HOMEBREW Digest #5850 Thu 23 June 2011

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  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 Return to table of contents
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