Does microwaving food cause nutrient loss?

Posted at 6:44 AM, Sep 26, 2018

Microwave ovens provide an easy way to quickly heat food when hunger strikes. But does the convenience of microwave cooking come at a cost by zapping nutrients out of foods?

The answer may surprise you.

Any kind of cooking method will result in some nutrient losses, so a better way to look at the issue is to what degree nutrients are depleted, explained Scott A. Rankin, professor and chair of the Department of Food Science at the University of Wisconsin-Madison. And "typical microwave heating results in very minimal loss of valuable nutrients in food," Rankin said.

The factors that affect nutrient losses in foods during cooking include time (the longer the food is cooked, the more nutrient loss); temperature (the more heat, the more losses); and the amount of liquid the food is cooked in (the more water, the more nutrients you will probably lose as they leach into the cooking liquid).

The effects of those wavelengths

Microwave cooking emits wavelengths that are absorbed by water molecules in food, and the molecules generate heat as they resonate with those wavelengths in food, Rankin explained. But heating food in a microwave often requires very little or no liquid at all. Compare microwaving to cooking broccoli in a pot of boiling water: The water turns green when water-soluble nutrients like vitamin C leach from the broccoli into the water.

Additionally, in a conventional oven, hot air penetrates food from the outside in; thus the exterior of foods can be exposed to excessive heat by the time the food reaches its target temperature, and this increases the potential for nutrient loss. With microwave cooking, the temperature raises more uniformly throughout the food; thus the point at which all particles of the food have reached the desired temperature is achieved with little damage, Rankin said.

What's more, heating food in a microwave often takes just a few minutes. Microwave cooking can have a shorter "come-up time," the time it takes a food to reach its proper cooked temperature and a factor that affects nutrient survival. "The advantage of microwaving is that the come-up time is quick, and so it takes less time to reach a target temperature of food," Rankin said.

So microwave cooking positively influences the time and temperature variables at play, and this bodes well for nutrient retention. But even still, if you conventionally cook foods to what Rankin refers to as a "reasonable degree of doneness," like steaming broccoli on a stovetop, then "the differences in nutrients are minimal at best," he said.

"In contrast, if you boil broccoli in a big pot of water for an hour, you will significantly deplete the nutrient profile," Rankin said.

Steaming is superior

Steaming in the microwave is preferable to submerging foods in water, which can result in a loss of nutrients. "You can use a microwave steaming tray with water in the bottom, or simply add your vegetables and a small amount of water to a microwave-safe bowl and cover with microwave-safe plastic wrap, leaving one corner open to allow air to escape," said Whitney Linsenmeyer, a registered dietitian and spokeswoman for the Academy of Nutrition and Dietetics.

Studies show that microwave cooking results in more moisture loss from foods (and explains why heating cold pizza in a microwave for 30 seconds results in a rubbery crust), but that doesn't result in significant differences in terms of nutrient retention in foods, Rankin explained.

So whether you choose microwaving or conventional cooking methods, keep your cooking times low when possible (don't overcook that broccoli). It's also best to use methods that require minimal added liquid; however, you can salvage nutrients that leach into cooking water when boiling foods by using the cooking liquid as a vegetable stock instead of pouring it down the drain, Linsenmeyer said.

And don't forget the big picture. "Especially when we're talking about fruits and vegetables, eat them any way you like them! Whether it's microwaved, steamed, roasted or raw ... more is better," Linsenmeyer said.