The common belief that freezing food sterilizes it by killing all bacteria is a misconception that can compromise food safety. In reality, freezing serves primarily as a method of preservation, which microbiologically is known as a bacteriostatic effect. This means the low temperature prevents bacterial growth and multiplication, but it does not achieve the complete cell death necessary for sterilization, which is a bactericidal effect. The bacteria that were present on the food before freezing remain dormant, ready to resume activity once temperatures rise.
Freezing Stops Growth, Not Killing
Freezing halts the life processes of microorganisms by slowing the movement of their molecules to an almost complete stop. Temperatures consistently maintained at 0°F (-18°C) or lower cause microbes to enter a state of deep dormancy, inactivating their ability to reproduce or metabolize nutrients. This state of suspended animation is why frozen food can be safe for extended periods; bacteria are prevented from increasing their numbers.
The bacteria remain viable, often for years, and the population reduction achieved by a standard household freezer is insignificant from a food safety perspective. While some cells may be injured or die, a large percentage of the original population survives in a dormant state. Once the food is thawed and returns to warmer temperatures, the surviving bacteria quickly resume their normal metabolic functions and reproduction. This means a food item contaminated before freezing will still be contaminated upon thawing.
Physical Damage from Ice Formation
Although the primary effect of freezing is to stop growth, a portion of the bacterial population does die due to two distinct mechanisms of cellular injury. The first is physical damage caused by the formation and growth of ice crystals. When water inside the cell freezes, the jagged edges of the crystals can rupture the delicate cell membrane and walls, leading to irreversible structural damage and cell death.
The second, and often more damaging, mechanism is osmotic stress, sometimes called a “solution effect.” As water freezes outside the cell, it forms pure ice, effectively removing water from the surrounding liquid environment. This causes the concentration of salts and other solutes remaining in the unfrozen liquid to increase dramatically. This highly concentrated external environment draws water out of the bacterial cell through osmosis, leading to severe dehydration and physiological damage to cellular components like proteins and enzymes.
Variables That Affect Survival Rates
The percentage of bacteria that survive freezing is not uniform and depends heavily on the freezing process and the specific type of microorganism. Death rates are often highest at temperatures just below the freezing point, such as between 31°F and 23°F (-0.5°C and -5°C). This occurs because the cells are exposed to the damaging effects of concentrated solutes and dehydration for a longer period as the freezing process slowly progresses.
The speed of freezing plays a significant role in determining the mechanism of injury. Slow freezing, typical of a home freezer, maximizes osmotic stress by giving water time to leave the cell before ice forms internally, resulting in more deaths from dehydration.
Conversely, very rapid freezing, such as flash-freezing at ultra-low temperatures like -35°C, can better preserve microorganisms. The water is solidified too quickly to form large, damaging ice crystals or cause significant osmotic stress. Furthermore, certain species exhibit enhanced cold tolerance; Gram-positive bacteria are generally more resistant to freezing damage than Gram-negative bacteria, and bacterial spores are extremely resilient.
Safe Handling of Frozen and Thawed Foods
Since freezing only inactivates and does not sterilize, the period during and after thawing presents the highest risk of bacterial growth. Bacteria multiply rapidly when food temperatures enter the “danger zone,” defined as the range between 40°F and 140°F (4°C and 60°C). Allowing food to thaw on a kitchen counter means the surface quickly reaches this range, providing an environment for dormant bacteria to multiply to dangerous levels.
To minimize risk, food should be thawed using controlled methods that keep it out of the danger zone. The safest method is slow thawing in the refrigerator, which must maintain a temperature below 40°F (4°C). Alternatively, food can be thawed under cold, running water, provided it is sealed in a leak-proof package and the water is changed every 30 minutes. Microwave thawing is also acceptable, but the food must be cooked immediately afterward because uneven heating can cause sections to enter the danger zone.
Freezing does not destroy any toxins that bacteria may have produced before the food was placed in the freezer. Some bacteria, such as Staphylococcus aureus, produce heat-stable toxins that cannot be eliminated by cooking or freezing. Consequently, if food was already spoiled or contaminated before freezing, those toxins will remain present, and the food may still cause illness even if properly thawed and cooked.