Tom yum (tom = boiled, yum = mixed, as in the tossed salad preparations) is central Thai in origin, and its combination of lemongrass, galangal, and kaffir lime in a clear broth is possibly the most internationally recognised Thai flavour profile. Thompson treats the soup as a straightforward preparation that requires quality ingredients rather than complex technique — the test of the cook is in the freshness of the prawns, the quality of the lemongrass, and the precision of the final seasoning.
A hot, clear, aromatic broth — not a coconut milk soup but a broth — of lemongrass, galangal, kaffir lime leaves, and lime juice, with prawns and mushrooms, and a finish of fish sauce and fresh chillies. Tom yum's character is entirely in the broth's aromatic quality and its balance between hot (chilli) and sour (lime) against the prawn's sweet, marine depth. The broth is thin, clear, and brilliant — not thickened, not enriched, not complicated. Its flavour comes from the quality of the aromatics and the freshness of the prawns, and nothing else.
Tom yum's aromatic architecture operates entirely through the volatile aromatic compounds of the three key herbs — lemongrass, galangal, kaffir lime leaf — all released into a broth that has no fat phase to carry them and therefore presents them as pure, volatile top notes rather than fat-dissolved base notes. As Segnit notes, lemongrass and lime is a pairing of complementary citrus compounds: both citral (lemongrass) and limonene-citral (kaffir lime) belong to the terpenoid family, while lime juice's citric acid provides an entirely different (non-aromatic) sour note at the palate level. The combination creates a three-level citrus experience: aromatic from lemongrass (volatile top note), aromatic from kaffir lime (volatile mid note), and physical sourness from lime juice (non-volatile, palate-level).
**Ingredient precision:** - Prawns (kung): large, head-on fresh prawns. The heads contain the tomalley (hepatopancreas) and the fat of the prawn — they are the flavour foundation of the broth. Split the heads and press them into the broth. Medium prawns in a head-off prawn soup produce a broth of far less depth. - Lemongrass: 2–3 stalks, bruised with the back of a knife and cut into 4cm pieces. The bruising (rather than slicing) releases the aromatic oils while keeping the fibres intact — in a broth, the lemongrass is not eaten, merely perfuming. - Galangal: 4–5 slices, bruised. - Kaffir lime leaves: 4–5, torn in half (tearing releases more aromatic compound than leaving whole). - Mushrooms: straw mushrooms (hed fang) or oyster mushrooms — Thai straw mushrooms are traditional and have an earthy, slightly nutty flavour profile that complements the broth's aromatic base. - Bird's eye chillies: 5–10, lightly bruised (not chopped — bruised chillies release heat gradually into the broth rather than the sharp immediate spike of cut fresh chilli). - Fish sauce: for salt. - Lime juice: added off heat. - Nam prik pao (roasted chilli paste): the tomyum kung of Bangkok street food typically includes a spoonful of this dark, roasted chilli paste — it adds a smoky, slightly sweet depth and a slight thickening to the broth. For a cleaner, more northern-style soup: omit. 1. Bring good quality chicken or prawn stock to a simmer (or water for the purest version — Thompson specifies water in some iterations). 2. Add lemongrass, galangal, and kaffir lime leaves. Simmer for 5 minutes. 3. Add the prawns with their heads split open and pressed. Simmer very gently for 3–4 minutes — the prawns cook quickly and must not be overcooked. 4. Add mushrooms 2 minutes before the prawns are done. 5. Off heat: add lime juice, fish sauce. Taste and adjust across all four registers. 6. Add fresh chillies at this stage — off heat, so they contribute sharp heat rather than cooked heat. 7. Scatter fresh coriander if desired. Serve immediately. Decisive moment: The moment the prawns are correctly cooked — and the immediate removal of the soup from heat. The prawn cooks from translucent grey to opaque pink over 3–4 minutes at a gentle simmer. The instant the entire prawn is opaque throughout: the soup is done. Every additional 30 seconds tightens the prawn's protein structure and moves it from just-cooked to rubber. Taste the broth for seasoning the moment the prawns are done — adjust fish sauce and lime juice off heat — and serve immediately. Sensory tests: **Sight — the prawn doneness:** The prawn at the correct point: fully opaque throughout, curled to approximately a C-shape (indicating the protein has cooked and contracted, but not a tight O-shape which indicates overcooking). The tail end of the prawn colour should show an even pink-orange with no remaining grey. **Smell — the broth quality:** A correctly made tom yum, when the lid is removed at service, should produce an immediate aromatic release: lemongrass's citral, galangal's resinous depth, kaffir lime leaf's limonene, and the marine sweetness of the cooked prawn heads. This is one of the most complex aromatic moments in Thai cooking — all of these compounds are highly volatile and are released simultaneously from the hot broth. The absence of any component is immediately detectable. **Taste — the hot-sour balance:** Tom yum's balance is different from a curry's: here, hot (chilli) and sour (lime) are the primary registers, with the fish sauce providing the salt foundation and the palm sugar's sweetness at a lower level than in a curry. The broth should taste bright and sharp first, with the sustained chilli heat arriving immediately after. The galangal's slight medicinal bite and the kaffir lime's aromatic depth should be perceptible as a background complexity behind the bright top notes.
- The head-on prawn is not merely a romantic touch: the hepatopancreas and the prawn's head fat provide inosinic acid and glutamic acid compounds that are the entire depth of the broth. Without head-on prawns, the soup tastes bright but thin - Thompson's nam prik pao (roasted chilli paste) addition is the distinction between the hotel-style pale tom yum and the more complex Bangkok street food version — one tablespoon adds a smoky-sweet layer that transforms the preparation - The kaffir lime leaves, lemongrass, and galangal are not eaten — but they must be bruised rather than merely added. An un-bruised kaffir lime leaf sitting in a broth perfumes less than a torn one by approximately 40%
— **Rubbery prawns:** Overcooked. The timing is 3–4 minutes maximum at a gentle simmer — never a rolling boil. — **Flat broth without aromatic depth:** The lemongrass and galangal were not bruised (merely added whole) and did not release their aromatic oils into the broth. Bruise with intent — the back of a heavy knife, a firm strike. — **Over-acidic, lime-dominant result:** Lime juice was added while the broth was still boiling — the volatile citric acid evaporated and the non-volatile bitter citrus compounds remained, making the soup taste harsh rather than bright. Add lime only off heat.
David Thompson, *Thai Food* (2002); *Thai Street Food* (2010)