Why Starch Gets Crispy When Fried

By Cook's Illustrated Published March 2014

We explain why we often dip food in a starchy coating before frying.

While you can certainly fry food in hot oil as is (think skin-on chicken pieces), we often dip food in a coating first. Such coatings provide a few benefits: They help protect the food from moisture loss, and they shield the food from direct contact with the hot frying oil for more gentle cooking. And perhaps most important, we know that these coatings—starchy coatings, specifically—become incredibly crispy when fried. But until now we’ve never really asked ourselves the deeper question: What exactly is happening that makes starch the key?

Here’s what we’ve learned. First, the starch granules in the coating absorb water, whether from the wet surface of the food itself or because they are combined with a liquid to make a slurry before coating the food (as we do for our Thick-Cut Sweet Potato Fries; see related content). The hydrated granules swell when they are initially heated in the oil, allowing the starch molecules to move about and separate from one another. As water is driven away during the frying process, these starch molecules lock into place, forming a rigid, brittle network with a porous, open structure.

Furthermore, the two types of starch molecules (amylose and amylopectin) form some cross-links with one another at high frying temperatures, further reinforcing the coating’s structure. Thus, the molecules in this porous network have room to compress and fracture, providing the sensation of crispiness. Interestingly, cornstarch contains 25 to 28 percent amylose, which is higher than the amount in wheat or potato starch (which are 20 to 22 percent amylose), and this is why cornstarch works the best for making crispy coatings on fried foods.