A pathogen is a microorganism, such as a bacterium, virus, or parasite, capable of causing disease. When considering how freezing affects these organisms, the central question is whether the extreme cold eliminates them entirely. Freezing generally forces pathogens into an inactive or dormant state, but it does not reliably sterilize a product or eliminate all disease-causing agents. Freezing is a method of preservation that stops growth, not a method of destruction.
Freezing: Inhibition, Not Elimination
The primary effect of subjecting a food item or material to temperatures below 32°F (0°C) is the cessation of microbial activity. Pathogens enter a state of stasis where their metabolic processes slow dramatically, preventing them from multiplying. This inhibition preserves food for extended periods by stopping the growth of both spoilage organisms and disease-causing pathogens.
The organisms remain viable in this frozen, dormant state, unable to reproduce or generate harmful toxins. This is fundamentally different from sterilization, which requires the complete destruction of all living cells. Once the temperature rises above freezing, the surviving microorganisms quickly become active again, resuming multiplication.
The Cellular Mechanisms of Survival
Pathogens survive freezing through physical and chemical resistance mechanisms. One potential source of damage is the formation of ice crystals outside and inside the cell. While sharp ice crystals can physically puncture cell walls, many microorganisms avoid lethal damage by rapidly expelling water from their interior.
This rapid water loss leads to the second major survival challenge: osmotic stress. As the water surrounding the cell freezes, the remaining unfrozen liquid becomes highly concentrated with salts and solutes. This severe environment draws water out of the microbe, causing cellular dehydration that many pathogens are adapted to withstand.
To minimize damage, some microbes produce compounds known as cryoprotectants, such as sugars and certain proteins. These substances act like a biological antifreeze, lowering the freezing point of the internal cellular fluid and protecting internal structures. Other adaptations include specialized cold-shock proteins and changes to the cell membrane’s lipid composition, which help maintain flexibility in the extreme cold.
How Different Pathogen Types Respond to Cold
The ability to survive freezing varies widely across different types of pathogens. Bacteria, such as Salmonella and E. coli, often see a reduction in population upon freezing, but a significant number of cells typically survive. Certain bacterial spores, which are dormant, highly resistant structures, are particularly resilient to cold.
Resistant Bacteria
The psychrotrophic bacterium Listeria monocytogenes is notably resistant, exhibiting exceptional tolerance to standard freezer temperatures. Studies show that Listeria populations on foods may decline only slightly, even after months of storage. This persistence is attributed to physiological adaptations like changes in cell membrane fatty acids and the uptake of natural cryoprotectants.
Viruses and Parasites
Viruses are generally well-preserved by freezing and are often stored long-term in laboratory freezers for research purposes. As non-living particles, they are not susceptible to the physical damage from ice crystals that can harm bacteria. Conversely, freezing is an effective tool for destroying certain parasites found in meat, such as Trichinella larvae in pork.
For freezing to be effective against parasites, it must be performed under specific, sustained, and very low temperatures. These conditions are usually only achieved under government-supervised commercial conditions. Home freezers cannot be relied upon to eliminate parasites, highlighting the variability in cold tolerance.
Applying Freezing Knowledge to Food Safety
The knowledge that freezing inhibits but does not reliably kill most pathogens is crucial for safe food handling. Freezing serves as a means of preservation, stopping the multiplication of organisms that cause spoilage or illness. It should never be used to make food that was already unsafe—such as food left out too long—safe for consumption.
The danger arises when frozen food is thawed, as the surviving pathogens reactivate and begin to multiply rapidly. Pathogen growth is accelerated within the “Danger Zone” temperatures, which range between 40°F and 140°F (4°C and 60°C). For safety, perishable foods must be thawed safely in the refrigerator, under cold running water, or in the microwave.
The only guaranteed method for eliminating most pathogens is thorough cooking, which uses heat to destroy the microorganisms. Freezing stops the clock on microbial growth, but cooking destroys the bacteria. This two-step process of preservation (freezing) followed by sterilization (cooking) is necessary to ensure food safety.