How air fryers work, scientifically speaking
Here’s how hot air can “fry” food.
In recent years, air fryers have exploded in popularity thanks to their promise: You can make crispy, fried food with little to no oil. But what’s really happening inside those countertop machines?
Frying generally refers to the process of cooking food in some amount of oil. When you heat up oil in a pan, the “strings” that form are a telltale sign of convection, or “movement of fluid due to temperature and density differences,” says Kevin Keener, an engineer at the University of Guelph who studies thermal food processing methods. As the oil swirls, it slips through the food’s crevices, transferring heat. The temperature of the food rises to the point that the water inside it boils, causing dehydration, a key step in achieving crispiness.
Air fryers emulate this process with air rather than oil. A fan situated above the food tray (near the heating element) blows hot air around the small chamber, accelerating convection and increasing the amount of heat that gets transferred to the food. This transfer is called heat flux: the amount of energy in a unit of area and time. The heat flux of convection that occurs naturally in a traditional oven might peak at 50 to 100 watts per square meter, Keener says. But adding a fan, like that of an air fryer, could increase the heat flux to a few hundred, or even a few thousand watts per meter, he says.
In a sense, both deep-frying and air frying work through convection, but with different fluids: oil and air. And that’s a key difference, Keener says. The heat flux of deep-frying is around 30,000 watts per square meter, whereas with hot air, the best you might get are values in the range of 5,000 watts per square meter. That’s because an air fryer can’t go above 450 or 500 F for safety reasons, so the heat flux is capped. This, in turn, affects how crispy your food can get.
“Crispiness is directly correlated to heat flux value. Higher heat flux, the crispier it’s going to get,” Keener says.
Although an air fryer’s heat flux and crisping effect don’t match that of deep-frying, that particular quality may be less important to home cooks for some foods, like a steak, Keener says. “You would still want to be at 5,000, but you don't necessarily need to be at 30,000,” he says. “Because people don't generally eat steak because of the crispiness.”