Fermented black beans are one of the most popular seasonings in Chinese cooking. Not to be confused with the black turtle bean used in Latin American cooking, dòu chĭ is made from soybeans, and has such a powerful, piquant, salty flavor that it is used only in very small amounts to flavor dishes, and never—unlike other fermented soybean products such as Japanese natto—eaten as a food. So strong are fermented black beans that some cooks wash them before using them. They often appear as a leading sauce ingredient for stir-fried dishes, especially those featuring seafood such as clams and shrimp

In the sixth century, Jia Sixie, in his book Essential Skills for Common Folk, set out the medieval process for making fermented black beans in quantity. First, a straw-roofed hut was built in a shady spot in cool weather, and a pit several feet deep dug inside. 500-600L of beans were boiled just until swollen, and had a little give when pressed, then spread outside to cool. The beans were heaped in the pit, and twice a day someone crawled into the hut and checked the temperature of the pile by thrusting his arm all the way into it, to see if the beans were as warm as his armpit. If they were, he turned the pile, then sealed the opening up again to keep out insects and rodents. Soon the beans were covered with white mold, which then turned yellow, and the height of the pile decreased a bit with each turning. When the mold had done its job, the beans were moved outside again and the mold dispersed by fanning, after which the beans were rinsed to remove any remaining mold, and allowed to dry. Now the pit was lined with chaff and clean straw mats, and the beans piled back in, and stamped down, after which more mats and more chaff were pressed on top. After ten to fifteen days, depending on temperatures, the beans were ripe, strong-flavored, and almost black. They would keep for a year. This particular recipe was unsalted, but Jia mentions eating salted black beans (now the standard) as well. Apparently he was also slightly embarrassed about the homely subjects covered in his now 1500-year-old Essential Skills for Common Folk, worrying, “I hope that future readers will not laugh at me.”