Microwave Cooking: Fast but Safe?
This blog posting discusses microwave cooking in commercial food service facilities. There are still numerous questions. How does microwave cooking work? Is the process safe from two perspectives: does the irradiation create human health hazards; and does the cooking destroy microorganisms?
Microwave ovens were invented in 1945 but became popular in the 1970s. By 1986 approximately one in four American homes owned one, and by 1997 nine out of ten American homes had a microwave oven.
How does a microwave oven ‘cook’? The oven uses radio waves similar to radar. The frequency is 2500 MHz, more powerful than frequencies used to transmit radio, television and cell phone signals. However, its power is much weaker than ultraviolet, visible light, and x-rays, and millions of times weaker than gamma radiation. Microwave radiation is too weak to damage human genetic material; it can generate heat, however, by rearranging water, fats and sugars in food. The radiation quickly dissipates, in a millionth of a second, once the oven turns off.
This heat generation is uneven, however, and that must be a factor when cooking. (This problem with uneven cooking is also a good reason why microwave ovens cannot be used to pasteurize or irradiate foods.) In one study involving a 9-pound pork roast, live trichina worms were recovered after cooking the pork well past 165 degrees F. Thus, the legal requirement is to cook all foods to 165 degrees F., but also rotate the food and check temperatures in two parts. Microwave cooking may also be too fast to allow for the heat to settle throughout the food; it is important to allow food to sit in the oven and continue cooking. Use of microwaves in combination with conventional heating methods will result in more uniform heating of foods and destruction of bacteria.
In another study, researchers found that microwave-heated soup had different temperatures and different bacteria kill rates, at the top, bottom and middle. The failure to provide even-heating can be unsafe since bacteria might survive cooking. It can also irritate customers served a plate of food with cold spots.
This problem is affected by different types and shapes of food. Several foods (tomato soup, beef broth, vegetable soup, hot dog, hamburger and fish) inoculated with E. coli were cooked by both conventional and microwave procedures to compare the relative efficiencies of the two methods in reducing microbial numbers. Conventional broiling, broiling-plus-microwave, and microwave-alone cooking of fish fillets and beef broth were all successful in reducing bacterial counts in sampled regions to a nondetectable level. However, microwave processing of the other foods yielded higher levels of survivors than were found following conventional cooking of these items.
The type of food container and heat loss during cooking are also concerns. The use of plastic food wrap has been recommended to insulate the food and prevent heat loss. The use of unapproved containers can cause leakage into the food.
There is no evidence to support leakage from microwave ovens. In 2000, Health Canada (the Canadian equivalent of the Department of Health and Human Services) tested 60 new microwave ovens and 103 used ovens. None of the new ones, and only one of the used ones, exceeded Canada’s stringent leakage limits, which are similar to the U.S. limits. The one oven that failed was 23 years old. “Leakage of microwaves can only occur if the cooking chamber—including the metal grid over the front window and the seals around the door—has holes in it,” says physicist Louis Bloomfield. If a damaged oven should leak, however, radiation cannot travel far, perhaps a foot, before it dissipates.
Note: Always purchase commercially approved equipment that carries an NSF seal; this assures you the equipment has been evaluated by established standards.
Here are some tips for cooking in a microwave oven, gathered from the USDA FSIS web site (https://www.fsis.usda.gov/node/3355 ‘Cooking Safely in the Microwave Oven’):
Microwave Oven Cooking
• Arrange food items evenly in a covered dish and add some liquid if needed. Cover the dish with a lid or plastic wrap; loosen or vent the lid or wrap to let steam escape. The moist heat that is created will help destroy harmful bacteria and ensure uniform cooking. Cooking bags also provide safe, even cooking.
• Do not cook large cuts of meat on high power (100%). Large cuts of meat should be cooked on medium power (50%) for longer periods. This allows heat to reach the center without overcooking outer areas.
• Stir or rotate food midway through the microwaving time to eliminate cold spots where harmful bacteria can survive, and for more even cooking.
• When partially cooking food in the microwave oven to finish cooking on the grill or in a conventional oven, it is important to transfer the microwaved food to the other heat source immediately. Never partially cook food and store it for later use.
• Use a food thermometer or the oven’s temperature probe to verify the food has reached a safe minimum internal temperature. Cooking times may vary because ovens vary in power and efficiency. Always allow standing time, which completes the cooking, before checking the internal temperature with a food thermometer.
• Cook all foods to at least 165 degrees Fahrenheit and check temperatures in at least two places. Never cooked stuffed meats in a microwave, for fear of uneven heating. Cook immediately after thawing.
• Only use approved containers; some may melt or warp.
• Read the manufacturer’s guidelines for servicing and maintaining the oven.
https://www.cspinet.org/sites/default/files/attachment/microwavemyths.pdf David Schardt ‘Microwave Myths: Fact Vs. Fiction’ Nutrition Action Healthletter April 2005. Jim Felton, Lawrence Livermore Radiation Laboratory and Gary Zeman of Health Physics Society are cited
“The lifetime of microwaves in the oven is something like millionths of a second,” says physicist Louis Bloomfield
Daniel Y. C. Fung and F. E. Cunningham ‘Effect of Microwaves on Microorganisms in Foods’ Journal of Food Protection. Vol. 43, No.8, Pages 641-650 (August, 1980) Copyright © 1980, International Association of Milk, Food, and Environmental Sanitarians 641