Q: How do I get roasted potatoes extra crispy?
A: I've got two simple tricks, starting with a little precooking.
First, parboil them before roasting. Simmering starchy baking potatoes in water (seasoned with salt) gelatinizes a layer of starch on their exterior surfaces. This layer will then dehydrate and brown as the potato roasts, creating a thick, crisp shell.
It's easy to accidentally overboil baking potatoes, causing them to fall apart. To prevent this, add a tablespoon of plain white vinegar to each quart of water. The now lower pH of the water keeps pectin—a gluelike carbohydrate that holds plant cells together—from breaking down. Start potatoes in cold water, bring them to a boil, and cook for about 5 minutes or until they're completely tender.
The second trick: Increase surface area. After parboiling, add potatoes to a bowl with a bit of fat (olive oil works great), season with salt and pepper, and then roughly toss them, trying to smush up their surfaces. As you toss, you'll notice a thin layer of mashed potato forming a coating around each chunk. It's this layer that crisps up into the kind of crunchy shell that only the best French fries have.
Once your potatoes have been parboiled and tossed, all that remains is to transfer them to a lightly greased rimmed baking sheet, put them in a 475° oven, and roast about 45 minutes or until crisp all over, flipping once or twice to make sure all the edges get browned.
Kenji Lopez-Alt is the chief creative officer of Serious Eats, where he writes The Food Lab, unraveling the science of home cooking.