Controlling bacteria in food through temperature
Bacteria are a natural part of our world and we consume millions or billions every d ay without ill effects. For instance, some cheeses contain billions of harmless b ac teria, which give the cheese its flavor. But some bacteria can harm humans . There may be as many as 80 million cases of food-borne disease in the United States every year, though the majority of infected people have mild symptoms and don't seek medical attention.
The bacteria that make you sick cannot be detected in or on food, except by special laboratory tests. Bad-sme lling food, full of bacteria that cause spoiling, may be quite harmless. Conversely, the best-looking and - smelling food may contain harmful bacteria. Some bacteria, such as the E. coli strain that has recently made news, don't have to be able to grow (multiply) in the food, they just have to be there alive, and in very low numbers, to cause illness. Others, however, need to multiply to numbers sufficient to cause illness; lower numbers can be ingested without apparent ill effects. With some strains, the bacteria themselves are harmless, but the toxins they produce can make you sick. Most foods can support the growth of some bacteria. Your best bet is to treat all foods, both raw and cooked-particularly those of animal origin-as though they do harbor harmful bacteria.
Use extremes of hot and cold-You can prevent bacteria from becoming a problem by controlling the temperature at which food is s tored, cooked, and reheated. To deal effectively with those bacteria that must multiply in food to cause illness, keep food out of the "danger zone," the range of temperatures at which bacteria multiply-between 41 ° and 1 40°F. Below 41 °, most bacteria are still alive, but they're not multiplying-or not multiplying very fast. When food is heated above 140°, most bacteria are killed and are therefore no longer a problem. Between 41 ° and 140°, most bacteria live quite comfortably and reproduce vigorously. At typical summer temperatures, bacteria multiply very rapidly; some can double their populations every 10 or 15 minutes.
If food has been cooked, cooled, and reheated, it has passed through the danger zone twice-once as it cooled and once as it warmed up again-so any bacteria that survived the initial cooking or got in the food after it was cooked has had two chances to multiply and accumulate. Also, not all toxins produced by bacteria can be destroyed by heat--Dne more good reason to prevent bacteria from multiplying in the first place.
Quick chilling-To minimize time spent in this bacteria-friendly zone, chill food quickly. For nonliquid dishes, first quickly cool them to room temperature by spreading the food out in a thin layer on a plate or baking sheet, and then cover it and put it in the refrigerator. For stocks and soups, put the whole pot in an ice bath, and stir to quickly bring the temperature down before refrigerating.
Killing bacteria with heat-While cold temperatures keep the bacteria from multiplying, high temperatures are needed to actually kill them. Bacteria succumb to heat for the same reasons that any living cell does. Heat does physical damage to the bacterial cell's protective membranes, but more importantly, heat denatures proteins. To carry out the functions of life, bacteria and other cells depend on proteins called enzymes. Enzymes are special proteins that catalyze chemical reactions. The sum of thousands of these re actions, catalyzed by hundreds to thousands of enzymes, are what define the organism's life. When the reactions stop, the organism dies. Heat disrupts the special three - dimensional structure of the bacterial cell's enzymes and renders them inactive. If enough enzymes are destroyed, the bacterium dies.
While most bacteria will die when they reach 140°, many can survive higher temperatures. Even bacteria of the same species (of which there are thousands) die at different rates. Bacteria are well adapted to survive. This is the reason that vigilance in cooking is stressed. Time is importa nt, too-Vigilance means controlling not just temperature, but time, too. Most recipes specify one cooking temperature and one fixed time. The U.S. Food and Drug Administration's Food Code 1993 now specifies flexible time/temperature combinations.
Some foods can't be cooked at temperatures high enough to kill bacteria and still retain their sensory appeal. For example, the Food Code directs that ground meats should be cooked to heat all parts of the food to a temperature of 155 ° for 15 seconds. But to allow for flexibility in the kitchen, time/temperature combinations that are equivalent in their lethality for bacteria can also be used. The Food Code says that ground beef can be cooked to 150° for one minute, or to 145 ° for three minutes. For roast beef, equivalent lethality is delivered to bacteria when the roast is cooked so that all parts a re heated to 145 ° for three minutes, or at the other time/temperature extreme, 130° for 121 minutes. Again, these two time/temperature combinations are equal in their ability to kill bacteria. Rare roast beef can be safely prepared by cooking at the lower temperature, but for a longer time. The chef's best insurance for proper cooking is a well-calibrated oven and a reliable cooking thermometer.
Microwave ovens cook differently than conventional ovens, and not every spot in the food may be equally heated by microwave cooking. That's why the Food Code specifies that the food must be rotated or stirred during cooking to compensate for uneven heat distribution. Food that is to be cooked in a microwave should also be covered to retain surface moisture because bacteria on a dry surface are some times more resistant to heat; the food should be heated to temperatures 25° above the recommended conventional oven temperatures. After cooking, it's extremely important to let microwaved food stand covered for two minutes to let the temperature even out.
This assures delivery of an adequate time/temperature combination. The best and most current compilation of these time/temperature combinations and other food safety guidelines is the new Food Code, a guide for restaurants and retail outlets. This 430-page document is available on hard copy, on computer diskette, and through a computer modem from National Technical Information Service (NTIS) at 703/487 -4650. The more you know about food-borne illness, the less likely it will affect you.
