Because starch will cause the potatoes to clump together and when starch turns to sugar, which happens when potatoes are stored below 40 degrees Fahrenheit, the fries will turn dark before they are fully cooked. You need to strike a careful balance between high and low starch, because low starch can be a culinary disaster. The best way to remove starch is to soak potatoes in water. It’s difficult to remove too much starch—a potato is about 80% water and 20% starch. When you soak cut potato pieces in water, you are only removing the outer surface of the potato.