Vicinal diketone production and amino acid uptake by two active dry lager yeasts during beer fermentation

Amino acid consumption and vicinal diketone (VDK) production of two different strains of commercially available active dry lager yeasts (ADLYs) were compared with freshly propagated cultures of the corresponding yeast strains. Rehydrated dry yeasts and freshly propagated yeasts were pitched in all-malt wort and allowed to ferment at 14 degrees C. Dry yeast strains took 15-26 hr longer to attenuate the wort to final gravity and 24-48 hr longer to significantly reduce wort amino acid content. A new indicator was introduced as a means to quantitatively characterize VDK production and reduction profiles. This indicator effectively demonstrated a large discrepancy in the diacetyl profile between the fresh and dry cultures: fermentations made with dried yeasts resulted in 1.5-7 times higher diacetyl indices. With one of the dry strains, diacetyl concentrations remained over the taste threshold value after 7 days of fermentation. These differences could be linked to a reduced ability of the rehydrated mother cells to absorb wort constituents caused by cell membrane damage occurring during the ADLY production process. Overall, dry yeasts resulted in a reduced level of performance compared with freshly propagated cultures, suggesting that the direct utilization of ADLY as an inoculum for beer fermentation at the industrial scale is not recommended.