Bacteria do not completely die when frozen; instead, freezing primarily inhibits their growth and activity. This common misconception often leads people to believe that freezing sterilizes food, but it is more accurately a method of preservation. While some bacteria may be killed during the freezing process, a significant portion can survive in a dormant state.
Freezing’s Impact on Bacterial Life
Freezing temperatures cause bacteria to enter a dormant state, significantly slowing or halting their metabolic processes. This inhibition of growth is why freezing effectively preserves food by preventing bacteria from multiplying to harmful levels. However, this differs from sterilization, which eliminates all living microorganisms. Many bacterial cells can endure freezing temperatures and remain viable.
Even though bacterial activity is suppressed, freezing does not destroy all bacterial cells. The effectiveness in reducing bacterial populations varies with the type of bacteria, food, and freezing rate. Consequently, food contaminated before freezing will still contain bacteria upon thawing.
The Science of Bacterial Survival
Bacteria can survive freezing due to several protective mechanisms and the nature of ice formation. One factor is ice crystal formation, which can cause physical damage to bacterial cell walls. However, not all bacteria are killed by this mechanism, and some can even produce antifreeze proteins to prevent internal ice crystal growth or lower the freezing point of water within their cells.
Another factor is the reduction of available water, or water activity. As water freezes, it becomes unavailable for bacterial metabolism, dehydrating the cells. This dehydration can be damaging but also contributes to dormancy, aiding survival. Metabolic processes also slow at low temperatures; some bacteria, like psychrophiles, even remain active in sub-zero conditions. Resilient bacteria can also form protective structures like spores, resistant to extreme conditions including freezing.
Reactivation After Thawing
Once frozen food begins to thaw, dormant bacteria can reactivate and resume metabolic functions. As temperature rises and water becomes available, these bacteria can multiply rapidly. This multiplication is concerning if food enters the “danger zone,” where bacteria thrive and reproduce quickly. This zone is between 40°F (4°C) and 140°F (60°C).
Thawing does not reduce the initial bacterial load from before freezing. If food was contaminated with harmful bacteria, they can become active and multiply once thawed. This highlights the importance of proper thawing and cooking for food safety.
Freezing for Food Safety
Freezing is an effective food preservation method, inhibiting microorganism growth and slowing spoilage. It does not sterilize food, only pausing bacterial activity. Therefore, safe handling practices before and after freezing are essential to prevent foodborne illnesses.
Food should never be left at room temperature when thawing, as bacteria multiply rapidly. Safe thawing methods include defrosting in the refrigerator, under cold running water, or in the microwave. Food thawed in cold water or the microwave should be cooked immediately. Cooking food to its safe internal temperature is the only way to kill any bacteria that survived freezing.