27. How do lactic bacteria affect sourdough bread?

13 Feb 1997 10:49:32 +0100

Dear Daniel Wing!

Your letter to Prof. Hammes has reached Hohenheim, and Prof. Hammes
has asked me to take care of the communication. I am a Ph.D.
candidate in Hammes' lab working on the physiology of sour dough
lactobacilli.

Please feel welcome to address questions to us concerning sour dough
microbiology and technology! I will mail two recent publications or
our lab concerning the physiology of sour dough lactic acid bacteria
by mail, but as they may take a week or longer to reach you, I will
give a few comments on the questions in your letter:

- yeasts do not produce appreciable amounts of either lactic or
acetic acids, their main metabolites are ethanol and CO2. If
acidification of the dough is desired or required (e.g. if rye flour
is used), lactic acid bacteria or organic acids (most commonly lactic
or citric acids) are added.

- homefermentative lactic acid bacteria do produce solely lactic acid
from maltose or glucose under anaerobic conditions (as they are
prevailing in sour dough fermentations). Thus, doughs acidified with
homofermentative lactic acid bacteria (LAB) contain but little acetic
acid. As homofermentative lactic acid bacteria do not produce CO2,
yeast must be added to ensure leavening of the dough.

- In sour doughs with a tradition of continuous propagation (such as
the San Francisco French Bread Sour Dough process, German rye sour
doughs or sour dough employed in Pannettone production in Italy),
heterofermentative lactobacilli, especially L. sanfrancisco, are
dominating the fermentation. Heterofermentative lactobacilli produce
lactate, ethanol, and CO2 from hexoses (most strains do not ferment
pentoses), HOWEVER, if additional substrates are present that serve
as electron acceptor to balance, acetate is produced instead of
ethanol. I do not know whether or not you are familiar with the
concept of the  "redox balance": Degradation of hexoses via the
pentose-phosphate pathway as employed by heterofermentative LAB
results in phosphorylation of ADP to ATP, and in the reduction of NAD
to NADH. As there is no use for NADH, it must be oxidized to NAD
again. In the absence of other substrates, acetyl-Phosphate is
reduced to ethanol, with two NADH becoming oxidized to HAD in the
process. If either fructose, oxygen, citrate or malate are present,
these become reduced to mannitol, H2O, lactic and acetic acid, and
succinate, respectively, and acetyl-P is dephosphorylated to acetate.
(This explanation may not be very straightforward, I hope we did a
better job in the publications I`m about to send you; these also
include a diagram showing the metabolic pathways of L. sanfrancisco).
The consequence for the molar ration of lactate:acetate (fermentation
quotient, FQ) in sour dough fermentations is, that acetate in
produced only if one or more of the above mentioned co-substrated is
present. Oxygen is present only in the beginning of the fermentation,
and the amounts of oxygen are too low to result in significant
amounts of acetic acid, though, in principle, it is possible to
increase the acetate content by aeration of dough. Fructose is
present in sucrose and other glucofructans with higher molecular
weights. Fructose is released from these compounds by cereal or dough
enzymes (many strains of L. sanfrancisco don`t even cleave sucrose)
and consequently reduced to mannitol by L. sanfrancisco. The ration
of mannitol : acetate in sour dough fermentation is approximately
2:1, suggesting that fructose is the most important electron
acceptor. Furthermore, citrate and malate are present in the dough in
amounts less than 10 mmol/kg, these are utilized also.

Thus, the effect of substrates and oxygen on the FQ is nicely
explained by the metabolic characteristics of the dominating
fermentation organisms. Dough yield (=kg dough per 100 kg flour) and
temperature also influence the FQ. Spicher reports that softer doughs
lead to an increased FQ; an increase in temperature results in higher
amounts of lactic acid, while the amount of acetic acid remains more
or less the same, thus, the FQ is increased again. I do not have a
straightforward explanation for these phenomena, but changes in dough
yield and temperature will result in changes in buffering capacities
of the dough, modified activities of cereal and microbial enzymes, as
well as a changed ration of yeasts : lactobacilli counts, all of
which are likely to influence the FQ.

Yours

Michael Ganzle

--

Dear Michael Gaenzele

Thank you for sending one of the most gracious letters I have ever
received in response to any kind of an inquiry. Since I wrote to
Prof. Hammes I have been able to copy a number of articles from
English language publications by Drs. Brummer, Spicher, Vogel, and so
forth. Unfortunately, some of them have been in non-technical
journals and were thus short on details, and even the less technical
ones were not as clearly and idiomatically written as your letter. I
DID have a hard time understanding what was meant by Dough Yield, for
instance, although I had figured it out before I got your letter. I
am still not sure I understand some of the statements those authors
made about the acid content of doughs (such as the units of
measurement), but I have been piecing things together by looking at
all the articles cumulatively. Your letter has clarified a great
deal. I will put stars next to my current questions to make THIS
letter easier to answer. Like this *.

One problem for me was that I did not realize how predominant rye
flours were in German sourdough baking. I know that typical rye
pentose is about 8% and that pentose viscosity is important in
gas-trapping in rye doughs (He and Hoseney, 1991) but I still don't
know how an acidified rye dough behaves differently from a more
neutral one. *Does it affect viscosity somehow? He and Hoseney
studied neutral doughs only.

I also do not understand why Brummer says "Anstellgut" is a
non-translatable term. *What do you think it translates as? *I take
it that this a very ripe starter, very acid, maintained at room
temperature at some infrequent rate of refreshment? *Is it always rye
based? *Always a high-ash flour? *How is it different from the type
of French and American wheat starters that are refreshed 1:1 every
eight hours, or 1:4 every 12 hours? *What is its consistency, pH,
Total Titratable Acid? *My assumption is that my lack of
understanding comes from the German use of sourdough as primarily
acidification, whereas here we look for a little acidification, a
good flavor, and good leavening power.*Do German bakers ever make
wheat breads leavened with higher starter percentages than those
Brummer cites, for example 20% or 30% starter? *Or do they acidify
with very ripe starters and leaven with commercial yeast?

I am curious about the flavor/sensory aspects of the FQ: *When a
bread is fairly sour (SF Sourdough, some rye breads) is the perceived
sourness mostly lactate, mostly acetate, or due to the pH or TTA of
the bread? Calvel brings this subject up, but does not resolve it to
my understanding.

As for your answers to my previous questions, thank you -- I will
look this material over again, and let you know if I have questions.
*Do you mind if I put the text of your letter (with attribution) on
the internet as a posting to the newsgroup Rec.Food.Sourdough? I will
NOT put your address or email address in the posting, unless you want
me to. Please let me know, as I think it might become part of the FAQ
file there (Frequently Asked Questions). I will forward your entire
letter to a very few people in academia here who have been helping
me, so you might hear from one of them.

Dan Wing

--


14 Feb 1997 15:50:30 +0100

Dear Dan Wing!

I do not mind if the answer is posted to the rec.food.sourdough: I've
also been browsing in that newsgroup.

To answer a few of your questions:

I) There is no rye bread without acidification of the dough. Rye
flour does not contain gluten (or a different type of gluten that
does not have the gas-retaining properties), so that the structure of
rye bread relies mainly on gelatinized starch. Rye flour does have a
higher amylase activity than wheat flour, furthermore, the
gelatinization temperature is a few degrees lower than that of wheat
starch. Thus, with the temperature optimum of rye amylase being about
50 - 52C (with substantial activity up to temperatures of 70C) and
starch gelatinization starting at 55C, starch is degraded during the
baking process UNLESS the amylases are inactivated by lowering the pH
below 4.5. The situation is exacerbated if there was wet weather
during the harvest, as germinating rye has higher amylase activities
and the starch granules are damaged, thus facilitating hydrolysis.

II) "Anstellgut" is more or less the same as the continuously
propagated wheat starters of the SF sour dough bread, so no harm is
done if it is translated as "starter sponge" or something like.
German sourdoughs usually are rye based for two reasons: 1) Due to
the climatic conditions in Germany, especially in the northern and
eastern parts that make it difficult to grow wheat, rye flour is just
as important for bread production as wheat flour. 2) As these is no
necessity to acidify wheat flour (though it enhances the flavor),
most bakers do not use sour dough to produce  wheat bread. Starter
sponges are not necessarily propagated separately. If the dough is
taken care of according to traditional methods, it is re-inoculated
three times to produce bread dough (reading Bruemmer and Spicher, you
probably have already encountered the "three stage sour dough
method." A part of the bread dough is used to prepare the sour dough
for the next day. This makes 3 - 4 inoculations a day, the ratio of
sour dough to fresh dough being approximately 1:3. One has to make a
point of it: there is no typical sourdough without continuous
propagation! The microflora of these rye starters is actually the
same as for wheat starter in SF or Italy: Lactobacillus sanfrancisco
and Candida milleri or Saccharomyces exiguus. The pH of a ripe sour
dough will be between 3.6 and 4.0 (L. sanfrancisco does not grow
below pH 3.6). The total titrable acidity (TTA) depends on the flour
employed: as the lactobacilli acidify to pH 3.6, flours with high
buffering capacity (amount of acid required to lower the pH), e.g.
whole flours, have a higher TTA than white flours with a low
buffering capacity. Furthermore, if "hard" water with high
concentrations of Me2+ CO3- is used, the TTA will be higher.

3) Acidification vs. leavening: As mentioned above, rye flour or
mixtures of rye and wheat flours containing more than 20% rye must be
acidified in order to get bread. As the propagation of sour dough is
very time consuming if the full leavening capacity of the organism is
to be obtained, quite a few processes have been developed in Germany
that ensure that the dough is acidified (or that the sour dough added
to the bread dough contains enough acid to bring the pH of the bread
dough below ca. 4.5), but no leavened by the sour dough microflora.
Leavening is achieved by bakers yeast. Basically, there are three
possibilities: 1) Dried sourdough with a high TTA (>20) is added to
the bread dough, there are no lactobacilli involved in the
fermentation (sometimes they are present in the dried sour dough
preparation anyway, as in Germany, something called sour dough must
contain viable lactic acid bacteria. The dried dough is sold much
more readily if it can be called sourdough). 2) A sour dough is kept
at room temperature for up to one week. The TTY of that dough is high
enough to use it for baking, but as the organisms are rather stressed
in such an environment, they will not contribute to the leavening of
the dough. Such doughs do not contain lactobacillus sanfrancisco, but
other lactobacilli that are more acid tolerant (the ph of such a
dough reaches 3.4 - 3.6 after one day, and stays there for the four
or five more days that the dough is kept).  3) One stage or two stage
processes with starter sponges. One or two stage processes usually do
not ensure that the lactobacilli in the dough are fully metabolically
active if the bread dough is prepared, thus, the leavening capacity
is rather poor, but enough acid has been produced. As far as I know
(I never made a survey, though), only few bakers make bread with
traditional processes without bakers yeast added to leaven the dough.
Acidification of the bread dough with sour dough is rather common,
and the sensory quality of such bread is quite close to that of bread
made without bakers yeast. Straight processes with bakers yeast and
chemical acidification (citric, lactic, and acetic acid, or mixtures
thereof) are also quite common to produce rye bread.

4) Lactic acid and acetic acid will change taste and flavor of bread
beyond the decrease of pH: the taste buds (sour, bitter, sweet,
salty) are on the tongue, any other aroma is perceived with the nose;
therefore, the aroma compounds must be volatile. Acetic acid is more
volatile than lactic acid, thus, it's impact on the flavor is more
pronounced than that of lactic acid. Spicher says that a ratio of 20
acetate to 80 lactate is optimal. It must also be taken into account,
that the lowering of the pH influences the formation of other aroma
compounds during the baking process. The acetic acid is furthermore
important as growth of spoilage organisms such as molds or rope
causing bacilli (Bacillus subtilis) is inhibited by high acetic acid
concentrations.

I hope that I could answer your questions

With kind regards

Michael Ganzle