Whenever I try to make sweet potato fries at home, I always end up with flaccid, thick-cut fries that taste great but don't have the satisfying crunch I was hoping for. And when I try to be healthyish and make hash browns out of sweet potatoes instead of regular potatoes, I end up with a mushy heap of grated sweet potato that doesn’t stick together. Why is that? Why do russet (baking) potatoes crisp up so much better? Both potatoes are tubers and both contain starch, but there is a key difference between sweet potatoes and regular potatoes that you need to account for when cooking—especially when making sweet potato hash browns.
Sweet potatoes have a slightly higher moisture content and a slightly lower starch content. This changes the gelling effect of the amylopectin and amylose in the two stem tubers (that’s right, potatoes are NOT roots). Sweet potatoes also contain more sugar, causing the outside of the potato to brown before the inside has sufficiently cooked. But the main thing to keep in mind when roasting or frying is moisture. When water is heated, it evaporates in the form of steam. If this occurs in a food like a potato, that will ideally develop crisped layer of skin when cooked, it makes potatoes mushier and the skin less crispy.
There are a few constants when cooking hash browns: grating by hand (or in a food processor), frying in a mixture of olive oil and butter, and using a cast iron skillet. The biggest difference is, with regular hash browns, you want to soak the grated potato in water and then squeeze them dry in a clean hand towel. This will remove the excess amylose and prevent an overly sticky, gelled texture. Then you want to remove as much moisture as possible—remember that moisture is the enemy of crispiness.
Whereas with grated sweet potatoes those steps are unnecessary. You should toss them in potato or cornstarch to make them crispier and hold their forms better. Pro tip: After soaking your white potatoes in water, you can pour out the water and re-add the potato starch that settled to the bottom of the bowl to make your hash browns similarly crispy.
The same principle applies to French fries. A lot of recipes recommend parboiling your slices of russet potatoes in a vinegar and water mixture prior to deep frying them, but simply slicing and frying is sufficient for sweet potatoes. Lightly dredging both types of potatoes in potato starch will again help the exteriors become crispier, and in the case of the par-boiled French fries, you might even get that McDonald’s-esque crunch-factor.
The best piece of advice I can offer you is that time is your enemy. Make sure you have your seasonings and cooking stations set up because the longer your hash browns sit at any stage, the stickier and less appealing they will get, despite all your thoughtful preparation. Even with sweet potatoes, it's best to move as quickly as you safely can.