Choose the freshest sweet potatoes you can find. They should be free from blemishes or gashes, and uniformly colored. Blemishes lead to early decay and may indicate a spoiled interior. If possible, let freshly harvested potatoes cure, or sit in a room hotter than 80 degrees Fahrenheit, for one week before boiling them for freezing. This allows the enzymes that create the potatoes' sweet flavor to develop.

Pack the mashed sweet potatoes into freezer-safe containers. Leave a 1/2-inch space at the top of each container for expansion. Seal the containers completely, covering, if needed, with a layer of plastic wrap or freezer paper. This will help seal the container and preserve moisture, the loss of which leads to freezer burn. Freeze the containers immediately.

To retain more of the potatoes' nutrition, which can leach out during boiling, consider steaming the potatoes before freezing. Bring about 2 inches of water to boil in a large pan. Slice potatoes into 1/2-inch pieces and place them in a steamer (metal or bamboo) that fits into the pan without letting the potatoes touch the boiling water. Steam them for 7 minutes. For extra flavor, steam garlic and onions with the potatoes. While mashing for freezing, consider adding spices such as cinnamon, nutmeg or cloves.

Sweet potatoes will keep best in a freezer that stays at 0 degrees Fahrenheit and has no defrosting cycles. Boiled and mashed sweet potatoes will maintain color, flavor and overall quality for up to 12 months in the freezer. The USDA recommends using one of three safe methods for thawing frozen foods: in a refrigerator, in cold water or in the microwave. Only these methods will keep the food at a temperature that inhibits bacteria growth. Frozen sweet potatoes can, however, be cooked without thawing, though the process takes about twice as long.