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Contents:
Re: Yeast propagation (Fred L Johnson)
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Date: Thu, 23 Jun 2011 07:07:42 -0400
From: Fred L Johnson <FLJohnson52 at nc.rr.com>
Subject: Re: Yeast propagation
BobZ (Robert Zukosky) has been propagating his yeast using low gravity wort
with aeration with the idea of avoiding the Crabtree effect and hence produce
more yeast with less wort. This method of yeast propagation intrigued me
greatly a number of years ago, and I tried my hand at it quite a bit. I
certainly was able to produce a significant mass of yeast, but I had lots of
problems with foaming in the starter. Some of the yeast from these
propagations looked pretty bizarre--pronounced fusiform shapes--and, more
importantly, I thought some of the beers made from these starters were not
very good, so I abandoned the method. My experience is NOT what has been
reported. Over 15 years ago, researchers were reporting that they could
produce excellent beer with yeast propagated aerobically in low glucose
media. I think I could do a better job with this method today than I did
years ago, and I may give it another try.
I'm not certain that Bob's 4P wort is at a low enough concentration to avoid
the Crabtree effect. The threshold for the Crabtree effect appears to be
about 0.4% glucose and a typical wort can contain 1.5% glucose (so I've
read), in which case a 4P wort would contain about 0.6% glucose, shifting the
yeast into fermentation mode. I would like to see the cell counts of Bob's
propagations to know if he is getting cell counts that are significantly
better than the standard step-up propagation method using the same amount of
sugar in lower volume. The aerobic (non-fermentative) method must provide
sugar at low concentrations by continuously infusing concentrated wort/sugar
into a volume that dilutes the sugar to a level below the threshold level of
the Crabtree effect. As cell numbers increase in the vessel, the rate of
medium infusion is increased proportionally.
I also was never able to find information on how quickly the disaccharides
in wort (like maltose) would be split into monosaccharides to further
increase the glucose level in the medium. (If someone has some figures on
this, I'd love to see them.) To use the method properly, one needs to monitor
glucose levels in the culture--something I never did. I suppose one could use
a medium containing only glucose as the sugar to avoid the potential problem
of having glucose levels rise from conversion of maltose and other
dissacharides, but I wonder if this would generate a yeast that would not use
the maltose in the wort into which it is ultimately pitched.
All of the propagation is to be conducted in an aerobic environment, which
Robert is doing. The air Robert is providing certainly helps produce some
healthy yeast, but I'm not certain that he is getting any more yeast than he
would if he added the same amount of sugar substrate using a higher gravity
wort (in a smaller volume).
Bob: If you could provide the cell counts from some of your propagations,
that would be informative to the discussion. If you would like to send me
small samples from a well-suspended culture, I could send you the cell
concentrations.
Fred L Johnson
Apex, North Carolina, USA
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End of HOMEBREW Digest #5850, 06/23/11
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