Street food has been a centuries-old offering in Japan, a land where tradition matters, and charcoal-roasted sweet potatoes probably date back furthest as farmers hawked their crops to passing travelers. Sweet potatoes, or Satsuma-imo, are made into myriad of dishes, both sweet and savory, but the simplest way they are enjoyed is roasted over hot coals until their flesh is soft and tender and their natural sugars caramelize.
There are only a few tips to consider when roasting sweet potatoes. Older potatoes tend to be dry and mealy so select plump, fresh sweet potatoes for the best flavor. (Remember that yams have orange flesh while sweet potatoes tend to have white flesh.) The sweet potato variety available in Japan is a purple-skinned type with a nutty flavor. Although you can use the light-colored sweet potato type, it wonít be exactly the same as those sold in Japan. Wash them well, keeping them in their skins. Start a charcoal fire and then allow the flames to ebb so you can use the radiant heat from the coals. This will cook the insides before the outsides char. Place a lightly oiled-grill just about 6-8 inches above the coals. Arrange the whole, washed sweet potatoes on the grill, leaving at least 4-5 inches of space around each one. Close the grill with its lid. Let the sweet potatoes roast, turning every 10 minutes, until tender. Use a thin skewer or cake tester to pierce through the center to check for softness. There are too many variables to give you the cooking duration (size of sweet potato, intensity of heat/number of coals).
To enjoy the cooked spud, split it down the center and enjoy the fluffy interior that is so sweet from the caramelization process that no additional condiments are necessary. Yaki imo makes a great snack, light meal or okazu with other dishes.
To duplicate this process without a charcoal grill, you can use an oven but the results will be very different as hot coals impart a nice smoky flavor and turn the starchy sugars into a sweet syrup. However, you can simply bake the sweet potato as you would a regular baked potato by pricking the skin with a fork and baking in a 400 degree oven for about 45 minutes to an hour. Some people place the potatoes in aluminum foil but this steams the potato instead of baking/roasting it. You can dilute a drop or two of liquid smoke (mesquite or hickory) with some water and brush it onto the potatoes. Then, finish off the potatoes under a broiler to get some caramelization and char going on the outside.
It isnít the same, but it will suffice in a pinch when youíve got a craving for some yaki-imo.