Not to be confused with the dark soy sauces of China or Japan, black soy sauce is a hyper-sweet condiment that hails from Thailand. Both salty and sweet, this dark and syrupy sauce is created by adding extra molasses or sugar to an already-dark variety of soy sauce. When added to a stir-fry sauce or curry, it helps chefs quickly obtain a diverse flavor profile.
A basic soy sauce, called shoyu, is made by fermenting soybeans, wheat, water and salt with a healthy enzyme agent called Aspergillus oryzae. The more wheat a sauce contains, the lighter, sweeter and less salty it typically will be. By contrast, less wheat usually means a darker and saltier sauce. When no wheat is used at all, this traditional soy sauce is called tamari.
When the darkest and sweetest soy sauce is desired, many chefs will turn to black soy sauce. To get it, some make their own by heating a dark soy sauce and dissolving molasses or palm sugar into it until left with the consistency of syrup. Others turn to a manufactured blend like those from the Dragon Fly® or Pearl River Bridge® brands. In 2011, the Koon Chun® brand even promises a double black soy sauce experience.
Using this type of sauce helps cooks create in their recipes what is known in many Asian cultures as umami. Originating from the Japanese way to say "delightful savory flavor," this culinary quest attempts to add a fifth category of flavor provided by the compound glutamate. This distinctive flavor bolsters the four main flavor groups: bitter, salty, sour and sweet. Many chefs would also add spicy to this list, for a total of six basic taste possibilities.
Using black soy sauce is one of several traditional ways that Thai cooks have attempted to quickly satisfy this diversity. Oyster or fish sauce is frequently used to add the same blend of sweet, salty and earthy umami to various dishes. Black soy is regularly substituted for these seafood sauces in vegetarian fare. Other common Thai sauce bases are tamarind or shrimp paste, chili jam or sauce, and coconut milk.
A close relative of black soy sauce is an Indonesian version called kecap manis. Imbued with the same salty and sweet flavor, this sauce is often sweetened with palm sugar to create a less-syrupy product. It is also frequently augmented by other ingredients like star anise and, more frequently, garlic.