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Contents:
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
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End of HOMEBREW Digest #5845, 06/14/11
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