How does fat create crispiness?

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.

2 thoughts on “How does fat create crispiness?”

  1. Hi Joe,

    Can you say more about how oil helps heat transfer in the oven? It makes sense that in a pan on the stove, oil facilitates heat transfer by providing a contact medium between food and pan, i.e. filling microscopic insulating air pockets between food and pan.

    But in the oven, how does that work? Whether or not something like a potato is coated in oil before going in the oven, it’s still getting heat transfer by radiation and convection. What role is the oil serving?


    1. Hello Evan!

      You’re pushing my physics knowledge here, brother, but I’ll do my best! The answer has to do with something called specific heat capacity, which refers to the amount of energy it takes to cause a substance to increase in temperature. Oil has a specific heat capacity that’s lower than water, which means that as you put heat energy into it, its temperature rises faster. Thus even in an oven, oil is a help when it comes to absorbing heat energy (whether radiant, conductive or convective) and transferring it to food. Particularly on the surface of a food, that has implications for browning and crispiness.

      Thanks for the question, Evan!

      – Joe

      That obviously has implications for cooking and browning, which I discussed in the post.

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