Presence and persistence of Pseudomonas sp. during Caspian Sea-style spontaneous milk fermentation highlights the importance of safety and regulatory concerns for traditional and ethnic foods

Abstract The aim of this study was to evaluate the performance of Caspian Sea-style spontaneous milk fermentation to improve the quality of pasteurized milk containing high levels of Pseudomonas contamination, with a focus on microbiological safety and stability of the final product. Bacterial diversity of pasteurized milk, fermentation process, and after 60 days of storage was analyzed by Illumina-based sequencing, and presence of viable taxa was confirmed by culturing on selective media. Low quality pasteurized milk harbored mainly Gram-negative bacteria, markedly dominated by Pseudomonas. Following fermentation, lactic acid bacteria rapidly became dominant with maximum population of 10.15 log CFU/mL at 18 h, represented mainly by Lactococcus. However, sequences related to Pseudomonas, and to a lesser extent for enterobacteria, remained constant throughout the fermentation process. The cultured-dependent approach confirmed the presence of viable Pseudomonas, with a final population of 5.60 log CFU/mL. Biochemical transformations were further analyzed, indicating lactic acid as the main end-metabolite produced (maximum concentration of 5.93 g/L at 24 h). In addition, the increase of 2-nonanone can be correlated as a volatile biomarker of P. aeruginosa and related species. Altogether, the results demonstrated that natural milk fermentation may often not inhibit the development of pathogens and food spoilage microorganisms.