I've harvested yeast from the primary after 4 or 5 weeks with very good results; the dead yeast drops out with the trub when you do the water rinse thingy
Originally Posted by Laughing_Gnome_Invisible
I've been wondering about this too lately. If you secondary, then that yeast is supposed to be lower quality and not worth harvesting. If however, you do not use a secondary, then that same yeast will be present in the final harvest? Or are all the yeast in the extended primary all the the same? Hmmmmm. Yeah, I need to know too. I'm wondering if you can harvest a better quality from the primary, then racking.
it's mostly a matter of selection - if you harvest from secondary, you're selecting for poor flocculating yeasts; so your next batch with that yeast may have issues clearing (and IIRC, those poor flocculating yeast also tend to be more highly attenuative (? was it more or less? in any case, they change in other ways as well).
If you harvest from the primary, you'll still get some of those yeasts, but you get them in more-or-less the same ratio to all the other yeasts that you started with, so the population as a whole acts the way it should terms of flocculation/attenuation/etc..