Dark soy sauce, also called lao chou in China and koi-kuchi shoyu in Japan, is a version of soy sauce that is aged longer than the light — or sheng — variety, giving it a more robust flavor, thicker body and darker color. A certain amount of caramel and molasses is added to dark soy sauce, balancing the inherent saltiness and giving it a milder and richer flavor overall. It is used very commonly throughout much of Asia but is particularly popular in China and Japan, earning note as the dominant type of soy sauce sold commercially in Japan. Depending on the country in which dark soy sauce is produced, it can have a unique and distinct taste, from being very sweet to having a mushroom flavor or cornstarch mixed in. For the most part, dark soy sauce is used in cooked meals and is added early in the dish to allow the flavors to develop, although it can be used in uncooked dips or other presentations.
The production of dark soy sauce begins just like the production of light, or regular, soy sauce, with the fermentation of soy beans. The difference occurs when the beans are allowed to ferment for a longer period of time. Additionally, caramel can be added to help counter the saltiness, and molasses is added to thicken the consistency. In Japan, the soy sauce is actually sweeter than it is in China. The Indonesian variety has palm sugar added, so much so that some mixtures are nearly half palm sugar. Cornstarch or other thickeners also can be added to the sauce so it becomes even denser, in which case the sauce can be called thick soy sauce, although it is different than the condiment of the same name.
One of the most prominent attributes of dark soy sauce is its deep color. When added in moderation, it can darken the color of light foods such as rice. If it is added in large quantities, then it can make all of the food in a dish a very deep, almost black color. The taste of the dark sauce is generally very balanced, rich and savory, with the heat from cooking allowing its sugars to develop. If used in a large volume or cooked for too long, however, the sauce can take on an undesirable bitterness.
In recipes, dark soy sauce can be added early on as a flavoring or coloring for the dish. It also can be used as a marinade for vegetables or meat. In countries such as Indonesia, it is commonly seen as an element in dipping sauces for grilled or fried foods. Dark soy sauce also is an important ingredient in recipes for Shanghai red-cooked recipes.