Isobutyric acid (CAS 79-31-2) — Musky Top to mid Note Fragrance Ingredient
Isobutyric acid
CAS 79-31-2
What Is Isobutyric acid?
Isobutyric acid is a sharp, pungent organic acid found naturally in some cheeses and fermented foods. It’s also synthesized for use in flavorings and fragrances. This ingredient matters because it adds a distinct sour, cheesy note that perfumers use to create animalic or buttery accents in small doses, though it’s overpowering in large quantities.
Safety Profile
USE WITH AWARENESSWhat Does Isobutyric acid Smell Like?
Isobutyric acid assaults the nose with an intensely sharp, sour-cheese aroma that borders on vomit-like at full strength. The top note is aggressively acidic, evoking spoiled milk or unwashed feet. As it dries down, it mellows slightly into a sweaty, animalic character with faint butter undertones. In extreme dilution (below 0.1%), it can contribute a desirable fermented cream nuance to gourmand compositions, but remains a challenging material requiring careful balancing.
In Famous Fragrances
Fragrance associations may not reflect actual formulations.
2D Molecular Structure
SMILES: CC(C)C(O)=O
Chemistry, Properties & Perfumer Guide
The Chemistry
Isobutyric acid (2-methylpropanoic acid) is a branched-chain carboxylic acid. While it occurs naturally in some fermented products and mammalian sweat, most fragrance applications use the synthetic form produced by oxidation of isobutyraldehyde. The molecule’s volatility and intense odor stem from its small size (C4) and free carboxylic acid group. Unlike its straight-chain counterpart (butyric acid), the branched structure modifies both odor character and physical properties.
Physical & Chemical Properties
| Boiling Point | 154-155 °C |
|---|---|
| Density | 0.949 g/cm³ |
| Vapor Pressure | 1.5 mmHg at 20°C |
| Solubility | Miscible with water, ethanol, ether |
Perfumer Guide
| Application | Typical % | Range | Notes |
|---|---|---|---|
| Fine Fragrance | 0.01-0.1% | Up to 0.3% | Only for bold animalic effects |
| Functional Fragrance | 0.001-0.01% | Up to 0.05% | Masking agent in harsh products |
Classic Accords
Tip: Always pre-dilute to 1% or lower before incorporating into blends.
Alternatives & Comparisons
Straight-chain version with even more intense fecal character, used when maximum animalic impact is desired.
Similar cheesy-sweaty profile but with added foot odor nuances from the longer carbon chain.
Safety, Regulatory & Sustainability
⚠ Regulatory Disclaimer
General reference only. Consult current IFRA Standards Library before formulating.
IFRA Status
Not restricted by IFRA, but self-limiting due to odor intensity.
GHS Classification
RIFM Assessment
RIFM assessment confirms safe use at current levels in fragrance, but notes extreme skin irritation potential at higher concentrations.
Sustainability
Synthetic production from petrochemical feedstocks dominates supply. While energy-intensive, the small quantities needed make environmental impact minimal compared to natural isolation from biological sources. No known ecological toxicity concerns at fragrance use levels.
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References
- Burdock, G.A. (2010). Fenaroli’s Handbook of Flavor Ingredients. CRC Press.
- Arctander, S. (1969). Perfume and Flavor Chemicals. Allured Publishing.
Data: PubChem (NIH), PubMed, RIFM, IFRA. Last reviewed: Apr 2026.
Report a data errorPerfumer’s Notes
FEMA #: 2222 | IOFI #: Nature Identical
Isobutyric acid has strong penetrating odor of rancid butter. The odor and flavor are similar to n-butyric acid.
Odor: [‘cheesy’, ‘fruity’, ‘sour’]
MW: 88.11
LogP: 0.8
Ingredient Data Sheet
CAS 79-31-2Physical Properties
| Molecular Weight | 88.11 g/mol🔬 PubChem |
| LogP (Octanol-Water) | 0.8🔬 PubChem |
| Boiling Point | 153.9 °C🔬 EPA CompTox |
| Vapor Pressure | 1.81 mmHg @ 25°C📊 OPERA |
| Flash Point | 55.6 °C🔬 EPA CompTox |
| Involatility Index | 0.2078💻 Calculated |
| log Kp (skin permeability) | -2.669💻 Calculated |
| SMILES | CC(C)C(=O)O🔬 PubChem |
Volatility & Performance
| Fragrance Note | Top💻 Calculated |
| Volatility Class | Moderate💻 Calculated |
| Persistence Score | 0.5 / 5💻 Calculated |
Odor & Flavor
| Primary Descriptors | fruity• leffingwell |
| “Powerful, diffusive sour (acid) odor, slightly less repulsive, and also less buttery than the n-Butyric acid. In extreme dilution the odor becomes almost pleasant, fruity.”📖 Arctander | |
| Isobutyric acid has strong penetrating odor of rancid butter. The odor and flavor are similar to n-butyric acid.📖 Fenaroli | |
Flavor Notes (Arctander)
| “Soluble 20% in water, miscible with Propylene glycol, alcohol and most perfume and flavor oils. The taste is, in proper dilution and with adequate sweetening, pleasant creamy-fruity, while buttery-cheesy notes are dominating in the absence of sweeteners. Sodium chloride enhances the buttery-cheesy f”📖 Arctander |
Sensory Thresholds
| Odor Detection Threshold | 0.5127 ppm (n=23)📖 van Gemert |
Regulatory Status
| FEMA Number | FEMA 2222⚖️ FEMA GRAS |
| GRAS Status | Generally Recognized as Safe⚖️ FEMA GRAS |
| IOFI Classification | Nature Identical📖 Fenaroli |
Physical data: PubChem (NIH/NLM), U.S. EPA CompTox Dashboard, EPA OPERA models, RDKit. Odor & flavor: Arctander (Perfume & Flavor Chemicals), Fenaroli's Handbook of Flavor Ingredients, Leffingwell. Thresholds: van Gemert (Compilations of Odour Threshold Values). Regulatory: IFRA Standards 51st, FEMA GRAS. Trade names: Surburg (Common Fragrance & Flavor Materials). All data compiled and cross-referenced for perfumertools.com.
Physicochemical Properties
DTXSID: DTXSID4021636
Physical Properties
| Molecular Weight | 88.106 g/mol🔬 EPA CompTox |
| Density | 0.949 g/cm^3🔬 EPA CTX |
| Boiling Point | 154.131 °C🔬 EPA CTX |
| Melting Point | -47.641 °C🔬 EPA CTX |
| Flash Point | 60.593 °C🔬 EPA CTX |
| Refractive Index | 1.408 Dimensionless📊 OPERA |
| Molar Volume | 89.565 cm^3/mol📊 OPERA |
Partition & Solubility
| LogP (Octanol-Water) | 1.78 Log10 unitless🔬 EPA CTX |
| LogD (pH 5.5) | 0.223 Log10 unitless📊 OPERA |
| LogD (pH 7.4) | -1.546 Log10 unitless📊 OPERA |
| LogKoa (Octanol-Air) | 4.1 Log10 unitless📊 OPERA |
| Water Solubility | 1.93 mol/L🔬 EPA CTX |
| Henry's Law Constant | 0 atm-m3/mole🔬 EPA CTX |
Transport Properties
| Vapor Pressure | 1.746 mmHg🔬 EPA CTX |
| Viscosity | 1.688 cP📊 OPERA |
| Surface Tension | 28.091 dyn/cm📊 OPERA |
| Thermal Conductivity | 141.305 mW/(m*K)📊 OPERA |
Molecular Descriptors
| Topological Polar Surface Area | 37.3 Ų💻 Computed |
| H-Bond Donors | 1 count💻 Computed |
| H-Bond Acceptors | 1 count💻 Computed |
| Rotatable Bonds | 1 count💻 Computed |
| Aromatic Rings | 0 count💻 Computed |
| Molar Refractivity | 22.104 cm^3/mol📊 OPERA |
| Polarizability | 8.763 Å^3📊 OPERA |
Data Sources:
🔬 EPA Experimental data from U.S. EPA CompTox Chemicals Dashboard & CTX APIs. 📊 OPERA Predicted using EPA's OPERA QSAR models. 💻 Computed Calculated from SMILES using RDKit.
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