Effects of Temperature on Food

Prepared by Pamela Schmutz, HGIC Food Safety Specialist and Elizabeth Hoyle, Extension Food Safety Specialist, Clemson University. (New 01/99. Revised 6/11.)

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Background

"Last night I left cooked roast beef on the counter to cool before refrigerating but fell asleep and discovered it this morning. I immediately put it in the refrigerator. Since the meat is cooked, shouldn't it be safe to eat?"

The answer to this question is that the roast beef should be thrown out. Why? Because leaving food out too long at room temperature can cause bacteria - such as Staphylococcus aureus, Salmonella enteriditis, Escherichia coli O157: H7, and Campylobacter - to grow to dangerous levels that can cause illness.

Bacteria exist everywhere in nature. They are in the soil, air, water and the foods we eat. When the bacteria have nutrients (food), moisture, time and favorable temperatures, they grow rapidly increasing in numbers to the point where some can cause illness. Therefore, understanding the important role temperature plays in keeping food safe is critical. If we know the temperature at which food has been handled, we can then answer the question, "Is it safe?"

The Danger Zone (40 to 140 °F)

Bacteria grow most rapidly in the range of temperatures between 40 and 140 °F, doubling in number in as little as 20 minutes. This range of temperatures is often called the "Danger Zone." That's why perishable foods should never be left out of refrigeration over two hours. If the temperature is above 90 °F, food should not be left out more than one hour.

Cooking

Raw meat and poultry should always be cooked to a safe internal temperature. When roasting meat and poultry, an oven temperature no lower than 325 °F should be used. Use a meat thermometer to assure that meat and poultry have reached a safe internal temperature.

Cook beef, lamb, pork and veal steaks, roasts and chops to a safe minimum internal temperature of at least 145 °F, then allow to rest for 4 minutes before carving or eating. Cook longer if you prefer your meat more well-done. Cook ground meats (beef, veal, lamb and pork), mechanically tenderized, or injected meats to an internal temperature of 155 °F. All poultry is safe if it reaches a minimum internal temperature of 165 °F throughout the product. For reasons of personal taste preferences, poultry may be cooked longer.

If raw meat and poultry have been handled safely, using the above preparation recommendations will make them safe to eat. If raw meats have been mishandled (left in the Danger Zone too long), bacteria may grow and produce toxins, which can cause foodborne illness. Cooking does not destroy toxins that are heat-resistant. Therefore, even though cooked, meat and poultry mishandled in the raw state may not be safe to eat even after proper preparation.

Storing Leftovers

One of the most common causes of foodborne illness is improper cooling of cooked foods. Bacteria are everywhere, even after food is cooked to a safe internal temperature, and they can be reintroduced to the food and then reproduce. For this reason leftovers must be put in shallow containers for quick cooling, and refrigerated immediately or within two hours of preparation.

Reheating

Foods should be reheated thoroughly until hot and steaming, and to an internal temperature of 165 °F. In the microwave oven, cover food and rotate so it heats evenly. Follow manufacturer's instructions for stand time for more thorough heating. In the absence of manufacturer's instructions, at least a two-minute stand time should be allowed.

Cold Storage Temperatures

Properly handled food stored in a freezer at 0 °F will always be safe. Freezing keeps food safe by slowing the movement of molecules, causing bacteria to enter a dormant stage. Once thawed, these bacteria can again become active and multiply to levels that may lead to foodborne illness. Because bacteria on these foods will grow at about the same rate as they would on fresh food, thawed foods should be handled as any other perishable food.

A temperature of 34 to 40 °F should be maintained in the refrigerator. In contrast to freezer storage, perishable foods will gradually spoil in the refrigerator. Spoilage bacteria will make themselves known in a variety of ways. The food may develop an uncharacteristic odor, color and/or become sticky or slimy. Molds may also grow and become visible. Bacteria capable of causing foodborne illness either don't grow or grow very slowly at refrigerator temperatures. A refrigerator/ freezer thermometer should always be used to verify that the temperature of the unit is correct.

Safe food-handling practices are a good defense against foodborne illness. Because we know how different temperatures affect the growth of bacteria in our food, we can protect ourselves and our families from foodborne illnesses by proper handling and cooking, and by storing foods at safe temperatures.

For more information see HGIC 3580, Cooking Meat Safely and HGIC 3587, Food Thermometers: A Key to Food Safety.

  1. USDA Food Safety and Inspection Service. How Temperature Affects Food. August 2006. http://www.fsis.usda.gov/Fact_Sheets/How_Temperatures_Affect_Food/index.asp
  2. FDA. Food Code 2009. http://www.fda.gov/Food/FoodSafety/RetailFoodProtection/FoodCode/FoodCode2009/ucm186451.htm
  3. USDA Food Safety and Inspection Service. May 2011. USDA Revises Recommended Cooking Temperature for All Whole Cuts of Meat, Including Pork, to 145 °F. http://www.fsis.usda.gov/News_&_Events/NR_052411_01/index.asp

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