Black bean sauce, also known as douchi sauce, is a popular condiment in Chinese cuisine. It's made with fermented and salted black soybeans, which are used to give the sauce a strong, savory, and slightly bitter flavor. The beans are typically mashed and mixed with other ingredients such as garlic, ginger, sugar, and soy sauce to create a rich, thick sauce.

Black bean sauce is often used in stir-fry dishes, marinades, and as a base for other sauces. It's particularly known for its use in dishes such as mapo tofu, black bean chicken, and jjajangmyeon, a Korean-Chinese noodle dish. The unique taste of black bean sauce brings a depth of flavor to dishes that is often described as "umami," a fifth basic taste that is best described as savory.

Also known as Chinese fermented black beans, douchi is an indispensable part of Chinese cuisine. The black soybeans are heavily salted and fermented, resulting in an extremely salty ingredient with a robust and pungent flavor.

They are rarely eaten plain and are usually used in classic Chinese dishes as a condiment or a seasoning. Since their taste resembles a mix of soy and fish sauce, they are often added to provide a unique flavor to Chinese culinary classics. In the cooking process, they develop a slightly sweet taste, which is a perfect match for stir-fry dishes, rice, and various Chinese sauces. 

Furu or fermented bean curd is a traditional condiment made from soybeans. Soybeans are soaked, ground, and then boiled to create soy milk. Coagulants are added to the soy milk to curdle it, forming tofu, which is pressed into blocks and dried.

The semi-dry tofu blocks are then inoculated with a fermenting agent, which is often a type of mold, like Actinomucor elegans. The tofu is then left to ferment for days to months, depending on the desired flavor and texture. After fermentation, the tofu blocks are submerged in a mixture of salt, rice wine, and sometimes other seasonings and preservatives.

This mixture not only adds flavor but also helps preserve the tofu.