isobutyric acid

sour acid Key food odorant

Found in 143 ingredients across the CompKitchen flavor graph.

Look up another compound — search by name, CAS number, PubChem CID, or FEMA #.

2D Lewis Structure

Formula: C4H8O2
Molecular weight: 88.11 g/mol
PubChem CID: 6590 — full record on PubChem

Key food odorant (individualist)

Dunkel et al. 2014

Identified in the meta-analysis of 227 food samples as one of 151 individualist odorants — appears in <5% of foods, carries the diagnostic signature of specific ingredients.

Source: Angew. Chem. Int. Ed. 2014, 53, 7124–7143

Properties

Odor description: rancid, butter, cheese
Odor threshold — orthonasal, in clean water: 3040 ppb – 8100 ppb median 5570 ppb · 2 published values · see all 11 with sources
Volatility: high
CAS number: 79-31-2
Retention index: 1563

Not in FEMA GRAS publications 1-33. Of 3,367 listed flavoring substances, none match this compound name. May still be food-safe under other regulatory routes (FDA 21 CFR 172, EFSA, JECFA) — check femaflavor.org for the authoritative current list.

How this molecule is described

Sources disagree about odour more often than they disagree about chemistry. Rather than show one answer, here is every description on file with how many ingredient records carry it.

Also filed as

2-methyl propanoic acid 2-methyl-propanoic acid 2-methylpropanoic acid

Same molecule, unified here — previously counted and displayed as separate compounds.

Descriptions on file

Ordered by how many independent descriptor streams give each wording, then by how often it appears. The bar is still replication — one source copied onto many ingredients scores high there — so the two can disagree.

rancid, butter, cheese shown above Flavornet Cas, Flavornet Name, Flavordb Odor Column 3 sources agree

—

butter, cheese, rancid, strawberry FlavorDB 2.0

acidic sour cheesy limburger cheese dairy creamy FlavorDB 2.0

buttery FlavorDB 2.0

sour/rancid FlavorDB 2.0

undesirable FlavorDB 2.0

Bars are the share of ingredient records carrying each description. High agreement means the sources replicate each other — not necessarily that they are right.

Published thresholds

11 measured values from the literature. Grouped by what was actually measured — a threshold smelled in clean water, tasted in the mouth, or measured in a real food are three different quantities and are not pooled.

Orthonasal, in clean water

3040 ppb – 8100 ppb

The reference measurement — smelled in water. OAV is computed against this.

Threshold	Matrix	Source	Paper
3040 ppb SIDA	water	Rychlik 1998 · 3 studies	—
8100 ppb	water_odor	Leffingwell	—

In a food matrix

0.205 ppb – 33.2 ppb

Measured in a real food, where the matrix shifts detection — not a clean-water value.

Threshold	Matrix	Source	Paper
0.010 ppb	water (food: wine)	Literature corpus	32657431
Flagged: 210× from the median of this group. Excluded from the range above pending re-check against the source publication — shown rather than hidden.
0.016 ppb	water (food: beer)	Literature corpus	17499755, 33889833
Flagged: 134× from the median of this group. Excluded from the range above pending re-check against the source publication — shown rather than hidden.
0.205 ppb	water (food: rice)	Literature corpus	26973621
4.0 ppb	water (food: unknown)	Literature corpus	19753606, 32517941, 33575277 +2
13.3 ppb	water (food: tea)	Literature corpus	37188725
33.2 ppb	water (food: bread)	Literature corpus	28274420, 28490062

Retronasal / in-mouth

1000 ppb

Tasted rather than smelled — a different sense, not comparable.

Threshold	Matrix	Source	Paper
1000 ppb SIDA	water_retronasal	Rychlik 1998 · 1 studies	—

In air

1.5 ppb

Airborne detection — not comparable to a water threshold.

Threshold	Matrix	Source	Paper
1.5 ppb	air	Nagata 2003	—

Other matrix

325 ppb

Measured in a non-aqueous medium (oil, starch, cellulose).

Threshold	Matrix	Source	Paper
325 ppb	oil (sunflower oil) (food: gouda cheese)	Literature corpus	17499755, 19753606, 26973621 +9

Only values we can attribute are listed. Where a compendium's licence does not permit public attribution its figures are omitted here, so every number on this page traces to a citable source.

Where you'd actually smell it

Odor Activity Value = measured concentration ÷ odor threshold (8.6625 ppb, the engine's canonical value — see the 11 published values). OAV ≥ 1 = perceptible.

Food	Concentration (ppb)	OAV	Source
gouda cheese	2600.0	300 ⚡	Corpus paper
tea	26.7	3.09	Corpus paper
bread	22.2	2.57	Corpus paper
rice	0.2	0.02	—
beer	0.0	0.00	—
wine	0.0	0.00	—

Concentrations from PubMed Central Open Access papers (free literature corpus, MIT-style licensed). Copper = OAV ≥ 10 (strongly perceptible). Green = OAV ≥ 1 (perceptible).

Found in

Measured impact first, then profile share

Every ingredient known to contain isobutyric acid. Ingredients with a measured concentration come first, ranked by odour activity; the rest follow by how large a share of that ingredient's profile this molecule is. The figure beside each is how many different compounds that ingredient has in total — not the concentration of isobutyric acid in it. Full concentrations are in the OAV table above.

617 cmpds

210 cmpds

432 cmpds

26 cmpds