Usukuchi Soy Sauce (“Usukuchi shoyu”) is a Japanese soy sauce.
It is lighter in colour than other soy sauce, but not lighter in taste — it’s actually saltier than regular soy sauce (which is referred to as “koikuchi shoyu.”)
It is made from equal amounts of soybeans and wheat. The wheat isn’t roasted as deeply as it is for regular soy sauce.
During and after simmering of the soybean and roasted wheat, the water is changed. A different strain of Aspergillus mould than is used for regular soy sauce is added to the mash, along with more salt and water than for regular soy sauce. It’s also fermented for less time. Before the mash is pressed, amazake is added to help balance the salty taste. After pressing, it’s pasteurized for at 140 F (60 C) for 25 minutes.
It turns out a light-amber colour.
It is preferred in the Kansai region of Japan.
Usukuchi shoyu is not used as a dipping sauce as regular soy sauce is, because of its saltiness. It is useful as an ingredient in dishes where you wouldn’t want the dark colouring of regular soy sauce.
Reputedly, Usukuchi was first made in Tatsuno, east of Kyoto, in 1666, but it appears that large-scale commercial production only began between 1809 and 1830 in the city. Production is now estimated to have been at the time 36,000 litres a year.
The Japanese government ordered production of it stopped during World War II and directed that regular soy sauce only be produced, for economy of scale.
“Usukuchi” means literally “pale mouth.”