Bitter tasting yogurt or cheese may not make it to your refrigerator, but it is produced and the result of pesky bacteria. The microbial composition of raw milk impacts the quality, shelf life, and safety of processed milk and other dairy products. Controlling the quality of these products is tricky—bacteria can enter milk on the farm, during transport, storage, and processing. While pathogens are destroyed by pasteurization, not all bacteria are eliminated and some can cause defects, such as bad tastes or holes in cheese, which can lead to food waste.
“The food that we get in our supermarket is the best of the best. Right now, there is a lot of mystery about why dairy products spoil or have defects. There doesn’t seem to be a logical pattern or some problem in the facility that can explain why certain products are defective and others are not," said Maria Marco, PhD, associate professor, Department of Food Science & Technology, University of California-Davis.
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. California is the largest producer of milk in the United States, producing 20% of the total U.S. milk production. 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.
“This study was an exploratory mission to find out what types of bacteria are in our raw milk and what happens to them when they reach the built environment," said Dr. Marco. "We now need to tackle the bigger problem of how can we control those microbes in an effective way. The ultimate goal in all of this research is to get dairy products with longer shelf life, less spoilage, and less waste."
By knowing the types of microbes present in foods, scientists can devise ways to get rid of them early, so they don’t get into the final product and cause quality problems.
For example, now that researchers know that certain microbes are more prevalent in certain seasons in California, taking additional steps during these seasons may improve dairy food quality.
Dr. Marco said the study findings might also make those who drink raw milk wary. While all milk sold across state lines must be pasteurized, consuming raw milk is legal in all 50 states.