Microwave Cookery Explained
1. Microwaves are electromagnetic, short length, high frequency, radio waves which vibrate at a frequency of two thousand four hundred and fifty times per second. In a microwave oven these waves are generated in a valve known as a magnetron and then directed via a wave guide into the oven where they are reflected around and establish an alternating electromagnetic field. It is an interaction between this alternating field and the food components which generates heat and cooks the food. Microwaves penetrate food and generate heat to a depth of about 25mm then heat is transferred to the remaining areas of the food by conduction and convection and this completes the cooking process. It is important to understand that a microwave oven does not cook or reheat food in the same way as a conventional oven.
APPLICATION OF MICROWAVE COOKERY
2. Microwaves are suitable for cooking, reheating or defrosting foods and will give comparable results in considerably reduced cooking times. They do not however give colour to the cooked surfaces of the food, to achieve this the food must be subjected to a period of conventional cooking, ie grilling or baking in a hot oven. Some microwave ovens have a combined microwave and conventional facility and are known as combination ovens.
3. Reheating chilled and frozen prepared food is greatly facilitated in a microwave oven. All food undergoing this process must be heated to reach a temperature of 70°C for two minutes at its centre and this should be checked with a temperature probe.
4. Defrosting frozen foods is achieved by microwaving for short periods interspersed with periods of rest to allow the heat to penetrate the food by conduction. Modern domestic and industrial ovens normally have a facility which allows automatic programming of this function.
MICROWAVE OVEN DESIGN
5. There are two grades of microwave oven, domestic and industrial:
a. Domestic microwave ovens are usually rated at 650w or below and are frequently equipped with a turntable to help ensure that foods cook evenly.
b. Industrial microwave ovens are rated from 700w to 1400w and are designed and built to more robust electric and mechanical standards to cope with much greater use. They are not equipped with turntables but have fans built into the base or top of the oven, sometimes both. These are known as stirrers and help to achieve an even distribution of the microwaves.
6. Both domestic and industrial ovens usually have variable power control systems. Because microwaves can pass through almost any substance except metal the oven cabinets are metal lined to provide a safe barrier. All models have a tight fitting door which is equipped with a device which automatically switches off the power if the door is opened. Doors are usually fitted with a glass panel reinforced with metal mesh to prevent microwaves escaping. Because cooking times are extremely critical an automatic timer is essential.
FACTORS WHICH INFLUENCE HEATING
7. In general the lower the rated power output of the appliance, the slower it heats, but it is important to assess the performance of the oven on loads representative in mass of the foods which will be heated in it.
8. The lower the starting temperature, the longer it takes to heat.
9. Although it takes longer to heat dense foods through to the centre, some dense foods can reach high surface temperature very quickly and can be a fire hazard. Each food has a finite depth to which the heating effect of microwaves will penetrate. Beyond this, all heating is by the methods of conduction and or convection.
10. Irregular shapes and large variations in component size should be avoided if consistent reheating/cooking both within and between different batches of a given product are to be achieved.
11. For a given food, the greater the quantity in terms of mass, the longer will be the heating time. The relationship between heating time and increased mass is not generally proportional.
12. Containers either transmit (glass), absorb (some ceramics) or reflect (foil or metal) microwaves. This influences greatly the heating of foods within these packs. The shape of the pack also influences the reheating within the pack.
13. For products which are normally presented moist, covering the product assists faster, more even heating, with greater moisture retention and less likelihood of products being spilled or splattered in the oven.
14. The specific heat of a product is of great influence in product reheating. Those with low specific heat capacity eg oil, although poor absorbers, can reach high temperatures very quickly, whilst high heat capacity materials, even if good absorption, rise in temperature more slowly. Electrolytes, such as salt, can lead to areas of overheating in products and, because of their more rapid absorbance of energy, lower penetration depths into the food.
USING THE MICROWAVE
15. Manufacturrers of microwave ovens produce user instructions and receipes written for their own machines and the best results are achieved by conforming with these. All the major oven manufacturers have demonstration units at which courses for microwave cookery are available and attendance on these is very worthwhile.