Bacteria are microscopic, single-celled organisms found in nearly every environment, including food. While many are harmless, some can cause illness. Understanding how temperature influences bacterial growth and survival is important for safety.
Bacterial Behavior at Different Temperatures
Temperature significantly impacts bacterial activity, influencing their growth and survival. Bacteria multiply most rapidly within the “danger zone,” typically between 5°C and 60°C. Within this zone, particularly around 37°C, bacteria can double quickly, increasing the risk of foodborne illness. Cold temperatures, such as refrigeration (4°C or colder) or freezing (-20°C or less), inhibit bacterial growth by slowing metabolic processes. However, these low temperatures do not kill most bacteria; they merely put them into a dormant state, allowing them to become active again once temperatures rise.
High temperatures eliminate bacteria by causing irreversible damage to their cellular components. Heat denatures proteins, which are crucial for bacterial structure and function, causing them to lose their shape and become non-functional. This process also alters bacterial cell membranes, leading to cell breakdown and death. This mechanism underpins the effectiveness of cooking and other heat treatments for food safety.
Temperatures for Eliminating Bacteria
Achieving specific internal temperatures eliminates harmful bacteria in food. For poultry, an internal temperature of 74°C is recommended. Ground meats, such as beef or pork, should reach 71°C to kill bacteria distributed throughout the product during grinding. Whole cuts of beef, pork, veal, and lamb can be cooked to 63°C, followed by a three-minute rest period that allows the temperature to continue rising and ensures pathogen elimination.
When reheating leftovers, heat them thoroughly to at least 74°C. This temperature should be reached throughout the entire food item to destroy any bacteria that may have grown during cooling or storage. Bringing gravies, soups, and sauces to a rolling boil is an effective way to ensure they reach a safe reheating temperature.
Pasteurization, a process used for liquids like milk, involves heating to specific temperatures for defined durations to destroy harmful microorganisms. For milk, common methods include 63°C for 30 minutes (low-temperature long-time) or 72°C for 15 seconds (high-temperature short-time). Ultra-high-temperature (UHT) pasteurization heats milk to 138–150°C for one to two seconds, extending shelf life.
Factors Affecting Bacterial Elimination
Beyond target temperature, several factors influence bacterial elimination. The duration of heat exposure is significant; maintaining a specific temperature for a longer period can compensate for a slightly lower temperature. Different microorganisms exhibit varying heat resistance; some, like spore-forming bacteria, are more tolerant and may require higher temperatures or longer heating times for inactivation.
The initial bacterial load, or the number of bacteria present before heating, also plays a role. A higher initial count may necessitate more rigorous heat treatment to reduce the population to safe levels. The moisture content of the food or environment impacts bacterial heat resistance. Bacteria in low-moisture environments often exhibit increased thermal resistance, requiring higher temperatures or longer exposure times for elimination.
Applying Temperature Knowledge for Safety
Using a food thermometer is the most reliable method to confirm food has reached a safe internal temperature. Visual cues like color or texture are not accurate indicators of doneness or safety. Insert the thermometer into the thickest part of the food, avoiding bones, for an accurate reading.
Proper cooling and reheating practices are essential to prevent bacterial growth. Cooked foods should be cooled rapidly: from 60°C to 21°C within two hours, then to 5°C or colder within an additional four hours. Dividing large portions into smaller, shallow containers facilitates quicker cooling. When reheating, all parts of the food must reach 74°C.
Preventing cross-contamination is important. Keep raw meats and their juices separate from ready-to-eat foods. This includes using separate cutting boards and utensils for raw and cooked items. Regular hand washing with soap and hot water after handling raw food minimizes bacterial spread.