The original type of sushi, known today as nare-zushi (馴れ寿司, 熟寿司,) was first made in Southeast Asia. Fish was salted and wrapped in fermented rice, a traditional lacto-fermented rice dish. Narezushi was made of this gutted fish which was stored in fermented rice for months at a time for preservation. The fermentation of the rice prevented the fish from spoiling. The fermented rice was discarded and fish was the only part consumed. This early type of sushi became an important source of protein for the Japanese. The term sushi comes from an antiquated grammatical form no longer used in other contexts, and literally means “sour-tasting”, a reflection of its historic origin as a fermented food. The oldest form of sushi in Japan, narezushi, is still made by wrapping fish in soured fermenting rice, which causes the fish proteins to break down into their constituent amino acids. The fermenting rice and fish have both a sour and an umami taste.
Contemporary Japanese sushi has little resemblance to the traditional lacto-fermented rice dish. Originally, when the fermented fish was taken out of the rice, only the fish was consumed while the fermented rice was discarded. The strong-tasting and smelling funazushi, a kind of narezushi made near Lake Biwa in Japan, resembles the traditional fermented dish. Beginning in the Muromachi period (1336–1573) of Japan, vinegar was added to the mixture for better taste and preservation. The vinegar accentuated the rice’s sourness and was known to increase its shelf life, allowing the fermentation process to be shortened and eventually abandoned. In the following centuries, sushi in Osaka evolved into oshi-zushi. The seafood and rice were pressed using wooden (usually bamboo) molds. By the mid 18th century, this form of sushi had reached Edo (contemporary Tokyo).