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Risk in Brief

Introduction and Microwave Cooking Principles

1. Microwave is a form of electromagnetic energy that has many uses such as applications in detecting speeding cars and radio and television communication. However, application of microwaves for cooking through microwave ovens is most commonly used by consumers.

2. In microwave cooking, the microwaves generated in microwave oven excite molecules inside the food such as water and fat which would rotate and collide with each other and generate heat to cook the food.

3. In general, the microwaves inside the microwave oven are not absorbed by glass, plastic and ceramic, and are reflected by metals.

Issues Related with Microwave Cooking

There are concerns that microwave cooking may be associated with some food hazards. Some of the important issues are discussed below:

Chemical

1. High temperatures are known to induce potential carcinogens in foods which include heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs). Nitrosamines, probable human carcinogens, are formed in certain foods after curing. However, scientific studies showed no evidence that microwave cooking would increase the amount of such substances as compared with conventional cooking. Conversely, results of studies showed that the levels of HCAs, PAHs and nitrosamines formed during microwave cooking is less than those form by frying, roasting and grilling. This is probably due to the lower cooking temperature and shorter cooking time in microwave cooking process.

Microbiological

2. Results of many studies showed that effectiveness of microwave cooking in killing microbes is comparable with conventional cooking methods provided that the appropriate temperature-time requirement is reached. The same temperature-time relationship applies to both cooking methods, i.e. it is generally advised that raw animal food should be heated to an internal temperature of at least 75oC for 15 seconds to kill any foodborne pathogen that may be present in the food.

Nutrition Quality

3. Available scientific evidence indicates that nutrient contents including those of proteins, fats, minerals, vitamins in microwaved foods are not less than those in foods cooked by conventional methods.

Food Contact Materials

4. Plastic, glass and ceramic are common materials used for the manufacture of containers for microwave cooking. Since the food cooked inside a microwave oven can reach a temperature of more than 100oC, plastic containers used for microwave cooking should be made with materials that have a high melting point such as polypropylene.

5. Metals are not suitable materials in microwave cooking as metal containers would reflect the microwave and the food inside would not be properly cooked. Containers with metal decorations when used inside a microwave oven may cause sparks and may cause damage to the microwave generator inside the oven.

6. A study also showed that foods wrapped with waxed paper or wax bags may be contaminated with waxed hydrocarbons after microwaving cooking. Therefore, not all types of papers are suitable for microwave cooking.

7. As for plastic films used for wrapping or covering food during microwave cooking, evidence showed that although there was migration of some of the chemicals used for making the films into food, the amount of migrated chemicals was generally low and does not pose a threat to health. Nevertheless, it is not recommended to use plastic wraps for prolonged microwave cooking of foods, especially high-fat or high-sugar foods.

Others

8. Heating a cup of pure water in microwave oven excessively may result in superheated water (water over boiling temperature without an appearance of boiling). When the superheated water is disturbed, the boiling water would erupt out of the cup and may cause injuries.

9. Heating intact shelled eggs in microwave ovens would result in building up of steam inside the shell and subsequent explosion of the egg.

10. Microwave ovens in the market are so designed that when the door is open, the power will be cut off and that when in operation, no radiations or microwaves will be leaked out unless the door is damaged.

Advice to Users

To use microwave ovens safely and to microwave food properly, adoption of the following techniques and practices are recommended -

General

1. Buy microwave ovens made by reputable manufacturers; read and follow operation instructions recommended by the manufacturer.

2. Close the door properly before turning on the microwave oven.

3. Keep the microwave oven clean and conduct preventive maintenance to ensure it functions properly.

4. Observe good food hygiene and personal hygiene, such as washing hands before and after handling foods, when using microwave oven for cooking.

Cooking wares and Food Wraps

5. Use microwave-safe cooking wares and food wraps for microwave cooking.

6. Do not use containers with metal decorations, plastic storage/grocery bags, foam trays or aluminium foils for microwaving cooking.

7. Do not reuse containers for convenience foods as they are normally for one-time use.

Cooking

8. Cover foods with a microwave-safe lid/plastic wrap; leave a small part uncovered to allow steam to escape; avoid direct contact between the food and the plastic wrap during microwaving.

9. Cook large pieces of meats at medium power for a longer period to enable heat to reach the food centre without overcooking the outer.

10. To avoid uneven cooking, debone large pieces of meats, cut foods into smaller pieces, place foods evenly on the dish with larger/thicker parts on the outskirts, stir or rotate foods several times during cooking, and let food stand for two minutes after microwaving.

11. Cook food thoroughly, especially poultry and meat which should be cooked until their juices run clear and no pink colour remains.

12. Do not over-heat water or liquids so as to avoid the potential danger caused by the superheated product (water or liquid over the boiling temperature without an appearance of boiling).

13. Do not cook eggs with intact shells so as to avoid explosion due to the building up of steam inside; remove or crack the egg shell, pierce the egg yolk/white several times before cooking the egg inside microwave oven.

Defrosting

14. Before defrosting food by microwaves, remove it from its package and place it on a microwave-safe cookware.

15. Defrost food completely before cooking it by microwave ovens.

16. Defrosted foods should be cooked as soon as possible.

Reheating

17. Cover foods with a microwave-safe lid/plastic wrap for uniform heating.

18. When covering foods by plastic wraps, give a vent at the corner as some wraps tend to tighten and split upon heat.

19. Leave a gap between the wrap and foods, in particular those with high fat or sugar contents, as the hot food might melt the wrap.

20. Reheat foods inside microwave ovens thoroughly.

Risk Assessment Section
June 2005

For the study report, please click the following website : Microwave Cooking and Food Safety

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