In March 2024, when the first cases of highly pathogenic avian influenza were identified in dairy cows in Texas, Cornell researchers - led by Diego Diel, associate professor of virology in the College of Veterinary Medicine - immediately began assessing how long the virus survived in the raw milk of infected cows.
At the same time, Nicole Martin, assistant research professor of food science and director of the Milk Quality Improvement Program in the College of Agriculture and Life Sciences (CALS), started getting calls and messages from dairy industry manufacturers worried about food safety.
Martin and Diel teamed up to provide answers.
"Industry was looking for scientific information to make sure their processes were safe," Martin said. "So, led by Diel's lab, we started a series of studies driven by the fact that industry needed these answers."
In the first of their studies related to the behavior of the virus in milk, published April 7 in Nature Communications, the team found that the avian flu H5N1 virus, present in high amounts in the milk of infected cows, persisted in raw milk for as long as eight weeks at refrigeration temperatures. But the researchers also tested how the virus responded to pasteurization and several other subpasteurization conditions, finding that pasteurization processes mimicking those recommended by the Food and Drug Administration (FDA) and several subpasteurization conditions completely inactivated the virus.
"That means if you drink pasteurized milk, even if it accidentally comes from a contaminated farm, you are safe," said first author Mohammed Nooruzzaman, assistant research professor of population medicine and diagnostic sciences (CVM). "Pasteurization completely negated the virus."
Researchers heated raw milk spiked with the virus, and milk from infected cows, to 63 degrees Celsius (145 degrees Fahrenheit) for 30 minutes and 72 degrees Celsius (162 degrees Fahrenheit) for 15 seconds - conditions which mimic FDA pasteurization - and the virus did not survive.
The team also found that the virus was inactivated at temperatures below pasteurization levels, using a subpasteurization method called thermization. Testing at various temperatures, they found that the virus is inactivated when the milk is heated to 60 degrees Celsius (140 degrees Fahrenheit), with 54 degrees Celsius (129 degrees Fahrenheit) for 10 minutes being a lower limit.
"Given the present situation, you should avoid drinking raw milk," Nooruzzaman said. "But if you treat the milk to 54 degrees Celsius for 10 or 15 minutes, the virus is completely inactivated. And if you want to be on the safe side: At 60 degrees Celsius, the virus is inactivated in five seconds."
In addition to validating the effectiveness of pasteurization processes, the research provides guidance to the dairy industry on how to dispose of infected milk without further spreading the virus.
The research also gives artisan cheese makers, who use raw milk to make certain cheeses, options to safeguard their customers; in another study, the pre-print of which published March 14, Diel's team found that the virus can persist in raw milk cheeses for 60 days, dispelling the idea that the aging process kills the virus.
"But even without full pasteurization, those subpasteurization temperatures are efficient in inactivating the virus," said Diel, senior author of the studies. "This research provides an alternative for the artisan cheese industry to treat the milk prior to making the cheese."
Previous studies had cast some doubt that the pasteurization process inactivated HPAI H5N1, but Diel said the tools used in those studies may have inaccurately measured the temperature of the milk. The Cornell team validated the temperature with a submerged coil system developed by Martin's lab. The new results also corroborated tests conducted by the FDA and U.S. Department of Agriculture (USDA) using commercial-scale pasteurizers.
The team will continue to research the virus in milk as new strains - and new questions from industry - emerge. Industry stakeholders, from big companies to small processors, have benefited from monthly virtual food safety office hours offered by the Cornell Institute for Food Safety, where Martin and Diel have presented the team's work and answered questions directly.
"That the industry knows they can come to us for that guidance is really important," Martin said. "And it is really about guidance - this is not industry-ending. They want to know the best practices, and our studies take those first couple of steps in the right direction."
Co-authors of the study include Renata Ivanek, professor of population medicine and diagnostic sciences (CVM); Samuel D. Alcaine, associate professor of food science (CALS); research associate Lina M. Covaleda; postdoctoral associate Pablo Sebastian Britto de Oliveira; and doctoral student Katherine Koebel.
Support for the study was provided by the New York State Department of Agriculture and Markets and the FDA.
