Open a bottle of fish sauce and the smell that emerges — sharp, pungent, marine, and unmistakably animal — sends a clear biological signal: something here has fermented, broken down, passed beyond its prime. Every instinct says to put it away. But add a small amount to a stir-fry, a braise, a dressing, or a broth, and the dish becomes something it was not before. Deeper. More complete. More satisfying in ways difficult to locate.
This contradiction — repellent raw, transformative in use — is not an accident of culture or acquired taste. It is chemistry. The smell and the flavour of fish sauce come from different compounds, operating through different pathways, and responding very differently to heat. Understanding the gap between them is understanding why this ingredient sits at the foundation of Thai, Vietnamese, Isaan, and many other Southeast Asian cuisines — and why the Romans made something nearly identical, called garum, and poured it over nearly everything they ate.
The smell signals decomposition. The taste delivers depth. They are produced by different molecules, and heat eliminates one while preserving the other.
Volatile compounds and the signal of fermentation
The pungency of fish sauce comes primarily from a group of volatile organic compounds produced during fermentation — chiefly short-chain fatty acids including butyric acid and propionic acid, alongside various nitrogen-containing compounds such as trimethylamine and indole. These molecules are lightweight and easily airborne, which is why they reach the nose before anything else does.
Many of these compounds are the same ones the human nose has evolved to detect at low concentrations as warning signals — they are produced by the breakdown of proteins in ways that can indicate spoilage. The brain receives the signal and prepares for something that might be harmful. The instinct is rational. The conclusion, in this case, is wrong.
Because fish sauce is not spoiled. It is fermented — a controlled process in which small fish (typically anchovies) are layered with salt at a ratio of roughly 3:1 and left for six to eighteen months in large vessels. The salt concentration is high enough to prevent the growth of harmful bacteria while allowing specific halophilic (salt-loving) microorganisms and the fish's own enzymes to break down proteins and fats into their component amino acids and fatty acids. The volatile compounds are byproducts of this process. The glutamates are the primary product.
Glutamate and the umami that heat reveals
While the volatile compounds produce the smell, the dominant taste compounds in fish sauce are glutamates — specifically free glutamic acid, released as proteins break down during fermentation. Fish sauce is, in this sense, a highly concentrated delivery system for umami. The longer the fermentation, the higher the free glutamate content, and the more intense and round the flavour becomes.
When fish sauce is added to a hot pan, something important happens: the volatile aromatic compounds — the ones responsible for the pungent smell — evaporate almost immediately. They are lightweight molecules with low boiling points, and the heat drives them off within seconds. What remains in the pan are the non-volatile compounds: the glutamates, the salts, and the deeply savory residue of fermentation. The smell disappears. The flavour intensifies.
This is why experienced cooks in Vietnamese and Thai kitchens add fish sauce early in the cooking process rather than at the end. Adding it at the beginning allows the volatiles to cook off and the glutamates to caramelise slightly against the hot surface, deepening their flavour further. Adding it at the end — as one might add lemon juice or a final seasoning — preserves some of the raw pungency, which can be useful in dressings and sauces served cold, where the smell is part of the intended experience.
The parallel with aged Parmesan and miso is instructive. All three are fermented protein products. All three produce high concentrations of glutamate. All three carry aromatic compounds that some people find initially off-putting. All three, when used correctly, produce the deepest savory flavour available to a cook.