Converts acetolactate into acetoin, which can be excreted by the cells. This may be a mechanism for controlling the internal pH of cells in the stationary stage. (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. Strongly induced under anaerobic conditions. Activated by ResDE, Fnr and ArfM. Belongs to the alpha-acetolactate decarboxylase family.