Are foods cooked in the microwave safe and nutritious? According to Irwin H. Rosenberg, MD, Professor of Nutrition, Friedman School of Nutrition Science at Tufts University, foods cooked in a microwave oven actually keep more of their vitamins and minerals because microwaves can cook food more quickly and without adding water or fat. At worst, microwave cooking reduces nutrient levels in food no more than conventional cooking.
As far as dangerous byproducts occurring during the cooking process, this is also an urban legend. Microwaves cause water molecules in food to vibrate, producing heat that cooks the food. Foods high in water content, such as fresh vegetables, can be cooked more quickly than other foods. The microwave energy is changed to heat as it is absorbed by the food and does not make the food "radioactive" or contaminated.
Microwave ovens should not be used in home canning, because they do not produce or maintain temps. high enough to kill harmful bacteria. Which is also why frozen foods and leftovers need to be thoroughly heated to make sure raw foods are cooked and pathogens are destroyed.
The main concern these days involve the containers used for microwave cooking. Some plastics can be toxic, while others can melt from the high temps. given off by the food. Metals will reflect the electromagnectic waves and damage the appliance, and possibly cause a fire. Use glass containers such as Pyrex, wax paper, paper plates, or items labeled "Microwave safe." Always use products according to the manufacturers' directions.
~ Tufts University, 2014