All sweet potatoes were first cultivated in South America, but they were brought to Japan by the Portuguese in the 1600s. Known as satsumaimo (サツマイモ) in Japan, they quickly became a staple crop, due to their ability to make up for any shortfalls in the rice harvest. Through selective breeding, Japanese sweet potatoes have become a unique set of cultivars characterized by red skin and pale yellow flesh that becomes vibrant yellow as they're cooked. They tend to be sweeter than Western varieties, but they also have a lower moisture content, making them dry if they're not prepared properly.