3-Hydroxy-2-butanone
[CAS 513-86-0]

Identification

• Classification: Organic raw materials >> Ketone compound
• Name: 3-Hydroxy-2-butanone
• Synonyms: Acetyl methyl carbinol; Acetoin
• Molecular Formula: C₄H₈O₂
• Molecular Weight: 88.11
• CAS Registry Number: 513-86-0
• EC Number: 208-174-1
• FEMA: 2008
• SMILES: CC(C(=O)C)O

Properties

• Density: 1.0±0.1 g/cm³ Calc.^*, 1.013 g/mL (Expl.)
• Melting point: 15 °C (Expl.)
• Boiling point: 145.4±8.0 °C 760 mmHg (Calc.)^*, 148 °C (Expl.)
• Flash point: 49.7±11.0 °C (Calc.)^*, 47 °C (Expl.)
• Solubility: water soluble (Expl.)
• Index of refraction: 1.408 (Calc.)^*, 1.417 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS02;GHS05;GHS07 DangerGHS02;GHS02
• Risk Statements: H226-H228-H315-H318-H319
• Safety Statements: P210-P233-P240-P241-P242-P243-P264-P264+P265-P280-P302+P352-P303+P361+P353-P305+P351+P338-P305+P354+P338-P317-P321-P332+P317-P337+P317-P362+P364-P370+P378-P403+P235-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Flammable solids	Flam. Sol.	2	H228
Flammable liquids	Flam. Liq.	3	H226
Serious eye damage	Eye Dam.	1	H318
Acute toxicity	Acute Tox.	3	H331
Specific target organ toxicity - single exposure	STOT SE	2	H371
Specific target organ toxicity - repeated exposure	STOT RE	2	H373
Flammable solids	Flam. Sol.	1	H228
Specific target organ toxicity - single exposure	STOT SE	3	H335

• Transport Information: UN 2621

chemBlink Chemical Story

3-Hydroxy-2-butanone is better known as acetoin, and its chemistry is inseparable from the smell of fermentation. It is a small alpha-hydroxy ketone produced by many bacteria and yeasts during carbohydrate metabolism. In dairy fermentation, acetoin and its oxidized relative diacetyl contribute to the buttery aroma associated with cultured cream, butter, and some cheeses.

Microorganisms often make acetoin through the acetolactate pathway. Carbon derived from pyruvate is combined to form alpha-acetolactate; decarboxylation then gives acetoin, while oxidation of alpha-acetolactate can generate diacetyl. Acetoin can in turn be reduced to 2,3-butanediol. This network is not merely a flavor factory. It can help cells redirect pyruvate away from strongly acid-producing pathways and manage redox balance when growth conditions change.

The sensory consequences are striking because closely related molecules have different odor impact. Diacetyl has an intense buttery note at very low concentration, while acetoin is softer but still contributes to creamy, buttery aroma. Fermentation technologists therefore care about the balance among alpha-acetolactate, diacetyl, acetoin, and 2,3-butanediol. Oxygen availability, pH, temperature, strain selection, and enzyme activity can shift that balance and change final flavor.

Modern biotechnology has turned the same pathway into a production platform. Researchers have selected or modified Lactococcus and other microorganisms to route more carbon toward acetoin from lactose, glucose, or low-value food side streams. One study used a natural dairy Lactococcus lactis isolate with reduced lactate dehydrogenase activity to increase acetoin production. The logic is classic metabolic engineering: suppress a major competing outlet for pyruvate and more precursor becomes available for the desired aroma chemical.

Acetoin is also chiral. The molecule contains a stereogenic carbon and can occur as different stereoisomeric forms, while microbial enzymes may produce them with differing selectivity. For flavor work the total concentration often receives more attention than enantiomeric composition, but stereochemistry is a reminder that even a four-carbon fermentation product has structural complexity. In biocatalysis, control of stereochemistry can become important when acetoin is used not as an aroma compound but as a chiral building block for synthesis.

Acetoin matters because it connects metabolism to human perception. A few enzymatic steps deciding the fate of pyruvate can change whether a fermented food smells sharply buttery, mildly creamy, or relatively neutral. The same molecule also shows how biotechnology can repurpose an old fermentation pathway for modern manufacturing. What begins as a microbial strategy for handling carbon and redox balance becomes, to us, part of the aroma of butter.

References:

1. Xiao Z, Lu JR. Biotechnology Advances. 2014;32:492-503. DOI: 10.1016/j.biotechadv.2014.01.002.

2. Journal of Agricultural and Food Chemistry. 2020;68:5891-5899. DOI: 10.1021/acs.jafc.0c00882.

3. NIST Chemistry WebBook. Acetoin, CAS 513-86-0.

4. Food flavor literature on diacetyl, acetoin and 2,3-butanediol.

Read more stories