yeast calculations

[brewinhard]

Here is something that I have been pondering for a bit and was looking for some insight from the army.

If I harvest some slurry of ale yeast (50 mL after compaction) from a primary and store it for a month (or so), then make a 2 qt. starter, cold crash, then use that slurry (200mL) to pitch into a 3 qt. starter all on a stirplate, then how old is the yeast considered to be for determining pitch rates for instance on Mr. Malty?

Do you calculate using the original harvest date or from the last starter time?

[spiderwrangler]

I would consider it the most recent propagation, as that is going to produce new cells.

[Ozwald]

Since the Mr. Malty calc is using that date to estimate viability (which isn't going to be all that accurate by nature), I'd agree that your closest bet would be the last prop. Depending on how/if you're rinsing, you may have more non-yeast in there though. The only real way to know would be to do a stain, which will get you a lot closer but still isn't 100% precise.

[spiderwrangler]

My understanding has been that doing a rinse/settle before stepping up 'old' yeast will help to pull out the better yeast to go into the starter to be stepped up.

[Ozwald]

That's pretty much what I do. When I'm being real picky (as opposed to just the regular level of picky) I'll rinse down to ~5% of what I originally collected (calculated after the initial rinse to get the major stuff out) & grow from there. I've even grown it up 4 steps & rinsed again with excellent results, but it's a ton of work.

[brewinhard]

Agreed it is a lot of work and time. I am sure there has to be some non yeast material in there along with some dead cells. Although the last stepup I did resulted in a much lighter color, cleaner looking yeast than when I started.