diff --git a/book/basics/how-sourdough-works.tex b/book/basics/how-sourdough-works.tex index 120e140..e1b4ce3 100644 --- a/book/basics/how-sourdough-works.tex +++ b/book/basics/how-sourdough-works.tex @@ -323,11 +323,12 @@ so that they could penetrate the tissue of the plants. \section{Bacteria} -The other more dominant microbial antagonist in your sourdough -are bacteria. They outnumber the yeast population in your sourdough -starter by 100 to 1. The bacteria is mostly responsible for creating -the sour flavour that sourdough bread is typical for. The acidity -is responsible for increasing the shelf life of sourdough breads. +The other most dominant microbial antagonists in your sourdough are bacteria. +In fact, they are so dominant that they outnumber the yeast in your dough 100 +to 1. Whereas yeast provide leavening power, bacteria create the distinct +flavours for which sourdough has been named. These are due to the acidic +byproducts that result from bacterial feeding. As an added bonus, these acids +can significantly increase the shelf life of sourdough breads. \cite{shelflife+acidity} \begin{figure} @@ -336,97 +337,103 @@ is responsible for increasing the shelf life of sourdough breads. \label{lactobacillus-franciscensis-microscope} \end{figure} -The bacteria in your sourdough mostly creates lactic and acetic acid. Lactic acid -has a dairy profile. Whereas the acetic acid has a more pungent -stronger vinegary profile. The bacteria are categorized into -two categories. First you have homofermentative lactic acid bacteria. -Homofermentative describes the fact that during fermentation -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. +There are two predominant types of acid produced in sourdough bread: lactic and +acetic. In terms of flavor, lactic acid has clear dairy notes, while acetic +acid tastes of vinegar (of which it is, in fact, the primary ingredient!) These +acidic byproducts are produced by both \emph{homofermentative} and +\emph{heterofermentative} lactic acid bacteria. -Both the yeast and bacteria compete for the same food source: sugars. -Some scientists reported how bacteria would mostly consume maltose -while the yeast consumes the glucose. Some scientists reported -how the bacteria consumes some of the compounds created by the -yeast fermentation. Similarly some of the yeast consumes left -over compounds of the bacterial fermentation. This makes sense -as nature does a very good job of composting and breaking down -everything at some point \cite{lactobacillus+sanfrancisco}. -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. +\emph{Homofermentative} means that, during fermentation, the bacteria produce +a single compound: in this case, lactic acid. \emph{Heterofermentative}, on +the other hand, means that other compounds are also produced: in this case, +not only lactic acid, but also acetic acid, as well as ethanol and even some +carbon dioxide, two byproducts ordinarily associated with yeast. One quite +famous strain of lactic acid bacteria, \emph{Fructilactobacillus sanfranciscensis}, +derives its name from the equally famous San Francisco style sourdough bread. +The first isolated culture came from a bakery in this city, hence the name. -One additional trait of the bacteria is its ability to break down -and consumes proteins in your dough. If you have baked a sourdough -bread before chances are you experienced this at first hand. After -a while wheat based doughs start to break down. They seemingly become -very sticky. It becomes almost impossible to handle the dough. This -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. +Yeast and bacteria both compete for the same food source: sugar. Some scientists +have reported that bacteria consume mostly maltose, while yeast prefer glucose. +Others have reported that bacteria feed on the byproducts of yeast and vice +versa. This makes sense, as nature generally does a superb job of composting +and breaking down biological matter \cite{lactobacillus+sanfrancisco}. -This to me is the amazing process of fermentation. -When you are eating a sourdough bread you are no longer eating raw flour. -You are eating the produce of bacteria and yeast. Because of this sourdough -bread also typically -contains less gluten than a plain yeast based leavened dough -\cite{proteolysis+sourdough+bacteria}. Furthermore the bacteria -also metabolizes the ethanol produced by the yeast microorganisms and other -lactic acid bacteria. In both cases most of the resulting compounds -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. +I have yet to find a proper source that clearly describes the symbiosis between +yeast and bacteria, but my current understanding is that they both co-exist and +sometimes benefit each other, but not always. Yeast, for example, tolerate the +acidic environment created by the surrounding bacteria and are thus protected +from other pathogens. Meanwhile, however, other research demonstrates that both +types of microorganisms produce compounds that prevent the other from +metabolizing food---an interesting observation, by the way, as it could help to +identify additional antibiotics or fungicides \cite{mold+lactic+acid+bacteria}. -Depending on which flavor you like you can adjust which organic acids -you would like your sourdough to produce. Production of acetic acid -requires the presence of oxygen. By depriving your sourdough starter -of oxygen you boosting homofermentative lactic acid bacteria in your -starter. 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 cultured strains. Generally the bacteria -work best at the same temperature used to initially setup your sourdough -starter. This has been the optimal temperature at which your strains -were set up. In another experiment scientists analyzed lactic acid 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 -before \cite{temperature+bacteria+corn}. This confirms that there is a -large variety of different bacteria -strains living on the leaves of the plant. You could probably reproduce -that experiment if you started a sourdough starter at lower temperature. -Your starter's microbiome would be more adapt to fermenting at lower -temperatures. The microorganisms that best thrive at the lower temperatures -will start to become dominant. It would be an interesting experiment -to see if this could actively influence the taste of the sourdough -bread. +In the past, I've tried cultivating mushrooms and observed the mycelium +attempting to defend itself against the surrounding bacteria; both types of +microorganisms actively produced compounds to combat each other. And yet, +after a while, the fight seemed to reach a standstill, as the mycelium had +fully grown around the bacterial patch, preventing it from spreading further. +I imagine a similar scenario could be playing out in our sourdough starters, +although, given that the sourdough environment tends to be more liquid, this +fight would have to take place everywhere in the dough and not just in an +isolated patch. More research on this topic is required to better understand +the details of the relationship between yeast and bacteria. + +One other interesting trait of sourdough bacteria worth mentioning is their +ability to break down and consume the proteins in your dough. If you've baked +sourdough before, chances are you've experienced this first hand. You'll recall +from the \emph{Enzymatic reactions} section that protease breaks down the +gluten network in your dough, resulting in a sticky mess if left unbaked for +too long. The bacteria, too, consume and break down the gluten in your +dough through a process called \emph{proteolysis}. + +This, to me, was a great riddle when I first started working with sourdough. +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. \ No newline at end of file