Part One of the Dry Yeast Series: Introduction
Part Two of the Dry Yeast Series: Rehydration
Part Three of the Dry Yeast Series: Temperature Differences
My latest post about dry yeast rehydration raised a rather unexpected wave of interest with surprising number of links from various websites and forums I didn’t even know existed. In about a week it became the most viewed post after Selective Medium. One of the readers (Luke) raised an interesting point regarding temperature difference between the yeast and the wort they’re pitched into. While I haven’t come across any studies of effects of this on flavor profiles, I found some whispers that pitching “hot” yeast into cool wort causes portion of the population to die. So I decided to do another mini study with dry yeast and whether or not this is true.
The experiments were designed as follows:
– Danstar Nottingham dry yeast were rehydrated in water at 32˚C (90˚F) for 20 minutes. Viability was assessed with Trypan Blue for all samples in the study.
– Samples of the resulting culture were placed into wort at various temperatures at dilution of 1:11 (0.1mL yeast into 1mL of wort) and incubated at those temperatures for 20 minutes, followed by viability assessment. Results summarized in Figure 1.
– After samples were taken, the culture was placed at 4˚C (39˚F) to passively cool until yeast began precipitating and culture started clearing up at the top (approx. 40 minutes).
– Viability of the resulting slow cooled culture was assessed, samples of it were placed into worts at various temperatures and treated as described above to observe the effects on yeast in a scenario of a washed yeast cake (since that’s done with water and stored in water) pitched straight into beer right out of the fridge. Results summarized in Figure 2.
Results:
Figure 1. Temperatures in ˚C correspond to following in ˚F:
90 | 0 (90 to 90) | 13 (90 to 77) | 29 (90 to 61) | 51 (90 to 39)
As seen in Figure 1, rehydration at 32˚C (90˚F) resulted in ~96% viability, which is consistent (though better) with the results of the previous study, validating it. Placing the yeast into isothermal wort results in retention of viability (~97%, higher due to random sampling). Rapid cooling from 32˚C to 25˚C and 16˚C (90˚F to 77˚F and 61˚F, respectively) resulted in ~20% viability loss in both cases, with statistically insignificant difference between the two. Rapid cooling from 32˚C to 4˚C (90˚F to 39˚F) resulted in ~40% viability loss.
Figure 2. Temperatures in ˚C correspond to following in ˚F:
39 | 0 (39 to 39) | 22 (39 to 61) | 38 (39 to 77) | 51 (39 to 90)
Results in Figure 2 show retention of ~96% viability in the culture that’s been passively cooled from 32˚C to 4˚C (90˚F to 39˚F). Transferring yeast to isothermal cold wort resulted in a significant viability loss of ~50%. Transferring yeast to wort at 16˚C and 25˚C (61˚F and 77˚F) resulted in ~30% viability loss, with statistically insignificant difference between the two. Transferring yeast to wort at 32˚C (90˚F) resulted in the most significant viability loss of ~70%.
Overall, both series largely imitate each other with major difference in initial isothermal transfer.
Discussion:
It looks like dry yeast really dislike the cold. In the previous post I showed that cold rehydration both in water and wort is detrimental to yeast health and this appears to support that notion. Transition from water to wort seems to be more detrimental in the cold rather than warm conditions. Percentage of cell death appears to decrease as the receiving wort gets warmer, ending with full viability potential at 32˚C (90˚F). As seen from gradual cooling of the yeast from initial rehydration temperature all the way down to 4˚C (39˚F), which retains full measure of viability, gradual cooling is a lot less detrimental than rapid. Can you pitch “hot” rehydrated yeast into your low-60s wort? Sure! Even with the loss of ~20% of cells, you’ll still probably have enough yeast there to ferment an average brew just fine. However, these data suggest that gradual cooling prior to pitching would be better. So if you’re willing to take that extra step of enjoying a pint or cleaning your gear as you let the yeast cool before pitching, I say “do it!” I think there are two factors playing here: 1 – temperature difference, and 2 – water to wort transition. Which is more important? Can’t say. Most likely it’s a combination of both. On one hand, we see that viability doesn’t change at all when you transfer from water to wort at the high end of the temperature series, but then when the same is done at the lowest end the effects are profound. Mid-range temperature difference is not all that horrifying, while large difference has profound effect in both cases. Perhaps the yeast dislike cold wort density and its dynamics. I don’t think it’s the weak cell walls because by that time they’ve had ample time to rebuild and utilize resources they have from dry preservation. Probably just general change of environment shock exacerbated by sudden low or high temperature that’s responsible for these results.
Effects of gradual cooling as well as equilibration with wort prior to pitching will be studied in the next part of the dry yeast series. Stay tuned.
Personal Notes:
– I originally planned to do the graduate cooling and equilibration study together with this, but as my time is limited and I just found out I have to present my research progress in 4 days due to sudden schedule change, I decided to put up these results now and finish the second part later.
– If anyone is interested in repeating these experiments in their home labs and validating or disputing my findings, please do. That way we can discuss our data and come up with more serious and credible results that would benefit the brewing community.
BKYeast 