This week I cover the topic of osmotic shock in yeast, and how it can impact the fermentation performance of very high gravity beers, meads, wines, and concentrated seltzers.
Osmotic Shock in Yeast Cells
I had never heard of the term osmotic shock until I started making very high gravity meads. Some of my large melomels (fruit meads) had starting gravities in the 1.160 (36 Plato) range with alcohol content higher than 15%, which is high enough to create osmotic shock or stress.
Osmotic shock, or osmotic stress occurs when there is a sudden change in the concentration of the solution surrounding a cell (Wikipedia). In the case of high gravity beverages, we are typically introducing either dry or liquid yeast into very high gravity solutions. The difference in sugar concentration inside and outside the cell can cause a very rapid movement of water across the cellular membrane. This rapid change can cause the cell to rapidly shrink or expand, and in extreme cases either burst or die due to compression (apoptosis). This can kill a significant portion of your yeast cells off, resulting in poor yeast viability and poor yeast performance during fermentation.
Single cell organisms like yeast, tend to be more vulnerable to osmotic shock as they are directly exposed to the solution, and lack a hard cellular wall. Introducing dry yeast directly into a high gravity solution can add additional stress as they yeast is simultaneously trying to hydrate and cope with a very high sugar concentration before it has had time to regulate its membrane.
When Osmotic Shock Might be a Concern
The risk of osmotic shock is highest for very high gravity solutions such as very high gravity beers, high gravity wines, many meads, ciders and even seltzers which are often fermented at a high sugar concentration and then diluted post fermentation. Examples include barley wines, high gravity stouts, sack meads, big fruit meads, and concentrated seltzers.
As I mentioned earlier if you are direct pitching dry yeast into your wort or must without hydrating it you run a higher risk of shock. However, even liquid yeast can be affected at high gravity wort or must.
Since each yeast strain has a slightly different level of tolerance, it is hard to define a hard cutoff where osmotic shock becomes a factor. My personal experience is that somewhere in the 1.100 (23.7P) original gravity range I start to think about osmotic shock as a factor and take steps to mitigate it.
Mitigating Osmotic Shock
Nor surprisingly the yeast cells have mechanisms in place to deal with osmotic shock, and they can regulate significant changes in their environment as long as it is done over a long enough time frame. They key here is to introduce the yeast to the high gravity environment over time rather than shocking it by dumping it in all at once.
- For dry yeast, this means properly hydrating your yeast up front. If you follow the steps mentioned here, you will give your dry yeast the proper starting environment and a chance to hydrate and regulate their cellular membrane before dropping them in a high gravity solution.
- Whether you are using dry or liquid yeast, the next step is to SLOWLY introduce the yeast to your wort or must. You do this by adding a small amount of wort or must and mixing it into your yeast starter. Then you let it sit for 5-10 minutes before introducing some more wort or must. You should repeat this several times until the sample has a substantial portion of wort/must and then finally pitch it into your fermenter.
The above process slowly raises the gravity of the yeast starter solution over time, allowing the cellular membranes to properly regulate and balance to avoid osmotic shock.
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