Inside the sourdough, part 1 — what is a sourdough?
As the name suggest, sourdough is nothing more than a dough which is sour. But why is it sour if two basic ingredients inside are only flour of some kind and water? And why is it bubbly and has a funky smell? Neither flour not water is sour, bubbly or have funky smell and mixing two together doesn’t result in any chemical reaction.
The answer is both very simple and very complex at the same time. Flour, like everything else in this world, contains many microorganisms which are hibernating on the surface of flour particles. Once one adds some water to flour, these microorganisms wake up and start eating, multiplying and, well, simply living their life. Let’s take a look at who they are and what exactly are they doing inside our sourdough.
There are plenty of different microorganisms around us and even inside of us. The same is true for flour and water used to create sourdough (or any any other product for that matter). Not all of these microorganisms are good. Putrefying bacteria can cause a serious disease or even death. The same is true for many fungi. But some of them are good and helpful. A properly managed sourdough usually contains colonies of lactic acid bacteria and yeast.
Lactic acid bacteria
Lactic acid bacteria (also known as lactobacilli or LAB for short) is a huge family of bacteria which can thrive in a variety of environments on many different food sources. Home bakers usually have mesophilic lactobacilli in their sourdoughs. This type of bacteria thrives at lower temperatures (below +40C) and is easier to maintain in home setting. Another large group is thermophilic LAB. These guys thrive at temperatures above +45C and are primarily used in wine and dairy making. Their maintenance can be quite tricky at home without temperature regulated incubator.
Lactic acid bacteria are responsible for transforming carbohydrates present in flour into lactic and acetic acids which give sourdough its sourness. They produce other aromatic and non-aromatic compounds as well, but in smaller quantities. Different families produce different aromatics and different ratios of acids, thus making different sourdoughs taste different.
Yeast
Yeasts are very simple single-celled microorganisms with a very limited variety of less than 2,000 identified species. The most common species found in sourdough are called Saccharomyces Cerevisiae also known as baker’s yeast or beer yeast. Different strains of S. Cerevisiae are used in virtually all fermented product.
Yeasts also feast on carbohydrates and primarily produce alcohol and CO2 with different strains varying in ratios between alcohol and CO2 production. CO2 is a gas which makes sourdough bubbly and gives rise to the bread.
Other microorganisms
Everyone else is undesired and is viewed as a contaminant. Thankfully combination of lactobacilli and yeast can create a self-preserving environment which can easily kill all other microorganisms. Both LAB and yeast are acid and alcohol resistant while dangerous fungi and putrefying bacteria are not. This is why sourdough usually doesn’t require sterilisation unlike other fermentation processes like cheese production. For example, cheeses fermented with mesophilic LAB should be started in a sterile environment to avoid any contamination. The same is true for beer brewing where yeast alone can be an easy target for invaders.
Bakers say that sourdough is ready to be used when it matures. That happens when a small set of bacteria species and yeast strains kill every other microorganism inside flour and water mixture. There is a common misconception that sourdough contains many different microorganisms and they somehow become better and more diverse with time. In reality mature sourdough contains 1-3 species of LAB and 1-3 species of yeast and they won’t change unless food source or environment changes. And the best sourdoughs out there are actually monocultures.
Further reading
Sourdough Technology—A Traditional Way for Wholesome Foods: A Review is a good read for those interested in a science behind sourdough and microorganisms living inside of it. It’s a big review of scientific studies and publications in related fields with many references. Reading everything is a time consuming task, but it is worth it if you’re into science behind casual things.
Upcoming articles in Inside The Sourdough series will talk about LAB and yeast in more detail covering topics like metabolism, environmental impact on their wellbeing, interactions between them and so on. The aim of this series is to present scientific knowledge and reasoning behind complex bread making technologies to the reader in a more friendly form and I hope you will find this series useful and interesting.