Primarily used in the Kansai region of Japan, usukuchi has a lighter colour than koikuchi, and is the second most popular shoyu in the country. Despite its light colour, usukuchi is actually saltier (18%-19% salt volume) than its ubiquitous counterpart (16% salt volume). Usukuchi is also made with soybeans and roasted wheat, except the latter is lightly roasted so as to keep the flavour mild. Plus, it boasts the addition of mirin, a sweet rice wine that gives the sauce its slightly sweet flavour. In terms of production methods, usukuchi has a relatively shorter fermentation window, which adds to its unique mellowness.
Historically, usukuchi was used in refined imperial court cooking – koikuchi was deemed a common condiment for the masses at the time. Today, because of its delicate flavour and light colour, usukuchi is often the shoyu of choice when it comes to preserving the natural colours of ingredients (koikuchi darkens the colours), like those in braised dishes and soups.
A common misconception about usukuchi lies in its label. Known as light soy sauce, particularly in the West, usukuchi is often mistaken to be a healthier alternative to koikuchi. The word glighth, however, merely refers to the colour and nothing else!