The effect of wort composition on brewer's yeast metabolites and fermentation performance in a continuous gas lift bioreactor

Yeast metabolite yields and fermentation performance measures were studied in a 60-L pilot-scale gas lift continuous bioreactor using commercial lager wort (control medium) to produce beer. Comparisons of the results were made to those from an industrial batch fermentation process as well as those from the 60-L continuous system using seven different experimental feed worts with compositional changes to better understand yeast metabolism and find strategies for altering the organoleptic qualities of the beer.;For the 60-L pilot-scale continuous system, under standard operating conditions (0.27m3/hr total volumetric gas flow rate, 7wt.% air and 93 wt.% nitrogen; 26 wet wt. % cell loading and a dilution rate of 0.045 hr-1) the respiratory quotient (RQ) was 47, indicating a fermentative mode, the ethanol yield was 0.48 g/g-fermentable sugar and the biomass yield was 0.088 g/g-glucose. Compared to the industrial batch process, the 60-L continuous system had double the specific free amino nitrogen (FAN) utilization resulting in increased by-products of oxidative catabolic amino acid metabolism and considerably higher ethanol productivity yet slightly lower ethanol yield. Very high total diacetyl concentrations in the continuously produced beer, which could be controlled by valine supplementation, indicated increased amino acid anabolic metabolism. For the 60-L continuous system, only large changes to the feed wort carbohydrate composition affected the fermentation performance, such as heightened catabolite repression with supplementation of high maltose corn syrup. Metabolite yields were more sensitive to free amino nitrogen (FAN) compositional changes and a positive correlations was found between the feed wort FAN concentration and the bioreactor FAN utilization for all worts, independent of other wort compositional changes. Increased FAN utilization resulted in increased by-products of amino acid catabolism (oxidative and reductive pathways) presumably through altered redox potential of the yeast. Valine supplementation to commercial lager wort in the 60-L continuous system was successful for controlling total diacetyl in beer and could reduce its levels to as low as 22.2 mug/L (normalized to 5 v/v% ethanol).