This is a vegetarian style of cooking known in Japan as shojin ryori, a Buddhist vegetarian temple cooking. It is known for using ingredients found seasonally to prepare simple vegetarian temple style of cooking. Japanese sweet potato is a bit different from the American version and if you can find it, please use Japanese sweet potato which is known as satsuma imo. But, if you cannot find it, any sweet potato will work fine here since the idea is to boost the nutritional content of white rice in this recipe as well as give steamed rice some sweetness. You can alternately use combination white rice and brown rice as well. I normally cook all my rice recipes using my electric rice cooker.