What is sourdough and why should I bake with it?

If you are new to the idea of fermentation and do not know how it works, I encourage you to read my other article What is fermentation and why fermenting?
If you want to start a sourdough starter, check the article Gluten-Free Rice Sourdough Starter to learn how to make one like in the pictures above and below.

Sourdough starters, called also frequently levain (from the French tradition) or lievito madre (from the Italian tradition, although less used internationally), are a culture of microorganism that include wild yeasts and lactic acid bacteria.
The difference between yeasts in levain/sourdough and yeasts in commercial baker’s yeast (be that instant, active dry, or fresh) is that the latter includes only a single species of yeast (Saccharomyces Cerevisiae), which was isolated and produced industrially because it is quick-acting and strong. It is the work of Louis Pasteur that made the cultivation of pure strains of yeasts and bacteria possible, in the late 1800s. But hey, there is evidence of fermented bread being made at least since the Ancient Egypt¹. How were they doing that? There are different strains of wild yeast living, dormant, on grains. We can create an environment that is good for them to activate and thrive by mixing water and flour and let it ferment, which is what it seems people were doing in Ancient Egypt and other civilisations.

But what is the advantage of using wild yeast instead of “domesticated” and “pure” baker’s yeast?

• Eating wild-yeast-fermented bread (let us call it sourdough) will make your diet more diverse, considered most breads are leavened with baker’s yeast. Moreover, different yeast will produce different flavours as a by-product of fermentation, and guess what… it can be delicious!
• As sourdough is less “efficient” than baker’s yeast, it allows for longer fermentation, which brings more flavour development. We can play with the variables affecting fermentation also in baker’s yeast, or by making a pre-ferment. But once again, different yeasts, different flavours. Read the article How I choose how long to ferment to learn more about affecting fermentation time.
• Other very important microorganisms join the party! As the sourdough starter ferments, other microorganisms join the party. Specifically, we are talking about L.A.B. (lactic acid bacteria). This is an important point because L.A.B. produce enzymes that break down starches and therefore will ferment your bread thoroughly and make more nutrients available, instead of simply rising it/leaven it/inflate it. This is a major point for me, because it can change bread’s digestibility.
  I believe we can help doughs made with baker’s yeast doing some steps in this direction by: 1) playing with the variables at our disposal in order to make the fermentation longer, and 2) making pre-ferments with baker’s yeast (instead of using it directly in doughs). You will rarely see me adding 7g instant yeast to 500g flour and raise a bread in 2 hours! Unless I really must. That is the reason why most of my recipes will ferment for a long time, but I will provide also with some quick bread recipes for emergencies.
  There are a lot of claims that long fermentation processes through sourdough also lower the glycaemic index of the bread, although there is some research doubting there is any clinical significance (i.e., doubting it makes enough a difference to reduce your chances of developing diseases like diabetes). However, we can always play with our flour blend to make sure we make large steps in the direction of making our bread healthier, in conjunction with the use of fermentation.
• Finally, although it is not an advantage, not all sourdough bread is very sour in taste. We can control the variables affecting fermentation, and how we feed the sourdough starter in order to reduce sourness in the final bread. Sourness is a flavour profile you can achieve with sourdough, if you like it. But it is not something that must be there. I tell you more, in some bread traditions (e.g., French and Italian), a very sour bread would be probably considered simply an over fermented bread, which needs some correction (as stated by Ken Forkish in his New York Times best-seller “Flour Water Salt Yeast”).

¹ https://h2g2.com/edited_entry/A2791820