Viruses that cause foodborne illness, such as Norovirus and Hepatitis A, are not reliably destroyed by freezing, presenting a unique food safety challenge. Unlike bacteria, viruses are obligate intracellular parasites; they cannot grow or multiply outside of a host cell. Freezing temperatures do not kill these viral particles but preserve their structure and infectivity, putting them into a state of suspended animation. Understanding why these viruses survive the cold and how to eliminate them is important for preventing illness.
Viral Survival in Frozen Environments
The resilience of foodborne viruses to freezing is rooted in their simple, non-metabolic nature. Viruses are packages of genetic material encased in a protective protein shell, or capsid. They lack the delicate cellular machinery that freezing temperatures easily damage. Freezing halts all activity, preserving the viral structure and allowing them to remain infectious when thawed.
This stability contrasts with many bacteria, where freezing often causes ice crystals to rupture cell walls, leading to die-off. For hardy, non-enveloped viruses like Norovirus and Hepatitis A, the cold environment acts as a preservative, extending their viability. Frozen produce, such as berries, has been implicated in outbreaks because these viruses can remain infectious for months or years while stored. If contaminated food is consumed without proper cooking, the preserved viral particles can infect a host and cause disease.
How Viruses Contaminate Food
Foodborne viruses primarily enter the food supply through the fecal-oral route, which highlights poor hygiene practices. An infected person, often a food handler, can shed high concentrations of viruses in their stool or vomit, even before symptoms appear. If they fail to wash their hands thoroughly after using the restroom, viral particles transfer directly to food or food contact surfaces.
Contamination also occurs at the source, particularly through water tainted with sewage. Shellfish, such as oysters and clams, are filter feeders that concentrate viruses from contaminated water. Fresh produce irrigated or washed with contaminated water can also harbor viral particles on its surface. Since these viruses require a host to replicate, they cannot multiply in the food itself, but only a few particles are necessary to cause an infection.
Effective Methods for Viral Inactivation
The most reliable method for neutralizing foodborne viruses is the application of high heat, a process known as denaturation. High temperatures physically damage the viral structure by causing the protective protein capsid to break down. This destroys the virus’s ability to infect a host cell. The destruction of these viruses depends on achieving a specific temperature for a sufficient duration.
For common food items, cooking to an internal temperature of at least 70°C (158°F) for two minutes is often sufficient to inactivate most viruses. More stringent guidelines exist for resistant viruses like Hepatitis A, suggesting 85°C (185°F) for at least one minute, especially in complex foods like berries or shellfish. While home cooks rely on thermal inactivation, commercial processors utilize other methods, including chemical sanitizers or ultraviolet (UV) light, which are less practical for residential kitchens.
Preventing Viral Transmission Through Food
Preventing viral illness from food focuses on interrupting the contamination cycle through rigorous hygiene. The most effective action is meticulous handwashing, performed with soap and water for at least 20 seconds before, during, and after handling food. This practice physically removes viral particles acquired from surfaces or during preparation.
Consumers should prevent cross-contamination by keeping raw produce and ready-to-eat foods separate from surfaces or utensils that have touched raw meat or poultry. All kitchen surfaces, cutting boards, and utensils should be sanitized after use to eliminate residual viral contamination. Finally, ensuring that all food is cooked to its recommended internal temperature guarantees any viruses that survived initial handling are destroyed before consumption.