Bacteria are microscopic organisms found nearly everywhere, including in and on food. While many types are harmless or beneficial, pathogenic bacteria can cause illness if consumed. Applying heat is a primary method to eliminate these harmful microorganisms, making food safe. Understanding how heat affects bacteria is fundamental for food safety.
How Heat Kills Bacteria
Heat eliminates bacteria by disrupting their cellular structure and function. The most significant effect is protein denaturation. When exposed to high temperatures, bacterial proteins lose their specific three-dimensional shape, rendering them non-functional. This irreversible process causes the cell to cease vital activities.
Heat also inactivates enzymes, specialized proteins that catalyze essential biochemical reactions. When these enzymes denature, the microorganism can no longer carry out basic metabolic processes, leading to its death. High temperatures can also damage the bacterial cell membrane, a protective barrier. The membrane’s lipids and proteins lose structural integrity, causing cell contents to leak out, resulting in cell death. The effectiveness of heat depends on both temperature and duration, as different bacteria have varying tolerances.
Temperatures for Killing Common Bacteria
The effectiveness of heat is quantified by concepts like thermal death point (TDP) and thermal death time (TDT). TDP is the lowest temperature to kill all microorganisms in a liquid suspension within a specific time, usually 10 minutes. TDT is the minimum time needed to destroy a specific population of microorganisms at a given temperature. These values vary depending on the microorganism type and environmental factors like pH and moisture.
For example, Listeria monocytogenes, a bacterium that can cause severe illness, can be substantially reduced when a temperature of 70°C (158°F) is reached and maintained for at least two minutes throughout food. Pasteurization, a common heat treatment, aims to destroy specific pathogens like Salmonella and E. coli while minimizing damage to food quality. Milk pasteurization, for instance, typically involves heating to 72°C (161°F) for 15 seconds or 63°C (145°F) for 30 minutes to control bacteria like Salmonella.
Practical Food Safety Temperatures
Adhering to recommended cooking temperatures translates scientific principles into practical food safety. The United States Department of Agriculture (USDA) provides guidelines for safe internal temperatures to destroy harmful bacteria. Using a food thermometer is the only reliable way to confirm food has reached a safe minimum internal temperature. Color or texture are not trustworthy indicators.
For poultry, the safe minimum internal temperature is 74°C (165°F). Beef, pork, veal, and lamb should be cooked to at least 63°C (145°F) and allowed to rest for three minutes. This rest time allows the temperature to remain constant or continue to rise, further destroying microorganisms. Ground meats have a higher recommended temperature of 71°C (160°F) because grinding can distribute bacteria throughout. Leftovers and casseroles should always be reheated to 74°C (165°F).
Beyond cooking, maintaining safe temperatures for hot and cold foods is important. Hot foods should be held at or above 60°C (140°F) to prevent bacterial multiplication. Cold perishable foods must be kept at or below 4°C (40°F). Foods should not remain in the “danger zone” between 4°C (40°F) and 60°C (140°F) for more than two hours.