For The Crispiest Oven Fries, Steam Them First
Think of the crispiest french fries you've ever had. Chances are they were made in a deep fryer at a restaurant, not in an oven at home. While it's technically possible to bake french fries instead of frying them and get edible results, the method doesn't always yield the best texture. Either the french fries will be limp and soggy, or they'll be hard all the way through (via NPR). If all you do is slice your potatoes into matchsticks and place them in the oven, it's hard to get french fries that are crispy on the outside yet soft on the inside.
According to A Couple Cooks, you can achieve a more fry-like texture by doing things like flipping each one part way through baking or spacing them out evenly on your baking sheet. But while following these tips can definitely make a difference, a much more effective way to go about it, per Real Simple, is to steam your potatoes before you bake them.
What happens when you steam potatoes before baking them?
Instead of soaking your french fries in cold water beforehand or putting them straight in the oven, Real Simple suggests steaming your spuds. By blanching the french fries with boiling water, starch is pulled out and ensures the insides will remain soft when they eventually come out of the oven. And since the potatoes are already partially cooked, you're able to take them out of the oven right when the outside is perfectly crisp, rather than potentially overcooking them while waiting for the insides to finish.
Blanching is so effective that it's what Alex Guarnaschelli uses when making the fries she serves at Butter Restaurant (via Food Network), though she opts for a quick dunk instead of making a spud sauna. And rather than use plain boiling water, the celebrity chef also adds vinegar, a tip she says she learned from one of J. Kenji Lopez-Alt's Serious Eats articles. The addition of vinegar, Lopez-Alt wrote, allows the french fries to hold their shape better, which means you won't have to worry about them being limp even when baking them in the oven.
Lifehacker, invoking cookbook author Patricia Wells, also advocates for the steam-roast oven fry method. Wells notes that thanks to steaming, the drawn-out starch "will stiffen and brown and crisp, and help keep the potatoes from sticking to the baking sheet too."
Steaming potatoes before baking them is a lot like frying them
The reason why an oven and a pot of oil produce different fry textures comes down to the way they cook food. When food gets fried, Discover explains, the hot oil not only cooks it, it also creates what's called a steam barrier. This steam barrier is what prevents the inside of a fry from drying out while its starches gelatinize. That's why french fries have a crisp outside but fluffy inside.
When you bake french fries, however, there's no steam barrier because it's heat transfer, not hot oil, that cooks the potato. Without the steam barrier, there's nothing preventing the inside of the fry from drying out, which is why baked french fries normally turn out the way they do. But if you take the extra step to steam them before you bake them as Real Simple suggests, you're essentially replicating what naturally happens during the frying process, which means they'll be a lot crispier.