What is the secret to surviving the heat of a Tokyo summer? For some it's ice cream or beer. Traditionalists might say broiled eel or tokoroten jelly. But there are a growing number of people for whom the answer is vinegar; not any old acetic acid, but the molasses-black vinegar known as kurozu.
Traditionally, kurozu is brewed in large ceramic ewers left to age for a year or more. The only ingredients are water and brown rice or wheat that has been inoculated with the magic mold spores known as koji-kin, the secret ingredient also used in producing sake, soy sauce, miso and other traditional Japanese fermented foods.
The resulting vinegar is nothing like the light, water-clear product used by sushi chefs and in most Japanese cooking. Instead, it has a dark-brown color and an appetizing fragrance in which the acetic sourness is balanced by a rich undercurrent of flavor.