Fermentation process for improved glycerol and acetic acid conversion

A process for producing ethanol, the process comprising:

• a) inoculating a diluted lignocellulosic hydrolysate, in a reactor, with a yeast that is capable of consuming sugar, glycerol and acetic acid and aerobically propagating the yeast to generate a propagation broth;
• b) fermenting the diluted lignocellulosic hydrolysate anaerobically with at least a portion of the propagation broth in a fed batch mode to generate a fermentation broth, wherein the pH of the fermentation broth is controlled by adding a mixture comprising lignocellulo sic hydrolysate and glycerol to the reactor in amounts sufficient to maintain a constant pH in the fermentation broth, in which mixture the molar ratio of glycerol to acetic acid is in the range of 1.8 to 2.2;
• c) after sufficient fermentation time, isolation of ethanol from the reactor;
• d) optionally keeping the remaining fraction after isolation of c) as spent broth;
• and
• e) optionally using a part of the spent broth of d) to dilute the lignocellulosic hydrolysate of a);
• wherein the yeast is genetically modified comprising:
• one or more nucleotide sequence encoding a heterologous NAD+-dependent acetylating acetaldehyde dehydrogenase (E.C. 1.2.1.10);
• one or more nucleotide sequence encoding a homologous or heterologous acetyl-CoA synthetase (E.C. 6.2.1.1);
• one or more nucleotide sequence encoding a heterologous glycerol dehydrogenase (E.C. 1.1.1.6);
• and
• one or more nucleotide sequence encoding a homologous or heterologous dihydroxyacetone kinase (E.C. 2.7.1.28 or E.C. 2.7.1.29).