The influence of yeast format and pitching rate on Scotch malt whisky fermentation kinetics and congeners

Yeast format and pitching rate are variables which can be easily manipulated in a distillery environment but are seldom altered. Fermentations using dried and liquid yeast were studied at laboratory scale and compared by application of a 4-parameter logistic model to measurements of the decline in apparent extract during fermentation. Congener analysis of new make distillate allowed comparison between compounds of interest produced during fermentation.  The liquid yeast format demonstrated a significant reduction (p<0.05) in lag time, which was 68% shorter than dried yeast. Despite this, longer overall fermentation times were observed due to a slower exponential phase as compared to the dried yeast format. Alteration of the yeast pitching rate using dried and liquid formats, demonstrated that high rates resulted in reduced lag times. The maximum fermentation rates (Vmax) were assessed from the fermentation models and no consistent trend could be identified. Dried yeast was observed to achieve its maximum fermentation rate when underpitched. Substantially higher Vmax ­values were obtained with overpitched liquid yeast fermentations, resulting in faster fermentations, compared to underpitched liquid yeast fermentations. The liquid yeast format created more esters compared to dried yeast. The concentration of ethyl esters generally trended downwards for fermentations pitched with less yeast which fully attenuated. By combining the results of these studies, distillers can make informed decisions to optimise spirit character, quality, and distillery production.