New test results released by the US Food and Drug Administration on Friday found that bird flu virus is making its way from dairy farms and into milk processing plants but also confirmed that the commonly used flash pasteurization method fully neutralizes the virus.
The FDA collected and tested 275 bulk samples of raw milk collected from farms in four states where herds had tested positive for H5N1, or bird flu. The samples were collected between April 18 and 27.
Half of those samples were positive for traces of influenza. A quarter of those positive samples also proved to be infectious, meaning the virus grew when it was inoculated into fertilized chicken eggs, the gold-standard test for determining whether a virus is viable and could make someone sick.
Dr. Don Prater, acting director of the FDA’s Center for Food Safety and Applied Nutrition, noted that the milk the agency tested was bound for pasteurization and was not going directly to store shelves.
But he says he is aware that some states allow the sale of raw milk for human or animal consumption.
“That’s why we put out the messaging that we have around raw milk, because it could be a potential route of exposure,” Prater said.
The FDA has long recommended against consuming raw milk because of potential contamination. It urged states this month to to warn the public more strongly about the dangers of raw milk and to test herds that produce it for sale. The FDA also recommended that states use their regulatory authorities to stop the sale of raw milk within the state or in areas where dairy herds have tested positive.
Prater says the FDA tested the raw milk to get an idea of how much virus might be in milk that was making it to pasteurization plants.
Based on the concentrations of virus in the samples they collected, FDA scientists next took uninfected raw milk from the University of Georgia dairy and laced it with live H5N1 virus. They ran it through a machine they built that replicated all steps of flash pasteurization, where milk is heated to 161 degrees Fahrenheit for 15 seconds.
Their testing included a preheating step that was missing from earlier studies that had tested the same times and temperatures to kill live virus.
In fact, the preheating step fully neutralized all the infectious virus, Prater said, proving that it was a critical element of the process.
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“We built an instrument so that we could sample milk as it goes through these different stages” of the pasteurization process, Prater said, “and what we found is that the virus was actually inactivated even before it got into the holding tube.”
Prater says the new study really helps explain earlier research that found that 1 in 5 dairy products pulled from store shelves contained inert fragments of the H5N1 virus.
“This information really helps explain what’s happening via commercial processing,” Prater said, and confirms that pasteurized milk is safe to drink.
The FDA has announced that it will do another round of sampling of dairy products purchased at grocery stores, including cheese made from raw milk, but has not released the results of that testing.
The FDA released its latest study as a preprint, ahead of peer review, and said it had submitted the research to the Journal of Food Protection for publishing consideration.