Culturable psychrotrophic bacterial communities in raw milk and their proteolytic and lipolytic traits

During cold storage after milk collection, psychrotrophic bacterial populations dominate the microflora, and their extracellular enzymes, mainly proteases and lipases, contribute to the spoilage of dairy products. The diversity, dynamics, and enzymatic traits of culturable psychrotrophs in raw milk from four farms were investigated over a 10-month period. About 20% of the isolates were found to be novel species, indicating that there is still much to be learned about culturable psychrotrophs in raw milk. The psychrotrophic isolates were identified and classified in seven classes. Three classes were predominant, with high species richness (18 to 21 species per class) in different seasons of the year: Gammaproteobacteria in spring and winter, Bacilli in summer, and Actinobacteria in autumn. The four minor classes were Alphaproteobacteria, Betaproteobacteria, Flavobacteria, and Sphingobacteria. The dominant classes were found in all four dairies, although every dairy had its own unique "bacterial profile." Most but not all bacterial isolates had either lipolytic or both lipolytic and proteolytic activities. Only a few isolates showed proteolytic activity alone. The dominant genera, Pseudomonas and Acinetobacter (Gammaproteobacteria), showed mainly lipolytic activity, Microbacterium (Actinobacteria) was highly lipolytic and proteolytic, and the lactic acid bacteria (Lactococcus and Leuconostoc) displayed very minor enzymatic ability. Hence, the composition of psychrotrophic bacterial flora in raw milk has an important role in the determination of milk quality. Monitoring the dominant psychrotrophic species responsible for the production of heat-stable proteolytic and lipolytic enzymes offers a sensitive and efficient tool for maintaining better milk quality in the milk industry.