IS MICROWAVE COOKING SAFE?
An Answer to a Global Controversy About the Safety of Cooking in Microwave Ovens
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- WHAT ARE MICROWAVES & WHERE ARE THEY USED?
- HISTORY OF MICROWAVES & ITS USE
- HOW DO MICROWAVE OVENS WORK?
- MYTHS ABOUT MICROWAVE OVENS
- SAFETY ASPECTS TO BE KEPT IN MIND WHILE USING A MICROWAVE OVEN
WHAT ARE MICROWAVES & WHERE ARE THEY USED?
Use of microwaves for cooking, reheating, baking, defrosting etc. is quite common nowadays both in home and commercial settings. However, on and off, the safety of microwave use in food has raised concerns.
First let’s understand what are microwaves. When we talk about the energy spectrum in the mother nature. Acc. to a 2014 study in the Journal of Advances in Biology, by an Egyptian research team, led by Nawal Ahmed El Ghazaly, from Alexandria University (https://bit.ly/3ikvw96), this spectrum contains frequencies with wave length from the longest to the shortest: radio waves, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. High frequency electromagnetic field (EMF) is generated from different sources such as radar installations, radio and television transmitters and microwave ovens.
A wave can be identified by its wavelength (distance from peak to peak) and by its frequency (the number of peaks that pass a stationery point every second). High frequency radiation has a small wavelength, and low frequency radiation has a long wavelength. Frequency is measured in cycles per second. One cycle per second is one hertz (Hz).
One million cycles per second is a megahertz (MHz), and one thousand million cycles per second is a giga hertz (GHz). Microwave radiations range in frequency from 300 MHz to 300,000MHz or 300GHz, and from 1 metre to 1 millimetre in wavelength.
Microwaves can pass through materials like glass, paper, plastic and ceramic, and
be absorbed by foods and water; but they are reflected by metals.
It is said that, it was during the World War II, when scientists found that birds that collided with radar masts would drop to the ground, become sizzling and well cooked. From then the idea of cooking food with microwaves emerged. Shortly after the War, microwave oven was introduced to the public.
Microwaves have many applications. They are used to detect speeding cars, send telephone, radio and television communications and treat muscle soreness, dry and cure plywood, cure rubber and resins, raise bread and doughnuts, as well as cook potato chips. Microwaves are also used for TV broadcasting, radar for air and sea navigational aids, and telecommunications including mobile phones.
Microwaves are reflected, transmitted or absorbed by materials in their path, in a similar manner to light. Metallic materials totally reflect microwaves while non- metallic materials such as glass and some plastics are mostly transparent to microwaves.
HISTORY OF MICROWAVES & ITS USE
Acc. to a 1980 study in the journal Annals of Science, a US research team, led by Harold J. Cook, from the University of Michigan (https://bit.ly/2DMcBVy), the history of research on the biological effects of microwave radiation effectively begins with the development of radar early in World War II, and the concerns that arose thereafter within industrial and military circles over the possible deleterious effects this new source of environmental energy could have on personnel. Prior to this time, the energy levels at which microwaves had been produced were not sufficient to cause widespread concern about harmful effects.
Before the invention of radar, artificially produced microwave energy was not a general environmental problem.
Interest among researchers in the effect of electricity on biological systems arose almost as soon as electricity could be generated in a controlled form. This same interest rapidly shifted to research on the biological effects of electromagnetic radiation when, during the years 1885-1889, Heinrich Rudolf Hertz demonstrated a technique for propagating electromagnetic energy through space. Efforts continued to generate shorter waves at higher energy and to explore ways of putting these new electromagnetic waves to use in medicine.
Well before the invention of radar, medical researchers had been interested in the controlled effect of radio-frequency energy on living things and the capacity of radio waves to generate heat in body tissue. The ensuing technique, called 'diathermy' (or 'diathermotherapy'), spread rapidly and was greatly improved by the invention of the magnetron tube in 1920. The magnetron proved to be a
convenient device for inducing local heating in tissue and was adopted as a recognized piece of medical apparatus.
Once it was discovered that radio waves could be used to heat body tissue, research was undertaken to study how such heating took place and its effect on the whole organism. Research on the new, ultra-short waves soon sparked a controversy that has enlivened the microwave field ever since. It was well known by the early 1920s that radio waves induced local heating, but the question soon arose whether heating was the only effect to be expected.
Between the early 1940s and 1960, research on the biological effects of microwave radiation slowly shifted from its medical context and the search for benefits to a military-industrial context and the search for hazards.
The phenomenon of frictional heat caused by microwaves was discovered accidentally in 1946 (if not earlier) during an experiment with a magnetic field tube. One of the researchers, Dr. Percy Spencer, noticed that a piece of chocolate he had been carrying in his jacket pocket had melted although he had not been aware of any heat, when working on radar for Raytheon Co. In 1945 Spencer filed the first patent concerning the use of microwaves to treat food. Raytheon went on to market commercial microwave ovens in 1947, but these were unreliable, complex, cumbersome, and expensive" and did not attract the attention of consumers to any significant degree.
By the late 1950s the technology had developed sufficiently to enable its use in specific commercial applications. Ovens produced by Toshiba were installed in the restaurant cars of the Japan National Railway in 1961. Street vendors used microwave ovens to heat hot dogs and hamburgers. and they quickly became com- monplace in Japanese cafeterias - albeit essentially as gimmicks to attract customers." In the USA microwaves were used for in-flight food service (heating pre prepared food) on domestic airlines in the early '60s. Japan led the way in domestic acceptance of microwave technology from 1966, following the development of a low cost, compact and reliable magnetron, and an internal power supply capable of coping with the fluctuating voltages of domestic electricity.
HOW DO MICROWAVE OVENS WORK?
In a 2017 study in the journal Food Control, a Malaysian research team, led by C.Y.
New, from Universiti Putra Malaysia (https://bit.ly/33h57T0), as food technology advances, the innovation of the microwave oven in the mid-20th century has made its significance in food preparation. Microwave oven becomes an indispensable tool due to its ability to reheat or cook food within minutes. The volumetric heat generation of microwave heating is the most important characteristic for rapid food reheating.
Acc. to the World Health Organization (WHO) (https://bit.ly/2DGWVTk), domestic microwave ovens operate at a frequency of 2450 MHz with a power usually ranging from 500 to 1100 watts. Microwaves are produced by an electronic tube called a magnetron. The magnetron inside the oven converts the electric power into very short radio waves.
Once the oven is switched on, the microwaves are dispersed in the oven cavity and reflected by a stirrer fan so the microwaves are propagated in all directions. They are reflected by the metal sides of the oven cavity and absorbed by the food.
Uniformity of heating in the food is usually assisted by having the food on a
rotating turntable in the oven. Water molecules vibrate when they absorb microwave energy, and the friction between the molecules results in heating which cooks the food.
Unlike conventional ovens, microwaves are absorbed only in the food and not in the surrounding oven cavity. Only dishes and containers specifically designed for microwave cooking should be used. Oven manufacturers do not recommend operating an empty oven. In the absence of food, the microwave energy can reflect back into the magnetron and may damage it.
Generally, the alternating electromagnetic field generated inside the microwave oven would lead to excitation, rotation/collision of polar molecules and ions inside the food. These molecular frictions would generate heat and subsequently lead to temperature rise. Thus, heat is generated throughout the food rapidly.
The major concern with using microwave technology for food safety applications is the survival of foodborne pathogens, due to uneven heating, including surface cooling effects.
Acc. to the 2005 report by Food and Environmental Hygiene Department, The Government of the Hong Kong Special Administrative Region (https://bit.ly/337n1Ys); the two major mechanisms, namely dipolar and ionic interactions, explain how heat generated inside food:
Dipolar interaction - Once microwave energy is absorbed, polar molecules such as water molecules inside the food will rotate according to the alternating electromagnetic field. The water molecule is a “dipole” with one positively charged end and one negatively charged end. Similar to the action of magnet, these
“dipoles” will orient themselves when they are subject to electromagnetic field.
The rotation of water molecules would generate heat for cooking.
Ionic interaction - In addition to the dipole water molecules, ionic compounds (i.e. dissolved salts) in food can also be accelerated by the electromagnetic field and collided with other molecules to produce heat.
Hence the composition of a food will affect how it will be heated up inside the microwave oven. Food with higher moisture content will be heated up faster because of the dipolar interaction. As the concentration of ions (e.g. dissolved salts increase, the rate of heating also increases because of the ionic interaction with microwaves. Even though oil molecules are much less polar than water molecules
and are non-ionic, food products with high oil content has a fast heating rate because the specific heat of oil is about less than half that of water.
Food cooked in conventional oven is heated by surrounding hot air whereas food cooked in microwave oven is heated as a result of the alternating electromagnetic field. The electromagnetic field generated is not uniformly distributed inside the cooking cavity and hence it leads to uneven heating of food. Because of the potential for uneven distribution of cooking, food heated in a microwave oven should rest for several minutes after cooking is completed to allow the heat to distribute throughout the food.
Fat will be heated more quickly than water because of its relatively low heat capacity. On the other hand, food of high fat content can be heated at a temperature greater than 200deg C whereas food of high moisture content would be cooked at temperature no greater than 100deg C unless all water was evaporated. It is because water has a lower boiling temperature. Generally speaking, the time and temperature of the heating process depends on a number of factors including composition, size, quantity, shape, density and physical state of the food item. The depth of penetration of microwaves decreases when the degree of absorbency increases. Food with higher water or salt content would have greater heating at the surface as it tends to absorb more microwaves and limits the penetration of
microwaves. The heating is also greater at the defrosted portion of a frozen food as water has higher microwave absorbency than ice
Inside the conventional oven, heat is lost when the hot air inside the oven escapes to the outside, whereas for microwave cooking, heat is produced inside the food and there is less energy loss. On average most microwave oven takes only about 20% of time required by conventional oven and saves at least 20% of energy depending on the food type.
Microwaves may not cook the food properly from inside. Microwaves penetrate the food to a depth of 1 to 1.5 inches. In thicker pieces of food, the microwaves don’t reach the centre. That area would cook by conduction of heat from the outer areas of the food into the middle.
In a microwave oven, the air in the oven is at room temperature so the temperature of the food surface is cooler than food in a conventional oven where the food is heated by hot air. Therefore, food cooked in a microwave oven doesn’t normally become brown and crispy.
MYTHS ABOUT MICROWAVE OVENS
Just like stupid demonizing of various other concepts, microwaves have been shown in the bad light since a long time. But the theory behind this controversy is common in all the websites terming microwave ovens as another evil to human health.
It started from a story of the Swiss food chemist Hans Hertel. In the late 1980s, Hertel and seven fellow vegetarians locked themselves into a hotel room, where they performed a two-month “experiment” that consisted of eating foods prepared in the microwave and by other means. After two months of togetherness, Hertel emerged with a terrifying pronouncement: He had found changes that “appear to indicate the initial stage of a pathological process such as occurs at the start of a cancerous condition” in the blood of the men who had eaten microwaved milk and vegetables.
But, Hertel didn’t actually find that microwaved food caused cancer. And his
“study,” which no researchers have tried to reproduce, was never peer-reviewed or published in a scientific journal. Also, Hertel never produced the data and never replicated the study. Since then, Hertel is nowhere to be seen, but his story lives among the controversy creators.
Similarly, there are many other theories floating around, like the research of Russian scientists into microwaves – which supposedly led to banning microwave ovens in Russia. This was written by an unknown William Kopp, described only as a “U.S. researcher,” who wrote an article in 1996 claiming that Cold War research in the Soviet Union had proven the dangers of microwave ovens.
“People who ingested microwaved foods showed a statistically higher incidence of stomach and intestinal cancers, plus a general degeneration of peripheral cellular tissues and a gradual breakdown of the function of the digestive and excretory systems,” Kopp wrote (https://bit.ly/3hbf9dn).
The Soviet research was never published and the institute where it was con- ducted, in what is now the Republic of Belarus, no longer exists. (The former Soviet Union may have banned microwave ovens for a short period, but no countries ban them today.) Kopp himself reportedly changed his name and vanished, believing that the appliance industry was out to persecute him.
The frequency used in microwave ovens, about 2,500MHz, is more powerful than the frequency that’s used to transmit radio, television, and cell phone signals. But it’s thousands of times weaker than ultraviolet light, visible light, and X-rays and millions of times weaker than the gamma radiation that’s used to irradiate some foods. What 2,500MHz microwaves can do is get absorbed by the water, fats, and sugars in food. That generates heat, which cooks the food. Once the oven is turned off, it produces no more microwaves. And those that it did produce are long gone.
On the other hand, I couldn’t find one proper study which relates the use of microwave ovens to cancers. Then there are more statements like (https://bit.ly/3m9qLBA), “within two minutes of imbibing your coffee that has been heated with microwave current, the molecular structure of your blood changes.
Foods that are heated or cooked with microwave current become unrecognizable to your body and cannot be broken down. Microwaved food contains both molecules and energies not present in food cooked in the way humans have been cooking food since the discovery of fire.” Such statements have no logic or evidence.
Cooking processes, especially the high temperature ones (e.g. grilling, baking, etc.) are known to induce the production of potential carcinogens. There have been concerns that microwave cooking may also increase the production of carcinogens or mutagens in foods. Currently there is no scientific evidence that the production of any carcinogenic substances would increase upon the application of microwave heating.
Of the carcinogens, the formation of the chemicals heterocyclic amines (HCAs), polyaromatic hydrocarbons (PAHs) and nitrosamines are of particular concern.
Many studies have been conducted to compare the effect of microwave cooking with other conventional methods on the formation of these chemicals. But microwave cooking did not produce significant amount of HCAs, nitrosamines or PAHs in meat products. The use of microwave cooking to precook meats before grilling or barbecuing has in fact been recommended so as to minimise the formation of HCAs and PAHs. It is probably due to the lower cooking temperature (temperature of microwave cooking normally would not exceed 100deg C) and shorter cooking time of microwave cooking.
Nearly all foods may be contaminated by microorganisms to a certain extent.
Concerns have arisen regarding whether microwave cooking can kill the food- borne pathogens as effective as conventional methods since microwave cooking generally requires shorter times and may sometimes result in lower temperatures at the food surface. Results of many studies concluded that the effectiveness of microwave cooking in killing microorganisms and spores is comparable with conventional methods provided that appropriate temperature and time are reached.
Another controversy is that proteins would be denatured with the modification in molecular structure upon heating. The degradation rates depend on the heating time and temperature. It has been shown that the nutritive value of proteins in foods treated by conventional and microwave heating are comparable.
Various studies have been conducted to investigate the stability of lipids upon microwave cooking, including studying the hydrolysis of triglycerides in soya, egg yolk and meats; fatty acid profiles in chicken and beef patties, chicken fat, beef tallow, bacon fat, rainbow trout and peanut oil; peroxidation of polyunsaturated fatty acids in meat, egg yolk and chicken. Available evidence suggested that microwave cooking did not result in significantly more chemical modifications.
Studies have been conducted to compare the retention of vitamins in different types of meat and vegetables subject to conventional and microwave cooking.
Generally speaking, water soluble vitamins such as vitamin B and C are more susceptible to heat treatment. The retention of vitamins varies with size and shape of the food, cooking time, internal temperature, etc. Review of available literature showed that vitamin retention in microwaved foods is equal or better than conventionally prepared foods because of the shorter heating time of microwave cooking. Therefore, it can be concluded the nutritional values of food cooked by microwave would be comparable with those by conventional methods.
Let’s see some of the studies which are floating around on the net:
In a 2003 study in the Journal of the Science of Food & Agriculture, a Spanish research team, led by F.Vallejo (https://bit.ly/3m4Jdeu), evaluated the total flavonoid content in portions of freshly harvested broccoli before and after cooking and in the cooking water. High-pressure boiling, low-pressure boiling (conventional), steaming and microwaving were the four domestic cooking processes used in this work. The results showed large differences among the four treatments in their influence on flavonoid and hydroxycinnamoyl derivative contents in broccoli. Clear disadvantages were detected when broccoli was microwaved, namely high losses of flavonoids (97%), sinapic acid derivatives (74%) and caffeoyl-quinic acid derivatives (87%). Conventional boiling led to a significant loss of flavonoids (66%) from fresh raw broccoli, while high-pressure boiling caused considerable leaching (47%) of caffeoyl-quinic acid derivatives into the cooking water. On the other hand, steaming had minimal effects, in terms of loss, on both flavonoid contents. Therefore we can conclude that a greater quantity of phenolic compounds will be provided by consumption of steamed broccoli as compared with broccoli prepared by other cooking processes.
Thus, the study did show a loss of nutrients in all methods of cooking, but a tad much more by the use of microwaves. However, this is not done in a household microwave oven. The amount of microwaves used in this study are next to impossible to even think of in a household oven.
Acc. to WHO (https://bit.ly/3bE7D9U), “it is important to realize that food cooked in a microwave oven does not become "radioactive". Nor does any microwave energy remain in the cavity or the food after the microwave oven is switched off. In this respect, microwaves act just like light; when the light bulb is turned off, no light remains.
Several countries, as well as the International Electrotechnical Commission (IEC), the International Committee on Electromagnetic Safety (ICES) of the Institute of Electrical and Electronics Engineers (IEEE) and the European Committee for Electrotechnical Standardization (CENELEC), have set a product emission limit of 50watts per square metre at any point 5cm away from the external surfaces of the oven. In practice, emissions from modern domestic microwave ovens are substantially below this international limit, and have interlocks that prevent people being exposed to microwaves while the oven is on. Moreover, exposure decreases rapidly with distance; e.g. a person 50cm from the oven receives about one one- hundredth of the microwave exposure of a person 5cm away.”
SAFETY ASPECTS TO BE KEPT IN MIND WHILE USING A MICROWAVE OVEN
Nowadays, common materials for packaging or containing foods are plastic, paper, glass, ceramics and metal. However, not all of these materials are suitable for microwave cooking. Materials like plastics, paper, glass and ceramics are generally transparent to microwaves. Nevertheless, some of them may absorb certain amount of microwave energy and hence reduce the amount of energy to be absorbed by food. On the other hand, there have been concerns on the possibility of chemical migration from such food contact materials (e.g. plastics, etc.) into food during microwaving.
Plastic containers are commonly used for microwave cooking and re-heating food and it is getting popular nowadays for carrying take-away meals. Not all types of
plastic materials are suitable for microwave cooking. Even though high density polyethylene can be used for foods with high water content, it cannot be used for foods with high fat or high sugar content as these foods may reach temperature above 100deg C during microwave cooking.
Ceramics and glass are ideal for microwave cooking. Metals are not to be used in microwaves, because microwave energy would be reflected by metals and not be able to penetrate it. If you try and use metal in a microwave, sparking occurs. This is because of something called ‘Arcing Effect’. Arcing is pronounced ‘AR-king’.
Some foods such as raw carrots and hot dogs can cause arcing while being microwaved. In hot dogs, this can be due to the uneven mixing of salts and additives. In carrots, it can be due to the minerals in the soil in which they were grown.
The arcing effect results from reflection or bouncing-off microwaves from the metallic components. Then the air between two metallic components nearby would become ionised and electric current would in turn pass across the gap between the two components. Extensive arcing is undesirable because it would damage the magnetron inside the microwave oven.
Heating of only water in a clean cup using microwaves may result in superheated water, i.e. water reached temperature higher than the boiling point without appearing to boil. Any disturbance of the water, e.g. movement of the cup or addition of other ingredients, would lead to eruption of boiling water out of the cup and causing injuries. To avoid superheated water, one should avoid excessive heating of water or liquids in the microwave oven, or let the water stand for at least 30 seconds before moving it or put other ingredients into the water
Cooking an egg within its shell would lead to steam built-up inside the shell and subsequent explosion of the egg. To avoid this problem, eggs can be cooked in microwave oven when the shell is removed or cracked and the egg yolk / white is pierced several times.
There have been some concerns about leakage of microwaves from the microwave ovens. Generally speaking, microwave ovens are specially designed such that the power is cut off when the door is open. Microwaves may be leaked out if the door does not fit well or if it is damaged. On the other hand, various international organisations and regulatory authorities have laid down safety standard for microwave oven, including the amount of microwaves that can leak out from the oven, such that there will be little or no detectable leakage of microwaves if the
oven is in good condition and operates properly. The users are advised to stop using the microwave oven and arrange a qualified technician for further inspection when any problems arise.
Acc. to WHO (https://bit.ly/2ZjZSkm), “when it comes to microwave safety; the design of microwave ovens ensures that the microwaves are contained within the oven and can only be present when the oven is switched on and the door is shut.
Leakage around and through the glass door is limited by design to a level well below that recommended by international standards. However, microwave leakage could still occur around damaged, dirty or modified microwave ovens. It is therefore important that the oven is maintained in good condition. Users should check that the door closes properly and that the safety interlock devices, fitted to the door to prevent microwaves from being generated while it is open, work correctly. The door seals should be kept clean and there should be no visible signs of damage to the seals or the outer casing of the oven. If any faults are found or parts of the oven are damaged, it should not be used until it has been repaired by an appropriately qualified service engineer.
Microwave energy can be absorbed by the body and produce heat in exposed tissues.
Organs with a poor blood supply and temperature control, such as the eye, or
temperature-sensitive tissue like the testes, have a higher risk of heat damage.
However, thermal damage would only occur from long exposures to very high power levels, well in excess of those measured around microwave ovens.
Food safety: Food safety is an important health issue. In a microwave oven, the rate of heating depends on the power rating of the oven and on the water content, density and amount of food being heated. Microwave energy does not penetrate well in thicker pieces of food, and may produce uneven cooking. This can lead to a health risk if parts of the food are not heated sufficiently to kill potentially dangerous micro- organisms. Because of the potential for uneven distribution of cooking, food heated in a microwave oven should rest for several minutes after cooking is completed to allow the heat to distribute throughout the food.
Food cooked in a microwave oven is as safe, and has the same nutrient value, as food cooked in a conventional oven. The main difference between these two methods of cooking is that microwave energy penetrates deeper into the food and reduces the time for heat to be conducted throughout the food, thus reducing the overall cooking time.
Only certain microwave ovens are designed to sterilize items (for example baby’s milk bottles). The user should follow the manufacturer's instructions for this type of application.”
In a 2013 study in the journal Plos One, a Spanish & Hungarian research team, led by Gabor Geczi, from Szent Istvan University, Hungary (https://bit.ly/3heXGk8);
analysed whether the effects of microwave heating with continuous flow are equivalent to those of traditional heat transfer methods. In the study, the effects of heating of liquid foods by conventional and continuous flow microwave heating were studied. Among other properties, the stability of the liquid foods between the two heat treatments were compared. The goal was to determine whether the continuous flow microwave heating and the conventional heating methods have the same effects on the liquid foods, and, therefore, whether microwave heat treatment can effectively replace conventional heat treatments. The majority of the results indicated that the microwave method used here is equivalent to the conventional heating method based on thermal conduction and convection.