Oooh..COOL question, reader Monica! Whether you’re deep frying, sautéing, or coating a madeleine form to help create crispy edges, fat plays a key role in creating the sensation of crispiness I described below. At least in cooked foods.
The reason is that fat facilitates heat transfer. When we coat chunks of potato with oil before oven-frying them, or melt butter in a skillet before frying a steak, what we’re doing (other than lubricating the food so it doesn’t stick to the cooking surface) is creating the conditions for the quick migration of heat — from the oven air to the potato or from the pan surface to the meat.
As that heat enters the food, moisture leaves it, evacuating in the form of steam. That dries it out. And when something is dry and brittle, it’s pretty much crispy by definition.
Of course we don’t want the whole potato chunk or the steak to become completely dry, which is why we cook both at high heat. We want heat to penetrate the food as quickly as possible and cook it to doneness before all the moisture on the inside has a chance to leave.
This is why deep frying is such an efficient and tasty way to cook. All that hot oil dumps huge amounts of heat into a food item in a very short time. The massive heat dries and crisps the outside of the food, but because the cooking interval is relatively short, it leaves the inside moist and tender.
This process is imitated in air fryers, which attempt to effect a comparable rate of heat transfer using quick-circulating currents of hot air (air, being much less dense than oil, doesn’t transfer heat nearly as well). It could be because I haven’t really learned how to use my air fryer very well yet, but I’m not a fan of air fryers. Yes, in the best cases you get the drying effect, but you also get more overall food dryness. And without the residual fat on the outside of the food, is just doesn’t taste as good. Maybe I just need practice.