The effect of pitching rate on the production of higher alcohols by top-fermenting yeast in wheat beer fermentation

The level of higher alcohols on top-fermentation determines the flavor profile and is one of the most important elements dictating the favorable top-fermented wheat beer (Ale beer) development. The optimization of the pitching rate has been shown to be crucial for industrial beer brewing. This study focused on understanding the effect of the variable inoculum size on the synthesis of higher alcohols. We utilized sequencing to investigate the transcript changes under different inoculum sizes and link the results to fermentation performance. Variable cell inoculum density levels were linked with differences in higher alcohol production. Specifically, we observed significantly less higher alcohols produced at lower cell inoculum density during the stationary phase. Importantly, the accumulation of higher alcohols during the exponential growth phase was overall similar between different pitching rates. Moreover, free amino nitrogen (FAN) consumption and yeast cell viability were significantly decreased during stationary phase at the lowest inoculum density. Transcriptomic analysis revealed that amino acid metabolism genes ALD4 , ALD6 , ARO9 , ARO10 , and PUT1 were differentially expressed once the cells entered the declining growth phase at the lowest inoculum size. The results suggest that the variable accumulation of higher alcohols in the top-fermenting yeast at different inoculum sizes is mostly accounted for in the stationary phase. We discovered that lower pitching rate was associated with a negative effect on amino acid metabolism and synthesis of higher alcohols during the stationary phase, leading to the decrease in higher alcohol concentration at low inoculum densities. Overall, our study provides valuable insights that could benefit wheat beer production.