Pseudomonas spp. and Serratia liquefaciens as Predominant Spoilers in Cold Raw Milk

The storage of fresh raw milk at low temperature does not prevent proliferation of psychrotrophic bacteria that can produce heat-resistant proteolytic enzymes contributing to the reduced shelf life of dairy products. This study aimed to identify the dominant psychrotrophic proteolytic enzyme-producing population of raw milk from Brazil. Raw milk samples collected in 3 different cooling tanks in Brazil were stored at optimal (45 h at 4 degrees C followed by 3 h at 7 degrees C) and suboptimal (45 h at 7 degrees C followed by 3 h at 10 degrees C) conditions to simulate farm storage and transportation allowed by Brazilian laws. The highly proteolytic enzyme-producing strains isolated from stored cold raw milk were characterized by repetitive sequence-based Polymerase Chain Reaction (PCR) analysis. This clustering resulted in 8 different clusters and 4 solitary fingerprints. The most proteolytic isolates from each rep-cluster were selected for identification using miniaturized kit, 16S rDNA and rpoB gene sequencing. Serratia liquefaciens (73.9%) and Pseudomonas spp. (26.1%) were identified as the dominant psychrotrophic microorganisms with high spoilage potential. The knowledge of milk spoilage microbiota will contribute to improved quality of milk and dairy products. Practical Application This study evaluated the dominant psychrotrophic microbiota linked to milk spoilage. The results highlighting storage conditions of raw milk have an impact on shelf life of dairy products. Furthermore, this work demonstrated that Serratia liquefaciens isolates from raw milk produces heat-resistant proteolytic enzymes, which may lead to spoilage of raw milk and milk products.