Introduction
Microwave ovens provide a convenient method of cooking and reheating food. Their ease of operation and time-saving properties mean that their popularity is likely to increase for domestic use as well as in restaurants and institutions. While few people would dispute their convenience, consumers are sometimes concerned about the safety of microwaves and their effect on nutrients in food.
Microwave cooking and nutrition
The majority of reports published on the nutritive value of foods cooked in microwave ovens indicate that food prepared in this manner is at least as nutritious as comparable food cooked by conventional methods.
Most of these studies have concentrated on vitamin retention and indicate that cooking in minimal water for a reduced time, as occurs with microwaving, promotes the retention of the water- soluble vitamins particularly of vitamin C and thiamin. Microwave cooking is preferable to boiling to minimise the leaching of vitamins into the cooking water; in this regard it is similar to steaming.
For the same reasons given for vitamin C, microwave cooking enhances mineral retention in vegetables.
Studies have not revealed any non-heat related effects on the macronutrients of foods, proteins, fats and carbohydrates, when cooked in microwave ovens. There may be slight differences in denaturation rates of proteins when food is heated in a microwave oven compared with conventional heating but this is due to differences in the time and temperature to which the food is subjected.
There have been recent reports (2003) that cooking vegetables in a microwave oven leads to a greater loss of soluble phenolic antioxidant compounds than does conventional cooking. However this appears to have been at least partly due to the use of more cooking water than is necessary with microwaves. The role of these phenolic compounds in human nutrition remains an open question.
Far less information is currently available on the effect of microwave cooking on other food components such as carbohydrates, lipids and fat-soluble vitamins.
For the same reasons given for vitamin C, microwave cooking enhances mineral retention in vegetables.
The quality of protein is higher in microwaved than in conventionally cooked food as far less oxidation occurs in meat cooked in a microwave. Lack of browning is visible evidence that heating is gentler, and makes it likely that vitamins A and E are better retained than in conventional cooking. However these differences are likely to be slight and of little nutritional significance.
Reheating food quickly in a microwave retains more nutrients than holding food hot for long periods; this is significant in institutions and hospitals where food may be held hot for several hours in traditional catering systems.
The nutritional value of food does not depend only on the way in which it is cooked. Just as important are shopping wisely for quality products, correct temperature control during storage and preparation and serving food promptly after it is prepared. Leaching effects aside there seems to be little difference to the retention of nutrients between food cooked by microwaves or by conventional means, providing cooking time and temperature guidelines are carefully followed.
Microwaves and food
Effect on food
All food undergoes changes when heated; there is no firm evidence that microwaves cause any effect on food other than those due to rapid heating. Care should be taken to avoid overcooking.
Radiation and food
Food cooked in a microwave oven does not present a radiation risk. Microwaves cease to exist as soon as the power to the magnetron of a microwave oven is switched off. They do not remain in the food and are incapable of making either it or the oven radioactive.
Chemical changes in foods
Consumer concern has been caused by media coverage of isolated reports which suggest that microwave heating produces chemical changes in foods with the formation of potentially toxic compounds. The most widely reported of these was a letter which appeared in the reputable journal The Lancet in 1989. This work was reviewed by an expert committee of the National Health and Medical Research Council which concluded that the results obtained in the experiment were not relevant to the way food is prepared and consumed. A second more recent report in a little known Swiss journal also appears to be irrelevant to domestic use of microwave ovens.
Microwave ovens and uneven heating
Food cooked in a microwave oven does not heat uniformly and unwanted microorganisms may survive in portions of poorly heated food.
Manufacturers use stirrer fans and turntables and recommend standing times to help alleviate the problem of uneven heating. Many microwaveable meal packs carry the instruction to stir the food part way through the cooking process. Items such as lasagne that can't be stirred should be allowed standing time to allow the whole product to reach a uniform temperature.
How far microwaves are able to penetrate into the food will also depend on the thickness of portions and on the composition and moisture content of the food. When heating large quantities of food it is more effective to divide it into smaller portions for reheating than it is to heat a large amount for longer.
Care should be taken that frozen food has been completely thawed. Water absorbs microwaves far more easily than ice does; incomplete thawing will result in uneven cooking and the potential survival of undesirable microorganisms in those parts of the food which have been insufficiently heated.
A positive feature of microwave ovens with regard to food safety is that food can be taken from the freezer, thawed quickly, cooked and served without it spending long periods of time in the danger temperature zone between 4°C and 60°C, which provides favourable conditions for the growth of dangerous microorganisms.
Microwave ovens and burns
Microwave ovens are less likely to cause burns than are conventional ovens. However the potential hazard of burns associated with microwave cooking is not often considered, and many people allow young children to operate these appliances unsupervised.
Burns have occurred from the steam emitted from microwaveable popcorn bags and similar closed packages and from the boiling portions of foods which heat unevenly. An example of this is a jam-filled donut; the jam centre may exceed the boiling point of water while the donut itself is only warm. Frozen macaroni cheese is another example as the cheese reaches a high temperature more quickly and retains more heat than the macaroni.
Severe scalding has also occurred when babies have been given milk heated in a microwave oven.
When using new crockery for the first time in a microwave oven, use oven gloves to remove the item after heating,until you are aware of its heating characteristics. There have been instances when some types of crockery mugs have absorbed more heat than the liquid they contained causing unexpected burns.
Containers and films for microwave cooking
Only utensils designed for the purpose should be used in a microwave oven. However as there are no standards currently available for claims such as 'microwave-safe,' any concerns about the safety of such products should be referred to the manufacturer.
Some additives used in the manufacture of plastics, particularly those which make it pliable, may migrate into food, especially at high temperatures. Only those plastic containers which have been specifically designed for microwave cooking should be used, and they should be discarded when the surface shows any signs of breaking down.
When plastic films are used in microwave ovens it is preferable that they are not in direct contact with the food they cover. Meals to be reheated on a plate may be covered with clean white absorbent kitchen paper to prevent spatter.
It is very important that food containers which have been designed to package frozen or chilled foods such as ice cream or margarine, are not exposed to high temperatures in a microwave oven. The low melt temperatures of these plastics may result in migration of undesirable contaminants into the food or in physical disintegration of the containers themselves.
As migration is more likely to occur into hot fatty foods, glass containers are preferred to plastic for heating them.
Container shape may also influence the way a food reacts to reheating. Circular or oval containers help prevent edges of the food burning because energy absorption occurs evenly around the edges. Square containers tend to encourage burning on the edges of a product.
Shallow containers, because they provide a large surface area, are a good choice for heating foods.
Packaging for microwavable meals has been especially designed for use at high temperatures. This sophisticated packaging may incorporate susceptors (surface layers) to compensate for some of the limitations of microwave cooking. Susceptors consist of a plastic film metallised usually with aluminium and laminated to paper or paperboard to hold the required shape. They are designed to enhance browning and crisping of a product and to improve its texture. For example without the use of susceptors, pizzas heated in a microwave oven would be soggy.
Susceptors absorb microwave energy and heat food mainly by direct contact. Susceptor materials have been tested both for migration levels of undesirable chemicals and the release of any volatiles, but tests have not revealed that they pose any threat to consumer safety.
The packaging industry recognizes the problems of potential migration from packaging into food and constantly monitors and improves manufacturing processes.
Radiation and leakage
Microwave oven doors are designed with at least two features which ensure that power is cut off immediately the door is opened. However it is possible for microwaves to leak out around the edges of a badly fitting or damaged door. If a door does not fit squarely and operate smoothly or if it shows signs of corrosion or damage, the oven should be inspected by a qualified technician.
Samples of all models of microwaves are tested for leakage before sale as prescribed in Australian Standard 3801-1980, and the National Health and Medical Research Council has determined a standard of safety for the power flux density of radiation for microwave ovens which it believes safeguards public safety. Microwave oven leakage levels which exceed the recommended levels are extremely rare. An oven in good condition and used correctly is safe.
Most microwave oven repair shops will test ovens for leakage at a reasonable cost.
Leakage detectors for domestic use are available but only those which comply with Australian Standard AS2889-1987, Microwave oven leakage detectors for household use, should be purchased and their instructions followed carefully for an accurate result. The Australian Radiation Protection and Nuclear Safety Agency closely monitors potential radiation hazards in consumer products. This webpage outlines general precautions with regard to the possible hazard of microwave leakage from ovens.
Summary
Successful microwave cooking depends on understanding the limitations as well as the benefits of this type of cooking.
Correctly used, microwave ovens offer a convenient and safe method of food preparation, without any detrimental effects on consumer safety or nutrition.
