Purpose: The goal of this project was to 1) determine the primary bacterial groups responsible for PPC and fluid milk spoilage and; 2) evaluate various methods of rapidly detecting PPC in freshly pasteurized fluid milk.
Methods: Microbiological analyses were performed on 105 HTST pasteurized fluid milk samples from 20 dairy plants throughout shelf life using Coliform Petrifilm, EB Petrifilm, Crystal Violet Tetrazolium agar, and Standard Plate Count agar. Additionally, aliquots of each sample were subjected to two different testing parameters (13°C for 18 h and 21°C for 18 h) to enrich for low levels of PPC, prior to enumeration on selective media listed above.
Results: Over half (56%; n=59) of the 105 samples tested reached spoilage levels during shelf life due to PPC with gram-negative bacteria. Of these samples, non-coliform EB were responsible for under 3% (n=2) of the spoilage and coliforms for 20% of the spoilage (n=12). The remaining 76% (n=45) of the samples with PPC were shown to have spoiled due to non-EB gram-negative bacteria (i.e., Pseudomonas), which represented 51% of the total spoilage observed in this study. Comparison of enrichment protocols with shelf-life results indicate that 73% of the PPC could be detected within 72 h of processing.
Significance: Non-EB gram-negative bacteria represent a significant hurdle to reducing PPC in fluid milk. Rapid, accurate detection of PPC is critical in identifying contamination, minimizing food loss and producing high quality products.