Doufu ru (腐乳, literally fermented tofu, also known as Chinese fermented bean curd or Chinese cheese) is produced by inoculating cubes of fresh tofu with a mold (Actinomucor elegans or Mucor sufu), allowing the mold to grow on the surface for a few days, then submerging the mold-coated cubes in a brine of rice wine, salt, and various spice combinations for 1-6 months. The mold's enzymes slowly break down the tofu proteins into amino acids and peptides (umami compounds) and convert the fats into aromatic fatty acids, producing a soft, intensely flavoured, complex product. Two principal types: white doufu ru (bai doufu ru, 白腐乳) — mild, creamy, used in braised dishes and marinades; red doufu ru (nan ru, 南乳, or hong doufu ru, 红腐乳) — coloured and flavoured with red yeast rice (hong qu, 红曲), distinctly earthier and more complex.
Using white doufu ru in cooking: The primary function is umami enrichment in marinades and braises. 1-2 cubes of doufu ru, mashed, added to the marinade for Cantonese roast pork (char siu) or to the braising liquid for clay pot tofu. The fermented tofu breaks down completely during cooking and disappears into the sauce, leaving its complex savouriness. Using red doufu ru (nan ru): The defining ingredient in certain Cantonese preparations — most notably the red-braised pork belly (hong shao rou) in the Cantonese style (different from the Hunan or Jiangnan versions), and in certain dim sum dipping sauces for white cut chicken. Serving as a condiment: Doufu ru is also served directly as a condiment alongside plain congee — the intensely flavoured fermented cube is eaten in tiny amounts alongside the bland rice porridge.
Fuchsia Dunlop, Every Grain of Rice (2012); Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)