Heather Bedard, C.H.E.
The household microwave as we now know it, was first patented in 1946 by a man by the name of Percy Spencer. The story goes that while testing a magnetron (which is two permanent magnets on either side of a vacuum tube that produces electromagnetic radiation) a peanut candy bar began to melt in his pocket. Unaware of any heat, he became curious about why this happened and did tests with other food items with the same result; the heat that was generated within the food near the magnetron caused the food to cook. With many companies desperate to find uses for their products after World War II, this revelation was enthusiastically accepted by magnetron manufacturers and the microwave oven began to be designed and sold for public use. Consumers were a little less enthusiastic about this product, and understandably so. Up until this point microwave radiation was used primarily for radar and wartime communication. Who knew what the effect would be on food quality?
Although a tremendous amount of marketing and information campaigns have been done to assuage the public’s hesitancy about the safety of microwaves, the question still remains in many people’s minds of whether or not it is ok to microwave your food. Considering the fact that 90% of United States’ homes have a microwave, this could be a significant issue. Let’s look at what the research tells us.
The main concern about microwaves revolves around their use of radiation. To determine the safety of radiation we first need to understand what radiation is and the difference between ionizing and nonionizing radiation. Radiation is the emission of energy as electromagnetic waves or as moving subatomic particles that is in turn absorbed by another body or object. All solid objects emit radiation (energy) at some level and the difference is in whether that radiation causes a change, at the molecular level (i.e. ionizing radiation), to the absorbing body or not. If it doesn’t, this radiation is termed nonionizing. Nonionizing radiation transmits waves of energy that are low frequency thus producing lower energy. Ionizing radiation, on the other hand, has waves of energy that are high frequency and high energy. Microwave ovens operate with nonionizing radiation and therefore with a low output of energy. This type of radiation does not irradiate or have the energy to ionize the molecules of an object or food item.
To begin the process of heating, a microwave is outfitted with a magnetron. When a microwave uses a magnetron to begin the flow of electromagnetic molecules through the vacuum tube, the magnetic and electric fields build up electrons and radiation is produced. This low frequency energy is set to 2,450 hz and especially prefers to move through water molecules causing them to bounce around and create friction which generates heat. Through this mechanism the water molecules in the food generates heat which in turn heats your food quickly and efficiently. In this study titled, “Unlocking Potentials of Microwaves for Food Safety and Quality”, the authors found that microwave wavelengths are 10,000,000,000 times longer than the lengths of chemical bonds.[1] As a result, microwaves cannot create free radicals or disrupt molecular bonds in biological materials. The study states, “The only effects on foods are the results of thermal energy converted from microwave energy.” Critics of microwave ovens beg to differ. Studies on this topic are limited but one in particular studied the effects of microwave heating on rice protein radicals and their growth and stability.[2] This study found that although radicals were found in both heated and unheated samples, microwave treatment caused stronger radical signals. From this abstract we don’t know how much water was used or what the results were of the oven heated samples. This information would be helpful in determining the usefulness of this study. A clarifying statement on both sides might be that free radicals are created by the foods themselves, not released by the microwave. This type of reaction happens in any food when oxygen and light or heat react with a food molecule. One study done in July of 2014 on the consumption of microwaved food in rats, specified that usually this type of reaction is found in foods with polyunsaturated fats due to the way their double bonds accept oxygen.[3] The free radicals caused Reactive Oxygen Species (ROS), causing oxidative stress which then began to affect different biological systems in the rats. Although more research needs to be done on the creation of free radicals from microwave ovens in humans, it is known that free radicals in general do affect humans in this way. Based on the research it would seem that any method of cooking that allows oxygen and heat to react with food would cause the food to create free radicals and this effect is not particular to microwave ovens. Finally, both sides of the argument agree that the microwave oven will not cause your food to become radioactive.
One of the biggest concerns that critics have, is that the nutrients in the food are diminished or nullified completely when using a microwave. While the vitamin levels in foods change with any application of heat, the heat at which most microwaves cook food leaves the mineral content in the food unchanged.[4] The vitamin loss in the food during microwave cooking is primarily dictated by several factors including the length of time something is cooked, how it’s cooked, the intensity of the heat, and the makeup of the food itself. One thing to keep in mind is that while some vegetables lose antioxidant capabilities when heated, others increase their antioxidant levels.[5] In a comparative study on the effects of different cooking methods on nutritional and physiochemical characteristics of selected vegetables, the authors found that processed vegetables did not offer lower nutritional values and that in actuality, there are particular cooking methods that best preserve the nutrients of specific vegetables.[6] This was further confirmed by a research group who did a study on how thermal processing enhanced the nutritional value of tomatoes by increasing total antioxidant activity.[7] In this study, as the tomato was heated at 88 degrees for 2, 15, and 30mins the antioxidant availability was increased from 2.01 mg of trans-lycopene/gram to 5.29, 5.53, and 6.70 respectively. There are some limitations, however, on how this information can be applied to microwaves since the temperature of the food heated in a microwave is affected by the water content of the food. Microwaves do not cook at a specific temperature. One of the studies pointed to by many critics of the microwave is titled, “Phenolic Compound Contents in Edible Parts of Broccoli Inflorescences after Domestic Cooking”.[8] This study found that broccoli lost 97% of its flavonoids after 5 mins of cooking in a microwave. One of the limitations of this study is that there were different cooking times for the various types of cooking methods and no clear amounts of water were used. This would make it extremely difficult to determine if these percentages would change with less water and less heating time. Another factor contributing to the shortcomings of this study include the fact that antioxidants in any vegetable begin to degrade after harvesting.[9] We do not know at what point these vegetables were tested to know what their antioxidant properties were before being microwaved. Many instructions on cooking broccoli in the microwave, recommend anywhere from 2-4 minutes. So, the length of time given in this study may not be representative of how the general population would cook their broccoli.
One of the antioxidants contained in broccoli in high levels is vitamin C. Vitamin C is extremely water soluble and cooking it in water will cause most of the vitamin C to be leached out into the water. A study on the effects of domestic cooking on flavonoids in broccoli, shows that the length of time and amount of water play a part in nutrients being lost.[10] Another one of the studies used to prove that microwaving vegetables causes a loss of nutrients is an experiment on changes in texture and nutrition of asparagus during microwave blanching or cryogenic freezing.[11] While the study itself is straightforward, those who use it to support their claim on the dangers of microwaves fail to mention that vitamin C will leach out into water when using heat and not by freezing anyway. To use this study and not mention this aspect isn’t a fair comparison and leaves out the most obvious point that vitamin C leaches into water regardless of what heating method is used. As mentioned above, the amount of vitamin C that is lost will also be greatly affected by the amount of water that it is cooked in.[12],[13],[14] This also applies to many other fruits and vegetables that contain water-soluble vitamins. On average, the shorter the heating time, the fewer nutrients that are lost.
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[1]Tang J. Unlocking Potentials of Microwaves for Food Safety and Quality. J Food Sci. 2015;80(8):E1776-E1793. doi:10.1111/1750-3841.12959
[2] Fan, Daming & Hu, Bo & Lin, L.-F & Huang, L.-L & Wang, M.-F & Zhao, J.-X & Zhang, Hao. (2016). Rice protein radicals: Growth and stability under microwave treatment. RSC Advances. 6. 97825-97831. 10.1039/c6ra15402f.
[3] Ajayi, Folasade & Afolabi, Rufus & Chukwudozie, Ndudi & Osuntoki, Akinniyi. (2014). Consumption of Microwaved Food induces Oxidative Stress in the Liver and Brain of Sprague-Dawley Rats. IOSR Journal of Environmental Science, Toxicology and Food Technology. 8. 21-24. 10.9790/2402-08712124.
[4]Cross GA, Fung DY. The effect of microwaves on nutrient value of foods. Crit Rev Food Sci Nutr. 1982;16(4):355-381. doi:10.1080/10408398209527340
[5]Jiménez-Monreal AM, García-Diz L, Martínez-Tomé M, Mariscal M, Murcia MA. Influence of cooking methods on antioxidant activity of vegetables. J Food Sci. 2009;74(3):H97-H103. doi:10.1111/j.1750-3841.2009.01091.x
[6]Miglio C, Chiavaro E, Visconti A, Fogliano V, Pellegrini N. Effects of different cooking methods on nutritional and physicochemical characteristics of selected vegetables. J Agric Food Chem. 2008;56(1):139-147. doi:10.1021/jf072304b
[7]Veronica Dewanto, Xianzhong Wu, Kafui K. Adom, and Rui Hai Liu. Thermal Processing Enhances the Nutritional Value of Tomatoes by Increasing Total Antioxident Activity. Journal of Agricultural and Food Chemistry 2002 50 (10), 3010-3014, DOI: 10.1021/jf0115589
[8]Phenolic compound contents in edible part of broccoli inflorescences after domestic cooking,' by F. Vallejo, F. A. Tomás-Barberán and C. García-Viguera was published in the Journal of the Science of Food and Agriculture (2003) 83:1511-1516
[9]Villa-Rodriguez JA, Palafox-Carlos H, Yahia EM, Ayala-Zavala JF, Gonzalez-Aguilar GA. Maintaining antioxidant potential of fresh fruits and vegetables after harvest. Crit Rev Food Sci Nutr. 2015;55(6):806-822. doi:10.1080/10408398.2012.685631
[10]Wu X, Zhao Y, Haytowitz DB, Chen P, Pehrsson PR. Effects of domestic cooking on flavonoids in broccoli and calculation of retention factors. Heliyon. 2019;5(3):e01310. Published 2019 Mar 7. doi:10.1016/j.heliyon.2019.e01310
[11] Ulla Kidmose & Karl Kaack (1999) Changes in Texture and Nutritional Quality of Green Asparagus Spears (Asparagus officinalis L.) during Microwave Blanching and Cryogenic Freezing, Acta Agriculturae Scandinavica, Section B — Soil & Plant Science, 49:2, 110-116, DOI: 10.1080/09064719950135623
[12]López-Berenguer C, Carvajal M, Moreno DA, García-Viguera C. Effects of microwave cooking conditions on bioactive compounds present in broccoli inflorescences. J Agric Food Chem. 2007;55(24):10001-10007. doi:10.1021/jf071680t
[13]Yuan GF, Sun B, Yuan J, Wang QM. Effects of different cooking methods on health-promoting compounds of broccoli. J Zhejiang Univ Sci B. 2009;10(8):580-588. doi:10.1631/jzus.B0920051
[14]Zeng, C. (2013), "Effects of different cooking methods on the vitamin C content of selected vegetables", Nutrition & Food Science, Vol. 43 No. 5, pp. 438-443. https://doi.org/10.1108/NFS-11-2012-0123