Hot and Sour Soup (Suan La Tang)

Variations

• Suan la fen: the same broth poured over sweet potato starch noodles as a meal in itself, sold on the street in Chongqing and Xi'an. The broth is already thick, so it clings to the noodles.
• The American-Chinese version (photographed): much more starch, less vinegar, and chilli oil over the top. The photo here is that kind. This recipe follows the mainland balance, where the sour side stands further forward.
• Vegetarian: leave the pork out, double the shiitake, and build the broth on kombu and dried mushroom. Mapo tofu makes the same move, using meat as an aroma rather than as a body.
• Hotter: add more white pepper. Chilli oil adds colour and fragrance, but the heat in this soup belongs to the pepper.
• Without the egg: it still works. What is left is body and sourness, and it reads much lighter.

Tips

• The dish is the order of the last three minutes. Starch, then egg, then vinegar and pepper. Swap any two and you have made something else. The three notes below are why.
• Why the starch goes in before the egg. Thickening a sauce records that swollen starch granules get between the egg proteins and raise the egg's setting temperature. Egg poured into a thickened broth therefore sets slowly, and the broth is heavy enough that it does not sink while it does. Poured into a clear broth it seizes where it lands and drops — which is what tteokguk is describing when it drizzles egg into the boiling soup "for a softer finish". That gives clouds, not threads.
• Slake the starch cold and let it boil once. The same guide's base rule: dry starch tipped into hot liquid gelatinises on the outside first and seals dry powder inside a lump.
• The vinegar goes in off the heat. Boiled, its aroma leaves and a flat sourness is all that stays. This looks like the opposite of collagen and time's "acid early", but the subject is different: there the acid has meat to work on, and here nothing in the pot is waiting to soften, so the acid's only job is flavour. Chilli con carne writes the same distinction from the bean side.
• The pepper is also last, for a different reason. Cacio e pepe notes that in a sauce with no fat phase the water is what carries pepper's aroma. This broth has almost no fat either, so the condition is the same — but unlike a pan of pasta it simmers for a while. The pepper therefore goes in where there is no time left for that aroma to leave again.
• Chilli and heat does not transfer whole. That page is about capsaicin, which does not cook off, so it says the surest control is keeping it out of the pot. The heat here is pepper's, and pepper can be added later. Tasting and then raising the heat is available in this dish.
• Season hard before the vinegar arrives. As balancing flavour puts it with its dials, sourness covers salt. Add salt after the vinegar and you are late: you end up with too much of both.
• Matching the cut is what sets the cooking time. Nothing in this pot needs to soften. Everything is done in minutes, so uneven shreds mean some of it raw and some of it collapsed.
• Wood ear is in the bowl for the sound, not the taste. Staying crisp after soaking is its whole job, and that crunch between soft tofu and fibrous bamboo shoot is the frame the broth hangs on.