Freezing temperatures are a common method for preserving food and biological samples, yet a misunderstanding persists regarding their effect on bacteria. Many assume that freezing eradicates all bacteria, leading to a “clean slate” once thawed. However, freezing does not destroy most bacteria; instead, it largely inactivates them by halting their growth and metabolic activity. This article explores bacterial survival in freezing conditions, their resilience, food safety implications, and broader applications.
What Happens When Bacteria Freeze?
When bacteria are subjected to freezing temperatures, their physiological processes slow down or stop entirely, entering a dormant state. This dormancy allows them to survive the cold without actively growing or multiplying. The primary mechanisms of microbial damage during freezing include dehydration within the cells and physical damage to cell surface structures from ice crystal formation.
During slow freezing, rigid ice crystals form both inside and outside bacterial cells. These ice crystals can physically damage cell walls and internal structures like DNA, and the expansion of water as it freezes can also burst bacterial cells. While some bacteria may perish due to these stresses, a significant portion will survive. The goal of freezing for preservation is to inhibit bacterial growth and enzyme activity, thereby slowing spoilage, rather than achieving complete sterilization.
Bacterial Resilience to Freezing
Bacteria exhibit varying degrees of resilience to freezing due to several adaptive mechanisms. Some bacteria produce cryoprotective substances like glycerol or trehalose, which act similarly to antifreeze to protect cells from damage during freezing and thawing. These substances help to lower the freezing point of intracellular water and reduce the formation of damaging ice crystals.
Certain bacteria, such as Clostridium and Bacillus species, can form highly resistant structures called endospores. These dormant structures can withstand extreme temperatures, radiation, and desiccation, reactivating when conditions become favorable. Bacteria can also alter the composition of their cell membranes to maintain fluidity at low temperatures, preserving cellular integrity. Even non-spore-forming bacteria have diverse tolerances, with some psychrophiles adapted to grow and reproduce efficiently at temperatures as low as -20°C.
Freezing and Food Safety
Because freezing does not eliminate all bacteria, thawed food can still pose a safety risk if not handled correctly. Once thawed, surviving bacteria can become active again and multiply rapidly, particularly if the food is left at room temperature. The “Danger Zone” for bacterial growth is between 40°F (4°C) and 140°F (60°C). Perishable foods should not remain in this temperature range for more than two hours.
To minimize risk, frozen food should be thawed safely in the refrigerator, under cold running water, or in the microwave. Thawing food on the counter is not recommended, as the outer layers can reach unsafe temperatures while the center remains frozen, allowing bacteria to multiply. Cooking food to the appropriate internal temperatures, verified with a food thermometer, is the only reliable way to eliminate any reactivated bacteria. Refreezing food that has been thawed incorrectly or left at room temperature is unsafe and should be avoided.
Beyond Food Preservation
The ability of bacteria to survive freezing extends beyond household food preservation, finding applications in scientific and industrial fields. Scientists routinely use cryopreservation to store bacterial cultures long-term in laboratories. This technique maintains the viability and genetic integrity of bacterial strains for research, diagnostic purposes, and educational uses.
Cryopreservation involves storing bacteria at very low temperatures, in liquid nitrogen at -196°C, with the addition of cryoprotectants like glycerol. This principle applies in various biotechnological and industrial contexts, such as preserving starter cultures for fermented foods, probiotics, and other microbial products. The resilience of bacteria to freezing is not merely a challenge for food safety but also a foundational element for many biological advancements.