What makes deep-fried food greasy, and when?

From Food Arts,

Chris Young, co-author of Modernist Cuisine: The Art and Science of Cooking, headed The Fat Duck Experimental Kitchen for five years.

We may think of deep-fried food as being submerged in oil, but that is not exactly true. If the frying oil is in peak condition, the food will directly contact the oil for no more than half the frying time. The rest of the time, the streaming bubbles shove the oil aside. Because of this, and contrary to conventional wisdom, very little oil is absorbed into food while it’s being deep-fried. Oil is trapped and absorbed by the surface after the food is removed from the fryer and begins to cool. Indeed, this is why blotting excess oil off the surface of the food immediately after it’s removed from the deep fryer helps to make the food a lot less greasy.

Researchers have shown that the cracks, fissures, and holes created at the surface of deep-frying food create capillary forces that wick oil once the food begins to cool. While frying, escaping steam mostly keeps oil out of these nooks and crannies, but when the crust cools, this steam condenses, which creates a slight vacuum that helps to draw the oil in.
This turns out to be the reason that lower frying temperatures produce such a greasy crust. Deep-frying a crusty food in oil at 340°F, rather than 360°F or hotter, can increase the oil absorption by 40 percent. Cooler frying oil is more viscous and sticky and, thus, doesn’t easily drain from the food. Hence, it’s always best to deep-fry at the highest practical temperature because this tends to produce a thin, delicate crust that’s less greasy. But be reasonable with the oil temperature. It’s wasted effort if the food gets scorched and burned.
A coating of oil isn’t necessarily bad, and the goal shouldn’t be to get rid of all the oil on the surface. Deep-fried food simply wouldn’t be deep-fried without some of the flavorful oil coating the surface. The oil provides a lot of the flavor, texture, and mouthfeel of deep-fried food. Interestingly, these effects are entirely superficial. When we chew food, the surface is the first thing that our tongue, cheeks, and palate sense. The first bite leaves them coated with warm, aromatic oil that is mixed into the food as we keep chewing. So the goal of deep-frying should be to leave food anointed with just the right amount of oil, neither too little nor too much.


For more on the science of deep-frying, please click here.