Material guide: methyl mercaptan — an odour threshold of 0.02 ppb in water, and the main component of bad breath

About a 5 minute read.

The short version

• Name: methyl mercaptan, also methanethiol, CAS 74-93-1, FEMA 2716
• Formula CH₄S, molecular weight 48.1 — among the smallest aroma molecules in this database
• Boiling point 6.1 °C: a gas at room temperature; vapour pressure 1,901 mmHg (about 2.5 atmospheres)
• Odour threshold in water 0.02 ppb; flavour threshold 2 ppb
• Odour type: sulfurous; odour: cabbage, garlic, vegetable, oily, alliaceous, eggy, creamy, savory
• Suppliers: 50; normal use level in finished product 0.005–1 ppm

It is a gas

The appearance field: "colorless gas or liquid below 6 °C."

A boiling point of 6.1 °C means that unless your bench is inside a refrigerator, this does not sit quietly in a bottle. A vapour pressure of 1,901 mmHg is about 2.5 atmospheres — it leaves the moment you open it.

That explains the 0.01% evaluation concentration. As I measured last time, the database's evaluation concentrations occupy five decade rungs, and the 0.01% rung holds 107 records with sulfurous accounting for 29 of them. This is one of them.

What 0.02 ppb means

A specification sits inside the odor_descriptions field:

boiled cabbage. Odour threshold in water: 0.02 ppb.

0.02 ppb is one part in fifty billion. Put another way: twenty micrograms in a tonne of water and you can smell it.

And taste_descriptions gives a second number:

Flavour threshold in water: 2 ppb. Used occasionally in onion, garlic, vegetable and meat flavours. Normal use levels in finished consumer product: 0.005–1 ppm. Council of Europe limits: Foods (1 ppm); Beverages (1 ppm).

Odour 0.02 ppb, flavour 2 ppb — a hundredfold gap.

You smell it first and taste it later. And that gap is exactly what makes it hard to formulate with: by the time you have added enough to taste, your nose has had enough.

(The lower use level, 0.005 ppm, is 5 ppb — 250× the odour threshold and 2.5× the flavour threshold. The entire working range lives there.)

Two evaluations in one field disagree

odor_descriptions holds five entries; two of them state different concentrations:

Concentration	Description
0.01% (in propylene glycol)	decomposing cabbage, garlic
1.00%	vegetable oil, alliaceous, eggy, creamy with savory nuances (Mosciano, P&F 23(6):31, 1998)

The more dilute reading is the less pleasant one.

That runs against the usual expectation — most materials get prettier as you dilute them. I cannot resolve it from the fields: it could be two evaluators' vocabularies, it could be the solvent (one in propylene glycol, one unstated), or there could be a genuinely non-monotonic concentration effect.

I note it because it makes a point: align the concentrations before comparing evaluations — and even then they may not agree.

(I measured how the odour field's signatures are distributed earlier; disagreement between evaluators is normal in this database.)

It is in your mouth

What Stephen and colleagues did in the Journal of Periodontal Research in 2021: measured volatile sulphur compound concentrations in the oral cavity with a portable gas chromatograph, while collecting tongue scrapings and subgingival and interdental plaque for 16S sequencing.

Oral malodour is often observed in gingivitis and chronic periodontitis patients, and the tongue microbiota is thought to play a major role in malodorous gas production, including volatile sulphur compounds (VSCs) such as hydrogen sulphide (H₂S) and methanethiol (CH₃SH).

The sample: 22 healthy individuals, 14 with gingivitis, 15 with chronic periodontitis.

This material is one of the main components of bad breath, and it is also a FEMA GRAS flavouring.

Those are not in conflict. Concentration decides everything — 0.02 ppb is a faint savoury note; what bacteria in a periodontal pocket produce is orders of magnitude higher.

(The notes field adds more: it is an intermediate in jet fuels, pesticides, fungicides, plastics and methionine synthesis; "odor may cause nausea; narcotic in high concentrations; depresses urea biosynthesis.")

It is in nearly everything you eat

The occurrence field: red algae, asparagus, beef, wheat bread, cabbage, gruyere cheese, chicken, egg, roasted filbert, garlic, hop oil, kohlrabi stem, lamb, leek, milk, shiitake, onion, orange juice, pineapple, pork, potato, radish seed, shallot, wild strawberry, tea leaf.

Orange juice and wild strawberry on the same list. This field sweeps up everything anything has been detected in; it is not a list of sources you could buy from — the same point I made about isovaleric acid, whose list contained "dolphin fat."

And what Vichi and colleagues did in Food Chemistry in 2014 is the modern version of that same work: derivatisation followed by LC-HRMS on twenty-five virgin olive oils, in which methanethiol was identified in virgin olive oil for the first time, by non-target analysis.

"For the first time" is worth pausing on. Olive oil has been studied for centuries, and this molecule was only found in it in 2014 — because the amounts are low enough to need new analytical methods, while your nose had it all along.

How to use it

• It is a gas. Confirm what form you are buying. Most suppliers sell solutions.
• Evaluate at 0.01%, with ventilation.
• 0.005–1 ppm in the finished product; the Council of Europe limit is 1 ppm.
• Its job is the base of savoury, ripened cheese and allium notes, not a lead. The flavor field: sulfurous, alliaceous, creamy, cheesy, clean, savory, meaty.
• This is a flavour material. It has no place in perfume.
• The oral LD50 record is an unspecified mammal at 60,670 µg/kg (a 1972 Soviet journal); inhalation rat LC50 675 ppm.

What this doesn't establish

• I have not smelled it. This is fields, whole-database statistics, and two papers.
• 0.02 ppb and 2 ppb are numbers inside a field; I did not trace their sources or learn the measurement conditions (water temperature, method, panel size).
• Neither paper is fragrance research — Stephen is periodontology, Vichi is food analytical chemistry. I cite their account of this molecule's detection and origins.
• I have not explained the inverted evaluations. I listed three possibilities and confirmed none.
• The oral toxicity record does not name a species, its source is a 1972 journal, and I did not go to it.
• I checked no IFRA or other regulatory text (the Council of Europe's 1 ppm is quoted from the field).