all wearable rubber components replaced during PM as well as milk cartons produced pre- and post-PM were collected. The milk cartons were stored at 6°C for 14 days to simulate mild temperature abuse, and the components were enriched with brain heart infusion broth at 32°C for 24 hours. All samples were then streaked on standard methods agar and crystal violet tetrazolium agar (CVTA) for the assessment of PPC.

The results indicated 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, it was found that gram-negative bacteria were present in 0% to 99% of wearable components from each of the three visits. Bacterial isolates taken from the milk were most frequently identified as Pseudomonas and Microbacterium, whereas those taken from the components were primarily Staphylococcus. Overall, it was observed that PM did not reduce the rates of PPC, and the effectiveness of PM may have been influenced by employee behaviour and standard practices at the processing facilities.

Conclusions: In general, the study provided an important perspective on how practices surrounding preventative maintenance on filling equipment affect the contamination of finished products. However, although the results confirmed the role of the filling equipment as a causative agent of gram-negative PPC, several limitations should be addressed in future research. Primarily the limited replication and high variability of circumstances between visits mean that trials were not directly comparable and should be treated as individual case studies. Additionally, the contamination of milk and components either by pre-filler contamination or by practices associated with PM means that evidence regarding the relationship between milk quality and PM is limited. Despite these limitations, the outcome of the study demonstrated that the application of CVTA as a method to track PPC should be used by processors to validate cleaning and sanitation practices and to address overall product quality. Finally, the study showed the sometimes negative influence of employee behaviour during PM on the effectiveness of reducing quality issues in milk quality.