Deep frying (also referred to as deep fat frying) is a cooking method in which food is submerged in hot fat, most commonly oil, as opposed to the shallow oil used in conventional frying done in a frying pan. Normally, a deep fryer or chip pan is used for this; industrially, a pressure fryer or vacuum fryer may be used. Deep frying may also be performed using oil that is heated in a pot. Deep frying is classified as hot-fat cooking method. Typically, deep frying foods cook quickly: all sides of a food are cooked simultaneously as oil has a high rate of heat conduction.
Modern deep frying in the United States began in the 19th century with the growing popularity of cast iron, particularly around the American South which led to the development of many modern deep-fried dishes. Doughnuts were invented in the mid-19th century, with foods such as onion rings, deep-fried turkey, and corn dogs all being invented in the early 20th century. In recent years, the growth of fast food has expanded the reach of deep-fried foods, especially French fries.
Deep frying is done with a deep fryer, a pan such as a wok or chip pan, a Dutch oven, or a cast-iron pot. Additional tools include fry baskets, which are used to contain foods in a deep fryer and to strain foods when removed from the oil, and cooking thermometers, used to gauge oil temperature. Tongs, slotted spoons, wooden spoons, and sieves may be used to remove or separate foods from the hot oil.
Deep-fried foods are common in many countries, and have also been described as "a staple of almost all street cuisines on all continents". There are hundreds of dishes that are associated with deep frying as most foods can be deep-fried. Examples of food that can be deep-fried include meat, poultry, fish and vegetables. Fish and chips, for instance, combines deep-fried fish and deep-fried potatoes. French fries, doughnuts, onion rings, and hushpuppies are common deep-fried foods. Other common deep-fried foods include Chinese You Bing deep-fried pancakes, Southeast Asian Jin deui, and Japanese tempura. Less common deep-fried foods include maple leaves, peanut butter and jelly sandwiches, pizza, and Snickers bars.
Japanese tempura is a popular deep-fried food that generally consists of battered and fried seafood and vegetables. Japanese deep-fried dishes, or Agemono, include other styles besides tempura, such as Karaage, Korokke, Kushikatsu, and Tonkatsu.
Many countries in Europe use pure or hydrogenated rapeseed oil for deep-frying. The deep-fried Mars bar originated in Scotland, with The Carron Fish Bar in Stonehaven claiming to have invented it in the early 1990s. Fish and chips is a very popular deep-fried dish in England since it originated in London in the 19th century and became popular among the working class. Its popularity continues with 229 million portions of fish and chips being sold annually in England.
Novelty deep-fried foods are popular today in American fairs, especially those in the American South. Hundreds of items are served at these fairs. Some of them include deep-fried beer, butter, and bubblegum. Additionally, deep frying can be used as a form of artwork by frying non-edible objects, such as electronics. Artists such as Henry Hargreaves have deep-fried replicas of electronic items such as iPads, Game Boys, and laptops.
Deep fat frying involves heating oil to temperatures in excess of 180 ĄC in the presence of moisture and air. These conditions can induce a series of complex chemical reactions which may impact the quality of both the food and the oil it is cooked in. Examples of different chemical reactions include the production of free radicals, oxidation, hydrolysis, isomerization and polymerization. The exact reactions are dependent upon factors such as the oil type, frying conditions, and food being cooked. When frying, water can attack the ester linkage of triacylglycerols, resulting in di- and monoglycerols, glycerol, and free fatty acids (a type of hydrolysis reaction). The aforementioned hydrolysis reaction is enhanced by the produced fatty acids and other low molecular weight acid compounds.