The present invention provides a modified strain of the bacterium Lactococcus lactis obtainable by a method that comprises a step of fusion of two protoplasts from two Lactococcus lactis parental strains which, compared to the wild type strain of Lactococcus lactis from which they derive, show: (a) an increased ability to produce acetoin and/or 2,3-butanediol (2,3-BDO), and (b) a decreased ability to produce lactic acid, when cultured under aerobic conditions, and wherein the modified strain of Lactococcus lactis has an increased ability to produce 2,3-BDO of at least 20 times the amount produced by the wild type strain, an increased ability to produce acetoin of at least 20 times the amount produced by the wild type strain and a decreased ability to produce lactic acid of at least 10 times the amount produced by the wild type strain, when cultured under aerobic conditions. Also provided are methods to produce acetoin and 2,3-BDO.