More on Yeast Rehydration

The issue of dry yeast rehydration has been following me around since about the time I first wrote about it. A conversation here and there sparked up, often resulting in lengthy discussions on the topic of water vs wort and temperatures. Some took my suggestions and ideas readily, backed up with some info from a scientist who worked on these, while others were more skeptical. In either case it seems that people who tried rehydration in warm water preferred it to sprinkling straight into wort. The hardest issue for people to understand seems to be the matter of yeast viability. I find it very hard to explain that making a starter with dry yeast is pointless because osmotic pressure would kill a large portion of the initial culture and the time would be simply spent on rebuilding it to its original numbers. It doesn’t seem like it’s a very difficult concept to grasp, but the issue persists. Rather than debating and trying to back it up with simple scientific logic, I decided to conduct a little study to address the questions of temperature as well as sugar and hop compound concentration in dry yeast rehydration.

The original study was supposed to just be a temperature series in water, but right when I was about to start, it occurred to me that repeating the same series with different worts would also be interesting. The experiment consisted of three series:

Water – just plain sterile water.

Starter – a simple standard starter made with 100g DME per liter of water. OG ~1.040.

High OG/IBU – some wort I had saved in my freezer from an IPA I brewed some time ago. OG of 1.068 and 123 IBU.

The idea was to test the concept of temperature and osmotic pressure as well as assess the effects of high hop compound concentration on ensuing yeast viability after rehydration. For that purpose I used a packet of Danstar Nottingham that’s been sitting in my fridge for a while. It’s safe to assume that the viability of that culture is not at its optimum, but it’s what I had on hand and its response should be comparable to that of a fresh pack.

Experimental design was simple and straightforward:

Incubate yeast for 15-20 minutes in each medium at following temperatures: 35C (95F), 32C (89.6F), 25C (77F), 16C (61F) and 4C (39F). Stain with trypan blue and count dead and living cells. For each data point, between 120 and 1500 cells were counted, totaling in around 5000 cells. Let me tell you it wasn’t at all fun. In cases where cell viability was so low that counting would have been a waste of time (see example below), I assigned values of 1 and 5% based on qualitative observation.


An example of a sample with such low viability that counting was useless. Red arrow = dead cell. Green arrow = live cell.

The results were not completely unexpected. As expected, cell viability was much higher in yeast rehydrated in water rather than in wort. As the temperature decreased so did the viability, though more rapidly than I would have expected. This could be attributed to the age of the packet, but getting 85% viability in water suggests that they are still pretty healthy. What’s interesting is that starter wort largely imitates the water curve, but with significantly lower live cell counts, but high OG/IBU wort does not. It seems that in those conditions yeast get killed very quickly in warmer temperatures and they survive better in slightly cooler conditions. At around 17C (63F), which is a normal pitching temperature for most beers, the starter and high OG/IBU wort intersect and are almost the same as the temperature decreases. This suggests that when it comes to sprinkling dry yeast straight into wort it makes no difference if it’s a small or a big beer. Osmotic pressure will kill them just the same.


Final verdict: Warm water is better for rehydrating yeast than cool water, starter or pitching straight into beer.

I suppose the viability curves would be a bit higher if the pack was fresher and they vary a little from batch to batch, so I constructed a theoretical viability curve chart based on these findings, but with an increase of 10-15% so as to not surpass  100% viability and decrease the sudden drop below 30C.


I hope this little study has been helpful to you since some of you have been asking me to do a temperature series for a few months now. I also hope that this cleared up some doubts and concerns that you had regarding yeast rehydration.

Note: Don’t forget to equilibrate the rehydrated yeast prior to pitching into wort as a large temperature differential may kill or mutate a considerable portion of the yeast. Allow the rehydrated yeast to passively cool to temperature of your wort, or add small portions of the wort to the yeast until the temperatures are very close.

As always, comments, discussion, critique are always welcomed.




20 thoughts on “More on Yeast Rehydration

  1. This was very informative and helpful in calculating yeast viability. Are you aware of any studies that compare flavor/ester profiles of yeast rehydrated at various temperatures?

    Admittedly, I’m unfamiliar with dehydrated yeast as I mostly use liquid yeast, but it would seem that yeast rehydrated at higher temperatures that are then pitched into wort at pitching temperatures might get stressed out and potentially cause off flavors, mutate or die due to the temperature differential. I know with liquid yeast the labs state to not pitch the yeast into wort that is more than 5°F different in temperature.

    • You’re right, Luke. But then again you aren’t supposed to pitch the “hot” yeast into cold wort. The rehydrated mixture is slowly equilibrated either by adding cold wort little by little until it’s near wort temperature or allowing it to passively cool until pitching temperature. I imagine the shock of going from hot to cold would kill a portion of the yeast cells. Sounds like we have a topic for the next experiment!

  2. The big debate on Forums in the UK is why bother rehydrating when the yeast manufacturers say just pitch dry? Followed by lots of anecdotes about how their beer is great. Personally I have always rehydrated and followed Chris White’s technique in the Yeast book. What I would like to see you do is a straight experiment that compares rehydrating with this proven method against pitching dry. It would give me great pleasure to link to the results.

    • Hi Ashley!
      I don’t think I’ll do a study like this myself, or at least not in the near future. Partly because it’s time consuming, and partly because it’s been done already. As with everything in science, everything is a combined effort and no one group does everything. So in this case you can do the same. This little study shows that there is a difference in yeast viability between rehydration in water vs wort, meaning that a straight pitch into the wort will result in underpitching since a significant part of the yeast will die. Now, Jason did a pitching rate study a year ago ( showing how pitching rate affects the resulting beer.
      I think you can combine these to use in your argument and support your point.

  3. Wow! Your data shows effects of wort on viability very clearly! I’ve only tested hydration of yeast with water and that was over a much smaller temperature range. I would like to give this a shot and try to duplicate your results.

  4. Very nice results. This is an experiment that’s on my TO-DO list as well. I’m really surprsised to see the strong optimum for the water temperature. I’m with Steven that repeating this will be very interesting.

    I have been thinking about using 3M petrifilm to count live cells and get around the issue of the limitations that staining has. But a pack of 50 is $80.


    • Yes, Kai, repeat this when you get the time. It would be cool to see how our results correlate or differ.
      Petrifilms look a lot like pre-made mini agar plates. Not exactly the same, but same principle. Give it a try if you want. Or you can just make your own plates or buy pre-poured ones for around the same price.

  5. This was a great post. When I don’t have a lot of time, I’ve occasionally done BIAB 3.5 gal. batches, chilling partially and then letting my fermentation fridge do the rest of the cooling. The next morning, I would pitch a packet of dry yeast, expecting a lot to die off and have a decent pitching rate for 3.5 gal. From your data, it underpitched by a significant amount. Looking at my notes, all three beers I made using this method attenuated below or on the low end of the expected range. Now I understand why that might have happened. Thanks!

    • To be honest until last year I too used to just sprinkle it straight into wort, but since I started rehydrating and looking up more info on that I’ve become a convert and believe that warm rehydration is the way to go for a healthy pitch. These data strengthen my opinion, though I didn’t think that 32-35C was so different from room temp.

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  8. The practice of adding dry yeast directly into the wort is poor and thwart with dangers for yeast viability. Dry yeast should be rehydrated in 10 times its weight in water at around 40C. Sprinkle in without clumping (very gently mix) then leave undisturbed for 20 minutes before another stir. Lower the temperature slowly until it’s within 10C, or less, of the wort then add. Longer than 30 minutes and yeast viability declines. (I’m a winemaker of 40 years experience, and homebrewer, but principles are the same)

  9. 30-35 °C is actually the temperature range that Danstar recommends. So my first impression was, you just got the same answer that is already on the package. Then again, the package also says you can pour it directly into wort which is apparently a bad idea.

    So, do you think this temperature optimum does also apply to fermentis yeasts, especially the lager strains? If i recall correctly, fermentis recommends 27 +/- 3 °C for ale strains and 24 +/- 3 °C for lager strains. Since the drop in viability below 30 °C is pretty sharp, this would make quite a difference.

  10. This is a rather newbie question, but why does temperature matter when rehydrating dry yeast cells? Higher temperature increases osmotic pressure, so maybe between 30-35C is when the rate of osmosis is just right for the cell? If so, then why does a slower rate of osmosis kill the cells?

    Thanks so much for your work!

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  13. Great article thanks.

    There is no doubt on your viabilty curve that shows clearly 30-35 deg C is best rehydration temperature for the strain of yeast tested .

    However I ask whether the same applies for dried lager yeast, fermentis s-189 in particular where the manufacturer recommends 23 deg c as the optimal rehydration temperature. In such case, I am wondering whether the “peak” in the curve you have produced would be shifted 10 deg C to the right ?

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  15. Great article, and very informative. Just curious if you have any thoughts of difference in Ph from hydrating in fresh water, and pitching to wort. Will this cause stress similar to difference in temperature delaying yeast in activity?

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