Viability of Harvesting Yeast that Has Fermented High Gravity Worts
 

There has been some talk that one shouldn't harvest yeast that has fermented any wort above 1.060. What changes between yeast cultures that have fermented normally and cultures that have femented higher gravity worts? Both cultures are made of live and genetically identical yeast populations.

The biggest reason to not use the cultures that have been fermented above 1.060 is those yeast have been put through a more stressful ferment overall. More sugar to begin with, more alcohol to deal with later in the ferment. So these stressed yeast tent to throw out more off flavors.

That implies that a yeast permanently alters its metabolism after being subjected to a high gravity ferment. I'm not trying to prove I'm right or anything, but I'm quite skeptical of these claims that are not backed by any real scientific literature. The way I understand it is that the next generation will have identical DNA and have no "knowledge" of the previous generation's environmental conditions. The new generation responds to its current environment based on what its DNA tells it to do. This means that there will be no change in the flavor compounds produced between a normally fermented yeast strain and high gravity fermented yeast strain.

So the bad taste is likely from either from autolysis, or from spores... This is certainly interesting. If it is from autolysis, there shouldn't be a problem using the yeast again. But, if the yeast starts to sporulate and those yeast spores somehow alter the flavor of the beer in a bad way, then there could be a problem. I'm willing to to bet that a teaspoon of yeast taken from the trub of a high-gravity brew and then made into a starter will perform exactly the same way as the yeast that came from the LHBS.

I think your assumption that all yeast cells are identical to their parents is flawed. Yeast can mutate.

So you're implying that yeast that have undergone high gravity fermentation mutate and create off flavors. Am I getting this right?

I've read or heard that yeast mutations are highly exaggerated. Wish I could remember where I heard that. Something like 10 batches using the same yeast and there was no noticeable off-flavors. I doubt I'd ever be able to use it that long myself, but it was an interesting bit of info.

I am pointing out a flaw in your reasoning.

Every living thing mutates. When you put survival stress on things, you're more likely to see some of those mutations.

Yes. One musn't deal in absolutes. Especially when discussing biology. I value your input and thank you.


My organismic biology professor mentioned the same thing when discussing evolution and mentioned the increased mutation rates of E. coli. Mutation rates are suggested to be higher in a population that is outside of their optimal environment.

The thing I love about biology is that there are so many possible variables that our brains can get overwhelmed in trying to determine the cause of a phenomenon. So it helps to have a subjective perspective on such matters as this one. Sporulation, autolysis, mutation, giant flying spaghetti monsters... The Viability of Harvesting Yeast that Has Fermented High Gravity Worts is still open for debate.

As I understand it, along with potential for mutation under stress is the health of the yeast after fermenting a high gravity batch.

There are two risks to the health of the colony in a high gravity wort. The first risk is exposure to high levels of alcohol. Alcohol is in fact unhealthy for, and basically poisonous to yeast. The higher the concentration of alcohol that the yeast has lived in, the more unhealthy the surviving population will be.

The second risk is hops. In many, though not all cases, a high alcohol batch of beer also has a higher level of hops used in it. Hops have a natural antibacterial property that works in that the properties of the hops actually coat the cell walls of the bacteria interfering with the uptake of nutrients and the reproductive functions.

Yeast is coated and affected in this same manner, so if you had a wort with a lot of bittering hops in it, again the yeast surviving the fermentation would be at less than optimal health.

As the gravity and IBU of the wort increase, so does the detriment to the health of the survivng poulation. A gravity of 1.060 is a ballpark figure that is often used to apparently represent a base level where these affects are starting to become significant. I do not believe it is meant to be an absolute, just a level at which to begin a closer consideration of re-using your yeast.