Converts acetolactate into acetoin. (2S)-2-acetolactate + H(+) = (R)-acetoin + CO2 Polyol metabolism; (R,R)-butane-2,3-diol biosynthesis; (R,R)-butane-2,3-diol from pyruvate: step 2/3. Belongs to the alpha-acetolactate decarboxylase family.