🦠 Pathogenic viruses can survive for years on frozen fruits

Even when exposed to very low temperatures for long periods, pathogenic viruses found on the surface of small fruits can survive and retain their ability to cause infections in humans. This is what a research team from Université Laval demonstrates in a study recently published in the International Journal of Food Microbiology.


According to the World Health Organization and the Food and Agriculture Organization of the United Nations, small fruits rank second among fresh foods whose consumption poses the highest microbiological risks to humans. Frozen small fruits are no exception.

"They rank first for the transmission of hepatitis A virus and second for the transmission of human norovirus," emphasizes the study lead, Julie Jean, professor in the Department of Food Sciences and researcher at the Institute of Nutrition and Functional Foods at Université Laval.

It is known that freezing puts bacteria into dormancy, which extends the shelf life of foods, but it is not a safe method for destroying microorganisms, the researcher reminds. "We wanted to determine whether certain freezing conditions for small fruits could reduce the infectivity of certain viruses and, consequently, the risks associated with their consumption."

Professor Jean's team therefore deposited known quantities of hepatitis A virus and murine norovirus, a close relative of human norovirus, onto five types of fresh small fruits purchased from grocery stores (strawberries, blueberries, cranberries, raspberries, and blackberries). These fruits were first frozen at temperatures of -20, -80, or -196 degrees Celsius. Subsequently, they were stored at -20 degrees Celsius for periods of up to 24 months.

The research team observed a decrease in the number of viruses over time, up to 1000 times less in the case of murine norovirus and up to 100 times less for hepatitis A virus. The reduction was more pronounced for fruits first exposed to a temperature of -196 degrees Celsius.

However, regardless of the initial exposure temperature and storage duration, the viruses present on the surface of the small fruits remained sufficiently abundant to cause infections in humans. "With these viruses, ingesting between 10 and 100 viral particles is enough to trigger an infection. Freezing is therefore not sufficient to eliminate the transmission risk they pose."

In Canada, in recent years, food recalls of small fruits that caused infections would have exclusively affected products from other countries. "With the exception of cranberries, frozen small fruits sold in grocery stores are almost all imported products," Professor Jean points out. "It is possible that the hygiene conditions surrounding the harvesting or handling of these small fruits are less strict in these countries than in Canada. It is also possible that these viruses are present in the environment, particularly in the water used to irrigate the fields."

However, this should not deter people from consuming frozen small fruits, Professor Jean insists. The following measures can help reduce the risks, she continues.

"The first thing to do is to carefully monitor food recalls. Also, although the transmission risk is limited, it can be greatly reduced by cooking the small fruits. Simply heating them to 85 degrees Celsius (185°F) for one minute is enough to destroy these viruses. This precaution is particularly important for people with weakened immune systems. Vaccination against hepatitis A is also available."