Deep frying is a cooking method in which food is submerged in hot oil or fat. This is normally performed with a deep fryer or chip pan; industrially, a pressure fryer or vacuum fryer may be used.
If performed properly, deep-frying does not make food excessively greasy, because the moisture in the food repels the oil. The hot oil heats the water within the food, steaming it from the inside out; oil cannot go against the direction of this powerful flow because (due to its high temperature) the water vapor pushes the bubbles toward the surface. As long as the oil is hot enough and the food is not immersed in the oil for too long, oil penetration will be confined to the outer surface. However, if the food is cooked in the oil for too long, much of the water will be lost and the oil will begin to penetrate the food. The correct frying temperature depends on the thickness and type of food, but in most cases it lies between 175 and 190 °C (345–375 °F).
Some fried foods are given a coating of batter or breading prior to frying. The effect of these is that the outside of the food becomes crispy and browned, while the inside becomes tender, moist, and steamed. Some foods – such as potatoes or whole, skin-on poultry – have a natural coating and do not require breading or battering.