Pseudomonas fluorescens group bacterial strains are responsible for repeat and sporadic postpasteurization contamination and reduced fluid milk shelf life

Abstract

Postpasteurization contamination (PPC) of high temperature, short time-pasteurized fluid milk by gram-negative (GN) bacteria continues to be an issue for processors. To improve PPC control, a better understanding of PPC patterns in dairy processing facilities over time and across equipment is needed. We 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. Specifically, we collected 280 samples of high temperature, short time-pasteurized milk representing different products (2%, skim, and chocolate) and different fillers over 4 samplings performed over 11 mo at each of the 10 facilities. Standard plate count (SPC) as well as total GN, coliform, and Enterobacteriaceae (EB) counts were performed upon receipt and after 7, 10, 14, 17, and 21 d of storage at 6°C. We 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 GN, coliforms, or EB. Day-21 samples were also evaluated by a trained defect judging panel. By d 21, 226 samples had SPC >20,000 cfu/mL on at least 1 d of shelf life; GN bacteria were found in 132 of these 226 samples, indicating PPC. Crystal violet tetrazolium agar detected PPC with the greatest sensitivity. Spoilage due to PPC was predominantly associated with Pseudomonas (isolated from 101 of the 132 samples with PPC); coliforms and EB were found in 27 and 37 samples with spoilage due to PPC, respectively. Detection of Pseudomonas and Acinetobacter was associated with lower flavor scores; coagulated, fruity fermented, and unclean defects were more prevalent in d-21 samples with PPC. Repeat 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 repeat product contamination with Pseudomonas with identical sequence types. Our data show that PPC due to Pseudomonas remains a major challenge for fluid milk processors; the inability of coliform and EB tests to detect Pseudomonas may contribute to this. Our data also provide important initial insights into PPC patterns (e.g., line-specific contamination), supporting the importance of molecular subtyping methods for identification of PPC sources.

A century of gray: A genomic locus found in 2 distinct Pseudomonas spp. is associated with historical and contemporary color defects in dairy products worldwide

“…The efficacy of both approaches to acidification suggests that pH, rather than other fermentation-related changes in the milk, determines the ultimate appearance of the pigment. It is likely that modern high-temperature shorttime pasteurized milk appears gray when contaminated due to its near-neutral pH and its low prevalence of lactic acid bacteria that reach spoilage levels (i.e., reduce the pH) over shelf life (Reichler et al, 2018). By extension, it could be surmised that blue, rather than gray, pigment was produced on mozzarella cheese by these Pseudomonas because of the acidification of the cheese during production, having much the same ef-fect as has been reported in fluid milk.…”

“…This level of Pseudomonas can realistically be achieved in commercial fluid milk over 21 d of storage at refrigeration temperatures, particularly under conditions of slight temperature abuse. For example, Reichler et al (2018) observed that milk contaminated with Pseudomonas at the processing facility showed an average SPC of 8.23 log cfu/mL after 21 d of storage at 6°C.…”

Characterization of Bacterial Microbiota in Tilapia Fillets Under Different Storage Temperatures

“…The total bacterial count and TVBN value in Figure are very similar to those in Figure A and 4B (variance analysis showed no significant difference [ P > 0.05]), implying that P. fluorescens is the most important spoiler of tilapia fillets across all the storage temperatures investigated in this study (4, 0, and –3 °C). P. fluorescens has frequently been identified as one of the most dominant species in some animal foods, such as fish (Parlapani & Boziaris, ; Tryfinopoulou, Tsakalidou, & Nychas, ), milk (Reichler, Trmčić, Martin, Boor, & Wiedmann, ), beef (Chan, Joo, Faustman, Sun, & Vieth, ), and pork and poultry (Cheng, Doyle, & Luchansky, ). While Xie, Zhang, Yang, Cheng, and Qian () demonstrates that P. fluorescens displays strong spoilage activity in salmon at 4 °C, there are few reports on the spoilage activity of P. fluorescens at temperatures below 0 °C.…”

Paenibacillus odorifer, the Predominant Paenibacillus Species Isolated from Milk in the United States, Demonstrates Genetic and Phenotypic Conservation of Psychrotolerance but Clade-Associated Differences in Nitrogen Metabolic Pathways

“…Furthermore, the growing demand for extended shelf life (ESL) milk (12) imparts an additional hurdle for milk quality by increasing the period of time postpasteurization that the milk must remain of sufficient quality for consumption. Paenibacillus species isolates are frequently isolated from fluid milk (10,12,31), yet few studies have examined the genetic and phenotypic diversity among these isolates. To this end, we characterized a preexisting collection of 1,228 Paenibacillus species isolates collected from milk and dairy environments and found that P. odorifer is the predominant Paenibacillus sp.…”