Frying

• 1. Introduction

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  Frying is one of the oldest cooking methods humans have ever devised, and it works by doing something counterintuitive: it dries food out even as it immerses it in fat. Ancient Egyptian cooks were practicing it around 2500 BC. By the time Geoffrey Chaucer and Miguel de Cervantes were writing, frying had found its way into two of the most celebrated works of European literature. What exactly happens when food hits hot oil? Why does a crust form? Why does a doughnut end up as much as 35 to 40 percent pure frying oil by weight? And what does a 2021 meta-analysis say about what all that fried food is doing to the human heart? The answers reach from ancient India to the woks of late Han dynasty field kitchens, from the chemistry of the Maillard reaction to the smoke points of palm oil versus olive oil.

• 2. Ancient Origins

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  Evidence of frying in India dates to around 1500 BC, when ghee was used to fry appam. That makes frying in South Asia roughly contemporary with the later pharaohs of ancient Egypt, where the technique had already been practiced for about a thousand years. China's Book of Rites records a Zhou dynasty recipe for fried rice combined with fat from a wolf's breast, a detail that signals how far frying had spread across distinct culinary traditions long before recorded history connected them. Stir frying is thought to have emerged in field kitchens during the late Han dynasty, though it barely appears in the literary record because it was too informal a practice to draw scholarly attention. In the Hebrew Bible, the book of Leviticus draws a clear distinction between baked bread and bread fried in a pan with oil, suggesting frying was recognizable enough to require its own legal category. In Norway, frying pans begin appearing in grave goods as early as the seventh century, pointing to frying's steady westward reach. The Wife of Bath's Tale mentions frying, and Don Quixote makes a reference to fried eggs, two literary data points separated by roughly three centuries.

• 3. The Physics of a Fry

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  When food meets oil heated to between 150 and 200 degrees Celsius, water inside the food rushes toward boiling point and begins to vaporize. That rapid vaporization punches pores through the food's cell walls, and through those pores oil flows inward. Because the surface touching the oil is exposed to the most heat, water escapes from the outside first, forming a dehydrated crust that then acts as a barrier slowing further oil penetration into the center. The crust turns brown through two overlapping chemical reactions: caramelization and the Maillard reaction, both of which produce melanoidins, the compounds responsible for that characteristic color. Starch in the food gelatinizes and softens while the outer crust stays firm, producing the contrast between a crisp exterior and a tender interior that makes frying so appealing. Frying is technically complete only when all water content in the food has vaporized. Then comes a lesser-known phase: as a fried food cools below 100 degrees Celsius, water vapor inside condenses, internal pressure drops, and oil is pulled inward through a vacuum effect. That is why food absorbs more oil after it comes out of the fryer than while it is still cooking.

• 4. Techniques and Their Tools

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  Deep frying submerges food entirely in hot oil so that all sides cook at the same time, and it can be done in a home chip pan or in an industrial pressure fryer or vacuum fryer. Battering or breading food before deep frying adds both color and crunch. French fries, doughnuts, cong you bing, and tempura are among the most familiar examples of the deep-fried form. Shallow frying, also called pan frying, cooks food in a small amount of oil on one side until browned, then flips it to finish the other side. Sauteing takes minimal oil even further: food is tossed over high heat with just enough oil to prevent sticking, using a pan with higher sides to keep ingredients from escaping during the toss. Stir frying is a Chinese high-heat technique carried out in a wok placed directly over an open flame. The wok's concave shape allows flames to climb up around the bowl, and the base of the wok can reach 400 degrees Celsius. Because of those extreme temperatures, stir frying is finished in two to three minutes.

• 5. Oil, Oxidation, and Fire

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  Rapeseed oil, also marketed as canola oil, is the third most used frying oil in the world, especially in northern Europe, Japan, Oceania, and Canada. Despite being predominantly monounsaturated, its high linolenic acid content makes it prone to oxidation. Olive oil, by contrast, degrades two times as slowly as sunflower oil and ten times as slowly as linseed oil, owing to its high monounsaturated fat and antioxidant content. Researchers have theorized that olive oil's stability may explain why fried food consumption has not been linked to increased mortality in Mediterranean countries. Palm oil is the most commonly used frying oil globally, particularly dominant in sub-Saharan Africa, Southeast Asia, and parts of South America. Its saturated and unsaturated fat content are nearly equal, though it skews slightly more saturated, and its high palmitic acid content is associated with heart disease. Reusing oil in industrial frying reduces costs but accelerates oxidation, producing aldehydes that affect flavor and increase cancer risks; foods fried in reused oil also absorb more. On the safety side, any oil with a smoke point below 200 degrees Celsius may foam and spill, causing a fire. An oil whose flash point falls below 315 degrees Celsius may explode.

• 6. Health Consequences

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  A 2021 meta-analysis found that the highest category of fried food consumption, compared to the lowest, is associated with a 3 percent increase in the risk of all-cause mortality and a 2 percent increase in cardiovascular mortality. The same study found that people who ate the most fried food had a 37 percent increased risk of heart failure, a 28 percent increased risk of major cardiovascular events, and a 22 percent increased risk of coronary heart disease. The researchers identified high trans fat and calorie content as the primary drivers, along with the pro-inflammatory nature of fried foods. They also noted that the appeal of crisp, hyperpalatable fried foods may encourage routine overeating. A 2023 meta-analysis added another dimension: fried food consumption is linked to a 52 percent increase in the risk of stomach cancer. Frying also concentrates things that are not always wanted. Vitamins like thiamin, riboflavin, and vitamin C can be lost during water evaporation. Vitamin A is particularly vulnerable: 24 percent of it is destroyed during frying, compared to 14 percent during boiling, because of oxidation. And heavy metals found in some fish are concentrated in the flesh as frying drives out water. Toxic compounds including formaldehyde are produced during frying and evaporate with steam; the International Agency for Research on Cancer has classified frying fumes as probably carcinogenic to humans.