Study: Some raw milk cheese is vulnerable to avian flu

While avian influenza continues to spread to U.S. dairy farms, foods like milk, butter and cheese have largely been unaffected. Pasteurization, the process of heating raw milk to get rid of harmful bacteria before processing, has been proven to kill the virus.

But new research suggests avian flu could be a threat to specialty producers that rely on aging instead of heat to create unique cheeses.

Preliminary results from federally funded research at Cornell University found that the standard 60-day aging process used for raw milk cheese was not effective at killing avian flu.

The study used both laboratory models mimicking the cheesemaking process and samples of commercially made cheese from a dairy that inadvertently used contaminated raw milk.

Scott Rankin, food science professor at the University of Wisconsin-Madison, said it’s not a surprise that the avian flu virus has the ability to survive under some conditions.

Raw milk cheeses fall under a unique exception to food safety regulations, which largely require pasteurization for dairy products. Rankin said only certain low-moisture cheese types like cheddar can be made from raw milk if they are held for at least 60 days at a room temperature above 35 degrees Fahrenheit.

“At that time and at that temperature, there would be a continued consumption by the culture bacteria in the cheese to slowly continue fermentation, slowly continue to drop the pH,” he said. “The thought was that process would eliminate any pathogen that may have come in through the raw milk.”

In the last 30 years, Rankin said some in food science have questioned whether the aging process is an infallible approach. He said there is evidence that harmful bacteria can survive past the 60 days in certain circumstances. 

But Rankin said cheese producers who make raw milk products take extra precautions to prevent pathogens from being introduced. That includes paying special attention to dairy cow health and sanitation practices within their plants.

“Good manufacturers of raw milk cheese have a terrific track record of safe product, and I don’t think that’s changed based on the emergence of the avian flu virus,” he said.

Rankin pointed out that sampling of commercially available raw milk cheese by the U.S. Food and Drug Administration has not found evidence of the virus.

The agency reported Friday that of the 110 samples of raw milk cheese taken between January and early March, 96 tested negative for avian flu and the remaining 14 were still in process. The agency plans to take 189 more samples and the final results will be available in late spring.

Cheese makers hope to see more guidance on how to safely operate despite avian flu

Rebekah Sweeney, senior director of programs and policy for the Wisconsin Cheese Makers Association, said her group sees the new research as a first step in understanding the risks of avian influenza for these specialty producers. 

The Cornell study found the virus remained infectious in cheese that reached a pH of 6.6 and 5.8 during the aging process. But it did not survive in cheese where the pH was lowered to 5.0 after the aging process.

“I think the first question that any raw milk cheese maker would have would be: What is exactly the point of pH when that virus is inactivated?” Sweeney said. “What other strategies can they employ to ensure that that virus is inactivated?”

The FDA also reported that heating milk to a lower temperature than what’s used in pasteurization, a process called thermization, is effective at killing the virus. Sweeney points out that some raw milk cheese makers already use this technique, providing a potential path for them to ensure their products remain safe.

She said only a handful of Wisconsin cheese makers produce raw milk cheese, but the products tend to have a very loyal customer base.

The Wisconsin Cheese Makers Association has asked the FDA to clarify its guidance for raw milk cheese makers based on the results of the new study.

In addition to more research on the topic, Sweeney said they’ve asked federal regulators to offer testing for raw milk cheese that has already been produced and resources for cheese makers who want to change their practices in response to the virus.