This article is from the collection of recipes from the Sourdough Mailing List, by David Adams with numerous contributions by others.
Some members of this newsgroup will remember that I have posted some of
the content of my correspondence with Michael Ganzle, a German sourdough
researcher. He has recently reviewed a proof of a book I have written
about masonry ovens and naturally fermented bread, and has commented in
detail. Those comments will interest those of you who are interested in
the science and technology of sourdoughs. This post and others that follow
are for you. If you ARE NOT linterested in the subject, stop here, and
save yourself from confusion and frustration.
In each section Michael quotes a sentance from the book, and then responds:
"witness the profusion of instant yeast brands-- while the opposite is true"
I strongly appreciate the notion that the "time equals money equation" is
not true for sourdough bread or any kind of other fermented foods-- wine,
soy sauce, cheese, vinegar, fermented sausage: they usually get better if
they are fermented for a long time (the definition of "long" varies,
though, with the different foods).
"I triple it by mixing it with its weight of water and its weight of flour"
There is a microbiological explanation for the three stage sourdough
processes. Microbial growth can be divided in three stages. When the
organisms are transferred to a new environment (e.g. by refreshing a
sourdough that has been in the refrigerator), they take some time to
adapt; no growth occurs ("lag phase"). Once the organisms are familiar
with the new environment, they start to grow exponentially, meaning one
doubling of cell counts in a given time (generation time), so called "log
phase". Eventually, the culture will become stationary, i.e. the organism
have run out of food, or are inhibited by the metabolic end products. For
effective sourdough fermentation, one needs a lot of metabolically active
cells. After three or more refreshments, the organisms will reliably start
to grow soon after inoculation and will produce enough carbon dioxide.
Things are different with yeast dough, though: there simply are so many
cells that these have to cough only once to raise the dough.
"the time it was inoculated and to the temperature at which it is kept
than with the size of the inoculation.] Let's call this the second leaven"
Comment No1: 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: Gänzle 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:
A) The optimum temperature for sourdough lactobacilli is 32 - 33°C. At
37°C and 20°C, the generation time is twice as long.
B) At 39 and 15°C, the generation time is four times as long.
C) At 41°C and 4°C, no growth is observed.
For the yeasts, the figures are as follows:
A) 28°C(optimum growth)
B) 32/20 (double generation time)
C) 34/14 (fourfold generation time)
D) 35°C, 8°C: 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 characterised 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 extend 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
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
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.
This discussion continues in the next post-- DCW