Placing sweet potatoes in the refrigerator alters and changes the structure of their cell walls. That ends up making them harder to break down and thus turning them harder than normal. This in turn, will cause them to take longer to cook. For best results, sweet potatoes should be kept in a cool, well-ventilated, dark place. By doing this, it lets the fresh air cure them and they can harvest normally. Also, be sure to keep them away from places with a strong heat source. That means keeping them away from appliances which get hot or the sun. The temperatures should be anywhere between 57 and 63 degrees Fahrenheit. If you keep your sweet potatoes in a cool, dark place, they will end up cooking evenly and softer. Remember to keep an eye on them and try using them before they start to turn or go bad.