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	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
  Re: Yeast mass vs. Yeast count ("\\-s@roadrunner.com")


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Date: Fri, 17 Jun 2011 12:02:02 -0400
From: "\\-s at roadrunner.com" <"\\-s"@roadrunner.com>
Subject: Re: Yeast mass vs. Yeast count

Fred says ...
> 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), ...
>   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.

Very good stuff Fred.  Did you measure OG & SG then estimate a real
attenuation ?  Or just use the OG ?  Was the starter wort standardized
in some way or is it's attenuability & FAN variable ?


> 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's not surprising that yeast cells vary considerable in size.  I'd 
expect a lot
more varietal variation among ale yeasts.   There is also the funny business
of osmotic pressure impacting cell size.

At the end-point cells going dormant can accumulate a considerable mass of
trehalose, & glycogen as storage carbs.  This adds considerable to yeast
(dried) mass. Unclear about the impact on volume.  You may be catching
cells in various phases of carbo-loading, but your measured variation seems
too large for that.

> 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'd guess a higher rate of glycolysis in larger cells, but probably not
just proportional to mass.  Maybe with a term related to surface area;
roughly M^(2/3) or V^(2/3) as a naive guess.

Cells with lossy and larger membranes (per unit mass) expend a lot
more energy maintaining ion balance.  So smaller cell size with crummy
membranes should be the least efficient at producing biomass.  This may
help explain why wine yeasts varieties (S.cerevis, nominally ale yeasts)
can handle higher ethanol levels w/o a lot of pampering.  It *might* be
educational to test something like Wy4946, which is alcohol tolerant,
[not Wy4021 Champagne is likely an S.bayanus].

Other things being equal (which they are not) we might expect large celled
yeast to ferment more slowly than an equal mass or volume of small celled
yeasts due to their lesser surface area per unit mass(volume).

Also there are chemostat studies that show the *rate* of growth (the slope
of the log=exponential growth in cell-mass) depends significantly on yeast
variety. But this doesn't explain differences in endpoint mass or cell 
count.

> 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.

Did lager strain data cluster differently than ale ?  Are the various yeast
variety consistencies ?  Probably hard to tell with 19 data points.

> 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.

I think you've put your finger on an important and not well explored aspect
of practical yeast management.  The ancient stoichiometric eqn give yeast
mass yields as:

Aerobic Fermentation:
2g Sucrose + 0.104g ammonia _ 1.025g O2 =>
    1g yeast + 0.772g H2O + 1.456g CO2 + 16kJ

Anaerobic Fermentation:
20g Maltose + 0.1g  ammonia =>
   1g yeast + 9.75g EtOH + 9.36g CO2 + 24kJ

So the interesting bit is that a few percent of aerobic fermentation in your
stirred, aerated starter may be responsible for a considerable fraction of
yeast mass.  We also have to consider growth limiting factors.  After carbon
source and O2/sterol then nitrogen is at issue.  Wort has sufficient
ammonia/amino acids for anaerobic fermentation but should fall far short
for aerobic fermentation.  Unclear if your media has sufficient ammonia if
a few extra percent of sugars are respired.

The crabtree effect repression of respiration isn't perfect and in a glucose
media yeast still can respire ~2% of glucose and a somewhat higher rate for
fructose where repression is less effective - in ALE yeasts .  I don't have
comparable figures for lager yeast and we might expect considerable
differences.

We've all seen some general conclusions about yeast cell energy used for
growth vs maintenance.  That 'stuck'/slow fermentation may be due to lack
of growth factors or growth inhibition, but it's very hard to quantify 
and is
tied up with the complex issue of dormancy.

========

Stepping back to the big picture - the goal of starters is to grow 
viable yeast,
and the goal of fermentation is to attenuate wort while producing proper
flavors; two very different things.  Generally a higher fermented 
pitching rate
seems better in all respects until we reach truly excessive levels of yeast.
OTOH creating a clean viable slurry is expensive and time consuming.

So one useful question is "How little (mass or volume) of yeast meets the
minimal requirements for a good fermentation ?".  That is certainly 
dependent
on a lot of factors, yeast strain, temps, wort quality, water content, 
gravity,
attenuability, sanitation.  Another question is "How can we produce more
and better yeast starters at a modest marginal cost ?".

What we are trying to measure seems is some sort of activity - but 
reproduction
rate and (related) energetic requirements both play into this.  I 
*suspect* that
cell dry mass or volume or surface area is a much better estimate of 
specific
activity than cell count - but that's a guess.

 -S


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