Biodiversity of culturable psychrotrophic microbiota in raw milk attributable to refrigeration conditions, seasonality and their spoilage potential

Refrigerated storage of raw milk promotes the growth of psychrotrophic bacteria, some of which produce heat-stable exoenzymes causing dairy product spoilage. The effects of storage conditions and season on the biodiversity of psychrotrophs in raw milk were examined using matrix-assisted laser desorption time of flight mass spectrometry and 16S rRNA analysis. The ability of psychrotrophs to produce protease, lipase and phospholipase C was determined. The predominant genera found were Pseudomonas (19.9%), Bacillus (13.3%), Microbacterium (5.3%), Lactococcus (8.6%), Acinetobacter (4.9%) and Hafnia (2.8%); a considerable number of isolates were hitherto unknown species and genera. Diversity varied significantly (P < 0.05), depending on the storage temperature, time, initial microbiota and season. The predominant isolates showed significantly higher heat-stable exoenzyme activities after heating at 142 °C for 4 s. Improving the quality of milk products may require differential processing of raw milk depending on the type of microbiota present, storage temperature and seasonality.