Sushi did not become the staple food we eat today, with its vinegared rice and toppings eaten together known as hayazushi (fast sushi), until the mid-Edo period. Prior to this, sushi was usually fermented using lactic acid fermentation. As fermented sushi evolved from honnare (fully fermented sushi), to namanare (partially fermented sushi), to kairyō namanare (improved partially fermented sushi), the fermentation period became shorter. From namanare onwards, fermented sushi went from being simply preserved fish to a dish in which fish and rice are eaten together. Sushi then evolved into hayazushi, and in the late-Edo period, nigirizushi (hand-pressed sushi) was invented in the city of Edo (Tokyo). One of the things that supported the development of nigirizushi culture was red vinegar (aka-su) developed in Handa Bishu (located in modern day Aichi). Red vinegar gets its name from its red color and is made from aged sake lees.
The basic role of acid in sushi is to improve preservation. The antibacterial power of lactic acid in fermented sushi and acetic acid in hayazushi enhance the preservation of sushi. Nowadays, vinegar also plays an important role in improving the taste of sushi.
Sushi has developed greatly in Japan from its origins in Southeast Asia. It has changed according to the environment and time period, and continues to evolve.
