1200d Long Technical Post 4
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This article is from the collection of recipes from the Sourdough Mailing List, by David Adams with numerous contributions by others.
1200d Long Technical Post 4
Daniel WingSince many people new to natural leavens would like to bake San Francisco
sourdough, Desem bread, or German rye bread, let's look at some of their
characteristics, as determined by their leavens, ingredients, and
processes.
The microflora of German rye sour and Sanfrancisco sourdough is (almost)
identical. The difference is raw material and production process. Prof.
Hammes thinks that L. sanfranciscensis isolated from wheat or rye may have
different properties (e.g. degradation of arginine to ornithine, see
comment No3, or proteolytic activity (see comment No2: wheat has less
proteolytic activity by itself than rye). He still has to prove his point,
though.
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That yeast is also resistant to a natural antibiotic made by the bacteria.
The most "antibiotic" compound in sourdough is acetic acid. Although I
mentionned earlier that Candida milleri from Böcker Reinzucht Sauer (The
Saccharomyces exiguus described by Kline and Sugihara has been renamed to
Candida milleri as well) is more sensitive to acetic acid than the
lactobacilli, it certainly is much more resistant than baker's yeast.
Gobbetti says that L. sanfrancisco produces other organic acids that may
inhibit yeast growth, but I don't know wheter or not the concentration in
the dough is high enough to make a difference. As far as I know, no other
antimicrobial compound in dough has been characterised.
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Most German rye bread has at least 30 percent rye
I have the figures: 60% is "mixed rye bread" containing both rye and
wheat, but more of the former. As far as the bread goes, rye only about as
important as wheat only. The situation is different for bagels, pretzels,
and so on. There is increasing interest in wheat sourdoughs: the 1 - 5%
addition of sourdough, which is sometimes replaced by a dried and "dead"
sourdough works, but not quite as well as it could. Which is why industry
is funding flavor research at the Universities of Hohenheim and Munich...
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Vollmar and Meuser showed that the rate of bacterial reproduction after
inoculation is self-regulated, within limits: if you add a small inoculum,
the bacteria will multiply faster than they will if it is larger, so the
static population (say 1,650 million cells/cc) is reached at the same time
in either case, about three and one-half hours.
The Vollmar and Meuser sourdough machine is not a very good example: as
pointed out in comment No1, it operates with an inoculum of 50%, which
makes the dough so acid from the beginning on the the lactobacilli don't
like to grow fast. Between 1 and 20% inoculum, lactobacilli grow at the
same speed (giving rise to the dependency of fermentation time and
inoculum size explained earlier). The Vollmar and Meuser machine also has
a rather high yeast content (if you've read their publication in Cereal
Chemistry; yeasts are above 100 million or more than 10% or the total cell
counts, while "normal" starters such as the Sanfrancisco starter of the
Böcker Reinzucht Stater have only around 10 million or about 1% of the
total cell count.
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When cultures are fermented at higher temperatures, non-pathogenic
acid-tolerant contaminants such as Pediococcus (makes too much lactic
acid) and Acetobacter (makes to much acetic acid) can intrude and
dominate, affecting taste.
Pediococcus is probably less acid tolerant than L. sanfranciscensis, but
it grows at higher temperatures (as mentionned above, sanfranciscensis
does not like more than 35 - 37°C. Acetobacter is of no importance in
sourdoughs: it strictly requires oxygen for growth, and sourdough becomes
anaerobic (=without oxygen) very quickly due to the metabolism of yeasts
and lactobacilli. If you've ever seen a vinegar fermenter you will notice
that several hundred liter of air are pumped through a liter of vinegar
during an hour: it is almost impossible to aerate sourdough in such a way.
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Dr. Sugihara, who participated in the characterization of the flora of San
Francisco sourdough and several other cultures, was asked whether natural
sourdough cultures could be contaminated with commercial yeast. His reply
was no, not if you have a stable culture that is continuously maintained
with the same conditions and ingredients.
Dr. Sugihara is certainly right here. There was an experiment done by a
Dutch group: baker's yeast didn't survive more than two refreshments. I
think that it's the acetate that kills the yeast as its less acetate
tolerant than sourdough yeasts.
And to the margin note right next (CONCERNING THE ABILITY OF BACTERIAL
FERMENTATION TO RAISE A LOAF OF BREAD, WITHOUT YEAST): We've done the
experiments, it works quite well without yeast. The volume is somewhat
smaller, though. Markus Brandt has estimated the contribution of yeasts
and lactobacilli to gas production in a "normal" sourdough: about 50%
comes from lactobacilli and yeasts each. The yeasts are fewer in numbers,
but larger in size.
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Bakers are interested in the acids produced by leaven microbes because
much of the distinctive flavor produced by leaven microbes comes in the
form of organic acids that are products of fermentation.
The production of lactic acid in dough in determined mainly by the
buffering capacity of the flour, i.e. the ash content. Dough yield and
temperature are much less important; as far as Spichers investigations go,
I think that the higher lactic acid concent of doughs with higher
temperatures or higher dough yields he measured is due mainly to the
faster fermentation at these conditions. (this holds true if you calculate
the lactate produced on the amount of flour in the dough: this ratio is
fairly constant). The amount of acetic acid produced is controlled mainly
on the availability of fructose. L. sanfranciscensis produced lactic acid
and ethano (and carbon dioxide) from maltose or glucose. If the organism
wants to produce the more oxidized end product, acetic acid, another
substrate must be reduced. L. sanfranciscensis reduced 2 moles of fructose
to mannitol per mole of acetic acid formed. The ratio of mannitol to
acetic acid in dough os about 1.8, fairly close to the theoretical value
of 2 if fructose was the only co-substrate that is reduced. During
fermentation, L. sanfrancisco starts to produce lactic acid and acetic
acid first, and forms lactic acid and ethanol only if the fructose is
depleted. There is a lot of fructose in dough, but not all of it is
available for the lactobacilli. Yeasts liberate some of the fructose bound
in glucofructans that thus becomes available for the lactobacilli (there
is some nice work that has been done by the Sugihara group, Saunders et
al., cereal chemistry, 1972 or 1973). If you to too high with the
temperature, you slow down yeast growth, and the acetic acid levels in the
dough decrease. For bakers, an easy way to increase the acetic acid
content is to add sugar 8that is sucrose,a consisting of glucose and
fructose). This won't increase the total titrable acidity, though, as that
is determined by the buffering capacity. Sugar addition (not too much, 1
or 2%) may speed up fermentation in white wheat flours: as mentionned
above, in contrast to whole wheat flour and rye flours, the enzyme
activities and thus the sugar concentrations are rather low and may limit
microbial metabolism.
As far as the influence of acetic acid and lactic acid on flavor go:
lactic acid has no influence on aroma, only on taste, while acetic acid is
an aroma volatile. So, I think it is not so much the ratio of lactic to
acetic acid, but more simply the acetic acid content that matters.
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Natural leavens should be actively fermenting and reproducing when they
are incorporated into a dough
Yeasts in dough don't have to rely on oxygen for growth: if that were the
case, they woudn' t be there.
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The more accepted and consistently successful way to store a culture for a
month or so is to make a fresh and very stiff storage leaven, put it in a
well covered vessel ...
Such leavens may keep up to almost three month (my sister had a baby in
March and didn't use her starter for almost three month. It was stored the
way you described here, and did come out well upon refreshment. The Böcker
Reinzuchtsauer is also distributed as stiff, refrigerated product. I think
the company does not guarantee storage stability of more than 4 weeks,
though.)
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Still there may be someone out there who does need to start a leaven
because of some terrible misfortune--
I think it does not matter when the first batch of a new sourdough stinks
- the good bacilli will come out eventually, and they may come faster if
fermentation is done around 25 - 30°C (as mentionned earlier, the
temperature optimum of L. sanfranciscensis is 32 - 33°C). There has been
nice work done in Rudi Vogels lab on the microflora of a freshly started
sourdough: first, there are Enterobacteria (Escherichia coli, Salmonella,
Enterobacter), highly undesirable organism that stink terribly, then there
are homofermentative lactobacilli (good, but no gas production), then
acid-tolerant, heterofermentative lactobacilli. I think, this took about
48 hours at 30°C. The stink at the beginning does not matter as the
organisms will be diluted out or die eventually. No L. sanfranciscensis,
though, these will occur only after repeated refreshments. Peter Stolz of
the Böcker company told me that it takes about two weeks of repeated
inoculations to get a good "sanfranciscensis" sourdough. I don't know
whether or not this process was sped up in his case as, due to his
workplace, his skin is all covered with L. sanfranciscensis.
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My biggest disagreement with her, though (NANCY SILVERTON), is about the
amount of material one should use in a starter.
I agree with you: one g of dough is one billion lactobacilli and 10
million yeasts: more than enough. In the lab, I'm doing most experiments
on a 1/10 ml scale, for dough refreshments at home, it does not get much
smaller than 10 g: it's difficult to handle smaller amounts.
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If leaven refreshment intervals are excessive
The main criterion of sourdoughs containing L. sanfranciscensis is the
repeated, frequent refreshment (not counted the storage in the
refrigerator). Peter Stolz said that one every 24 hours will suffice, if
intervals are much longer than that (lets say more than 3 days),
different, more acid tolerant organisms may evolve (e.g. L. pontis as
found in the Vollmar and Meuser Breasd maschines: these are refreshed
frequently, but with a very high inoculum).
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Refreshment schedules are always dependent on temperature.
See my earlier comment on the temperature dependency of growth of L.
sanfranciscensis and Candida milleri. Most of the typical sourdough yeasts
resemble C. milleri with respect to the temperature sensitivity (i.e. no
growth at 37°C).
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Acidity can be expressed as flavor (an acid flavor), as pH, or as total
acidity.
That a good explanation of the total titrable acidity concept. (I find
students almost done with their degree still have difficulties with this
concept).
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At any given temperature the thinner starter will ferment faster and reach
a lower pH; but will not contain as much acid.
If you calculate the amount of acid produced on the weight of the flour
rather than the dough weight, the outcome -lactic acid per g flour -
should be pretty independent on dough consistency (not if very stiff
doughs are produced: the combined salt and acid stress leads to a
decreased acid production). Markus Brandt observed this in doughs (rye
flour, TA 180) if more than 2% salt were added.
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Together, caramels and Maillard products are responsible for much of the
flavor and aroma of fresh yeasted bread, although of these two, Maillard
products are moch more intensely aromatic.
This is right for both yeasted breads and sourdough breads, however, it is
important to note that whatever chemicals are reacting with each other
during baking must be formed during dough fermentation. (Schieberle in
Munich has done several nice studies: he supplied doughs with amino acids
and demonstrated that the levels of aroma compounds in the bread were
increased). So, formation of aroma precursors during dough fermentation is
crucial for the Maillard reaction.
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