The last time I published the results of some dry yeast viability testing, I made the assumption that the reduction in viability that resulted from rehydrating the yeast in wort rather than water would have flavor impacts similar to under-pitching a liquid yeast culture. Shortly thereafter, James Spencer of Basic Brewing and Chris Colby of BYO Magazine threw down the fungal gauntlet by soliciting home brewers’ tasting results using dry yeasts, and I decided to take the additional step of actually fermenting and tasting beers using both techniques.
After brewing a recent IIPA with an OG of 18.9°P (recipe below) one gallon of wort was split into three half-gallon growlers and fermented using US-05. Two of the beers (U and 2U) were rehydrated in room-temperature wort, and the third (R) in room-temperature water. Based on the results of a methylene blue viability count, the pitching rates were:
- R: Rehydrated, viable pitching rate 0.75 million/mL-°P
- 2U: Unrehydrated, viable pitching rate 0.73 million/mL-°P
- U: Unrehydrated, viable pitching rate 0.34 million/mL-°P
The fermenters were covered with aluminum foil and left at room temperature, which varied from 16-20°C over the course of fermentation. To minimize variations between the fermentations, they were not agitated or aerated.
Once again, I observed a substantial difference in viability, with the yeast rehydrated in water yielding a viability of 72.7%, and the wort-rehydrated yeast 48.8%. Interestingly, this is a higher viability than was seen with yeast rehydrated in lower-gravity wort, though it’s entirely possible that the variation is simply within the error associated with methylene blue testing.
Fermentations in all three beers proceeded similarly, although R began visible fermentation sooner than U or 2U. It did, however take the longest for krausen to fall in R – 11 days, versus 10 days in U and 8 days in 2U. This may be at least partially explained by the lower final gravity of R, as estimated by refractometer readings. U finished at 1.015, 2U at 1.014, and R at 1.013. There may be some inaccuracies introduced by the use of the refractometer formula, but since the OG of each beer was the same, the relationship between the FG values should be correct. Based on a hydrometer reading, the main “control” batch of beer fermented with Wyeast 1272 finished at 1.0145.
A blind tasting revealed that while similar, there were distinct differences between the three samples. All exhibited some degree of “musty” yeast off-aroma, with the smell being strongest in 2U and least prominent in U. 2U also had the highest degree of esters (particularly peach/apricot), and was the only sample to exhibit an acetaldehyde flavor. R was the cleanest overall, with the lowest level of “hot” alcohol character.
A substantial reduction in viability continues to be seen for dry yeast rehydrated in wort. There were also some of the characteristic effects of under-pitching in the wort-rehydrated beers, although the differences were less than in the liquid yeast pitching rate experiment.
These results were also featured on this week’s episode of Basic Brewing Radio.