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.

There are two forms in which yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple life cycle of mitosis and growth, and under conditions of high stress will generally simply die. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple life cycle of mitosis and growth, but under conditions of stress can undergo sporulation, entering meiosis and producing a variety of haploid spores, which can proceed on to mate. -Wikipedia

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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.

remilard said:

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

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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.

amercuric said:

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

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I am pointing out a flaw in your reasoning.

amercuric said:

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

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

remilard said:

I am pointing out a flaw in your reasoning.

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

Synovia said:

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

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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.

Don't forget, even though we like the alcohol they produce, it is actually excremented waste product from the cell. I think it was Jamil that equated it to us taking a bath in pee.

Mutations are often present in a gene pool. Sometimes these mutations aren't noticeable until an event occurs. Such as high levels of alcohol. The surviving yeast would display characteristics that would allow them to survive in these hostile environments. These mutations would thus be beneficial...this describes life in general. Beneficial mutations lead to the development of organisms...ie evolution.

Equate this to the fact that antibacterial soap no longer is going to work as bacteria are adapting. What happened? The antibacterial soap killed 99.9% of the bacteria. That .01 went on to reproduce and pass on its survival characteristics.

It's quite possible that the high gravity survivors could put out nasty flavors, however, without isolating and testing them I don't think you can say that they do for sure. They aren't in optimal conditions, however, some plants produce the best fruit under certain environmental stresses.

Anyway, what is the real gain of having such a strain of yeast?

Zen_Brew said:

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

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The question is, assuming minimal mutation, will making a starter from a portion of the slurry with a low gravity, un-hopped wort create a healthy yeast sample?

xalu said:

Mutations are often present in a gene pool. Sometimes these mutations aren't noticeable until an event occurs. Such as high levels of alcohol. The surviving yeast would display characteristics that would allow them to survive in these hostile environments. These mutations would thus be beneficial...this describes life in general. Beneficial mutations lead to the development of organisms...ie evolution.

Equate this to the fact that antibacterial soap no longer is going to work as bacteria are adapting. What happened? The antibacterial soap killed 99.9% of the bacteria. That .01 went on to reproduce and pass on its survival characteristics.

It's quite possible that the high gravity survivors could put out nasty flavors, however, without isolating and testing them I don't think you can say that they do for sure. They aren't in optimal conditions, however, some plants produce the best fruit under certain environmental stresses.

Anyway, what is the real gain of having such a strain of yeast?

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I was wondering the same thing. Why not take some of the fresh yeast and harvest that at the outset? At least if you are looking for consistency.

So, we have survival of the fittest plus the possibility of 'something else' being present in the left overs of the process. Most of us have to be getting something else in the way we aerate the wort. That something else may never flourish given the procedure but could be more prominent in harvesting the leftover and could make a decent percentage gain in the next batch.

AnOldUR said:

The question is, assuming minimal mutation, will making a starter from a portion of the slurry with a low gravity, un-hopped wort create a healthy yeast sample?

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Maybe.


Theoretically, this low gravity wort would select against exactly the sort of mutations that the high gravity wort had selected for. Who knows.

Depends on what you mean by high gravity. 1.120 is probably too high. Up to 1.090 I doubt there would be any issues. IBUs are the biggest thing I would worry about. A BGSA at 30 IBUs is going to have happier yeast than a 100+ IBU west coast imperial IPA.

If your harvested yeast cells are coated with the alpha acids, can't you remedy that situation by only harvesting a small amount and doing a starter?

Wouldn't this also help with "stressed" yeast or overworked/exhausted yeast?

There are really two things here. Firstly, "stressed" yeast. Secondly, mutated yeast.

"Stressed" yeast are purely physiologically in a sub-optimum (for growth) state. They are going to have limited viability. This means that they may not reproduce readily if you put them into a sub-optimum culture medium. If you harvest a "stressed" yeast and try to put it straight into a high gravity, high IBU, low oxygen wort, it might not get to take off. However, if you harvest viable stressed yeast (by yeast washing, for example) and allow them to "relax" by placing them into a nice 1.040 unhopped starter with lots of nutrient and oxygen, they will quickly re-adjust and do just fine. Stressed yeast can return to unstressed yeast, and make you great beer again.

Now, mutations are different. Every culture of yeast is going to harbor genetic variation. With enough reproduction cycles in an uncontrolled setting like a homebrewery, the "wrong" yeast are possibly going to start getting more of a foothold in your particular sub-culture. These yeast are going to give a slightly different malt profile, or have altered alcohol tolerance for example. I have heard that if you keep re-using yeast, it will last about 6 batches of beer before it starts to give subtly different profile to your beer. Some people claim that supposedly the same strain from White labs and Wyeast can have different profiles, possibly due to different culturing like this. Now, in a microbiology lab, the original strain can be kept pure by a variety of methods. But not so in "the wild" as it were.

SO what happens when you combine stress and mutations? Basically, a stressed out yeast is going to be less viable, for the reasons already mentioned. It will have to adjust its metabolism before it can start to grow. All of this gives "other" varieties within the same culture (i.e. you beer) a chance to grow a little bit better; reproduce a little bit more. Yeast which are fitter will have a slight competitive edge, and come to dominate. "Stress" (such as low oxygen, or high alcohol) is a selective pressure. The fittest yeast will grow fastest. The population will adapt, because of genetic drift. Basically, the more stressed a yeast is (the more selective pressure that is applied to it) the faster the population will evolve.

Did you realize that everytime you make beer, you are providing more support to the Theory of Evolution (first published 150 years ago today!)?

Now, the actual changes that take place in your brewery are going to depend on a lot of factors. Where did you get your initial batch of yeast from? How genetically pure was it then? What stresses did it get subjected to? How well did you recover active yeast? How many reproductive cycles has it had (what generation is it)? So to say NEVER use yeast from a 1.060 beer is being to absolutist. You need to judge a lot of other factors too.

Hermit said:

I was wondering the same thing. Why not take some of the fresh yeast and harvest that at the outset? At least if you are looking for consistency.

So, we have survival of the fittest plus the possibility of 'something else' being present in the left overs of the process. Most of us have to be getting something else in the way we aerate the wort. That something else may never flourish given the procedure but could be more prominent in harvesting the leftover and could make a decent percentage gain in the next batch.

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I sometimes do that. I will take a new vial of yeast and only pitch 50% of into the starter. So now I have ballpark 50% of the yeast in my starter, so to correct for the lower cell count in the starter I set the Mr Malty caculator age of yeast until it shows 50% viability then make that volume of starter.
Other times I just make maybe 100-200ml more starter than I need and pour some off into a sterile canning jar to save for other batches. I like this procedure because I know I have unstressed unmutated yeast for future batches.

Another way to get optimal yeast for future batches is to top crop from the primary. Of course this assumes top fermenting yeast. The bonus is you get 100% viable aggressively fermenting yeast at peak activity that have only been exposed to alcohol and hops acids for a limited amount of time. I have heard such top cropped yeast referred to as superman yeast as they apparently perform very well in subsequent fermentations/

There are also some advantages to getting yeast that have completed a fermentation. I have heard and read from several brewers that yeast sort of hits a stride around the 2nd or 3rd generation (assuming no extremely adverse conditions prior). The yeast start faster, finish cleaner and seem to attenuate better.

So there are definitely more than one way to skin a yeast. Remember no matter what other factors are at play, good oxygen up front, correct temperature, and lots of nutrients goes a long way towards a happy and healthy yeast colony.