What specific sensory attribute does the compound 'diacetyl' contribute to beer?

The specific sensory attribute that the compound 'diacetyl' contributes to beer is a buttery or butterscotch flavor and aroma. Diacetyl is a vicinal diketone (VDK) produced by yeast during fermentation as a byproduct of amino acid metabolism, specifically the synthesis of valine and leucine. At low concentrations, diacetyl can contribute a pleasant, creamy mouthfeel and subtle butterscotch notes, which are considered desirable in some beer styles, such as certain English ales and Scottish ales. However, at higher concentrations, diacetyl can become an off-flavor, imparting an overwhelming buttery or butterscotch taste that masks other desirable flavors and aromas in the beer. The acceptable level of diacetyl varies depending on the beer style and the brewer's intent. Brewers often employ techniques to minimize diacetyl production or to allow the yeast to reabsorb it during fermentation. These techniques include maintaining proper fermentation temperatures, ensuring adequate yeast health, and allowing for a diacetyl rest (raising the temperature slightly at the end of fermentation to encourage yeast to consume diacetyl). The detection threshold for diacetyl is quite low, meaning that even small amounts can be noticeable to the palate.