Until refrigeration became cheap and widespread in the 20th century, our forebears kept leftovers covered at room temperature in their pantries. Not surprisingly, sudden illnesses were also common in those days. In the modern era of powerful microscopes, homemakers now have the luxury of understanding the life cycle of bacteria and why it's dangerous to leave foods sitting out.
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Bacteria and Other Pathogens
Bacteria and other microorganisms are all around us, every day. Only a very few, referred to collectively as pathogens, are dangerous to humans. Most of them thrive at temperatures between 40 and 140 degrees Fahrenheit, referred to as the food safety "danger zone." Below 40 F or above 140 F, bacteria struggle to survive and have little energy to spare for reproduction. At temperatures of 170 F or above, most pathogens are killed, so when your roast comes from the oven it's pretty sterile. Unfortunately, that changes quickly once its temperature drops.
Time and Temperature
Time and temperature are key factors in food safety. Once your food drops back below 140 F and into the danger zone, it quickly becomes repopulated with bacteria. Bacteria reproduce quickly and can double their population in as little as 20 minutes. The longer they have to reproduce at room temperature, the more bacteria there are in the food. It takes a certain number of bacteria, referred to as the "infective dose," to make someone sick. That number varies from species to species, but invariably the more bacteria there are, the greater the risk.
Bacteria and microscopic pathogens are not the only risk factors for food left out. The longer your food sits, the greater the likelihood that it will become contaminated by dust, flies, pet dander, hair, insects, rodents, curious pets or your children's unwashed hands. A more serious risk is contamination from allergens or household cleaning products, which can be quite toxic if ingested.
To minimize the risk of foodborne illness, always follow the food safety guidelines laid out by the USDA's Food Safety and Inspection Service. Don't let your food cool for more than a half hour at room temperature before refrigerating it. Ideally, leftover meat should be reduced to 40 F or below within two hours. Neither refrigeration or freezing kills bacteria, but both prevent them from reproducing and arriving at an infective dose. Always package your leftovers in airtight bags or containers to prevent contamination by other foods or outside elements.