Studies on the effect of mechanical agitation on the performance of brewing fermentations: Fermentation rate, yeast physiology, and development of flavor compounds

The effect of agitation intensity during standard brewing fermentations, using a lager strain of yeast, Saccharomyces cerevisiae NCYC 1324, and a model lager wort was investigated. A series of reproducible small-scale (500-ml) fermentations was carried out in which agitation was applied using two standard Rushton-type impellers. The specific power input was held constant within individual fermentations, but was varied over the range from no stirring to approximate to0.25 kW/m(3) for replicate fermentations. Standard fermentation parameters (specific gravity, dry cell weight, fermentable sugars, and flavor compounds) were monitored. There was a threshold for specific power input of 0.03 kW/m(3) at, or below which there was no significant difference between stirred and unstirred (control) fermentations. This value is approximately equal to that found in 400-m(3) cylindroconical vessels at the maximum CO2 evolution rate. Above this threshold, however, fermentation rate increased and, hence, time to attenuation decreased from approximate to168 to approximate to100 hr. The rates of formation and maximum concentrations of esters, higher alcohols, and diketones also were affected by increased agitation. Above the same threshold, a decrease in the final concentration of esters was found, whereas the concentration of higher alcohols increased. It is concluded that the observed changes are associated with enhanced turbulence at the scale of cells, leading to higher mass transfer rates coupled to metabolic processes. The use of multiparameter flow cytometry indicated that, up to 4 W/m(3), there were 6% dead cells after 168 hr, at which time the attenuation limit had been reached. After 168 hr at the highest specific power input (250 W/m(3)), the proportion of dead cells was 17%. However, after 100 hr when, at higher agitation intensities, the attenuation limit had been reached, there were only 9% dead cells. It is proposed that, at these intensities, the increase in dead cells between 100 and 168 hr was enhanced by a combination of exposure to inhibitory concentrations of ethanol and exhaustion of substrates during this period, as opposed to the effect of (mechanical) damage due to agitation alone. The results suggest that mechanical agitation might lead to shorter fermentation times and increases in fermentation reproducibility at all scales.