Microwave Safety

In the United States, food grade containers are known not to leach harmful substances into the foods they hold, whether for storage or for microwave cooking. In most homes in the country, you’ll find a range of containers in the refrigerator, from plastics of known and unknown origin to paper to glass to metals to ceramics. Although many of us probably don’t, maybe we should care whether or not a container is safe for microwave use.

How does a microwave work?  

The oscillating waves produced by a microwave oven are similar to radio waves but much faster. They act mainly by energizing the water molecules present in a food, causing them to vibrate and to make heat. Because of the speed involved, food cooks faster than with conventional means, where heat is transferred from an external source to the material, working from the outside in by way of thermal conduction. Only substances that absorb microwaves can be heated by a microwave oven, with the food itself becoming the heat source for cooking. Heating metals in a microwave produces different, and sometimes unexpected, results. Low penetration depth results in reflection of the waves, setting up high voltage between the metal and the magnetron that is the heart of the system. When this voltage surpasses a threshold, sparks fly. Powdered metal probably would react differently. But we suggest you refrain from trying this unless you work in a science laboratory.

I heard that microwaves destroy the nutrient value of food.

Yeah, we heard that, too. The fact is that any cooking method destroys some character of a food. Using too much water in a pot to cook frozen vegetables, for example, will render water-soluble nutrients to the water, which often goes down the drain. Several studies have shown that microwave cooking, if used the right way, has no more adverse effect on food nutrition than conventional heating methods. In fact, probably because of shorter cooking time, there might even be a tendency for greater nutrient retention (Lassen, 1995). If there be a fault, it would be uneven heating. Moisture loss is more noticeable (Cross, 1982) (Quan, 1985), though, and that makes sense, since all those water molecules bumping against each other create friction, and friction creates heat.

Some studies examined the effects of microwaves on human milk. Besides the usual nutrients a baby needs, breast milk contains immunity factors, such as IgA. Microwaving to temperatures between 161°F and 208°F caused a marked decrease in anti-infective factors (Quan, 1992). We have questions about this. Who heats jarred or bottled baby food or formula hotter than the human wrist can tolerate, which is far lower than the temperature of your water heater? Doesn’t milk come from mom at about 98.6°? At temperatures up to 149°F, fatty acids, most vitamins and immunoglobin are safe (Ovesen, 1996). The hydroxo- form of Vitamin B12, which predominates in foods, appears to be degraded by microwave heating as evidenced in tests on B12-dependent organisms fed a microwaved diet (Watanabe, 1998). But this is only one such test. And most of us don’t put all our eggs into one basket. Because adults cannot metabolize the vitamin B12 from food sources anyway (we lack the gastric intrinsic factor required), we mention this study as a courtesy to the young readers. To overcome poor absorption, sublingual or injectable forms of B12 are available.

So, what should not go into the microwave?

There are some things to keep in mind when using the microwave. Most containers from the takeout place, water bottles, plastic tubs from margarine, yogurt, cream cheese, mustard and mayonnaise, and whipped toppings are not safe for microwave use. Some microwavable trays, such as those from frozen dinners, are designed for one-time use. It should say that on the package. Plastic bags of any kind belong in the trash. If the plastic containers you just bought at the dollar store do not say “Microwave Safe,” don’t use them. Choosing to microwave with a plastic lacking such a declaration doesn’t necessarily mean it’s unsafe, but it is missing the assurance of safety. The symbol on the bottom of the container means nothing in this case.

Plastic wrap—saran—helps to retain moisture but it should not touch the food. The wrap itself is not heated by microwaves, but it will conduct heat from warmed food, and it could melt. The result would have to be an acquired taste that may present toxicity issues. The box of wrap will tell you if it’s microwave safe. Don’t even think about Styrofoam cups and dishware unless it says otherwise.

How about paper?

Paper coffee cups are occasionally lined with wax, and sometimes plastic. Overheating is the worry here. Learn to control the microwave. Many papers are manufactured with chemicals you don’t want in your mouth. You have to read the label. The dyes from printed paper towels can contain toxins. White paper towels are usually safe, but reading that affirmation on the package lets you know for sure. Paper grocery bags—or paper bags of any kind, for that matter—may contain unwanted metals or be recycled from who knows what. Waxed paper and parchment are safe in the microwave. Except for those coated with wax or a plastic film, paper plates should not be a problem. The wrapping will tell you. But plain paper plates are flimsy. The big-name companies have microwavable dinnerware. There is always a bottom line, right? Here it is: the preferred options are glass and ceramic. Still, the best habit to cultivate is to become a label reader. We said that already, didn’t we?

Anna Angela Barba, Antonella Calabretti, Matteo d'Amore, Anna Lisa Piccinelli, Luca Rastrelli Phenolic constituents levels in cv. Agria potato under microwave processing Food Science & Techniology. Dec 2008; 41(10): 1919-1926

Cross GA, Fung DY. The effect of microwaves on nutrient value of foods. Crit Rev Food Sci Nutr. 1982;16(4):355-81.

Anne Lassen, Lars Ovesen Nutritional effects of microwave cooking Nutrition & Food Science, 1995; Vol. 95 Iss: 4: pp.8 - 10

López-Berenguer C, Carvajal M, Moreno DA, García-Viguera C. Effects of microwave cooking conditions on bioactive compounds present in broccoli inflorescences. J Agric Food Chem. 2007 Nov 28;55(24):10001-7.

Ovesen L, Jakobsen J, Leth T, Reinholdt J. The effect of microwave heating on vitamins B1 and E, and linoleic and linolenic acids, and immunoglobulins in human milk. Int J Food Sci Nutr. 1996 Sep;47(5):427-36.

Quan R, Yang C, Rubinstein S, Lewiston NJ, Sunshine P, Stevenson DK, Kerner JA Jr. Effects of microwave radiation on anti-infective factors in human milk. Pediatrics. 1992 Apr;89(4 Pt 1):667-9.

Fumio Watanabe, Katsuo Abe, Tomoyuki Fujita, Mashahiro Goto, Miki Hiemori, and Yoshihisa Nakano Effects of Microwave Heating on the Loss of Vitamin B12 in Foods J. Agric. Food Chem., 1998, 46 (1), pp 206–210