Study on bacterial diversity in milk aims to improve food safety

Milking MachineResearchers at the University of California-Davis recently found that fresh (or raw) milk transported by tanker truck for processing shows “amazing bacterial diversity,” which even varies by season.

Reported in mBio, the researchers explained that while microbes with the potential to cause disease are destroyed during milk pasteurization, not all bacteria and their associated enzymes are eliminated by the process. The remaining bacteria retain the ability to cause spoilage and quality defects and influence shelf life in dairy foods like cheese and yogurt.

For the study, the researchers sampled and analyzed raw cow’s milk from 899 tanker trucks as they arrived at two dairy processors in California’s San Joaquin Valley during the spring, summer and fall seasons. The samples were then analyzed using gene sequencing.

In all, the collection included 229 tankers filled in the fall of 2013 and another 264 and 406 tankers filled in the spring and summer of 2014, respectively. The larger set of samples collected in the summer included milk collected from two sampling dates one week apart.

The article from the University of California-Davis explains that the milk collected in the spring contained the most diverse bacterial communities with the highest total cell numbers and highest proportions of Actinobacteria, one of the largest known groups of bacteria.

The study also found that within those diverse microbial populations was a core community of microbes that existed in all of the raw milk samples. This core group of microbes represented 29 different bacterial groups and included high proportions of Streptococcus and Staphylococcus as well as Clostridiales, bacteria frequently associated with dairy cattle.

“The level of bacterial diversity that we discovered in these shipments of raw milk was amazing,” lead author and microbiologist Maria Marco, said in the article. “More than half of the bacterial groups identified represented less than 1% of the total microbial content.”

The broad mix of bacteria could be due to raw milk’s high nutrient content, as well as the many potential sources of bacteria associated with dairies. These include bacteria from the cows’ skin, feed, bedding and aerosols, and from human handlers and the equipment and containers used to collect, store and transport the raw milk.

The researchers also found that the bacterial composition of raw milk stored in silos at processing plants was distinct from that of the tanker trucks. One group of silos contained microbial populations similar in makeup to the milk from the tanker trucks, while the other group of silos had distinctly different microbial populations dominated by the Acinetobacter and Lactococcus bacteria.

“This finding demonstrates how the built environment in food-processing plants can have significant but still unpredictable impacts on the microbial quality of foods,” Marco said.

The article states that identification of these raw milk microbes and their abundance should help dairy processors develop new and more effective sanitation procedures and process controls to make sure the milk and resulting dairy foods are safe and of consistently high quality.

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