39. What factors affect microbial growth in sourdough?

We've been doing quite some work to figure out which factors affect
microbial growth in sourdough. I've done some work in vitro (which is
about to be published: Ganzle et al., Modeling of growth of
Lactobacillus sanfranciscensis and Candida milleri in response to
process parameters of the sourdough fermentation, Applied and
Environmental Microbiology, July 1998); and a colleague of mine,
Markus Brandt, has tried to figure out how my "model predictions"
work out during the actual dough fermentation. Taken together, one
can state the following:
For sourdough lactobacilli:

A) 32°C - 33°C (89.6F - 91.4F) -- optimum growth

B) 37°C & 20°C (98.6F & 68F) -- double generation time

C) 39°C & 15°C (102.2F & 59F) --  fourfold generation time

D) 41°C & 4°C (105.8F & 39.2F) -- no growth

For the yeasts, the figures are as follows:

A) 28°C (82.4F) -- optimum growth

B) 32°C & 20°C (89.6F & 68F) -- double generation time

C) 34°C & 14°C (93.2F & 57.2F) -- fourfold generation time

D) 35°C & 8°C  (95F & 46.4F) -- no growth.

So: if several refreshments are done above 32 C, the yeasts will drop
out eventually. The optimum pH for lactobacilli is 5.0 - 5.5 (which
is the initial pH of a sourdough with 5 - 20% inoculum), the minimum
pH for growth is 3.8 (they usually produce acid until pH 3.6 is
reached).

Lactic or acetic concentrations don't affect growth of lactobacilli
very much: this is the reason why the buffering capacity of the flour
is so important for the organism (a high buffering capacity in high
ash flours means that the lactobacilli produce much acid until the
critical pH is reached). It also means, that in doughs that are
continuously operated with a high inoculum (more than about 30%),
you'll find more yeasts and fewer lactobacilli. Eventually, the
lactobacilli flora may change, with more acid tolerant lactobacilli
(e.g. L. pontis) prevailing. Such a sourdough is found in the Vollmar
and Meuser continuous sourdough fermentation machines (there are 6
operating in Germany, and a diploma candidate in our department
characterized the microflora of several of these: as the machine is
operated with a 50% inoculum, the pH is never above 4.1 - 4.3, and no
L. sanfranciscensis is found in those doughs).

Yeasts are different: they don't mind the pH at all, but are strongly
inhibited by acetic acid, and to a much lesser extent by lactic acid.
Increasing salt concentrations inhibit growth of lactobacilli, but
yeasts tolerate more salt. No salt is added to the sourdough until
the final bread dough, but the dough yield affects the salt
concentration: with a low dough yield (little water), the salt (ash)
is dissolved in a smaller water volume, and the salt concentration
goes up: resulting in a slower fermentation.

So much for the "in vitro" theory. Surprisingly, Markus has found
most of the predictions to come true when he was looking at the cell
counts at different temperature, size of inoculum, salt
concentration, and pH in rye dough. The variation of the inoculum
size was interesting: If he reduced the inoculum size by 2, he had to
wait almost exactly one generation time (one doubling time of the
lactobacilli) longer until the dough has reached the same cell
counts, pH, titrable acidity, and so on as the dough with the higher
inoculum. This was true for inoculum sizes between 1% and 20%: at 50%
inoculum, the pH is so low that the lactobacilli don't really grow
well, and at an inoculum size of 0.1%, the pH and/or the oxygen
pressure in the dough are so high that the cells have a lag-time (see
above) of an hour. Thus, a scanty inoculum means one generation time
longer fermentation.

The generation time of L. sanfranciscensis in rye dough at 28 C is a
little less than an hour (figures may vary with different strains in
different flours, but it's not much more or less than that), so if
the inoculation size is reduced from 20 to 2.5%, it'll take about
three hours more until the dough is ripe.

The question is, whether these findings are true for all flours and
for all organisms. The strain isolated by Kline and Sugihara does not
differ very much from the two strains I've been looking at. All the
literature available tells me that - as long as we're looking at
sourdoughs with a tradition of continuous propagation - the system
behaves the same way. Differences may be between rye flour and white
wheat flour: in white wheat flour, the enzyme activities are so low
that the organisms may run out of food before the critical pH
(lactobacilli) or the critical acetic acid concentration (yeasts) is
reached.

-Michael