According to beer writer Michael Jackson, the Belgian ale Duvel is fermented with two yeasts, both originally isolated by De Clerck from 10-20 strains in McEwan's Scotch ale between the wars. See http://www.beerhunter.com/documents/19133-000020.html. Jackson writes "both [yeasts] are used in primary fermentation. The brew is divided into two separate batches, one for each yeast. These two batches are not of equal sizes." The separate batches are filtered and blended after secondary, and one of the strains reintroduced for bottling.

Why do you think this is done, rather than simply pitching them both into the same wort? Assuming that there is no K (killer) factor involved (since they both coexisted in the original McEwan's), how do you think the results would differ if the yeasts were both pitched into one wort in the same proportion as the sizes of the two batches at Duvel? I'd appreciate any further thoughts you might have regarding mixed cultures of yeasts in traditional and/or historic ales.

-Jeff Renner


RESPONSE:

Each strain has its own growth rate and nutrient requirement and can be controlled better when fermented separately. When fermented together, they may not remain 50/50 in the fermentation. One strain can easily out grow and dominate the other throwing the desired balance of flavor and aroma off balance.

In the wine industry, especially Champagne, it is not uncommon to ferment three or more strains separately then blend different preportions of each to achieve a desired mouthfeel, flavor and aroma profile.

Dr. Clayton Cone