Choose Containers for Microwave Cooking

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containersContainers have a great influence on the evenness of cooking and on the cooking times. They must be made of a material suitable for the microwaves to pass through and not include those that will react to microwaves. All forms of metal are excluded because items which have a metal trim will damage the magnetron (the microwave source) and possibly the oven lining.


Glazed pottery and china are excellent choices for use in the microwave provided that they do not have any metal decoration or printing above or underneath. However, some pottery has impurities in its manufacture and even a small fleck of metal may cause problems. To test for this put the empty dish in the microwave together with a glass half-filled with water. Switch on for about 30 seconds. If after this the water is hotter than the dish, then the dish is safe to use. If the dish is hotter it is not safe. If they are the same temperature, then the dish is not suitable for prolonged heating, but may be used for short periods of, say, 2 minutes.

Plastic dishes and containers now being sold specifically for microwave use have various advantages but have disadvantages too. The main advantage is that foods can cook more quickly as there is no obstruction to prevent the microwaves readily reaching them. The disadvantage is that the containers can become soft and distort from the effects of hot liquids following prolonged cooking. Most plastics will develop holes and sear if very hot fats or sugar syrups are cooked in them. Also, some plastic containers discolour if coloured substances, such as tomato sauces, are prepared in them.

Plastics are only suitable for microwaving when they can stand up to average cooking temperatures. Generally speaking you can use them for warming food.

Foam dishes should not be used because there is a possibility that they will melt. Foam also acts as an insulator, slowing down thawing if you attempt to defrost on dishes of this material.

Boilable plastics often sold in the form of pudding basins are excellent for all cooking, except when sugar or fat are in the ingredients.

There are ranges of thermo-plastics that can be used in the conventional oven, the microwave or the freezer and are indestructible unless dropped on a hard floor. Paper containers are also available, specially treated, so that they too can be used both in the microwave and a cool-to-medium conventional oven. Check the manufacturer's instructions before using.

Heat-resistant clear glass basins and mixing bowls are ideal for microwaving. A 2.8 litre (5 pint) bowl can be used to make about 2 kg (51b) jam or enough soup to serve twelve. Like any other product they will break if misused, for example placed on an ice-cold surface when just having been used for making caramel. Most ovenglass and ceramics are not suitable for use under the conventional grill and plastic and paper must never be used for this type of browning.

Specially designed glass ceramic ware is now available which is suitable for the microwave, the freezer, the refrigerator and on the hob as well as at a suitable distance under the grill. But remember that you should not put a small dish on the hob because the heat on either side would make it impossible to lift.

All forms of metal are excluded because items which have a metal trim will damage the magnetron (the microwave source) and possibly the oven lining. This is immediately detectable because sparks occur audibly and visibly. Secondly, dense metal, such as stainless steel, cast iron and enamelled dishes, do not allow microwaves to pass through them, so that food contained inside will not heat. Beware of casseroles that appear to be ceramic, when in fact they are enamelled cast iron. These should not be used.

Never use any container having metal in it and this includes lead crystal, gilt-edged glass and staples (sometimes used to secure straw baskets). Never use metal tags from polythene bags. The only time that aluminium foil can be used is to shield parts of the food, that would otherwise overcook and this can only be done provided your manufacturer's instruction book confirms its use. The aluminium foil is of a higher density metal than the fine particles of metal decorating plates or twist-ties and therefore can be used. The pieces of aluminium foil must not touch the sides of the oven. If in doubt, you can overwrap with cling film or greaseproof paper. When aluminium foil is used to shield the top of food, it is unlikely that it will reach anywhere near the top oven lining, but it must not stick up in a twisted point as this turns it into a sort of lightning conductor which will cause sparking. Aluminium foil dishes, provided they are shallow (less than 2cm (1 inch) deep), are completely filled with food and fit in with all theother requirements of using metal in the microwave, are safe, but any foil lid must first be removed. Sometimes ready-prepared dishes are sold in a foil dish inside a cardboard carton. To use these, take out the foil dish and remove the lid, then replace the foil dish in the carton but leave one end open. The cardboard will help to hold in the heat, making reheating faster but will also protect the foil. As described above for small foil pieces used to shield food, the foil carton must not touch the sides of the oven.

See-through roasting bags both hold in the heat and enhance browning, particularly if they are domed, so that the bag is not in direct contact with the food. Use them for poultry, when you will find it convenient to split the bag open to make a kind of tent, or use it for vegetables, sealing the top loosely with an elastic band. You must leave a sizeable gap at the neck of the bag for steam to escape.

Whenever you cook in a roasting bag make sure that it is resting on a lipped shelf or removable plate, because the bag becomes very hot and limp due to the heat from the contents and spillage can occur as you take it from the microwave.

Paper is useful as a covering, e.g. greaseproof, nonstick silicone and paper towels which will also absorb excess fat and moisture. Cling film is now regularly used for microwave cookery and also for freezer wrapping. When using it for cooking it should be vented to prevent ballooning. Cling film wraps snugly around all but plastic dishes. If you wish to use it in conjunction with these, you will have to wrap it so that the cling film overlaps and sticks to itself. Buy a large or catering pack if you are going to do a lot of microwaving. The weight of the pack ensures that it will remain steady when you tear off the cling film and this makes it much easier to use.

The shapes and sizes of dishes also have a marked effect on both the evenness and the speed of the cooking. Deep-sided dishes slow down the cooking times because the microwaves get weaker as they go through the food. They also allow room for boiling liquids and their rounded sides make it easy to stir and mix the ingredients thoroughly.

Shallow dishes are useful when little liquid is involved and also when drier results are needed, but moist items can be cooked in shallow dishes provided they are suitably covered. Casseroles also have their uses and a large chicken roasting dish with shallow lid is essential.

Always use deep dishes when making crumbles, for dishes with toppings which cover the food, such as Shepherd's Pie. and particularly when the base food is watery. Use jugs and bowls that are large enough to contain the food even after they have boiled up. Cake dishes should only be two-thirds filled.

When making cakes, quiches or custards choose ring moulds, round dishes no more than 23cm (9 inches) in diameter or rectangles that are about twice as long as they are wide. Cakes and tarts cannot easily be unmoulded when freshly cooked but after freezing they turn out without any trouble.

It is inadvisable to leave utensils, such as wooden spoons, in the bowl while cooking in the microwave oven. Painted wood handles will crack and the handles of ordinary wooden spoons can become very hot due to residual moisture or grease that has become ingrained. Plastic spoons may melt but there is a special range of utensils specifically designed for microwave use.

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