Exploratory Milk Mission Investigates Bacteria in Raw Milk

24 Aug 2016 --- Scientists are looking at how food companies can improve the quality of dairy products by studying the microbial composition of raw milk.

Different kinds of bacteria in raw milk impact on the shelf life, overall quality and safety of processed milk and other dairy products. It’s very rare for harmful bacteria to ever reach the consumer because it gets destroyed during the pasteurization process, but other bacteria can cause spoilage issues or defects like off-flavors in cheese.

Scientists have widely investigated the microbial ecology of fruit, vegetables and animal products, not much is known about the influences of transport, storage and processing facilities. 

"The ultimate goal in all of this research is to get dairy products with longer shelf life, less spoilage, and less waste. We don't know what kind of influences the environment has on the microbiome on our foods, and this study is a step forward in that direction," said Maria Marco, PhD, associate professor, Department of Food Science & Technology, University of California-Davis, and lead author of the paper. "If we can better understand and control the microbes coming into processing facilities, we can avoid some food waste."

Researchers aimed to identify the microbiota of raw milk collected for large-scale product manufacturing in California, the largest milk producer in the US with 20 percent of the total production. Scientists analyzed the bacteria in raw milk arriving in 899 tanker trucks at two different dairy plants in California Central Valley in the fall of 2013 and the spring and summer of 2014. 

There were seasonal difference and the bacteria found were diverse. Approximately half of the taxa present at less than one percent abundance. As a comparison, around 20 percent of human fecal communities are composed of taxa below one percent relative abundance. Milk also had a core microbiome composed of 29 different taxa, including Streptococcus, Staphylococcus, and unidentified Clostridiales.

Professor Marco says another interesting point is what happened to the milk after it got into 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 silos where the milk is stored before pasteurization. The effects of the processing facility outweighed the raw milk microbiome and the microbial composition changed distinctly within some, but not all silos, a short time after transfer.”

By knowing the types of microbes present in foods, scientists can devise ways to manage or get rid of spoilage microbes, so they don't make their way into the final product and cause quality problems. 

"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.”

"We now need to tackle the bigger problem of how can we control those microbes in an effective way."