Construction of a flocculent Saccharomyces cerevisiae fermenting lactose

A flocculent Saccharomyces cerevisiae strain with the ability to express bo th the LAC4 (coding for beta-galactosidase) and LAC12 (coding for lactose p ermease) genes of Kluyveromyces marxianus was constructed. This recombinant strain is not only able to grow on lactose, but it can also ferment this s ubstrate. To our knowledge this is the first time that a recombinant S. cer visiae has been found to ferment lactose in a way comparable to that of the existing lactose-fermenting yeast strains. Moreover, the flocculating capa city of the strain used in this work gives the process several advantages. On the one hand, it allows for operation in a continuous mode at high cell concentration, thus increasing the system's overall productivity; on the ot her hand, the biomass concentration in the effluent is reduced, thus decrea sing product separation/purification costs.