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Dry Yeast Viability, Take Two

Background

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.

Experimental Setup

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.

Results

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.

Conclusions

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.

RAWR! Recipe (PDF)

7 comments to Dry Yeast Viability, Take Two

  • Again, great stuff. Though, I continue to balk rehydrating in water because of worries about one more contamination point, you have the preliminary data to back up this additional step.

    Do you have a hypothesis to account for the difference in taste and viability?

    • Norm,

      I had expected the R and 2U samples to be the most similar, and for U to exhibit the most off-flavors. In hindsight, though, 2U had about twice as many dead cells pitched as the other two, and so it makes sense that it would have the most autolysis character.

      Sean

  • Travis

    Sean, nice work again. How did you obtain your pitching rates? I see you used methylene blue, but was the sample taken from inoculated wort?

    • Travis,

      Yes, the yeast was rehydrated in either water or wort and allowed to rest for 30 min before the samples were taken for cell counts. The yeast pitched into the beers were then measured out volumetrically from those same two sample containers.

      Sean

  • Terry

    Sean, great work with this experiment. You mention (R) was rehydrated in room temperature water. Rehydration instructions suggest 75-86 degrees F, but I am thinking your room temp is near 68 degrees well below the lower limit. In the BYO article, you rehydrated 2 samples in water, one at 80 degrees and one at 64 degrees and the results showed the cooler sample had a higher viability level. I was curious why you did not add a second water sample at the lower (room) temp in this “take two” experiment? If in fact I could rehydrate around 64 degrees, I would do it every time since my wort is chilled to around 64 degrees when I pitch.

    • Terry,

      In fact, my room temperature is rarely as high as 68°F; for this experiment it was probably about 60°F. I decided not to use warmer water simply to eliminate that as a variable. The viability of the yeast in the first round of trials showed little, if any, dependence on temperature.

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