Effects of CO2 on the formation of flavour volatiles during fermentation with immobilised brewer's yeast.

Immobilised-cell fermentors offer great benefits compared to traditional free-cell systems. However, a major problem is unbalanced flavour production when these fermentors are used for the production of alcoholic beverages. One of the keys to obtaining better control over flavour formation may be the concentration of dissolved CO2, which has inhibitory effects on yeast growth and metabolism. This article demonstrates that the presence of immobilisation matrices facilitates the removal of CO2 from the liquid medium, which results in a low level of dissolved CO2 during fermentation. Moreover, the formation of volatile higher alcohols and esters was greatly enhanced in the immobilised-cell system when compared to the free cell system. By sparging a CO2 flow (45 ml/min) into the immobilised-cell system, cell growth was reduced by 10-30% during the active fermentation stage, while the fermentation rate was unaffected. The uptake of branched-chain amino acids was reduced by 8-22%, and the formation of higher alcohols and esters was reduced on average by 15% and 18%, respectively. The results of this study suggest that mismatched flavour profiles with immobilised-cell systems can be adjusted by controlling the level of dissolved CO2 during fermentation with immobilised yeast.