Narezushi

Narezushi is the earliest form of sushi, in which fish is fermented with salt and cooked rice for months to preserve it. Wikipedia documents it as the ancestor of all modern sushi. The technique originated in Southeast Asia and spread to Japan around the Yayoi period (roughly 300 BCE to 300 CE). During fermentation, the rice breaks down into lactic acid, which pickles and preserves the fish. The rice is discarded; only the fermented fish is eaten. Modern sushi — which uses vinegar to simulate the sourness of narezushi without the months-long wait — is a direct descendant. Funazushi from Shiga Prefecture, made with crucian carp, is the best-known surviving narezushi tradition.

A piece of narezushi — specifically funazushi — is an acquired taste: the fish, after months of fermentation, has a pungent, cheesy, intensely sour aroma unlike any other Japanese food. The flesh is firm and slightly translucent, with a complex tang from the lactic acid. Eaten in thin slices with sake, it is a taste of medieval Japan that survives in a single prefecture.

Narezushi rests on lactic acid fermentation. The fish is salted, then packed with cooked rice; lactobacilli naturally present on the rice grain convert the rice's carbohydrates into lactic acid over months. The acid lowers the pH to a level that inhibits spoilage organisms, preserving the fish; it also denatures the fish proteins, firming the flesh and developing the characteristic sour, pungent flavor. The rice, now mostly decomposed into sour paste, is traditionally discarded — only the fish is eaten. This is the opposite of modern sushi, where the rice is the primary food and vinegar substitutes for the lactic acid that once took months to develop.