α-Acetolactate is the unstable metabolic intermediate that stands behind one of brewing’s most watched flavours: it is the precursor of diacetyl, the buttery compound that is a defect in most lagers above trace levels. Yeast excretes α-acetolactate during valine biosynthesis, and it spontaneously decarboxylates outside the cell into diacetyl, which yeast later reabsorbs and reduces if given a warm diacetyl rest. Brewers therefore manage fermentation temperature and time to ensure the precursor is fully cleared before packaging. The same chemistry matters in dairy fermentations, where controlled diacetyl gives cultured butter its character. It is a textbook case of a fleeting intermediate dictating final product quality.