Storage of raw milk in the bulk tank creates an environment which selects for psychrotrophic bacteria. Results from earlier studies suggested that the microbiota of bulk tank milk with high bacterial counts is dominated by single, cold-adapted species with high growth rates at low temperatures. We checked this assumption in more detail and analyzed the microbial diversity of 48 samples from bulk tank raw cow's milk with bacterial counts >100,000 cfu/mL from different geographic regions by culture-dependent and -independent methods. Contrary to presumptions from earlier studies, only the minority (24%) of samples was dominated by a single bacterial species and diversity was not correlated with bacterial counts. The dominating species in this group of samples were identified as psychrotrophic Acinetobacter and Pseudomonas species, related to poor hygiene and spoilage, or mesophilic, mastitis-related Streptococcus species and Escherichia coli. This shows that storage of raw milk under refrigeration does not always lead to a selection of cold-adapted bacteria. Approximately half of the raw milk isolates showed either lipolytic or proteolytic activity at 10 °C or 4 °C. Consistent or increased enzymatic activity at cold temperatures was detected for Acinetobacter spp. and Pseudomonas spp., but also for genera with minor abundance, e.g. Carnobacterium and Arthrobacter.
Keywords: Microbial contamination; Psychrotrophic bacteria; Raw milk; Spoilage.
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