Does a fishy smell mean my fish sauce has gone off?

Not by itself. Fishy and meaty notes led the aroma of every normal sample in the research. If the smell clearly changes from what the bottle was like, a film forms, or the bottle bulges, do not use it.

Why does every bottle smell a little different?

Fish source, species, fishing season, type of salt, fish-to-salt ratio and fermentation conditions all play a part. The 2025 study showed that the catch area alone changes the aroma, which is why producers have experts blend tanks to keep the smell consistent.

Is trimethylamine the main fishy smell in fish sauce?

Older work leaned on it heavily. Thai studies from 2015 and 2025 found acids, aldehydes and sulfur compounds at least as important. The 2025 study did not detect trimethylamine at all, probably because it evaporated during extraction.

What makes fish sauce smell: Thai research on its key odour compounds

The smell of fish sauce does not come from one compound. It is a blend of dozens. Thai researchers used an instrument that separates volatile compounds while a trained person sniffs the outlet, picking the aroma apart one compound at a time. They found that even when every production step was identical, fish sauce made from anchovies caught on different coasts smelled different enough for a trained panel to tell apart.

This is a deep dive into where the “fishy”, “meaty” and “cheesy” notes in fish sauce (nam pla, น้ำปลา) come from.

How Thai fish sauce is made, briefly

Traditional Thai fish sauce uses anchovies (Stolephorus spp., pla katak, ปลากะตัก) mixed with salt at roughly 2-3 parts fish to 1 part salt. It ferments naturally for 12-18 months, then the clear liquid is drawn off and ripened for another 2-12 weeks. During fermentation, enzymes in the fish and microbes break down protein and fat into both umami taste and aroma compounds.

The industrial problem is consistency. A producer needs a lot of fish from many sources, and each tank comes out smelling slightly different, so experienced blenders mix tanks to keep each batch smelling the same.

The experiment: three seas, one method

A team led by King Mongkut’s University of Technology Thonburi (KMUTT), with Chulalongkorn and Suan Dusit universities, published in 2025. They used anchovies caught in April 2022 from three areas:

West (WT): Prachuap Khiri Khan
East (ET): Trat
South (ST): Ranong, on the Andaman coast

All fish were sorted to under 12 cm, firm and fresh-smelling, with protein of 18.88-19.84% and histamine (a freshness indicator) of 16.7-36.4 ppm. Each batch was fermented the same way: 10,000 kg of fish to 5,000 kg of sea salt in a tank 2.5 m across and 2.5 m deep, at 28-35°C, for 18 months.

The finished sauces were nearly identical on paper: protein 11.65-12.31%, salt 22.95-23.16%, pH 5.27-5.66. The aromas were not.

What the panel smelled

Eight panellists with more than 20 hours of training rated six odours on a 0 (none) to 5 (very strong) scale, after sniffing reference compounds for each.

Sample	Anchovy source	Fishy	Meaty	Character
ET	Trat	Highest, above 4/5	Lowest, about 2/5	Fishy-led; several high-FD sulfur compounds
WT	Prachuap Khiri Khan	Lowest	Highest, about 3.5/5	Meaty-led
ST	Ranong	In between	About 2.5/5	Between the other two

Fishy and meaty led in every sample, followed by sweet, cheesy, sour and musty. Those last four differed only slightly, not enough to be statistically significant.

Same method for every batch, then each compound sniffed until dilution hides it

Sniffing one compound at a time with GC-O

The method is GC-O (gas chromatography-olfactometry). The instrument separates volatile compounds one by one over time; one end goes to a detector, the other to a human nose. A trained sniffer notes the second each odour appears and what it smells like. The team combined this with AEDA (aroma extract dilution analysis): the extract is diluted by half again and again and sniffed each time. A compound that can still be smelled after heavy dilution is likely to matter to the overall aroma.

Worked example: reading FD factors

The extract was diluted 1:2, 1:4, 1:8 and so on to 1:1024, ten steps, each half as strong as the one before.

Butanoic acid (cheesy, sweaty) could still be smelled at 1:1024 in all three sauces, giving an FD factor above 1024, the top of the range.
3-(Methylthio)propanal (meaty, cabbage-like) scored FD 512 in WT, above 1024 in ET and 128 in ST.
Comparing WT and ST: 512 ÷ 128 = 4, which is two dilution steps further for WT.

The catch: an FD factor is not an amount. It tells you how dilute that extract could get before the nose lost the compound. It is a screening tool for which compounds are probably important.

Of 68 odour compounds detected, 18 reached an FD factor of 128 or more in at least one sauce, and 16 of those 18 had different FD factors depending on where the anchovies came from.

Which compound smells of what

The standout compounds the team confirmed against pure standards fall into a few groups:

Acids: acetic acid, vinegar-like (FD 128-512); propanoic acid, sour and sweaty (FD 256 in ET); butanoic acid, cheesy
Aldehydes: 3-(methylthio)propanal, meaty; phenylacetaldehyde, flowery and honey-like (FD 256-512)
Sulfur compounds: ET had six with high FD factors, some fishy and some sulfurous, which the authors suggest may explain why ET smelled fishiest
Nitrogen compounds: such as 1-pyrroline, fishy with a cooked-rice note (FD 256-512)

Faecal-smelling indole and skatole were present, but with very low FD factors (1-8), so they played little part in these samples.

Why would fish from different seas differ when protein and freshness matched? The authors suggest the catch area may bring a different mix of species, amino acids, fish enzymes and microbes, none of which standard industry checks measure.

Where two studies disagree

A 2015 study by Thai researchers with the University of Illinois looked at premium Thai fish sauce, using both solvent extraction and sniffing the air above the sample (headspace). Six compounds had an odour activity value above 500: methanethiol, 2-methylpropanal, 3-methylbutanal, dimethyl trisulfide, 3-(methylthio)propanal and butanoic acid. Recombination and omission tests confirmed that acids, aldehydes and sulfur compounds shape the overall aroma.

The 2025 study confirmed five compounds that match the 2015 work: acetic, propanoic and butanoic acids, 3-(methylthio)propanal and phenylacetaldehyde. But there are clear differences.

Point	2015 (premium fish sauce)	2025 (anchovies from three seas)
Dimethyl trisulfide	Prominent, OAV above 500	FD of only 1, in one sample
Trimethylamine (salted-fish note)	Reported by earlier work	Not detected
Extraction	Solvent + headspace	Solvent + SAFE

The 2025 team explains that very volatile compounds such as trimethylamine may be lost during extraction and concentration, or the samples may simply differ. The safe conclusion: which compound ranks highest depends on how you measure, so no single compound should be called “the smell of fish sauce”.

Can the aroma be steered?

A 2016 Thai study from Rajamangala University of Technology Thanyaburi and Panyapiwat Institute of Management added two salt-tolerant enzymes during fermentation: 5´-phosphodiesterase from Bacillus pumilus at 1.5% and lipase from Halobacterium salinarium at 1.0% (weight for weight). The treated sauce had more 5´-GMP (an umami enhancer) and more volatile fatty acids than untreated sauce. The authors see it as a way to improve aroma and flavour, but it is experimental work.

Common mistakes

Assuming fishy means spoiled. Fishy is the main note of normal fish sauce. What to watch for is a change from how the bottle used to smell.
Blaming it all on trimethylamine. Both Thai studies point to acids, aldehydes and sulfur compounds as major contributors.
Reading FD factors as amounts. A high FD means the nose detects it even when very dilute, not that there is the most of it.
Calling one bottle better because it smells different. Aroma can differ from the catch area alone; it says nothing by itself about quality or safety.

Summary

Fish sauce aroma is a blend of dozens of compounds, led by fishy and meaty notes with sweet, cheesy, sour and musty behind them. The 2025 Thai study showed that anchovies from Trat, Prachuap Khiri Khan and Ranong, fermented the same way, still produce different aromas, while which compound matters most still depends on the method used. For the practical side of fish sauce at the table, see the four table condiments.

Sources

Ingredients and heat vary by kitchen and recipe. If you have a food allergy, ask before you order, every time.