Some textbooks mistakenly attribute butter’s aroma solely to diacetyl, a compound with a pair of ketone groups. Diacetyl does indeed have a buttery smell, but gas chromatography olfactometry reveals a more complete profile of the smell of butter. Complimenting diacetyl in sour cream butter are a pair of other key compounds, also formed by lactic acid- fermenting bacteria. Known as butanoic acid and δ-decalactone, they contribute cheesy and peachy notes, respectively. Sweet cream butter’s smell is defined by lactones and sulphurous compounds while aldehydes are found in butter oil’s aroma. Heat up the butter and the caramel-like furanone, the potato-like methional and then the cheesy 3-methylbutanoic acid will surface.

Small amounts of diacetyl are also found naturally in a variety of other foods aside from butter including cheeses and other dairy products, and it’s also in beer and wine.  Depending on the type of beer, diacetyl is not always desirable. In wine however it lends a smooth, buttery taste. Interestingly our threshold for detection of diacetyl in wine varies with the type. It’s low in Chardonnay (0.2 ppm) but higher in Merlots and Sauvignon. Apparently diacetyl  binds to sulfur dioxide, whose concentration varies from one wine to another.

But why is diacetyl’s presence fairly common? Glucose is life’s most important investment of chemical energy. But cells can’t burn it in a crude manner as if it were wood or fossil fuels in the hands of humans. That would release only heat, would be too disruptive and too limiting. Instead the energy has to be invested in other compounds such as ATP that can then both facilitate constructive reactions and release heat slowly. Whether or not oxygen is present, the 6-carbon molecule, glucose, is, in a series of steps, first converted to a pair of 3-carbon atoms known as pyruvate. In oxygen’s presence, pyruvate will enter the citric acid cycle and lead to the production of lots of ATP. On the other hand, the absence of oxygen will lead to less productive options known as fermentation. Fermenters start with pyruvate, obtaining it either from glycolysis or from citric acid. But certain bacteria, while investing in ATP also produce lactic acid, acetate, 2,3-butanediol or, like yeasts, even alcohol. Diacetyl trickles out of that reaction-ensemble, coming from a side-reaction that releases diacetyl and CO₂ from a 5-carbon molecule, which is in turn made partly from pyruvate.

As long as there’s more profit to be gained, industry is too often ready to cater to consumers’ laziness. Why oblige them to add their own butter to popcorn or flavors to coffee when part of butter’s aroma can be prepackaged? Unfortunately diacetyl has been shown to be an occupational hazard for workers in factories handling the compound. Repeated exposure to elevated concentrations of diacetyl leads to permanent shortness of breath from obliterative bronchiolitis, a condition involving scarring and constriction of the bronchioles. Some companies have stopped using diacetyl altogether. Those who persist have to make sure that a limit of 5 parts per billion for up to 8 hours a day and 40 hours per week is not surpassed. All the energy that goes into mass-producing diacetyl and all the physical suffering and regulations could be saved and avoided if people buttered their own popcorn or ate the unadulterated version while learning about diacetyl’s chemistry.

Sources:

• Aroma-active compounds of butter: a review.
  European Food Research and Technology 226(3):315-325 · January 2007
• Development & Modification of Bioactivity Leen C. Verhagen, in Comprehensive Natural Products II,
• The ‘buttery’ attribute of wine—diacetyl—desirability, spoilage and beyond. Eveline J.BartowskyPaul A.Henschke International Journal of Food Microbiology
  Volume 96, Issue 3, 15 November 2004, Pages 235-252
• Exposure to Flavoring Chemicals. Centers for Disease Control and Prevention
  https://www.cdc.gov/niosh/topics/flavorings/exposure.html and blogs.cdc.gov/niosh-science-blog/2016/01/25/coffee-workers/