HOMEBREW Digest #5845 Tue 14 June 2011

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  Yeast mass vs. Yeast count (Fred L Johnson)

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---------------------------------------------------------------------- Date: Tue, 14 Jun 2011 16:51:22 -0400 From: Fred L Johnson <FLJohnson52 at nc.rr.com> Subject: Yeast mass vs. Yeast count I have been propagating yeast slants for starters, and I have been curious about the number of cells that a starter will produce for a given amount of sugar (gravity), so I've been calculating the number of cells produced for worts of various specific gravities and for various volumes. My propagation method has been to step up the culture from a small sample obtained with a loop from a slant with something like 20 mL + 200 mL + 1800-2000 mL wort. The culture is performed using a spinner flask with filtered air pumped into the headspace constantly to the 200 mL and 2000 mL additions. I typically use wort with a gravity of 1.040 with added zinc at 0.2 mg/L and Fermax 0.25 teaspoons/L. But the wort gravity isn't strictly controlled in all cases (although always known), and some cultures have been performed with wort with a gravity as high as 1.060. I count the cells while they are still suspended at the end of the propagation, when all of the fermentation is complete and there is no or little growth. Nineteen propagations have been performed in which the cells were counted at the end of the propagationd. Seven different strains yeast strains were used. My hypothesis was that there would be linear relationship between the amount of cells obtained per mL and the gravity of the wort, i.e., that there would be a linear relationship between the number of cells produced and the amount of sugar provided to the cells. I have seen a fairly high variability of the number of cells I obtain from such propagations, mean=252.3 million cells/mL, S.D.=115.5 (coefficient of variation= 0.46), range=91-533. Thinking that this variability is at least partly due to the actual amount of sugar available to the cells (my hypothesis), I corrected the final cells counts for differences in wort gravity. However, contrary to my hypothesis, this did not appreciably reduce the variability in cell counts: mean=21.3 million cells/mL/degree Plato, S.D.=8.0 (coefficient of variation=0.38), range=8.5-46.7. I noticed that the size (mean cell volume) of the cells produced from the different strains is also considerably variable--something I had not ever heard expressed in the homebrewing literature. Some strains produce large cells, and other strains produce very small cells. (For example, Wyeast 3944 propagations resulted in lots of small cells compared to other strains.) It seems that for consistency of fermentations, the number of cells pitched may be less important than the total mass of cells pitched into a wort. Wouldn't one expect a large cell to metabolize more sugar per minute than a small cell? It seems possible that the total cell mass produced per unit of sugar may be less variable than the total number of cells, especially when comparing across a large number of strains and cell sizes. I have not attempted to quantify the cell mass of these starter cultures, but I am now sensitized to the possibility that the number of cells pitched is possibly less important than is the total cell mass pitched and that controlling for cell mass may be more desirable and will produce more consistent results than controlling the number of cells pitched. Has anyone ever heard of pitching rates expressed in terms of cell mass rather than cell number? I suppose that is essentially what is happening when brewers pitch by "volume" of packed cells, not even bothering to count the cells. In the case of Wyeast 3944, I estimate that the mean cell volume may be 1/3 that of other strains and that I would not be overpitching if I pitched three times the cell number of cells compared to other strains. Any thoughts on this issue? I'll probably also send this question to Wyeast and ask for their comments. Fred L Johnson Apex, North Carolina, USA Return to table of contents
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