Central Thai — though versions of the hot-sour prawn soup appear throughout Thailand, the Central Thai version with galangal-lemongrass-kaffir lime leaf is the canonical form
Tom yum kung is the most internationally recognised Thai soup and consequently the most frequently mis-executed. The broth is hot-sour-aromatic, not creamy — 'tom yum nam khon' (creamy, with evaporated milk or coconut milk) is a variation, not the original. The base is a clear broth built by simmering galangal, lemongrass, kaffir lime leaves, and sometimes crushed chilli in pork or chicken stock until fragrant, then seasoning aggressively with fish sauce and fresh lime juice added off heat. The prawns (or mushrooms, tofu, or mixed seafood) go in at the last moment. The critical rule: lime juice is added after the pot is off the heat — boiled lime juice becomes bitter and loses its fragrance.
Tom yum's sensory impact is immediate and multi-dimensional: the lime hits the sides of the mouth, the chilli warms the back of the throat, the galangal provides a piney top note, and the fish sauce binds everything into a coherent broth. The prawn is almost incidental to the flavour architecture.
{"Build the aromatic broth first — simmer galangal, lemongrass, kaffir lime leaves 5–8 minutes before adding anything else","Prawns added to simmering (not boiling) broth — they need 2–3 minutes maximum","Fish sauce added while cooking; lime juice ONLY after removing from heat","Nam prik pao (roasted chilli paste) is optional for the standard version; essential for tom yum nam khon","Bird's eye chillies — bruised, not sliced — for heat that blooms slowly rather than immediately"}
The restaurant technique for high-volume service: pre-make the aromatic broth and hold it at 80°C; cook prawns to order in 90 seconds of simmering; season and add lime off heat in the bowl or just before ladling. This produces a perfectly fresh tom yum every time without the degradation that comes from holding seasoned broth.
{"Adding lime juice while the broth is still on heat — citric acid breaks down under prolonged heat and becomes flat and bitter","Over-cooking prawns — they should be just-cooked and curl gently, not tight spirals","Using bottled lime juice — lacks the volatile aromatic oils that make fresh lime essential","Making the broth too thin by not properly infusing the aromatics in the first stage"}