Why tom yum smells before you taste it

Why does tom yum reach the nose first? It’s boiled with lemongrass, galangal and lime leaves, all rich in fragrant oils. Those oils evaporate easily, so the steam carries them out of the pot. It’s how lemongrass oil is extracted, too: by boiling it in water. So the scent reaches the nose before the spoon reaches the mouth. Then it’s smelled again from inside the mouth, through the back of the nose. Since December 2024, tom yum kung has been on UNESCO’s heritage list – aroma and all. We tell you everything you need to know about Bangkok. Follow for more!

Tom yum is boiled with lemongrass, galangal and lime leaves, all rich in fragrant oils that evaporate easily. The steam carries them out of the pot, so the scent reaches the nose before the spoon reaches the mouth – and it is smelled again from inside the mouth.

• Tom yum kung (ต้มยำกุ้ง) is a prawn soup boiled with herbs including lemongrass, makrut (kaffir) lime leaves, galangal root and shallots; UNESCO’s listing notes its “recognizable aroma” (UNESCO).
• Lemongrass stalks, galangal and makrut lime leaves all carry essential oils: citral in lemongrass, 1,8-cineole in galangal, citronellal in the lime leaves (Buathong et al., 2023). Citrus leaves keep their oils in tiny oil glands (Wang et al., 2024).
• These oils are volatile, and the essential oils released while the soup is made give tom yum its fragrance (Sattayakhom et al., 2021; Setareh et al., 2023).
• Lemongrass oil itself is extracted by hydro-distillation: the leaves are boiled in water and the oil comes off with the steam (Setareh et al., 2023).
• Smells reach the nose by two routes: in through the nostrils, and from the mouth through the back of the nose (Furudono et al., 2013).
• Tom yum kung was inscribed on UNESCO’s Representative List of the Intangible Cultural Heritage of Humanity on 3 December 2024, at the Committee’s 19th session in Asunción, Paraguay (UNESCO; Thai Government Public Relations Department).

Behind the Dish
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Sources:
– UNESCO Intangible Cultural Heritage: Tomyum Kung (inscribed in 2024, 19.COM, on the Representative List of the Intangible Cultural Heritage of Humanity), nomination file No. 01879: https://ich.unesco.org/en/RL/tomyum-kung-01879
– Thai Government Public Relations Department (December 2024): Tom Yum Kung inscribed as UNESCO’s Intangible Cultural Heritage of Humanity: https://thailand.prd.go.th/en/content/category/detail/id/2078/iid/346057
– Buathong, Duangsrisai et al. (2023): Plant ingredients in Thai food: a well-rounded diet for natural bioactive associated with medicinal properties. PeerJ 11:e14568: https://pmc.ncbi.nlm.nih.gov/articles/PMC9985418/
– Wang, Ren et al. (2024): Molecular regulation of oil gland development and biosynthesis of essential oils in Citrus spp. Science 383(6683):659–666: https://pubmed.ncbi.nlm.nih.gov/38330135/
– Sattayakhom, Songsamoe et al. (2021): Effects of Thai Local Ingredient Odorants, Litsea cubeba and Garlic Essential Oils, on Brainwaves and Moods. Molecules 26(10):2939 (Walailak University): https://pmc.ncbi.nlm.nih.gov/articles/PMC8156645/
– Setareh, Mohammadi-Ghermezgoli et al. (2023): The effectiveness of hot-air, infrared and hybrid drying techniques for lemongrass: appearance acceptability, essential oil yield, and volatile compound preservation. Scientific Reports 13:18820: https://pubmed.ncbi.nlm.nih.gov/37914737/
– Furudono, Cruz and Lowe (2013): Glomerular input patterns in the mouse olfactory bulb evoked by retronasal odor stimuli. BMC Neuroscience 14:45 (Monell Chemical Senses Center): https://pmc.ncbi.nlm.nih.gov/articles/PMC3626767/

Facts and sources: https://soibysoi.com/episodes/why-tom-yum-smells-before-you-taste-it

Illustrations: the scent is drawn as small characters riding the steam, not to scale.
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