Freezing is a widely used method for preserving food, often leading to the misconception that it eliminates all bacteria. However, freezing is primarily a way to halt bacterial growth and maintain food safety, rather than a sterilization technique. It significantly slows down processes that cause spoilage. Understanding how freezing impacts microorganisms is essential for safe food handling practices.
The Science of Freezing and Bacteria
When food is frozen, the low temperatures cause water within bacterial cells to form ice crystals, rendering it unavailable for cellular processes. This reduction in water activity, combined with slowed enzyme activity, forces bacteria into a dormant state where they cannot grow or multiply. While freezing can cause some physical damage to bacterial cell structures due to ice crystal formation and dehydration, this damage is often not lethal for all bacteria.
The extent of damage depends on factors like the cooling rate; rapid freezing can lead to smaller ice crystals and less cell damage, while slower freezing can cause larger, more destructive ice crystals. Even with cellular damage, many bacteria can remain viable, merely suspended in their activity rather than killed.
Bacteria That Survive Freezing
Many types of bacteria can survive freezing temperatures, posing potential risks if food is not handled correctly after thawing. Factors influencing bacterial survival include the specific species, the rate of freezing, the food matrix, and the duration of frozen storage. For instance, bacterial spores, such as those from Clostridium botulinum, are highly resistant to freezing and can remain viable for extended periods.
Vegetative cells of some bacteria, like Listeria monocytogenes and Staphylococcus aureus, are resilient to cold. Listeria monocytogenes, for example, can survive and even grow at refrigeration temperatures, remaining viable when frozen. While freezing reduces bacterial numbers over time, it does not eliminate all bacteria, especially those protected by a food matrix or as spores.
Thawing and Bacterial Growth
The process of thawing is an important stage where dormant bacteria can become active and multiply. When frozen food begins to warm, bacteria reactivate and can rapidly grow if temperatures enter the “danger zone,” which is between 40°F and 140°F (4°C and 60°C). In this temperature range, bacteria can double in number in as little as 20 minutes.
Improper thawing methods, such as leaving food at room temperature, allow the outer layers of the food to warm into the danger zone while the center remains frozen. This provides an opportunity for reactivated bacteria to multiply to dangerous levels. The number of bacteria present before freezing, combined with this rapid growth during thawing, can lead to foodborne illness if the food is not subsequently cooked to a safe internal temperature.
Best Practices for Freezing and Thawing Food
Proper freezing and thawing practices ensure food safety. Freeze food promptly to minimize the initial bacterial load, and cool any cooked dishes before freezing them. Packaging food properly in airtight containers or freezer bags helps prevent freezer burn and cross-contamination, while also reducing air exposure. Freezers should be set at or below 0°F (-18°C) for best preservation.
Safe thawing requires avoiding the temperature danger zone. The safest methods include thawing in the refrigerator, which requires planning due to the time it takes. Other safe methods include thawing under cold running water in a leak-proof bag, changing the water every 30 minutes, or using a microwave, provided the food is cooked immediately afterward. Cooking food to its proper internal temperature after thawing is important to kill any reactivated bacteria. Refreezing thawed food is generally not recommended for quality. However, it can be safe if thawed in the refrigerator and kept below 40°F (4°C) for no more than 3-4 days, or if raw foods were cooked before refreezing.