While scientists have intensively studied the microbial ecology of fresh produce and animal products, little is known about the influences of storage, transport, and processing facilities. To shed light on the issue, Dr. Marco and colleagues set out to identify the microbiota of raw milks collected for large-scale product manufacturing in California. The scientists analyzed the bacteria in raw milk arriving in 899 tanker trucks at two different dairy processors in the California Central Valley in the fall of 2013 and the spring and summer of 2014.
In a study published online in mBio, the researchers report that bacteria varied by season and were highly diverse, with roughly 50% of the taxa present at less than 1% relative abundance. As a comparison, roughly 20% of human fecal communities are composed of taxa below 1% relative abundance. Milk also had a core microbiome composed of 29 different taxa, including Streptococcus, Staphylococcus, and unidentified Clostridiales.
Another important finding was what happened to the milk after it got to the dairy processing plant. “We saw this interesting shift of the types of bacteria that are dominant in the milk when it goes from the truck to the silos where the milk is stored before pasteurization,” said Dr. Marco. The conditions or microbial exposures at the processing facility outweighed the raw milk microbiome, and the bacterial composition changed distinctly within some, but not all silos, a short time after transfer.