Electromagnetic waves from a power source magnetron are absorbed by the food and food becomes hot at once. Thus, microwaves do not require any medium of transfer of heat in cooking. The microwaves can be absorbed, transmitted or reflected. They are reflected by metals and absorbed by food when food is kept in the cavity of the microwave oven for cooking. The microwaves generated by the magnetron strike the food and metal walls of the oven. Microwaves that strike the metal walls are reflected and bounced back, so they disperse throughout the oven and accomplish uniform heating of the food.
Cooking with microwaves differs radically from conventional cooking methods because the heat is generated inside the food rather than being transferred to the exterior of the food by conduction, convection or radiation.
The energy of these electromagnetic radiations excite the water molecule in food which bear a positive electrical charge in one position and negative charge in other position of the molecule (dipole). When the electric field of the microwave interacts with the water dipole, the water molecules begin to vibrate very rapidly in food around 2,450 million times a second. This vibration produces friction that creates heat within the food as microwaves are able to penetrate.
The efficiency of microwave cookery depends heavily upon the constitution of the food being cooked. Different components of the food will interact with the microwave radiation at varying rates.
The most important material in any food is water. It is the major constituent of the cellular material and many foods contain over 70% of their normal weight as water. It is the water in cellular organic matter that converts microwaves to heat energy with great efficiency and rapidity whereas other components such as starch, cellulose and protein are nowhere near as efficient. Least effective as an energy converter is fatty tissue which contains a very small amount of moisture. Microwave cooking can be done on paper dishes, plastics, glass, chinaware and ceramics because these materials transmit the waves through them without absorbing them like water. Aluminium foil can also be used. But brown paper bags, stainless steel vessels, metal twist ties, conventional thermometers cannot be used.
After cooking in a microwave oven washing dishes is much easier as food does not stick to the sides of these vessels and the paper dishes can be thrown away. Cleaning the oven involves only wiping it dry with a damp cloth.
Microwave cooking enhances the flavour of food because it cooks quickly with little or no water and thus preserves the natural colour of vegetables and fruits. Leftovers from the previous days dinner can be heated in a minute and reused for breakfast or lunch, precooked processed foods available in Indian markets like tikkas, kababs, dhals and chicken curry can be on the table in minutes.
Following the instructions supplied by the manufacturers, home makers can make a cake in 8 minutes and chicken tikkas in about the same time. Stuffed capsicum are ready in 6 minutes only. Some microwave ovens have an infrared lamp fitted into the oven cavity so that exterior colouring can be included while the microwave heating is occurred.
An alternate method that is currently being developed in domestic microwave ovens is to lengthen the cooking time by reducing the microwave output. In this way the longer cooking time allows some surface colouring to occur so that the appearance of the food matches the conventional product more closely. The increase in cooking time (2-4 fold) still allows a significant saving over conventional cooking time.
An essential difference between microwave and conventional cooking is done by time rather than temperature in microwave cooking.