Microwaves have gotten a bad rap for being unhealthy, zapping the vitamins and other nutrients out of food, and leaching bad stuff out of plastic and into your lasagna, but is that reputation deserved? The answer is, well—complicated.
Whether you cook on a stove or in the nuker, heating food causes a certain loss of nutrients, period. Factors like cooking time, the amount of liquid, and cook temperature are the biggest factors in nutrition loss, not the source of heat. Overall, the healthiest way to cook food is to do so as quickly as possible.
As a microwave heats food, it instantly reaches molecules an inch below the surface of the food, rather than heating from the outside (as with conventional cooking), resulting in better nutrient preservation. For example, boiling spinach in a saucepan causes it to lose up to 70 percent of its folic acid, but zapping it in the microwave with a small amount of water retains almost all of it. Cooking bacon in a frying pan triggers cancer-promoting chemicals like nitrosamines, whereas cooking it in a microwave produces less. On the other hand, there are a few vegetables that are actually healthier when boiled on the stovetop (boiling carrots, for example, increases carotenoids like lutein and beta carotene).
In other words, nutrient loss according to heat source is pretty much give and take. Depending on what you’re cooking and how long you cook it, microwaving can actually be about the healthiest way to make dinner. Anyway, as Robert L. Wolke notes in What Einstein Told His Cook: Kitchen Science Explained, there’s really no harm in eating the occasional dish without all of the optimal nutrients intact, because you’ll most likely balance it out with another, more nutrient-rich dish during the day. At the end of the day it’s about balance—even when it comes to how you cook.
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