Douzhi

Byproduct of mung-bean fensi (vermicelli)/starch extraction; recorded as a folk food in the Beijing area since the Liao–Song period, drunk by the poor; promoted to imperial breakfast under Qianlong (Qing court records, 1753). The Huguosi Snack Shop and Jin Shengyong (锦馨) in Tianqiao are the surviving Beijing benchmarks.

Wang Zengqi wrote that not drinking douzhi means not understanding Beijing. The 1894 Tianqiao stalls priced it at 2 wen a bowl. Liang Shiqiu's Yashe Tan Chi describes it as the litmus test for Beijing locals — outsiders gag, locals queue.

Cloudy gray-green, smells like sour socks to outsiders — fermented mung-bean funk with a sharp acetic bite. Taste is thin, sour, slightly sweet at the back. Drunk warm in a thin bowl, dunk a fried jiaoquan (焦圈) ring, eat with shredded pickled mustard.

Mung-bean starch slurry left to lacto-ferment 24-48 hours at room temp; the supernatant after starch settles is the douzhi. Lactobacillus and acetic acid bacteria drive the funk. Reheat just to steaming — boiling makes the proteins curdle and separate, ruining the drink. 麻豆腐 (the second product) is not made this way: it is the fibrous mung-bean residue filtered out before the liquid douzhi is drawn off, then stir-fried.