In the 1940s, Percy Spencer at Raytheon was testing a magnetron — a device that generates microwaves — when he realized a candy bar in his pocket had melted.
This accidental discovery would lead him to develop what we now know as the modern-day microwave oven. Over the years, this kitchen device has become one more item that makes domestic work that much easier.
Yet questions surrounding the safety of microwave ovens remain. Is the radiation used by these ovens safe for humans? Is the same radiation destroying the nutrients in our food? And what about that study performed on plants fed microwave-heated water (more on this later)?
To answer some of the most popular (and pressing) questions surrounding microwaves, we asked the opinion of three medical professionals: Natalie Olsen, a registered dietitian and exercise physiologist; Natalie Butler, a registered dietitian; and Dr. Karen Gill, a pediatrician.
Here’s what they had to say.
What happens to food when it’s cooked in a microwave?
Natalie Olsen: Microwaves are a form of nonionizing electromagnetic radiation and are used to heat food rapidly. They cause molecules to vibrate and build up thermal energy (heat).
According to the FDA, this type of radiation does not have enough energy to knock electrons out of atoms. This is in contrast to ionizing radiation, which can alter atoms and molecules and cause cellular damage.
Natalie Butler: Electromagnetic radiation waves, or microwaves, are delivered by an electronic tube called a magnetron. These waves are absorbed by water molecules in food, causing [the molecules] to vibrate rapidly, resulting in heated food.
Karen Gill: Microwave ovens use electromagnetic waves of a very specific length and frequency to heat and cook food. These waves target specific substances, using their energy to produce heat, and it is primarily the water in your food that is being heated.
What molecular changes, if any, happen to food when it’s microwaved?
NO: Very minimal molecular changes happen with microwaving, due to the low energy waves given off. Since they are considered nonionizing waves, chemical changes in the molecules in food do not occur.
When food is heated in the microwave, energy is absorbed into the food, causing ions in the food to polarize and rotate [causing] mini-collisions. This is what generates friction and thus heat. Therefore, the only chemical or physical change to the food is that it is now heated.
NB: Water molecules in microwaved food vibrate rapidly as they absorb the electromagnetic radiation waves. Cooked and overcooked microwaved food will gain a rubbery, drier texture due to the rapid movement and accelerated evaporation of water molecules.
KG: Microwaves cause water molecules to move rapidly and cause friction between them — this generates heat. The water molecules change polarity, known as “flipping,” in response to the electromagnetic field created by the microwaves. Once the microwave is turned off, the energy field is gone and the water molecules stop changing polarity.
What nutritional changes, if any, happen to food when it’s microwaved?
NO: When heated, some nutrients in food will break down, regardless of whether it is cooked in a microwave, on a stove, or in an oven. That said, Harvard Health stated food that is cooked for the shortest period of time, and uses as little liquid as possible, will best retain nutrients. A microwave can accomplish this, as it is a faster method of cooking.
One 2009 study that compared the nutrient losses from various cooking methods found that griddling, microwave cooking, and baking [are the methods that] produce the lowest losses of nutrients and antioxidants.
NB: Water content within microwaved food is reduced as it rapidly heats. When cooked or overcooked in a microwave, food texture may become undesirable. Protein may become rubbery, crispy textures soften, and moist foods become dry.
Likewise, vitamin C is a sensitive water-soluble vitamin and is more prone to degradation by microwave cooking than in convection cooking. Yet, while microwave cooking can decrease the antioxidant (vitamin and phytonutrient concentrations of certain plants), they can preserve other nutrients better in the same plants than other cooking methods, like roasting or frying.
Microwaving, can also reduce the bacterial content of food, which can be a useful method of pasteurization and food safety. For example, microwaving red cabbage is superior to steaming for protecting anthocyanin but worse when trying to preserve vitamin C.
Microwaving better protects quercetin, a flavonoid in cauliflower, but is worse in protecting kaempferol, a different flavonoid, when compared to steaming.
Moreover, microwaving crushed garlic for 60 seconds greatly inhibits its allicin content, a powerful anticancer compound. It has been found, however, that if you rest the garlic for 10 minutes after crushing it, much of the allicin is protected during microwave cooking.
KG: All methods of cooking foods cause some loss of nutrients due to heating. Microwaving food is good for retaining nutrients because you don’t need to use a significant amount of extra water (such as with boiling) and your food cooks for a short time.
Vegetables are particularly suited for microwave cooking, as they are high in water content and, therefore, cook quickly, without requiring extra water. This is similar to steaming, but faster.
What are possible negative effects of microwaving food?
NO: The Scientific American offered an explanation from Anuradha Prakash, assistant professor in the Department of Food Science and Nutrition at Chapman University, which stated there is not sufficient evidence to support that a person’s health is negatively impacted by the microwave.
It was stated that, “as far as we know, microwaves have no nonthermal effect on food.” In other words, aside from changing the temperature of food, there is very little to no impact.
NB: Plastic food containers that are microwaved may leach toxic chemicals into the food and should thus be avoided — use glass instead. Radiation leakage may also occur in poorly designed, faulty, or old microwaves, so make sure to stand at least six inches from a microwave when cooking.
KG: There are no short- or long-term effects from microwaving food. The biggest risk with microwaving liquids or foods with high water content is that they can heat unevenly or to very high temperatures.
Always stir foods and liquids after microwaving them and before checking the temperature. Also, choose microwave-safe containers for heating and cooking.
It’s been suggested that plants given microwaved water don’t grow. Is this valid?
NO: The research on this wavers. Some studies have shown an impact on plants in a negative way when microwaved water is used. It has been shown that radiation on plants can affect their gene expression and life. This, however, is primarily seen with ionizing radiation (or higher energy radiation) [rather] than with the radiation that is emitted by microwaves (nonionizing, low energy).
NB: The original science fair project that studied the effect of microwave water on plants went viral back in 2008. To this day, microwaved water is still under question.
Microwaved water has been shown in some studies to actually improve plant seed growth and germination, like in the case of chickpea seeds, while it had the opposite effect on other plants, possibly due to changes in pH, mineral function, and water molecule mobility.
Other research also shows conflicting results on the chlorophyll content of plants: Some plants have decreased color and chlorophyll content when watered with microwaved water, whereas others exposed have increased chlorophyll content. It appears some plants are more sensitive to microwave radiation than others.
KG: No, this is not accurate. This myth has been circulating for years and appears to come from a child’s supposed science experiment. Water that has been heated in a microwave and then cooled is the same as that water before it was heated. There is no lasting change in the molecular structure of water when it is heated in a microwave.
Are there measurable differences between stove- or oven-cooked food and microwave-cooked food?
NO: Microwave ovens have better cooking efficiency since you are heating food from the inside out, rather than outside in, as is the case with a stove or oven. Therefore, the main difference between food cooked on a stove or oven versus a microwave is the cooking time.
According to the World Health Organization (WHO), food cooked in the microwave oven is just as safe and has similar nutrient values as food cooked on the stove.
NB: Yes, differences in food cooked in a microwave versus other methods could be measured by color intensity, texture, moisture content, and polyphenol or vitamin content.
KG: In general, no, there is not. The type of food you are cooking, the amount of water added to cook it, and the container you use can all affect cooking times and the amount of nutrients lost during cooking.
Microwaved food can often be healthier due to short cooking times and less of a need for extra fat, oil, or water needed for cooking.
Natalie Olsen is a registered dietitian and exercise physiologist specializing in disease management and prevention. She focuses on balancing the mind and body with a whole-foods approach. She has two Bachelor’s degrees in Health and Wellness Management and in Dietetics, and is an ACSM-certified exercise physiologist. Natalie works at Apple as a corporate wellness dietitian, and does consulting in a holistic wellness center called Alive + Well, as well as through her own business in Austin, Texas. Natalie has been voted among the “Best Nutritionists in Austin” by Austin Fit Magazine. She enjoys being outdoors, warm weather, trying new recipes and restaurants, and traveling.
Natalie Butler, RDN, LD, is a foodie at heart and passionate about helping people discover the power of nourishing, real food with an emphasis on a plant-heavy diet. She graduated from Stephen F. Austin State University in east Texas and specializes in chronic disease prevention and management as well as elimination diets and environmental health. She’s a corporate dietitian for Apple, Inc., in Austin, Texas, and also manages her own private practice, Nutritionbynatalie.com. Her happy place is her kitchen, garden, and the great outdoors, and she loves teaching her two kids to cook, garden, be active, and enjoy a healthy life.
Dr. Karen Gill is a pediatrician. She graduated from the University of Southern California. Her expertise includes breastfeeding, nutrition, obesity prevention, and childhood sleep and behavior issues. She has served as chair of the Department of Pediatrics at Woodland Memorial Hospital. She was a clinical preceptor with the University of California, Davis, teaching students in the physician assistant program. She now practices at the Mission Neighborhood Health Center, serving the Latino residents of the Mission district in San Francisco.