diacetyl, a product of malolactic conversion with a powerful butterscotch or butter aroma. The ability to detect diacetyl depends on its concentration and the wine type and style. The perception threshold varies from 0.2 mg/l for Chardonnay, 0.9 mg/l for Pinot Noir to 2.8 mg/l for Cabernet Sauvignon. At low concentrations, it may be perceived as nutty or toasty and add desirable complexity. In excess, as was once common in new world Chardonnays, it is perceived as distractingly obvious butteriness. The amount of diacetyl produced depends on the bacterial strain and the rate of progress of the malolactic conversion as well as temperature, oxygen availability, the wine’s pH, sulfur dioxide content, and, importantly, citric acid concentrations.
