Yakiimo

The sweet potato reached Japan from the Americas by way of China and the Ryukyus around the turn of the 17th century, and spread widely after Aoki Konyo promoted it as a famine crop in the 1730s; it is called satsumaimo ('Satsuma potato') after the southern Kyushu domain through which it entered the mainland. Roasting sweet potatoes over heat is old and has no single inventor, but the stone-roasting drum (ishiyaki-imo) became an Edo and then a winter-street institution, with vendors working from pushcarts and, later, small trucks that toured residential streets calling their wares. The familiar drawn-out cry and the kerosene-heated stone drum are 20th-century fixtures of the Japanese winter.

Split the skin and the flesh should be deep yellow to orange, moist and almost custard-like in the soggy-fleshed cultivars, steaming and fragrant. The taste is clean and honeyed, with no added sugar; a good one is sweet to the center, while an underdone or fast-cooked potato is starchy, dry, and flat. The blistered, faintly caramelized skin is edible and slightly bitter-sweet against the soft interior.

The sweetness is made during cooking, not present in the raw root. Sweet potatoes are rich in the enzyme beta-amylase, which breaks down starch into the sugar maltose — but only once the starch has gelatinized with heat and water, and only while the enzyme is still active. Beta-amylase works fastest with the core held around 60-70°C, and it is progressively destroyed above about 75°C. Slow roasting is therefore ideal: the core lingers a long time in that 60-70°C window, gelatinizing starch and converting it to maltose, so a stone-roasted potato comes out far sweeter than one blasted in a microwave or hot oven, where the center races past 75°C and deactivates the enzyme before much maltose can form. The bed of hot stones holds and radiates a steady, moderate heat that suits this slow conversion.