Discipline: contamination; Key words: post-pasteurization contamination, Pseudomonas, shelf-life, spoilage, crystal violet, tetrazolium agar.
Consumers complain about spoilage due to bacterial growth and spoilage also contributes to dairy product waste, which in a developing country we cannot really afford. We probably do not have figures in South Africa, but milk spoilage in the US amounts to $6.4 billion per year. Microbial spoilage is of particular concern and can be caused by either psychrotolerant spore formers, which likely originate from raw milk, or by post-pasteurization contamination (PPC). Whereas gram-positive bacteria can also re-contaminate milk after pasteurization, bacteria responsible for PPC resulting in milk spoilage are typically gram-negative and are thought to originate from the processing environment. Several recent studies have explored the on-farm sources of psychrotolerant spore formers. However, less information is available on sources of PPC in milk processing facilities. Defects associated with gram-negative bacterial PPC include coagulation, bitter and astringent flavours, rotten, barny, cheesy and fruity odours. Whereas the regulatory limit on bacterial counts in high temperature, short time-pasteurized (HTST) milk as done in the US, is 20,000 cfu/mL, sensory defects due to gram-negative bacterial PPC typically require bacterial growth to >5,000,000 cfu/mL, a level often reached during shelf life by milk with PPC. Dr S.J. Reichler and colleagues studied these problems further. Their study was published in the Journal of Dairy Science, Volume 101 of 2018, page 7780 to 7800, the title of their paper being: Pseudomonas fluorescens group bacterial strains are responsible for repeat and sporadic post-pasteurization contamination and reduced fluid milk shelf life.
The authors argued that in order to improve PPC control, a better understanding of PPC patterns in dairy processing facilities over time and across equipment is needed. They thus collected samples from 10 fluid milk processing facilities to (1) detect and characterize PPC patterns over time, (2) determine the efficacy of different media to detect PPC, and (3) characterize sensory defects associated with PPC. More specifically, they collected 280 samples of HTST milk representing different products (2%, skim and chocolate) and different fillers over four samplings performed over 11 months at each of the 10 facilities. Standard plate count (SPC) as well as total gram-negative bacteria, coliform and Enterobacteriaceae counts were performed upon receipt and after 7, 10, 14, 17, and 21 days of storage at 6°C. They used 16S rDNA sequencing to characterize representative bacterial isolates from (1) test days with SPC >20,000 cfu/mL and (2) all samples with presumptive gram-negative bacteria, coliforms, or Enterobacteriaceae. Day-21 samples were also evaluated by a trained sensory panel.
By day 21, 226 samples had SPC >20,000 cfu/mL on at least one day of shelf life. Gram-negative bacteria were found in 132 of the 226 samples, indicating PPC. Spoilage due to PPC was predominantly associated with Pseudomonas (isolated from 101 of the 132 samples with PPC), whereas coliforms and Enterobacteriaceae were found in 27 and 37 samples with spoilage due to PPC, respectively. Detection of Pseudomonas and Acinetobacter was associated with lower flavour scores; coagulated, fruity fermented, and unclean defects were more prevalent in day-21 samples with PPC. Isolation of Pseudomonas fluorescens group strains with identical partial 16S rDNA sequence types was observed in 8 facilities. In several facilities, specific lines, products, or processing days were linked to repeated product contamination with Pseudomonas with identical sequence types. The data show that PPC due to Pseudomonas remains a major challenge for milk processors.
Closing comments: Despite overall improvements in milk quality and shelf life, detection and control of PPC remains an important challenge in the production of high temperature, short time-pasteurized milk and similar products in other countries. Pseudomonas is not only the most common cause of PPC, particularly in facilities that have traditionally focused on control of coliforms, but also causes many severe defects, including coagulation (and flocculation can be added to that) as well as flavour, odour and colour defects. Consequently, according to the authors, there is a need to develop and apply improved approaches to detect PPC and identify PPC sources, including improved rapid-detection methods as well as sub-typing methods or whole genome sequencing. Sub-typing methods should greatly enhance the ability to identify PPC sources, such as specific fillers, lines, or tanks, facilitating corrective actions and long-term improvements, for example through improved sanitary equipment design.