Researchers at the University of California-Davis report that samples of raw cow’s milk shipped by tanker truck for processing show “amazing bacterial diversity” which varies by season. Their findings were reported Aug. 23 in mBio, the online, open-access journal of the American Society of Microbiology.
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 spring, summer and fall. The samples were analyzed using gene sequencing.
This 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 level of bacterial diversity that we discovered in these shipments of raw milk was amazing,” said lead author and microbiologist Maria Marco, an associate professor in the UC-Davis Department of Food Science and Technology. “More than half of the bacterial groups identified represented less than 1 percent of the total microbial content.”
She said 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.
Raw milk is known to harbor diverse strains of bacteria that strongly influence shelf life, sensory qualities and safety of fluid milk, as well as that of fermented dairy products such as cheese and yogurt.
While pasteurization of raw milk kills microbes which can cause disease in humans, not all bacteria and their associated enzymes are eliminated in the process. The remaining bacteria can still cause spoilage and quality defects in dairy foods.
Researchers found that a “core community” of microbes remains constant throughout the seasons and across farms where the milk is collected. This core community represented 29 different bacterial groups and included high proportions of Streptococcus and Staphylococcus, as well as Clostridiales, bacteria frequently associated with dairy cattle.
The study revealed that 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 group includes some pathogens.
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 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.
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, according to the report.
Marco’s fellow researchers were Mary E. Kable, Yanin Srisengfa, Miles Laird and Jose Zaragoza, all of UC-Davis, and Jeremy McLeod and Jessie Heidenreich of the Hilmar Cheese Co. of Hilmar, CA. Funding for the study was provided by the California Dairy Research Foundation.
California is the largest dairy producer in the U.S., providing more than one-fifth of the nation’s fluid milk production. In 2015, California dairy farms sold about $9.5 billion worth of milk, according to the Agricultural Issues Center at UC-Davis.
The Golden State is also the nation’s second-largest cheese producer. Wisconsin is number one.
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