Growth of Baking Yeast

Mick mick at blankley.prestel.co.uk
Wed Sep 10 02:57:36 EST 1997


Kenneth Sole wrote:

> > > I have noticed that if the first starter takes, say, 28 hours to
> > > double, all the following stages will take (pretty close to) that
> > > same amount of time.
> > >
> > > This conflicts with my intuition... (snip)

Marc R. Roussel replied:

> Note that this is just Malthusian growth.  Since gas evolution
> (which determines the macroscopic doubling time) is proportional to the
> population, your dough doubling time should be approximately constant
> provided the yeast are always well fed.
 
Well, may I point out that since the rate of rise of the dough is
proportional to the number of cells per unit volume of dough, then the
rate will only be constant if the number of cells per unit volume stays
the same.  This condition is met only if the dilution rate of the dough
(at the refreshment stages) is equal to the rate of reproduction of the
cells (the cellular doubling time).

But we know that this is not the case.  The purpose of the starter dough
is to increase the number of cells per unit volume of dough, from the
initially small, natural population, up to a working population. So the
rate of rise of the starter dough, at the very least, should be less
than the subsequent stages.  Note that although the cellular doubling
time is constant, the number of cells per unit volume of dough is
increasing, at least comparing the first with the subsequent stages.

What happens during the subsequent stages depends on the rate of
dilution, but since a conservative estimate of yeast doubling time is
2h, and the refreshment stages take place at 28h intervals, I suggest
the cellular growth rate should easily outstrip the dilution rate, even
ignoring the natural yeast that would also be present in the fresh
dough.  

So the rate of rise of the dough should accelerate. IF it does not, an
explanation is required, as Kenneth points out.

Michael Pocklington



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