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@@ -323,11 +323,12 @@ so that they could penetrate the tissue of the plants.
\section{Bacteria} \section{Bacteria}
The other more dominant microbial antagonist in your sourdough The other most dominant microbial antagonists in your sourdough are bacteria.
are bacteria. They outnumber the yeast population in your sourdough In fact, they are so dominant that they outnumber the yeast in your dough 100
starter by 100 to 1. The bacteria is mostly responsible for creating to 1. Whereas yeast provide leavening power, bacteria create the distinct
the sour flavour that sourdough bread is typical for. The acidity flavours for which sourdough has been named. These are due to the acidic
is responsible for increasing the shelf life of sourdough breads. byproducts that result from bacterial feeding. As an added bonus, these acids
can significantly increase the shelf life of sourdough breads.
\cite{shelflife+acidity} \cite{shelflife+acidity}
\begin{figure} \begin{figure}
@@ -336,97 +337,103 @@ is responsible for increasing the shelf life of sourdough breads.
\label{lactobacillus-franciscensis-microscope} \label{lactobacillus-franciscensis-microscope}
\end{figure} \end{figure}
The bacteria in your sourdough mostly creates lactic and acetic acid. Lactic acid There are two predominant types of acid produced in sourdough bread: lactic and
has a dairy profile. Whereas the acetic acid has a more pungent acetic. In terms of flavor, lactic acid has clear dairy notes, while acetic
stronger vinegary profile. The bacteria are categorized into acid tastes of vinegar (of which it is, in fact, the primary ingredient!) These
two categories. First you have homofermentative lactic acid bacteria. acidic byproducts are produced by both \emph{homofermentative} and
Homofermentative describes the fact that during fermentation \emph{heterofermentative} lactic acid bacteria.
they mostly produce a single compound: Lactic acid. The second
category contains heterofermentative lactic acid bacteria. They
produce lactic acid, acetic acid, ethanol and even some carbon
dioxide. A quite famous strain of bacteria is called
\emph{Fructilactobacillus sanfranciscensis}. The name derives
from the famous San Francisco sourdough bread. The culture has
first been isolated from a local bakery and was then named
after the city in appreciation.
Both the yeast and bacteria compete for the same food source: sugars. \emph{Homofermentative} means that, during fermentation, the bacteria produce
Some scientists reported how bacteria would mostly consume maltose a single compound: in this case, lactic acid. \emph{Heterofermentative}, on
while the yeast consumes the glucose. Some scientists reported the other hand, means that other compounds are also produced: in this case,
how the bacteria consumes some of the compounds created by the not only lactic acid, but also acetic acid, as well as ethanol and even some
yeast fermentation. Similarly some of the yeast consumes left carbon dioxide, two byproducts ordinarily associated with yeast. One quite
over compounds of the bacterial fermentation. This makes sense famous strain of lactic acid bacteria, \emph{Fructilactobacillus sanfranciscensis},
as nature does a very good job of composting and breaking down derives its name from the equally famous San Francisco style sourdough bread.
everything at some point \cite{lactobacillus+sanfrancisco}. The first isolated culture came from a bakery in this city, hence the name.
I am still yet to find
a proper source that clearly describes the symbiosis between
the yeast and bacteria. Based on my current understanding
they both co-exist and sometimes benefit each other. The yeast
for instance tolerates the acidic environment and thus benefits
from enhanced protection from other pathogens. Other research
has shown how both the microorganisms produce compounds
to prevent the other source from consuming food. This is interesting
as it could serve as a source to identify additional antibiotics
or fungicides \cite{mold+lactic+acid+bacteria}. I have had
occasions when trying to cultivate mushrooms where you could
see the mycelium trying to defend it self from bacteria. Both
of them were actively producing compounds to combat each other.
After a while the fight between seemingly came to a standstill.
The mycelium had fully grown around the bacterial patch preventing
it from spreading any further. I imagine the same scenario happening
in a sourdough starter. As the environment tends to be more liquid
compared to when growing fungi this fight is happening in more places
at the same time, not isolated to a single patch in your dough.
More research is needed on this topic to answer the details of the
relationship between the microorganisms.
One additional trait of the bacteria is its ability to break down Yeast and bacteria both compete for the same food source: sugar. Some scientists
and consumes proteins in your dough. If you have baked a sourdough have reported that bacteria consume mostly maltose, while yeast prefer glucose.
bread before chances are you experienced this at first hand. After Others have reported that bacteria feed on the byproducts of yeast and vice
a while wheat based doughs start to break down. They seemingly become versa. This makes sense, as nature generally does a superb job of composting
very sticky. It becomes almost impossible to handle the dough. This and breaking down biological matter \cite{lactobacillus+sanfrancisco}.
is because the bacteria starts to ferment the gluten inside of your dough.
The process is called \emph{proteolysis}. This to me was a great riddle
when starting to work with sourdough bread. Your dough becomes stickier
but at the same time it also becomes more extensible. As the gluten
is reduced it becomes easier and easier for the microorganisms to inflate the
dough. Imagine a car tire initially with thick rubber and then ultimately
a very fragile balloon. You can inflate the balloon a lot easier with your
mouth. In comparison the car tire is going to be impossible for you
to inflate. This process is further accelerated by the protease
enzyme breaking down the gluten to smaller amino acids.
This to me is the amazing process of fermentation. I have yet to find a proper source that clearly describes the symbiosis between
When you are eating a sourdough bread you are no longer eating raw flour. yeast and bacteria, but my current understanding is that they both co-exist and
You are eating the produce of bacteria and yeast. Because of this sourdough sometimes benefit each other, but not always. Yeast, for example, tolerate the
bread also typically acidic environment created by the surrounding bacteria and are thus protected
contains less gluten than a plain yeast based leavened dough from other pathogens. Meanwhile, however, other research demonstrates that both
\cite{proteolysis+sourdough+bacteria}. Furthermore the bacteria types of microorganisms produce compounds that prevent the other from
also metabolizes the ethanol produced by the yeast microorganisms and other metabolizing food---an interesting observation, by the way, as it could help to
lactic acid bacteria. In both cases most of the resulting compounds identify additional antibiotics or fungicides \cite{mold+lactic+acid+bacteria}.
are organic acids. All the resources in your sourdough are recycled
as much as possible by the microorganisms. They are trying to eat whatever
is available. With each feeding they will become more adapt at using
the available resources.
Depending on which flavor you like you can adjust which organic acids In the past, I've tried cultivating mushrooms and observed the mycelium
you would like your sourdough to produce. Production of acetic acid attempting to defend itself against the surrounding bacteria; both types of
requires the presence of oxygen. By depriving your sourdough starter microorganisms actively produced compounds to combat each other. And yet,
of oxygen you boosting homofermentative lactic acid bacteria in your after a while, the fight seemed to reach a standstill, as the mycelium had
starter. Over time they will become dominant and outcompete the acetic acid fully grown around the bacterial patch, preventing it from spreading further.
producing bacteria \cite{acetic+acid+oxygen}. The optimal fermentation temperature of your I imagine a similar scenario could be playing out in our sourdough starters,
lactic acid bacteria depends on the cultured strains. Generally the bacteria although, given that the sourdough environment tends to be more liquid, this
work best at the same temperature used to initially setup your sourdough fight would have to take place everywhere in the dough and not just in an
starter. This has been the optimal temperature at which your strains isolated patch. More research on this topic is required to better understand
were set up. In another experiment scientists analyzed lactic acid bacteria the details of the relationship between yeast and bacteria.
on corn leaves. They on purpose lowered the temperature from 20-25°C to around 5-10°C.
They were able to observe lactic acid bacteria that they had never seen One other interesting trait of sourdough bacteria worth mentioning is their
before \cite{temperature+bacteria+corn}. This confirms that there is a ability to break down and consume the proteins in your dough. If you've baked
large variety of different bacteria sourdough before, chances are you've experienced this first hand. You'll recall
strains living on the leaves of the plant. You could probably reproduce from the \emph{Enzymatic reactions} section that protease breaks down the
that experiment if you started a sourdough starter at lower temperature. gluten network in your dough, resulting in a sticky mess if left unbaked for
Your starter's microbiome would be more adapt to fermenting at lower too long. The bacteria, too, consume and break down the gluten in your
temperatures. The microorganisms that best thrive at the lower temperatures dough through a process called \emph{proteolysis}.
will start to become dominant. It would be an interesting experiment
to see if this could actively influence the taste of the sourdough This, to me, was a great riddle when I first started working with sourdough.
bread. On the one hand, it makes the dough stickier. On the other, it makes the dough
more extensible and easier to work with. As the gluten is reduced, the dough
becomes easier for the microorganisms to inflate, allowing it to rise. This
could be likened to the level of effort required to inflate a thick rubber tire
versus a thin and fragile balloon. The latter would be easy to blow up with
your mouth, while the former would not.
Unsurprisingly, proteolysis is further accelerated by the protease enzyme
previously discussed, which aids in the breakdown of gluten into smaller,
more easily metabolized amino acids.
This, to me, is the amazing process of fermentation. When you eat sourdough
bread, you are not merely consuming flour and water but the end result of
complex biological processes accomplished by the bacteria and yeast. Because
of the added bacterial component, sourdough bread typically contains less
gluten than a pure yeast-based dough \cite{proteolysis+sourdough+bacteria}.
Furthermore, the homofermentative bacteria metabolize the ethanol produced by
the yeast and other heterofermentative lactic acid bacteria. In both cases,
most of the resulting compounds are organic acids. Every natural resource in
your sourdough bread is recycled by the microorganisms inside, who are all
trying to eat whatever is available for as long as possible, and with each
feeding, they become more adept at utilizing these resources.
Depending on which flavour profile you prefer, you can select for one organic
acid or another. Acetic aacid production requires oxygen, and by depriving
your sourdough starter of it, you can boost the population of homofermentative
lactic acid bacteria. Over time they will become dominant and outcompete the
acetic acid-producing bacteria \cite{acetic+acid+oxygen}.
The optimal fermentation temperature of your lactic acid bacteria depends on
the strains you've cultured in your starter. Generally, they work best at the
temperature used to create your starter because you've already selected for
bacteria that thrive under that condition.
In one noteworthy experiment, scientists examined the lactic acid bacteria
found on corn leaves. They lowered the ambient temperature from 20-25°C to around
5-10°C and afterward observed varieties of the bacteria that had never been
seen before \cite{temperature+bacteria+corn}, confirming that there is, in
fact, a large variety of bacterial strains living on the leaves of the plant.
Incidentally, you could perform a similar experiment by kicking off a sourdough
starter at a lower temperature. In theory, the microbiome should adapt, as the
microorganisms that thrive the most at lower temperatures will start to become
dominant. It would be interesting to see if this could actively influence the
taste of the resulting bread.
One last footnote worth mentioning: Some sources say that fermenting at a
lower temperature can increase acetic acid production, while fermenting at a
warmer temperature can boost lactic acid production. I could not verify this
in my own tests. More research is needed on the topic.