One element that greatly affects the taste of sushi rice is the type of vinegar used to season it. Chefs use either rice vinegar (kome-zu), red vinegar (akazu), or a blend of the two when seasoning sushi rice.
Akazu (also called kasuzu - 粕酢) vinegar is made using sake lees (sake kasu - 酒粕) which is the solid left over from the sake production.
Aka-zu is typically aged for three to five years, it is more expensive to produce than rice vinegar. Its reddish-black color darkens the rice.
Akazu brings a deep, flavorful, refreshing umami flavour to sushi rice. Although red-rice vinegar is already a little bit sweet, this pre-made sushi vinegar has some sugar and salt added. It has aged for 3 years.