Preheat oven to 450 degrees with racks in the upper and middle positions. Line two baking sheets with parchment paper and rub with oil.
Put sweet potatoes in a microwave-safe container, cover, and microwave 2 minutes. Stir gently, cover, and microwave 1 to 2 minutes more until pieces are pliable. Let rest 5 minutes covered; pour onto a platter.
In a large bowl, whisk egg whites until frothy, add spice mix, and whisk to blend. Working in batches, toss the sweet-potato pieces in the seasoned egg whites, letting the excess liquid drip back into the bowl. Place in a single layer on prepared baking sheets. Bake 10 minutes, then flip pieces over with a spatula. Rotate baking sheets from front to back and from one rack to the other. Bake until dark golden brown, about 15 minutes. Serve immediately.