Diacetyl^[1]

Names
Preferred IUPAC name Butane-2,3-dione
Other names Diacetyl Biacetyl Dimethyl diketone 2,3-Diketobutane
Identifiers
CAS Number	431-03-8 Y
3D model (JSmol)	Interactive image
3DMet	B00164
Beilstein Reference	605398
ChEBI	CHEBI:16583 Y
ChEMBL	ChEMBL365809 Y
ChemSpider	630 Y
ECHA InfoCard	100.006.428
EC Number	207-069-8
KEGG	C00741 Y
PubChem CID	650
RTECS number	EK2625000
UNII	K324J5K4HM Y
UN number	2346
CompTox Dashboard (EPA)	DTXSID6021583
InChI InChI=1S/C4H6O2/c1-3(5)4(2)6/h1-2H3 Y Key: QSJXEFYPDANLFS-UHFFFAOYSA-N Y InChI=1/C4H6O2/c1-3(5)4(2)6/h1-2H3 Key: QSJXEFYPDANLFS-UHFFFAOYAX
SMILES CC(=O)C(=O)C
Properties
Chemical formula	C4H6O2
Molar mass	86.090 g·mol−1
Appearance	Yellow liquid
Density	0.990 g/mL at 15 °C
Melting point	−2 to −4 °C (28 to 25 °F; 271 to 269 K)
Boiling point	88 °C (190 °F; 361 K)
Solubility in water	200 g/L (20 °C)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Harmful, flammable
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H225, H302, H315, H317, H318, H331, H373
Precautionary statements	P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P272, P280, P301+P312, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P310, P311, P314, P321, P330, P332+P313, P333+P313, P362, P363, P370+P378, P403+P233, P403+P235, P405, P501
NFPA 704 (fire diamond)	2 3 0
Safety data sheet (SDS)	External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Diacetyl (/daɪjəˈsiːtəl/ dy-yuh-SEE-tuhl; IUPAC systematic name: butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH₃CO)₂. It is a yellow liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side). Diacetyl occurs naturally in alcoholic beverages and some cheeses and is added as a flavoring to some foods to impart its buttery flavor.

A distinctive feature of diacetyl (and other vicinal diketones) is the long C–C bond linking the carbonyl centers. This bond distance is about 1.54 Å, compared to 1.45 Å for the corresponding C–C bond in 1,3-butadiene. The elongation is attributed to repulsion between the polarized carbonyl carbon centers.^[2]

Diacetyl arises naturally as a byproduct of fermentation. In some fermentative bacteria, it is formed via the thiamine pyrophosphate-mediated condensation of pyruvate and acetyl CoA.^[3] Sour (cultured) cream, cultured buttermilk, and cultured butter are produced by inoculating pasteurized cream or milk with a lactic starter culture, churning (agitating) and holding the milk until a desired pH drop (or increase in acidity) is attained. Cultured cream, cultured butter, and cultured buttermilk owe their tart flavour to lactic acid bacteria and their buttery aroma and taste to diacetyl. Malic acid can be converted to lactic acid to make diacetyl.^[4][5]

Diacetyl is produced industrially by dehydrogenation of 2,3-butanediol. Acetoin is an intermediate.^[6]

Diacetyl and acetoin are two compounds that give butter its characteristic taste. Because of this, manufacturers of artificial butter flavoring, margarines or similar oil-based products typically add diacetyl and acetoin (along with beta-carotene for the yellow color) to make the final product butter-flavored, because it would otherwise be relatively tasteless.^[7]

Diacetyl is used as a flavoring agent in some liquids used in electronic cigarettes.^[8] People nearby may be exposed to it in the exhaled aerosol at levels near the limit set for occupational exposure.^[9]

In some styles of beer (e.g. in many beer styles produced in the United Kingdom, such as stouts, English bitters, and Scottish ales), the presence of diacetyl can be acceptable or desirable at low or, in some cases, moderate levels. In other styles, its presence is considered a flaw or undesirable.^[10]

Diacetyl is produced during fermentation as a byproduct of valine synthesis, when yeast produces α-acetolactate, which escapes the cell and is spontaneously decarboxylated into diacetyl. The yeast then absorbs the diacetyl, and reduces the ketone groups to form acetoin and 2,3-butanediol.^{[citation needed]}

Beer sometimes undergoes a "diacetyl rest", in which its temperature is raised slightly for two or three days after fermentation is complete, to allow the yeast to absorb the diacetyl it produced earlier in the fermentation cycle. The makers of some wines, such as chardonnay, deliberately promote the production of diacetyl because of the feel and flavor it imparts.^[11] Diacetyl is present in some chardonnays known as "butter bombs", although there is a trend back toward the more traditional French styles.^[12]

Concentrations from 0.005 mg/L to 1.7 mg/L were measured in chardonnay wines, and the amount needed for the flavor to be noticed is at least 0.2 mg/L.^[13][14]

Chronic industrial exposure to diacetyl fumes, such as in the microwave popcorn production industry, has been associated with bronchiolitis obliterans, a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles (small airway branches) are compressed and narrowed by fibrosis (scar tissue) and/or inflammation.^[15][16]

The European Commission has declared diacetyl is legal for use as a flavouring substance in all EU states.^[17] As a diketone, diacetyl is included in the EU's flavouring classification Flavouring Group Evaluation 11 (FGE.11). A Scientific Panel of the EU Commission evaluated six flavouring substances (not including diacetyl) from FGE.11 in 2004.^[18] As part of this study, the panel reviewed available studies on several other flavourings in FGE.11, including diacetyl. Based on the available data, the panel reiterated the finding that there were no safety concerns for diacetyl's use as a flavouring.^{[citation needed]}

In 2007, the European Food Safety Authority (EFSA), the EU's food safety regulatory body, stated its scientific panel on food additives and flavourings (AFC) was evaluating diacetyl along with other flavourings as part of a larger study.^[19]

In 2007, the Flavor and Extract Manufacturers Association recommended reducing diacetyl in butter flavorings.^[20] Manufacturers of butter flavored popcorn including Pop Weaver, Trail's End, and ConAgra Foods (maker of Orville Redenbacher's and Act II) began removing diacetyl as an ingredient from their products.^[21][22]

A 2010 U.S. OSHA Safety and Health Information Bulletin and companion Worker Alert recommend employers use safety measures to minimize exposure to diacetyl or its substitutes.^[23]

In 2016, diacetyl was banned in e-liquids/e-cigarettes in the EU under the EU Tobacco Products Directive.^[24]

• Acetylpropionyl, a similar diketone
• Acetoin
• Bronchiolitis obliterans

• Toxicology data
• NIOSH Alert: Preventing Lung Disease and Workers who Use or Make Flavorings
• A Case of Regulatory Failure – Popcorn Workers Lung, from www.defendingscience.org.
• Scientists Urge Secretary of Labor to Protect Workers from Diacetyl, a press release from defendingscience.org. Links to studies on the health effects of diacetyl, and to a variety of related documents including the recent OSHA petition and the scientists' letter of support may be found here.
• Flavoring suspected in illness, Washington Post, May 7, 2007.
• NIOSH International Safety Card for 2,3-butanedione
• National Institute for Occupational Safety and Health – Flavorings-Related Lung Disease
• IFIC – Diacetyl