While extreme cold might seem like a harsh environment for microorganisms, scientific evidence indicates that ice and freezing generally do not kill most germs. Instead, these low temperatures primarily inhibit microbial growth, placing them into a state of dormancy. This distinction is crucial for understanding true food safety.
Ice’s Impact on Germs
Freezing temperatures significantly affect microorganisms by causing them to enter a dormant state. At 0°F (-18°C) or lower, the metabolic processes and reproduction of bacteria, yeasts, and molds are largely slowed or halted. This occurs because the water within and around microbial cells turns into ice crystals, limiting the availability of liquid water necessary for cellular functions. The formation of these ice crystals can also cause physical damage to microbial cell walls and internal structures, which may reduce the number of viable organisms, but it typically does not eliminate them entirely.
When frozen food or water thaws, these dormant microorganisms can become active again. They can then resume multiplying if conditions, such as temperature and nutrient availability, become favorable. This means that while freezing is an effective method for preserving food and preventing spoilage by inhibiting germ growth, it does not sanitize or sterilize the product.
Germs That Endure Freezing
Numerous types of bacteria, viruses, and parasites are known to survive freezing temperatures, posing a potential risk if food is not handled properly after thawing. Listeria monocytogenes, for instance, is particularly resilient, capable of surviving and even growing at refrigeration temperatures as low as 32°F (0°C) and remaining viable when frozen. This bacterium adapts to cold by adjusting its cell membrane fluidity and producing cold shock proteins, which help maintain cellular function.
Salmonella and Escherichia coli (E. coli) are also common bacteria that can withstand freezing. While freezing may cause some reduction in their numbers, a significant portion can survive. Salmonella can enter a dormant state, slowing its metabolism and accumulating protective molecules to endure freezing stress. Noroviruses, frequently associated with foodborne illness, are highly resistant and can survive freezing temperatures. Certain parasites, such as Trichinella, can also survive freezing.
Effective Germ Elimination Methods
Unlike freezing, which inhibits growth, other methods actively eliminate germs by destroying their cellular structures. Heat is a primary method for killing microorganisms, particularly in food. Cooking to safe internal temperatures denatures microbial proteins and destroys their cells, effectively eliminating harmful bacteria and viruses. Poultry should reach 165°F (74°C), ground meats 160°F (71°C), and whole cuts of beef, pork, and lamb 145°F (63°C) with a three-minute rest time. Using a food thermometer ensures these temperatures are met throughout the food.
Chemical disinfectants are another effective means of germ elimination, primarily for surfaces rather than food. These agents work by coagulating proteins, disrupting cell membranes, or interfering with microbial metabolism. Common disinfectants like bleach and alcohol are effective against a broad range of bacteria, fungi, and viruses. Their effectiveness can vary depending on the specific microorganism and concentration. Proper cleaning techniques, which physically remove dirt and germs using soap and water, also reduce the microbial load on surfaces.
Ensuring Ice and Frozen Food Safety
Given that freezing does not kill most germs, proper handling of ice and frozen foods is important to prevent the growth and spread of microorganisms. To avoid cross-contamination, always thaw raw meats separately from other foods, ideally on the bottom shelf of the refrigerator in a container. Safe thawing methods include refrigeration, cold water, or the microwave, never at room temperature. Foods thawed in cold water or the microwave should be cooked immediately.
Proper storage temperatures are important; freezers should be kept at 0°F (-18°C) or lower to inhibit microbial activity. Regularly cleaning ice machines and ice trays prevents contaminant buildup. Do not refreeze foods that have fully thawed at room temperature for more than two hours. However, if food thaws in the refrigerator and remains at 40°F (4°C) or below for 3-4 days, it can be safely refrozen, though quality might be affected.