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Choose firm, dark, smooth sweet potatoes without wrinkles, bruises, sprouts, or decay. Even if cut away, a decayed spot may have already caused the whole potato to take on an unpleasant flavor.
Sweet potatoes spoil rapidly. To keep them fresh, store them in a dry, cool (55-60º) place such a cellar, pantry, or garage. Do not store them in the refrigerator, where they will develop a hard core and an "off" taste. If stored properly, sweet potatoes will keep for a month or longer. At normal room temperature, they should be used within a week of purchase. You may brush off any excess dirt before storing, but do not wash them until you are ready to cook them. It is the moisture from washing that will increase their spoilage.
From the Centers for Disease Control and Prevention (CDC), Updated 2005©1996-2005 WebMD Inc. All rights reserved.