Deep Frying: Convection
Frying is the cooking of foods in fat or oil in an open pan at a temperature that is well above the boiling point of water. The fats and oils used seem to vary from place to place, e.g. coconut oil is popular in Sri Lanka and South India, and butter and vegetable oils in Europe. Fats and oils can be heated to a very high temperature without evaporating, although they will eventually burn or decompose. The temperature at which any particular fat or oil will burn differs from type to type, but most vegetable oils will withstand temperatures over 200oC (392oF), which is much higher than the 165oC (374oF) usually needed for frying. The high temperature causes the proteins in food to coagulate quickly, then further cooking causes additional flavour changes and gives a crisp-textured food that has a pleasant mouthfeel.
During frying, when the food is fully submerged into hot fat, the water on its surface, and then further inwards, to instantaneously boil and become steam. There is a violent boiling and a coagulation of protein that seals the surface and the fat cannot enter the food through the pores that are left, because the steam is rushing outwards. Pretty quickly, as the temperature rises (having initially dropped when the foods was put in), starches and sugars in the outer layers dry out and then caramelize. The food gains a brown and crisp outer coating. This coating is important and great attention is given over to this by good cooks. Foods like chips (French fries), doughnuts and fritters are cooked in this way; there are also a group of fried confections and biscuits from the Middle East and India, e.g. jalebi.
The temperature of the fat is critical. If the temperature is too high, the exterior scorches and toughens, then the fats will burn and the food may carbonize before the centre is cooked. In contrast, if the fat is too cold, then the instantaneous coagulation of proteins with the rushing out of steam does not take place, and the fats can get into the food and make it soggy. A similar effect arises if too much food is put in, which cause the fat's temperature to fall too low.
Egg is a good coating for foods to be fried, because the egg proteins coagulate quickly and forma more of less impenetrable skin. Egg-based batters have a similar effect, but dusting food with flour or cornstarch alone does not make the food's surface impenetrable, although it may dry it up and help crisp them. Often, the foods are just dusted in flour or simply patted dry.
For deep frying, there must be enough fat and oils for the food to be totally immersed. When frying is done properly, the fat should not pick up tastes from the foods fried in it. Fat used for deep frying should be strained regularly to remove the crumbs and scraps that accumulate in it, or these will burn and spoil the flavour of the next food to be cooked in the fat. Because oils turn to become rancid more easily after heating, the fat or oil for deep frying should be kept in the fridge between uses; also, for health, it is much better to change the fat being used regularly.