Anuradha Prakash, assistant professor in the Department of Food Science and Nutrition at Chapman University, offers this explanation:
Image: UNIVERSITY OF OREGON
There is no evidence that eating microwaved foods is detrimental to humans or animals. Microwaves are low-energy waves that, like visible light, fall within the electromagnetic spectrum. Like all electromagnetic waves, they are composed of photons, but the photons in microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter--including those in food. They are non-ionizing waves and do not leave a residue.
When food absorbs the energy in microwaves, ions in the food polarize and polar food molecules rotate, causing collisions. It is these collisions that generate friction with the surrounding matrix; the friction quickly produces a lot of heat.
As far as we know, microwaves have no nonthermal effect on food. The only chemical and physical changes result from the heat generated. For instance, if the food is heated to very high temperatures, proteins and carbohydrates may be hydrolysed, certain vitamins will be destroyed, and sugars and proteins may interact; the same reactions occur in foods heated in regular ovens at high temperatures. In fact, some of these changes are less obvious in foods heated in microwaves because the temperature rise is much faster than it is in regular ovens. Longer heating periods can lead to greater physical and chemical changes. For this reason, the nutritional quality of microwaved foods is often superior. For example, when vegetables are steamed on a stove, the water used to cover the vegetables can leach away some of their valuable vitamins; in a microwave, though, additional water is not needed.
Peter McIntyre, physics professor at Texas A&M University, adds the following information:
When food is placed in a microwave oven, the electromagnetic fields from the oven induce electric currents within the water in the food. Because all of our food (like ourselves) is mostly water, this tactic is a pretty good way to generate heat uniformly throughout a serving of food. That is also why it is possible to heat food more quickly in a microwave oven than in a conventional oven, where the food must be heated from outside in. Microwaves do nothing more to food than heat it. There is no evidence that microwaves alter the composition of food or have any other detrimental effects.