大学芋, the autumn street snack of Tokyo: wedges of Satsumaimo deep-fried to a crisp golden exterior, then tossed in a dark soy-inflected caramel that sets to a glossy, cracking shell scattered with black sesame seeds, sweet, salty, smoky, and impossible to stop eating
Origin: Satsuma Domain and the Edo Period Heartland, Japan
From the journey of Sweet Potato.
The name daigaku imo (大学芋, university sweet potato) is usually explained by a story set in the Taishō era (1912–26): students in the Hongo district of Tokyo, where Tokyo Imperial University stood, sold candied Satsumaimo on the streets to fund their studies, and the dish acquired its name from the concentration of customers and sellers around the university quarter. Whether the story is strictly historical or is the kind of explanatory folklore that attaches itself to beloved street foods is debatable; what is beyond doubt is that the dish was already established as a Tokyo autumn and winter snack by the early twentieth century and has never lost that association. The Satsumaimo (Ipomoea batatas var., named after the Satsuma Domain of Kyushu through which the sweet potato entered mainland Japan around 1700) has a skin of deep red-purple and a flesh that is pale yellow and notably dense and sweet. It is the preferred variety for daigaku imo because its sweetness and starchiness are precisely calibrated for the caramel coating: a wetter, less starchy sweet potato would not hold the caramel well, and a less sweet variety would be overshadowed by the sugar rather than enhanced by it. The soy sauce in the caramel (typically just a small amount, added at the end of the caramel-making process) is what distinguishes daigaku imo from its Chinese counterpart bā sī hóng shǔ and from the Korean goguma mattang: the soy adds a savoury depth and a faint bitterness that prevents the caramel from being merely sweet, and gives the dish its characteristic dark colour.
Satsumaimo (with its distinctive red-purple skin and dense, pale yellow flesh) is available from Japanese grocery shops and some Asian supermarkets year-round in Japan and in autumn elsewhere. Regular orange-fleshed sweet potato can be substituted but will produce a moister, less starchy result and the caramel may not adhere as firmly. The skin must be left on for authenticity; it becomes part of the finished texture. Daigaku imo is best eaten within an hour of making; the caramel softens on standing. Leftovers can be reheated briefly in a 200°C oven for 5 minutes to re-crisp the coating.
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