Aging raw milk cheese does not inactivate avian flu, FDA-backed study reveals
USA – A recent study by Cornell University provided compelling evidence that the Highly Pathogenic Avian Influenza (HPAI) H5N1 virus is stable in raw milk cheese, surviving throughout its minimum required aging period.
The U.S. Code of Federal Regulations requires that cheeses produced from unpasteurized (raw) milk must undergo a curing process for a minimum of 60 days at temperatures no lower than 1.67 °C (35 °F) to inactivate bacterial pathogens.
The study, funded by the U.S. Food and Drug Administration (FDA) and New York State, evaluated the stability of highly pathogenic avian influenza (HPAI) H5N1 virus in raw milk cheeses using a mini cheese model prepared with HPAI-spiked raw milk under varying pH levels (pH 6.6, 5.8, and 5.0).
It also examined commercial raw milk cheese inadvertently produced with naturally contaminated raw milk.
They observed a pH-dependent virus survival, with infectious virus persisting throughout the cheese-making process and for up to 60 days of aging in the pH 6.6 and 5.8 cheese groups.
Meanwhile, pre-processing milk acidification to pH 5.0 (with lactic acid) effectively inactivated the virus. No infectious virus was detected in cheese curd immediately after cheese production and in raw milk cheese throughout the 60-day aging.
Alternative safety measures for cheese production
Their other recent findings indicated that an alternative of thermization (sub-pasteurisation) of raw milk at temperatures above 54°C successfully inactivates the virus within 15 minutes, offering an alternative safety measure for cheese production.
With the increased frequency of HPAI H5N1 virus outbreaks in U.S. dairy herds and the recent emergence of the new potentially more virulent virus genotype D.1.1 in dairy cattle, the authors are rightfully concerned about the need for alternative measures to ensure food safety in the dairy industry.
Their preprint study indicated that “implementing additional mitigation steps such as testing raw milk bulk tanks or using milk pasteurization, thermization or acidification before cheese making becomes crucial to ensure food safety.”
They further noted that this food safety concern may extend to other raw milk products as the virus can persist for up to 56 days in raw milk under refrigeration.
As of March 10, 2025, 96 out of 110 of the samples they have collected since the project started in January 2025 came out negative on polymerase chain reaction testing. Tests on 14 others are still in progress, with none yet to test positive for H5N1.
They had planned to collect 299 samples and complete the project by the end of March 2025, after which the agency will publish the final results.
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