Is Third-Generation Harvesting a Good Idea?

Since I've got my yeast washing operation down to a science, I was wondering how many generations I should go on a strain. Awhile back, I pitched a new packet of Forbidden Fruit into a wit. I harvested and cleaned it, and ended up with 4 jars in my fridge. I used one of them on a wheat last weekend, and the starter was very active; fermentation of the main wort took maybe 36 hours. I just racked it to my keg, and was wondering if I should bother washing & harvesting the third-gen cake in there. How many generations does it take for mutations to take place typically? Given that I still have 3 jars from the second-gen harvest, it may be overkill (and a waste of time & space) to harvest even more. I mean, the stuff's not gonna last forever, and I don't think I'll be brewing 8 beers for forbidden fruit any time soon.

Thoughts?

I remember a post from a while back that said that it took a minimum for 5-7 generations for noticable mutations to take place. You obviously have alot more experience then myself, But I have pitched on the same cake twice and it was fine. Original pitch, then pitched on that, then on it again.

Re-using yeast can save us a lot and I have made a lot of beer this way. If you take a sample of the yeast you washed and make a starter you can taste the beer produced and if it is clean tasting then I would trust it for another batch. Eventually it will start to produce a slight off flavor of some type and then I would give it the toss. I have made up to 6 brews from the same yeast but I do practice sanitary brewing so that sure helps. If you really want to make your yeast last then you should get some slants and petri dishes and start inoculating them with new yeast from a tube of White Labs or Wyeast and you can make up a few slants for starters and then you will know that it is truely clean yeast and not take as many chances. It's fun and saves money over all. I make large starters for my 12 gallon lagers in a carboy (2 to 3 gallons) using grain in a pan on the stove (with hops) and then pour off the cooled wort into a carboy and pitch yeast. When that is fermented I pour off most of the wort (which I save and bottle or keg) and pitch the yeast into the next brew.

I've talked to commercial brewers and they'll pitch onto the same yeast over a dozen times, so at 3 I don't think you have anything to worry about.

I think the mutations will depend mostly on how stressful a life the yeast have had. I have harvested 2nd generation yeast from a long drawn out fermentation, and it acted a bit strange the next time around. I won't do that again. Plus there is the sanitation factor - introduction of wild yeast. I'd think it would be great to use the whole cake for another quick brew if the last one off it tasted good, and then toss it.

mr x said:

I think the mutations will depend mostly on how stressful a life the yeast have had. I have harvested 2nd generation yeast from a long drawn out fermentation, and it acted a bit strange the next time around. I won't do that again. Plus there is the sanitation factor - introduction of wild yeast. I'd think it would be great to use the whole cake for another quick brew if the last one off it tasted good, and then toss it.

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Both beers have been relatively low-OG brews and neither have been unusual fermentations. This last one, like I said, took 36 hours tops, and the hydro sample I took today tasted very clean with no off-flavors to speak of.

As for pitching on the cake, I usually reserve that for ales where I'm not looking for much character from the yeast. With forbidden fruit, I expect plenty of character...but if I were to pitch on the cake, I'd be skipping the repro process and thus end up with a pretty neutral yeast character.

At my local brewery I talked to the head brewery (they give out free yeast to homebrewers ) He said that there yeast was originally wyeast 1056, but that it has basically turned into their own strain through mutations. I think he said that he is on generation 252 with the same yeast. He did also say though, that if a batch ends up having any undesirable traits, he throws out that one and re pitches with the previous gen the next batch......or something like that.

You could always haul some of the cake off when you rack.

mr x said:

You could always haul some of the cake off when you rack.

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True. The other problem with that, then, is that I'm not planning anything soon that would be good for Forbidden Fruit.

According to some of the discussions on Probrewer.com, yeast can be reused "hundreds" of times.............. When you're brewing barrels of beer at a time, saving a lot of money on yeast can certainly increase your bottom line.

Not to get all sciency either, but it should probably take many, many generations for any mutations to make a difference. The mutation rate in yeast is about 1 x 10^-10, meaning one mutation every 10,000,000,000 base pairs replicated. The genome of brewing yeast is 12,495,682 base pairs.. If you pitch 200 billion yeast cells, you shouldn't get more than 4 divisions, meaning 200 billion x 4 total cell divisions = 10 x 10^18 total base pairs replicated. That would mean A LOT of mutations in your wort. However, for a mutation to make a difference, it would have to be advantageous, not deleterious or silent, so as to increase the percentage of yeast cells with that mutation in the wort, as well as leading to some time of off flavor or other aspect of fermentation. The chances of that happening is incredibly low. So the take home message is ... mutations happen, but most, almost all, make no difference at all.

Science is fun.

I've used 5th gen successfully and 1 at the 6th, but no more.

Just throwing this out there from what I do:

When you grow a starter, save a portion of the starter before you pitch. Then just grow up a new started from that when you need it. Wash, rinse, repeat.

That way, you're always using first gen yeast.