A distinctive feature of diacetyl (and other 1,2-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.
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. 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
Diacetyl is produced industrially by dehydrogenation of 2,3-butanediol. Acetoin is an intermediate.
In food products
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.
In alcoholic beverages
At low levels, diacetyl contributes a slipperiness to the feel of
the alcoholic beverage in the mouth. As levels increase, it imparts a buttery or butterscotch flavor.
In some styles of beer (e.g. in most beers produced in Britain and
Ireland, such as India Pale Ale), 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.
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.
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. Diacetyl is present in some chardonnays
known as "butter bombs", although there is a trend back toward the
more traditional French styles.
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.
1-Hexanol and diacetyl are strong inhibitors of the CO2-sensitive neurons in the Drosophila melanogaster fruit fly and the Culex mosquito, a vector of several deadly diseases. Fruit flies tend to
avoid CO2, but exhaled CO2 is the main attractant for the Culex.
The United States National Institute for Occupational Safety and Health has suggested diacetyl, when used in artificial butter flavoring
(as used in many consumer foods), may be hazardous when heated and
inhaled over a long period.
Workers in several factories that manufacture artificial butter flavoring have been diagnosed with bronchiolitis obliterans, a rare and serious disease of the lungs. The cases found have been mainly in young, healthy, nonsmoking
males. As with other end-stage lung diseases, transplantation is
currently the most viable treatment option. However, lung
transplant rejection is very common and happens to be another
setting in which bronchiolitis obliterans is known to occur.
The disease has been called "popcorn worker's lung" because it was
first seen in former workers of a microwave popcorn factory in Missouri, butNIOSH refers to it by the more general term "flavorings-related lung
disease". It has also been called "flavorings-related bronchiolitis
obliterans" or diacetyl-induced bronchiolitis obliterans. People
who work with flavorings that include diacetyl are at risk for
flavorings-related lung disease, including those who work in
popcorn factories, restaurants, other snack food factories,
bakeries, candy factories, margarine and cooking spread factories,
and coffee processing facilities.
In 2006, the International Brotherhood of Teamsters and the United Food and Commercial Workers petitioned the U.S. OSHA to promulgate an emergency temporary standard to protect workers
from the deleterious health effects of inhaling diacetyl vapors.
The petition was followed by a letter of support signed by more
than 30 prominent scientists.The matter is under consideration. On
January 21, 2009, OSHA issued an advance notice of proposed
rulemaking for regulating exposure to diacetyl.The notice requests
respondents to provide input regarding adverse health effects,
methods to evaluate and monitor exposure, the training of workers.
That notice also solicited input regarding exposure and health
effects ofacetoin, acetaldehyde, acetic acid and furfural.
Two bills in the California Legislature seek to ban the use of diacetyl.
A 2010 U.S. OSHA Safety and Health Information Bulletin and
companion Worker Alert recommend employers use safety measures to
avoid exposing employees to the potentially deadly effects of
butter flavorings and other flavoring substances containing
diacetyl or its substitutes.
A preliminary in vitro study, published in 2012, suggests that diacetyl may exacerbate the
effects of beta-amyloid aggregation, a process linked toAlzheimer's disease. Research in rats and mice has shown that diacetyl can damage the
airway and damage cells that line the airway.
In 2015 there were allegations that the health of workers who roast coffee is threatened by diacetyl.
In 2007, the Flavor and Extract Manufacturers Association recommended reducing diacetyl in butter flavorings. 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.
In 2012, Wayne Watson, a regular microwavable popcorn consumer for
years, was awarded US$7.27 million in damages from a federal jury in Denver, which decided his lung disease was caused by the chemicals in
microwave popcorn and that the popcorn's manufacturer, Gilster-Mary Lee Corporation, and the grocery store that sold it should have warned him of its
European Union Regulation
The European Commission has declared diacetyl is legal for use as a flavouring substance in
all EU states. 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. 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.
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.
"The experts of the EFSA AFC panel and its working group on food
additives will look at this issue to see if new scientific evidence
is available that may require further actions. If the experts
conclude that consumer exposure to diacetyl can reach levels well
above those considered as safe and, that a possible health risk for
consumers cannot be excluded when inhaling diacetyl, EFSA will give
priority to the re-evaluation of this substance and provide
detailed scientific advice."
A 2014 publication found that diacetyl was present in samples of
many sweet-flavored electronic cigarette liquids, though at levels 100 times lower than that of tobacco smoke.
According to that research, diacetyl is approved for food use, but
is associated with respiratory disease when inhaled. The study
concluded that diacetyl is an avoidable risk for electronic cigarette liquid, and measures could be taken by the industry to eliminate its
usage, without limiting availability of flavors. In 2015, another
test of 51 purpose-selected liquids found diacetyl at trace levels
or higher in 39 of the liquids.