Engineering the cytosolic NADH availability in lager yeast to improve the aroma profile of beer

ObjectiveTo improve the aroma profile of beer by using metabolic engineering to increase the availability of cytosolic NADH in lager yeast.ResultsTo alter NADH levels in lager yeast, the native FDH1 (YOR388C) encoding NAD(+)-dependent formate dehydrogenase was overexpressed in the yeast strain M14, yielding strain M-FDH1. This led to a simultaneous increase of NADH availability and NADH/NAD(+) ratio in the M-FDH1 strain during fermentation. At the end of the main fermentation period, ethanol production by strain M-FDH1 was decreased by 13.2%, while glycerol production was enhanced by 129.4%, compared to the parental strain respectively. The production of esters and fusel alcohols by strains M14 and M-FDH1 was similar. By contrast, strain M-FDH1 generally produced less organic acids and off-flavor components than strain M14, improving the beer aroma.ConclusionsIncreased NADH availability led to rerouting of the carbon flux toward NADH-consuming pathways and accelerated the NADH-dependent reducing reactions in yeast, greatly impacting the formation of aroma compounds and improving the beer aroma.