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Okinawan sweet potatoes are purple relatives of the more common orange sweet potato, and were first cultivated on the Japanese prefecture of Okinawa. These tubers have a thin, light tan to brown skin, and a sweet, starchy, bright purple flesh used for both sweet and savory dishes. The best Okinawan sweet potatoes are small to medium in size with a firm texture and smooth, unwrinkled skin. Avoid specimens with soft spots, dark brown discoloration, or visible sprouts. You can roast or boil these unusual tubers for consumption on their own, or use them in pies, tempura, and meat dishes.
This tuber is about the size and shape of a conventional gold or orange sweet potato, with a slightly starchier texture when raw. While Okinawan sweet potatoes look a lot like Filipino ube, or purple yams, they are unrelated and have a sweeter flavor. You can identify these tubers in Asian markets or specialty grocery stores by the faint purplish tint that shows through thin-skinned areas, or by looking for broken tubers that have a purple center. Do not mistake them for purple-skinned sweet potatoes, which have white flesh and are less sweet.
Look for sweet potatoes between fingerling size and about 1 pound (0.5 kg), as larger specimens can be tough and woody. These Japanese purple potatoes should have smooth, unwrinkled skins with no darkening or discoloration, and a firm texture. Soft spots or darkened areas can indicate spoilage, even in a sweet potato that seems sound from the outside, while sprouts on the surface produce a less appealing tuber with reduced sweetness. Choose sweet potatoes of similar size for even cooking, and store them for no more than two weeks in a dark, dry, well-ventilated location.
Like other sweet potatoes, the Okinawan variety is inedible when raw, and must be boiled, baked, roasted, or grilled before consumption. When completely cooked, Okinawan sweet potatoes have a soft fluffy texture with a thin, flexible skin that may crisp slightly if baked. The flesh can be eaten with butter, coconut milk, or other condiments, or removed from the skin for use in souffles, mashed potatoes, and casseroles. Okinawan sweet potatoes are often eaten with meat, wilted greens, or onions. These naturally-sweet tubers also work well in pies, puddings, and other desserts, especially when combined with traditional Asian flavors such as coconut or ginger.