Dark soy sauce, also known as lao chou in China and koi-kuchi shoyu in Japan, is a type of soy sauce that has been aged longer than light soy sauce, giving it a richer flavor, thicker body, and darker color. Dark soy sauce is given a milder and richer flavor by adding a small amount of caramel and molasses to balance out the inherent saltiness. It is widely used throughout much of Asia, but it is especially popular in China and Japan, where it is the most common type of soy sauce sold commercially. Dark soy sauce can have a unique and distinct flavor depending on the country in which it is produced, ranging from very sweet to having a mushroom flavor or cornstarch mixed in. Dark soy sauce is typically used in cooked meals and added early in the cooking process to allow the flavors to develop, but it can also be used in uncooked dips and other presentations.
Dark soy sauce is made in the same way that light, or regular, soy sauce is made, with the fermentation of soy beans. When the beans are fermented for a longer period of time, the difference becomes apparent. Caramel can be added to help balance out the saltiness, and molasses can be used to thicken the sauce. Soy sauce in Japan is actually sweeter than soy sauce in China. Palm sugar is added to the Indonesian variety, to the point where some mixtures are nearly half palm sugar. Cornstarch or other thickeners can be added to the sauce to make it even denser, in which case it is referred to as thick soy sauce, despite the fact that it is not the same as the condiment of the same name.
The deep color of dark soy sauce is one of its most distinguishing features. It can darken the color of light foods like rice when used in moderation. It can turn all of the food in a dish a very dark, almost black color if used in large quantities. The dark sauce has a well-balanced, rich, and savory flavor, thanks to the heat from cooking, which allows the sugars in the sauce to develop. However, if used in large quantities or cooked for too long, the sauce can develop an unpleasant bitterness.
Dark soy sauce can be used as a flavoring or coloring ingredient in recipes. It can also be used as a marinade for meat or vegetables. It is commonly used in dipping sauces for grilled or fried foods in countries such as Indonesia. In Shanghai red-cooked recipes, dark soy sauce is also an important ingredient.