To Microwave or Not: The Great Debate

To Microwave or Not: The Great Debate

Joy Stepinski, MSN, RN-BC

February 11, 2023

Introduction

The microwave oven has been a fixture in the American kitchen for decades. Yet throughout the years, the safety of the microwave oven has been a hot topic. Often the appliance is found right above the oven, at head height. Some solely use the microwave for their cooking, as a convenience for not having to turn on the conventional oven or as a quick way to heat up a meal. Others take the argument that the microwave leads to severe health problems, such as claims that it causes radiation damage and more. So, what is truly the answer? Turning to the scientific literature, the history, function, and research are imperative to consider in order to weigh in on the debate. This discussion explores the safety, federal guidelines, and potential benefits and drawbacks of the microwave oven.

History and Function

The microwave oven became first known in the domicile in the mid-twentieth century [1]. Compared with the conventional oven, the appliance offered a very quick way to heat food. The first commercial microwave was produced in 1947 by Raytheon Corporation. Originally named “Radarange” and costing within a range of $2000 to $3000, the appliance was housed in cabinets the size of a refrigerator. While the current domestic version uses 1000 watts, this large microwave featured 1600 watts. In the mid-1950s, Tappan advertised the first household style costing $1295. About a decade later, a subdivision of Raytheon called Amana produced the countertop household version. Once the product became smaller and less expensive under $500, the appliance grew in popularity starting in 1971 [2].

Microwave ovens are one of the means for heating, along with direct resistance, induction, and radio frequency [3]. Microwaves, the type of electromagnetic radiation from which the appliance received its name, is produced by a magnetron [4], which operates at 2.45 GHz [5]. The distinct characteristics include reflection of metal; ability to traverse through plastic, paper, and glass; and absorption by food. A volumetric heating process is used, which means that heat is generated and distributed throughout the food, as a result from polar water molecules and charged ions in the food material [6]. The areas where greater moisture is found absorbs the energy directly, then converts that energy to heat. Types of material that can be used with microwave heating are glass, paper, certain plastics that will not melt, or ceramic. Aluminum foil and metal cannot be used to hold the food and may damage the appliance [7].

The microwaves are electromagnetic waves, which oscillate at 2450 times per second [8]. Water is an important component of the food because its molecule functions like a magnet. The hydrogen atoms are positively charged and the oxygen atom is negatively charged. The oscillating microwaves cause the water molecules to rotate. As the positively charged microwave attracts the negative charge within the water molecule, the negatively charged microwave likewise attracts the positive charge of the water molecule. This prompts the water molecule to match the oscillation with a rotation rate of approximately 2450 times per second. Friction is created from the colliding water molecules, which generates heat. The food warms due to the heat flowing by conduction, convection, or radiation. The microwave is reflected from the food material, and then back to the food from the metal walls [8]. A portion enters at a refracted angle, usually toward the center of the food. The wavelength is determined by the amount of moisture within the food. Salt is a factor, as well, with salty foods heating more quickly than unsalty. The sodium (Na+ ) and chloride (Cl- ) ions within the salt are charged and interact with the alternating electric field that will cause heat [9].

Microwave Regulations

Microwave manufacturers are required to be in compliance with certain regulations. According to the U.S. Food and Drug Administration (FDA) [10], that organization has been responsible for the regulation of microwaves since 1971. Overseen by the FDA, the Center for Devices and Radiological Health (CDRH) standards are created and enforced regarding radiation emission in an effort to avert public health hazards. Included in these standards are the limits to the amount of microwave radiation, which is 5 milliwatts of microwave radiation per square centimeter. This radiation specifically refers to the region two inches from the surface of the microwave, which would be the area of highest energy. Another requirement is two independent interlock systems. The locks are protective because once the latch is released or the door is opened, the microwave would stop producing microwaves. Furthermore, the appliance would not work if one of the locks fails. The microwave must be labeled stating that the safety standard requirements are met. The FDA also tests microwaves in their labs to validate the safety standard methods.

Manufacturers must meet specified safety performance standards and certify their products. The Federal Food, Drug and Cosmetic Act (FFDCA) [11] have guidelines to which they expect manufacturers selling products that emit electronic radiation sold in the United States to comply. Other regulatory agencies that play a role in microwave function include the United States Department of Agriculture (USDA) and Occupational Safety and Health Administration (OSHA). The USDA outlines information on how microwaves cook, along with consumer guidelines. For example, the organization claims that food may heat unevenly leading to “cold spots” where bacteria can thrive [6] and recommends that food is covered in plastic when warmed to distribute the heat more evenly. The USDA also advises the use of food thermometers to ensure that food is properly heated. OSHA does not have regulations specific for consumers; however, does offer guidance for employees who are exposed to microwaves in an occupational setting.

Benefits

Clear benefits of the microwave oven are documented in the literature. Consumers perceive the appliance very convenient due to the reduced heating time, leading to one of the most popular ways to heat food [8]. Within the food industry, microwaves help to decrease throughput time with the way food is thawed, pre-cooked, and pasteurized. They are also used for pasteurization and sterilization, which kills pathogens [7]. Energy requirements are less than conventional heating. Perhaps above all, the technology of microwaving preserves the character of the food such as color, taste, nutritional value, and overall quality through this heating method as compared with blanching, cooking, and baking.

One study funded by the USDA, presented by Wu and coauthors [12], investigated the effect of conventional cooking on broccoli flavonoids due to the longstanding belief that domestic cooking plays a role with influencing vegetable flavonoid contents. The outcome of the study showed that microwaving was best for preserving the flavonoids, followed by steaming, and then boiling. The biggest limitation was that broccoli was the only vegetable examined. Because different vegetables have varying profiles, testing an assortment would be important to gain a deeper understanding of the effect of conventional cooking on the nutritional content. In a separate research publication, Jiménez-Monreal and colleagues of a Spanish university [13] investigated the influence of cooking on the antioxidants within vegetables. Antioxidants may act individually or synergistically to inhibit the negative effects of free radicals, regulate inflammatory responses, and aid in detoxification. The research sought to examine the highest antioxidant retention among modern-day aggressive cooking methods. A variety of vegetables were used, like artichoke, beetroot, broad bean, broccoli, Brussels sprouts, carrot, cauliflower, celery, eggplant, garlic, green bean, leek, maize, onion, pea, pepper, spinach, Swiss chard, and zucchini. Their findings showed that the microwave heated most vegetables and maintained active components without losing much antioxidant activity, in comparison with other heating methods. Limitations included the use of olive oil with some cooking methods and water with others. Using oil may have affected the way that antioxidants were preserved or lost.

Boubaker, Mosbah, & Jaafar [15] researched the safety of thawing meat in a microwave versus conventional oven in Tunisia. The authors report that the microwave has been valuable due to its food processing and decontamination abilities. The purpose of the investigation was to review the effect of thawing meat due to the high demands of poultry during the holy month. In order to conduct the research, a high frequency microwave of 2.45 Ghz and 12.2 cm wavelength was used with both a control and experimental group. The findings of this study demonstrated that the microwave complies with microbiological safety for the consumer. Because the country of Tunisia often is faced with food shortages, the microwave helps to quicken the thawing process. The investigation showed that thawing took one-third of the time with microwaving, 15 hours, as opposed to conventional thawing of 48 hours. The water losses were overall less after microwaving the chicken. Shelf-life of the product was a beneficial finding. Some quality issues, such as the chemical stability in thawing, were not investigated. Another limitation included that only one type of food, chicken, was tested. Furthermore, disadvantages were changes in the appearance of the chicken, in which uneven cooking caused overheated reddened spots.

Drawbacks

Although these examples show that the microwave maintains nutrients of microwaved food and shortens thawing time is very beneficial to the consumer, other articles report that the microwave oven is not safe. If a general Google search on “dangers of microwave ovens” is performed, there are numerous articles written on the subject. For example, an article posted on the Health Research Funding site [15] claims that although microwaves are efficient and easy to use, they expose the human body to radiation that is generally not good to absorb. Moreover, the microwave can change food structure. Other claims include not heating the food evenly, leading to infant burns or the creation of carcinogenic compounds. Another assertion is that microwaved foods promote obesity because people become lazy and irresponsible to prepare their own food. The site states believes that almost all microwaveable foods contain fat, calories, chemicals, additives, and GMOs. Because this article was so opposed to the appliance, yet did not cite any reference to base their claims, the themes will be scrutinized further.

One Romanian publication by Panait and coauthors [9] investigated the electromagnetic fields generated by three microwave ovens with respect to human health. The authors relate that microwave ovens have a glass door, leading to another mechanism to trap the electromagnetic waves so that they do not leave the oven. As part of the experiment, the investigators used a Radio Frequency EMF Strength Meter to determine the radiation level in the action of being used and at rest. The results showed that the highest part of the electric and magnetic fields was in front of the oven. However, the data varied depending on the oven used. The magnetic field at 1 m away from the oven was the same at rest and when the microwave was turned on. The highest power density was noted 5 cm away from the front of the oven. After 10 minutes of being on, the magnetic field maintained a plateau of at least 88.27% of the value from when the microwave was first powered on. Once the microwave ovens were turned off, the magnetic field values decreased and reached the level prior to turning on after 15 minutes.

Regarding the effects of the human body, the authors discussed that in high doses humans may experience electric shock and burns due to the absorption of electromagnetic radiation. They shared that all domestic microwaves use the same frequency of 2450 MHz, which is not on the same frequency as most other devices. Because the microwave is typically placed on counter tops or suspended, the human body is close in proximity. Yet when comparing values of American and Canadian standards, the authors determined that the recommended limit of 5.0 mW/cm2 was not exceeded with the three ovens. When the microwave is powered on, the greatest amount of radiation was found at 5 cm in front of the oven. Some countries like Russia have adopted stricter radiation limit levels, due to temporary feelings of headache, fatigue, and dizziness that may result. The microwave discharges only 1.49% of all total radiation emitted. In conclusion of the study, the recommendation advised that people do not stand directly by the microwave when it is powered on. Additionally, unharmed and functional devices are recommended, along with decreasing the amount of total usage. The limitations of the study were that the direct effects of radiation were not measured on humans.

One hot topic is a chemical linked with cancer called acrylamide, of which the pre-cursor free amino acid is asparagine [16]. Acrylamide is reportedly found in tobacco smoke, as indicated by a hemoglobin (Hb) adduct. Hb adducts are biomarkers used to estimate chemical exposure. According to a study by Tareke and colleagues [17], acrylamide is formed by incomplete combustion or the heating of organic matter, as shown through animal experiments. High amounts of acrylamide are associated with heated starch-rich food, correlated with glucose and fructose. The acrylamide compound has been noted to be a neurotoxin and carcinogen [5, 16], evident in animal studies [5]. Barutcu and coauthors [16] discuss that various literature relates that the formation of acrylamide in food appears to be very complex because many factors may play a role, including time heated, temperature of heating, method used, location within the food of core versus the outer layer, water content, salt concentration, and pH value.

In a 2002 Swedish pilot study, Tareke and colleagues [17] studied the formation of acrylamide, through the use of certain solvents and chemicals by chromatography-mass spectrometry. Food material was analyzed for acrylamide content using various methods, including frying-searing, microwave, and oven. The authors found that no acrylamide was detected in the raw or unheated food material or in the boiled foods. Moderate levels, defined as 5 – 50 µg/kg, were found in protein-rich foods. High levels, defined as 150 – 4000 µg/kg, were noted in carbohydrate-rich foods, such as potatoes and beetroot. For the microwave specifically, acrylamide was not detected in fish, but a high amount of the chemical, median of 551 µg/kg, was found in the grated potato. The study found that the highest level of acrylamide was found after heating the potato for 150 seconds. However, after heating it for 100 seconds, the molecule presence was noted to be 100 times lower. This seemed to correspond with water loss. More water, about 40%, was noted in the food when heating for 100 seconds versus complete water loss when heating for 150 seconds. The authors proposed that there is a protective quality from the water, of which a potato contains approximately 79% water. As a pilot study, further analysis was needed to further solidify the effect of acrylamide on food consumption.

In a Turkish study, Barutcu and coauthors [16] observed the acrylamide content in the coating of chicken using different batter types during microwave frying. The results of the study showed that the moisture content decreased the greater the amount of time that the chicken was microwaved. As found in the previous study, the higher the moisture level, the lower the acrylamide content. The microwave led to lighter colors and lower acrylamide formation in the different flour coatings compared with the corresponding flours in the conventional frying. The authors reported that the amount of acrylamide was 13.8% less in the microwaved control batter than in the control used in the conventional frying method. Overall, less acrylamide was formed in each of the microwaved chicken compared with the conventional frying. The study concluded the microwave frying would be a better alternative to conventional frying for reduced frying time overall and less acrylamide formation. However, the research neglected to establish the relationship of acrylamide to the human body, other than to discuss that it is classified as a “probable human carcinogen” (p. 1).

In a review [5], the relationship of acrylamide formation with microwave heating was examined. The authors discussed that acrylamide was noteworthy because the compound was considered to be neurotoxic and carcinogenic, evident in animal studies. However, they also pointed out that the various thermal processes, also including baking, roasting, and frying, could affect food both beneficially and negatively. The negative effects included the formation of toxic compounds, not limited to acrylamide. Heating temperature and length of time heated, along with the particular heat methods, appeared to be the factors that can lead to these hazardous compounds. Increased content of acrylamide during microwave heating had been highlighted at a high-power level. However, research in many publications found both high and zero content in microwaved food. This could be due to microwave heating parameters utilized within the various studies, and extension of the microwave heating time leading to higher content. Blanching or pre-thawing frozen food led to minimal formation. The water content of food was also protective, along with salt content. When heating carbohydrate-rich food, the lowest level possible should be considered.

Another claim is that microwaved foods may not heat evenly, leading to burns especially in infants. A publication by colleagues at Children’s Healthcare of Atlanta and Emory University [18] reported that burns represented a category of preventable injuries among children, especially caused by hot liquid or steam. Scald burns were documented in approximately 100,000 children per year. One reason for the burns was due to a child opening a microwave and removing contents that led to scalding. Instant soup was a major factor, possibly due to flimsy packaging. In this study, the researchers reviewed data from the database of the Consumer Product Safety Commission’s (CPSC) National Electronic Injury Surveillance System (NEISS) to establish injury patterns. The data was taken from a sample of 100 hospital-based emergency departments (ED) within the United States, which the authors reported was a representative sample of all EDs. The study included retrospective data from 2006 through 2016 for scald burns in children aged 4 through 12, including the involvement of the microwave appliance. The authors identified 4518 cases in the sampled data, which was estimated to be 104,736 total cases or 9521 cases per year in all EDs.

The outcome of the study showed that younger aged children, average age of 7.5, were treated for a scald injury due to soup, while older children, average age of 7.9, were often seen for scald injuries non-related to soup. Trunk-related burns were more common with soup-related scald injuries, 43% versus 34%. The authors attributed the results of the study to the development age of the child and the ability to carry the soup. As a suggested way to address this problem, there were proposals of better packaging for spill-proof food, the counseling of parents on risks associated with carrying hot liquids, and the adoption of a childproof opening mechanism of the microwave. Limitations included the setting of the study, which was specific to the ED. Additionally, the authors did not specifically state that the research was directly as a result of a microwave, yet there were inferences within the article. The study would be more concrete if the method of heating the soup was specified.

Finally, Cowan and colleagues [19] perceived a high number of children admitted to the emergency room due to oropharyngeal burns. Pediatric burns were noteworthy due to the potential lifelong effect they have on families. As the most common injury among children, more than 90% of scald injuries were related to overheated food or liquid. With respect to oropharyngeal burns, the authors witnessed infant admissions due to over-heated microwaved formulas. These burns were particularly concerning due to the morbidity, including airway instability and alternative feeding needs. The reason for heating formula in the microwave was due to the parent perception that warming the milk may lead to improved taste and digestion. A retrospective 10-year review was completed between the years 1999 and 2009 for patients presenting with oropharyngeal burns. Seventy-five patients were identified as appropriate for the study, of which 50 were male. The median age was 2.7 years old. The injuries sustained were mostly to the lips and tongue, of which overheated liquids and solids accounted for 20 of the burns.

Within the study, the authors expressed that the microwave was related to fewer burns than expected. Although the literature reports scalding burns as the most common form of infant injury, the authors identified chemical burns as more significant. However, the overheating of food was second, specifically soup, milk, or formula. All of these burns occurred despite the supervision of a caretaker for microwave-related injuries, which shows that education is needed for caretakers. No medical benefit to the warming of formula actually existed. Microwaves varied depending upon the design, which explains the lack of universal guidelines for specific food warming recommendations. Limitations included the retrospective nature of the study and reliance on the caretaker history within the medical documentation. There also was not a clear scoring system used to identify burn severity.

Conclusion

Upon conclusion of this in-depth analysis, the microwave is a relatively safe and easy way for consumers to heat and thaw food. Nutrients appear to be mostly preserved upon using the appliance [11, 12] and do not cause greater microbial concerns [13]. Although there are claims that the appliance emits a high amount of radiation, compared with the total amount of exposure, the microwave is low [9]. The amount emitted is also within American and Canadian standards. Consumers could be educated on not standing directly in front of the microwave, especially within 5 cm [9]. With respect to carcinogens, the articles cited in this discussion observed acrylamide content in microwaved food [5, 15, 16], but did not establish how the chemical affected the human body within their research. Finally, scald injuries appeared to be the result of the child developmental age or education of the caregiver, but not specifically due to microwave functioning [17, 18]. Therefore, the great debate can end. The microwave oven can be recommended as a convenient method for reheating and thawing food.

References

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19. Cowan, D., Ho, B., Sykes, K. J., & Wei, J. L. (2013). Pediatric oral burns: a ten-year review of patient characteristics, etiologies and treatment outcomes. International journal of pediatric otorhinolaryngology, 77(8), 1325-1328. http://dx.doi.org/10.1016/j.ijporl.2013.05.026