diff --git a/README.md b/README.md index 03b306c..bcdb31f 100644 --- a/README.md +++ b/README.md @@ -31,7 +31,7 @@ The book is a work in progress. This represents the current status: * ✅ Intro * ✅ Enzymes -* ❌ Microorganisms +* ✅ Microorganisms * ✅ Making a starter * ❌ Sourdough starter types * ❌ Flour types diff --git a/book/basics/how-sourdough-works.tex b/book/basics/how-sourdough-works.tex index 0c2ef85..74fd3c5 100644 --- a/book/basics/how-sourdough-works.tex +++ b/book/basics/how-sourdough-works.tex @@ -312,4 +312,112 @@ sustained damages the yeast became opportunistic and started to attack the plant event producing hyphae to deeply penetrate the plants tissue. -\section{Bacteria} \ No newline at end of file +\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. +\cite{shelflife+acidity} + +\begin{figure} + \includegraphics[width=1.0\textwidth]{bacteria-microscope} + \caption{Fructilactobacillus Sanfranciscensis under the microscope} + \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. + +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. + +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. + +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. + +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. diff --git a/book/images/external/bacteria-microscope.jpg b/book/images/external/bacteria-microscope.jpg new file mode 100644 index 0000000..724660f Binary files /dev/null and b/book/images/external/bacteria-microscope.jpg differ diff --git a/book/references.bib b/book/references.bib index 08da546..3ce6f98 100644 --- a/book/references.bib +++ b/book/references.bib @@ -227,3 +227,42 @@ volume = {96,171-181} } +@article{lactobacillus+sanfrancisco, + title = {Lactobacillus sanfrancisco a key sourdough lactic acid bacterium: a review}, + author = {M. Gobbetti et al.}, + year = {1997}, + journal = {Food Microbiology}, + volume = {14,175-187} +} + +@article{proteolysis+sourdough+bacteria, + title = {Proteolytic activity of sourdough bacteria}, + author = {Gottfried Spicher et al.}, + year = {1988}, + journal = {Applied Microbiology and Biotechnology}, + volume = {28,487–492} +} + +@article{shelflife+acidity, + title = {The effect of pH on shelf-life of pork during aging and simulated retail display}, + author = {S F Holmer et al.}, + year = {2009}, + journal = {Meat Science}, + volume = {82,86-93} +} + +@article{temperature+bacteria+corn, + title = {Effect of temperature (5-25°C) on epiphytic lactic acid bacteria populations and fermentation of whole-plant corn silage}, + author = {Y Zhou et al.}, + year = {2016}, + journal = {Applied Microbiology and Biotechnology}, + volume = {121,657-671} +} + +@article{acetic+acid+oxygen, + title = {Effects of Oxygen Availability on Acetic Acid Tolerance and Intracellular pH in Dekkera bruxellensis}, + author = {Claudia Capusoni et al.}, + year = {2016}, + journal = {Applied Microbiology and Biotechnology}, + volume = {82,4673-4681} +}