Fermented Mung Bean Noodle Soup

Fermented mung bean noodle soup grew out of a frugal use of a by-product: when households ground mung beans or peas to settle out the starch (粉芡), the leftover whey would sour after a day or two, and rather than throw it away cooks used it to cook noodles. The dish is firmly associated with Luoyang, where the technique 老雒阳面食制作技艺, also called 浆面条制作技艺, is listed as a provincial-level intangible cultural heritage of the city's 西工 district. A local folk story attributes it to Liu Xiu, the Eastern Han founder, said to have been fed soured whey and dried noodles while fleeing his enemies, but this is told only as legend; the exact origin and date are unrecorded. A closely related Henan dish, 粉浆面条, is documented in nearby Fugou and Xihua counties.

The broth is the point: thick, pale, and milky, coating the noodles and clinging to the spoon, never a thin watery soup. The first taste is a soft lactic tartness — fresh and almost yoghurt-clean, not the sharp sting of vinegar — under a faint mealy sweetness from the beans. The noodles are slippery and a touch soft from simmering in starch; the diced celery snaps green against them, the soaked soybeans give a firm, nutty bite, and a spoon of chilli oil and chive-flower paste lifts the whole bowl. Done well it is glossy and integrated. Done badly the whey has 'broken' into watery liquid with curd-like flecks, or it has soured so far past its peak that the taste turns acrid and stale.

The whole dish rests on lactic fermentation and starch. Ground mung beans or peas left to stand are colonised mainly by lactic-acid bacteria and yeasts, which ferment the residual sugars and turn the whey sour over a day or two; that lactic acid, not vinegar, is what gives the clean tartness. The whey also still carries fine bean starch and protein in suspension. Heated toward boiling, at around 80°C the proteins begin to flocculate and a white foam rises; if left alone the whey curdles and 'breaks' (澥) into watery liquid and flecks, so it must be stirred constantly and the foam skimmed until the body turns smooth — a little sesame oil stirred in helps settle it and keeps it from boiling over. Once stabilised, the noodles are added and their starch, together with the suspended bean starch, gelatinises and thickens the broth into its signature milky body. The trick is heat control and an unbroken stir.