Pretty good question.. I'd say yes, each step, which would be a separate fermentation, would slightly change the yeast. How robust it is to change is probably strain dependent. Would that be considered a generation in the yeast.. I don't know. However, I've read that folks such as The Alchemist will keep track of the generation of their yeast, as they can see the decline in attenuation, and will know based on taste and numbers what gen they are at due to using it so much. So if they get to Gen 12, and it's done, they'll get a fresh batch. To get a step up to pitch on the commercial level would be huge, so by that definition even if they had a single colony and pulled from that to start over without any mutations, it by that definition of a fermentation being a generation wouldn't be nearly "new".
Hmmmm, what do you mean by first generation? Maybe it's semantics, but I wouldn't call any yeast first generation. They've all be propagated up from something, even before they were banked by a brewery, university, or yeast manufacturer . ETA: If you mean first generation as in 'packaged by yeast manufacturer', then I wouldn't worry too much about changes during starter step-ups. If done in reasonable steps, and harvested from each step in a non-selective way, your population ought to be pretty much the same as where you started, just bigger.
as stated above, i would not worry about mutations in growing a starter. I, myself, assume my yeast will change a little when i feed the sample i have kept and then put it back in the fridge. I would say a definitive generation is each batch. I have top-cropped a yeast on a wed. and pitched it that weekend and noticed a change in productivity, it could have been pitch size though. I might tend to call each feeding a portion of a generation, based on what the yast goes through before it gets to me. If you are decanting a stepped starter the only yeasts you are losing are the ones that don't floc well.
My previous hob was at one of the major yeast producers. We would step up yeast starting from a shake flask and then use a fed batch method to grow the yeast. That means that by the time you get the yeast it has already passed through many generations. If I was you I would only count a generation when the yeast has gone through an entire fermentation, ie lag phase, exponential growth, and stationary phase. When it is growing in a starter it is ideally being kept all in exponential growth phase. If you only count each time it divides then there would be many generations in every batch of beer. We would also use the same "seed" flask for a month at a time (ale) or even longer for a lager. Which meant that over that month each new batch of yeast that was started was started from an older and older seed flask. Despite this there was little change in the characteristics of the yeast at the final step. So in my opinion I would not worry about mutation all that much when propagating yeast at home.
Is there anything you can share about their measurements of die-off rates for dormant yeast? Here's part of an email I sent to White Labs, but haven't (yet) received an answer... ...would it be better to assume some daily die-off rate as a function of the remaining cells each day, or as a function of just the original count?
It depends in large part on how they are being stored. We would "feed" the yeast with a mixture of malt and sugar if we were going to keep them in cold storage for more than a day or two. We also would toss out yeast if it was older than 2 weeks, our guarantee was 99% viability upon arrival at the brewery. So I would assume a starter in your fridge after two weeks is still pretty healthy. In my experience the viability (percent alive and dead) stays pretty high if kept cold for a month. However, the vitality (the yeasts ability to perform a proper fermentation) drops off quickly after about two weeks. I plan on buying some yeast from White labs and Wyeast and performing a viability and vitality test on them. I will make sure when I do that I report the finding here along with the dates on the vials. So to answer your question I would assume a daily die off rate after 2-4 weeks that is based on the function of the remaining cells (say 2% of the population dies every day), but I have not tested this.
Good info, please report back what you find. Would say that the vitality of the yeast would be more robust if, after 2 weeks, you made a good sized starter with some conservative assumptions on the vitality of the yeast?
