Achieving the crispiest possible sweet potato fries can be challenging as their flesh is softer than regular potatoes. One ingredient that will get you there is cornstarch.
There's a sure way to use cornstarch when you make sweet potato fries that will produce the best results, and it's all about understanding the role of starch.
Recipes call for rinsing the potatoes first, but this removes some of the starch. Coating the sweet potatoes in cornstarch restores the starch to crisp up properly.
If frying, toss the potatoes in cornstarch and dry them out a bit. If baking, toss them in oil, then coat with cornstarch. Use a tablespoon for each pound of potatoes.