The Science of Double-Frying (Patreon)

If you want the crispiest, crunchiest deep-fried foods, fry, fry again.

Ever since man first dropped a chipped potato into a vat of hot oil, we've been chasing that perfect crunch. Not just for potatoes, but for virtually all fried foods. Fried chicken. Hush puppies. Tempura. Korean fried chicken. Chongqing-style hot chicken. Deep fried oreos.

And even when we manage to get there—when we pop that first french fry in our mouth and it shatters with a glassy crunch revealing a moist, fluffy interior—wait ten minutes, and it's turned soggy again.

So what's the secret to getting foods extra crispy and, more importantly, for that crunch to last? Getting that crunch just right is more than heat management or a good breading recipe. It has a lot more to do with what's going on below the surface of the food, and that puff of steam that escapes from a freshly-fried potato as you bite into it is a clue.

What is Frying?

What exactly goes on when we drop foods into hot oil? At first, it looks like the oil is boiling. In fact, we've probably even heard the phrase "cooked in boiling oil." But this is inaccurate. At normal atmospheric pressure, oil will not boil. This is because long before it reaches the high temperatures needed to transform liquid oil into a gas, it hits what's called its flash point. That's the temperature at which oil will spontaneously burst into flames. Not something you want to see in your kitchen. (At least not if you value your eyebrows and your lungs.)

What looks like boiling oil and what makes the steady sizzling, bubbling sound of deep frying is not oil vaporizing. It's actually water from the food transforming into steam and escaping from its surface.

Frying, more than anything else, is a dehydration process. When you fry something, three main things are happening at the same time:

Browning: The Maillard reaction creates flavor and color that add complexity to an otherwise bland crust.

Setting the crust: Proteins and starches firm up and create basic structure, the same way that dough firms up while baking.

Dehydration: Water evaporates from the surface, allowing the crust to become crisp.

That last one is the most important. Crispness depends on dryness. The drier the outer layer gets, and the thicker that outer layer is, the crunchier the food becomes.

With a typical fry, hot oil will do a good job of browning, setting, and dehydrating the very surface of the food. When the food comes out of the oil, it will be crisp, but there’s a problem: there's still moisture just under the surface.

Here's the thing. Water has a tendency to diffuse evenly through a porous medium. That is, it will move from areas of high concentration to lower concentration. We can see this in the way that a drop of water will spread through a paper towel, or in the way that a sponge can get fully wet even if only its corner is submerged. Because the layer of dehydrated crust on fried food is so thin and the interior remains so moist, that spread happens pretty rapidly, and water is the enemy of crispy crusts.

What started as a crunchy crust quickly absorbs moisture from the interior of the food—like a paper towel—and softens up.

So how do we solve this problem? Double-frying.

Fry Hard 2: Fry Harder

By allowing fried foods to rest until that moisture migration has stabilized, then returning it to hot oil, you end up removing more moisture overall. Moreover, the structure of that crust has actually transformed between the first and second fry.

During that first fry, steam will form in the food and travel outward through the coating as it expands, creating a network of tiny channels. As the food cooks, these pathways become increasingly open and interconnected before eventually setting into shape. It's these very pathways that allow hot oil to enter deeper and for moisture to escape more easily the second time the food is fried.

In other words, the first fry prepares the food for the second fry.

Because there’s less water left overall, and because the dehydrated layer becomes thicker, the crust on double-fried foods will be significantly crispier and will stay crisp much longer. In the case of dishes like General Tso's Chicken, that extra fry can help chunks of breaded chicken stay crunchy even when doused in a sticky sauce.

This is why French fries are traditionally fried twice. It’s why at my restaurant, Wursthall, we pre-fried all of our fried chicken and potatoes. It’s why reheating fried food in an air fryer often makes it seem even crispier than the first time. It even works with leftovers. That leftover fried chicken leg or the fries that have gone soggy? Give them a minute or two in hot oil and they will be crisper, crunchier, and tastier than they were when you first bit into them.

Why I Like Frying in a Wok

One other thing that helps a lot with frying at home is using a wok instead of a straight-sided pot. When you drop food into hot oil, the bubbling can make the oil level rise quickly. In a tall pot, that expansion goes straight up, which makes boil-overs more likely.

In a wok, the sides slope outward, so as the oil bubbles, it spreads instead of climbing. That gives you more headroom, more surface area for bubbles to break, and a much lower chance of overflow. The wide shape also makes it easier to stir and flip the food while it fries, which keeps the oil temperature more even and prevents hot and cool spots from forming which can result in an uneven or sub-optimal fry.