Trying to understand yeast viability

[jmf143]

I pitched a Wyeast Propagator that was about 4 1/2 month old into a liter of starter. Due to the age of the yeast obviously I wouldn't have 100% viability - assume for discussion sake its only 20%. I'm going to step it up by decanting the liquid and re-pitching the slurry. Since I'm pitching the yeast that grew from the original Propagator is it safe to assume this "2nd generation" of yeast has closer to 100% viability?

 

[ajf]

I'd make a very small starter for the first stage, say 1/4 - 1/2 pint. Then leave it about 2 - 3 days. This will let the yeasts wake up and start budding. Then just add more wort to bring the volume up to to the required final volume without decanting. You will need to add at least 1 liter/quart for this second step. If you decant and pitch the slurry, you will lose a lot of your most viable yeast which will still be in suspension. When the second stage is finished, then you can decant, because adding a large spent starter to the brew could result in adding some undesirable flavors.

-a.

 

[jmf143]

I should mention that I'll be cold crashing prior to decanting. Would I still lose yeast in suspension?

 

[Ichthy]

You'd still lose some yeast, but I would recommend following ajf's instructions if you want the largest number of yeast.

 

[Got Trub?]

I do a variation on ajf's recommendation. I "wake up" the yeast in the pack. I leave it 3, 4 5 days, whatever it takes to swell the pack, then pitch it into a liter starter. I'm lazy...

GT

 

[devilishprune]

I pitched a Wyeast Propagator that was about 4 1/2 month old into a liter of starter. Due to the age of the yeast obviously I wouldn't have 100% viability - assume for discussion sake its only 20%. I'm going to step it up by decanting the liquid and re-pitching the slurry. Since I'm pitching the yeast that grew from the original Propagator is it safe to assume this "2nd generation" of yeast has closer to 100% viability?

I think that the generation statement refers to yeast that have been exposed to entire batches of beer. In my opinion, your yeast would still be first generation.

To answer your question, yes. The point of making a starter is to increase th amount of viable yeast you pitch. Your yeast from your starter will have a much higher viability approaching 100%, but it will never truly be that high. You're always going to have dead yeast cells in your slurry that will lower the "actual" value (quotation marks because there's not a really practical way of measuring it). Just assume 100% viability after you make a reasonably sized starter.