Douchi (豆豉, fermented black beans) are soybeans that have been fermented in salt and dried, developing an intensely savoury, slightly pungent, deeply umami flavour. They are one of the oldest soy fermentation products in China — excavated douchi have been found in Han dynasty tombs. Douchi are a versatile seasoning: added whole to Sichuan preparations (mapo tofu, hui guo rou), mashed and fried as a flavouring base in black bean sauce, or used to make douchi chilli sauce. The most famous douchi is Yang Jiang douchi (阳江豆豉) from Guangdong — considered the finest quality, used in Cantonese stir-fries and black bean preparations.
Using douchi in cooking: Douchi are typically rinsed briefly to remove surface salt, then either used whole (added during the qiang guo aromatic stage) or roughly chopped (to release their interior more quickly). They should be fried in oil until fragrant — this activates their aromatic compounds. Douchi + garlic + ginger in oil is one of the classic Chinese aromatic bases, the foundation of black bean sauce. Making black bean sauce: 50g douchi, 3 tbsp neutral oil, 4 garlic cloves (finely chopped), 1 fresh chilli (finely chopped), 1 tbsp Shaoxing wine, 1 tbsp soy sauce, 1 tsp sugar. Rinse and roughly mash the douchi. Fry the garlic and chilli in oil briefly. Add the mashed douchi. Fry 2-3 minutes until fragrant. Add the soy, wine, and sugar. Cook 2 minutes more. This is the base black bean sauce used across Cantonese cooking.
Fuchsia Dunlop, Land of Fish and Rice (2016); Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)