The microwave may have done more to revolutionize cooking than the original discovery of how fire affected raw food. Convenient and fast, microwave ovens have spawned an entire industry devoted to allowing the cook to have dinner on the table in minutes. Although they are so widespread, questions and concerns about the safety and nutrition of microwaved foods do surface from time to time.
- How Microwaves Work – Unlike traditional methods of cooking, which heat foods directly from a heat source, microwaves use energy in a form similar to that of a radio. The waves primarily affect water and some other molecules, causing them to vibrate. It is the vibration that produces the heat that actually cooks the food.
- What Destroys Nutrients? – All forms of cooking change the food in some way. Heat denatures (breaks down) proteins. Cooking in water can leach vitamins. Vitamin C readily breaks down when exposed to any form of heat. Generally speaking, cooking something quickly retains more nutrients. However, slow braising and stewing, which are important techniques for tougher cuts of meat, allow many nutrients to leach into the cooking liquid, which is then eaten.
- Timing Matters – Extended exposure to heat causes many negative changes in food. A tender steak can become tough. Roasted meats may dry out in the heat of the oven, and eventually, almost any food can become charred even if exposed only to low heat but over a long period. Short cooking times help foods retain nutrients – particularly vegetables, which don’t usually need long cooking times in the first place.
- Water Matters – The more liquid used in cooking, the greater the amount of leaching. Foods will retain the most nutritional value if cooked with little or no water. Broccoli, for example, loses a sulfur-containing compound called glucosinolate that provides anti-cancer properties when it is boiled. For vegetables, steaming is probably the best method of cooking, as it is quick and uses minimal water.
- Cooking vs. Warming – This is really a matter of degree. You can warm raw food without fully cooking it. The food does undergo some chemical and structural changes, but they are not sufficient to bring the dish to a state at which you could safely eat it. When you’re warming food, however, it’s already been cooked. Most of the vitamin loss and other changes that will occur have already occurred.
- The Bottom Line – Microwave cooking, like all cooking, does cause loss of some nutrients. Some nutrients, like the lycopene found in tomatoes, are actually more concentrated when cooked. Once a food is fully cooked, however, reheating it could cause the loss of heat-sensitive nutrients like vitamin C. Reheating can also make foods tough and unpalatable, even if it doesn’t affect the nutritive quality of the food.
There’s no evidence that microwave cooking destroys any more nutrients than conventional cooking, whether during the cooking or reheating process. For that matter, since cooking foods like vegetables quickly in small amounts of water is one of the best ways to retain nutrients, microwaves generally get a gold star from most nutritionists and dietitians. Do cook in a microwave-safe container, stir the food and let it stand to distribute the heat evenly for best quality.