There is so much talk about sourdough, but why does it matter and how is it different from yeast based bread?
Let’s start with some background on yeast:
Yeast is all around us, in our air and on our food, for example on the fresh vegetables you buy at the store or farmers market. Luckily yeast is beneficial.
Yeast has been used for millennia in bread making and to ferment beer and wine. The earliest documented yeast leavened bread comes from ancient Egypt around 3000 years ago. Early breads used to be unleavened flat bread, until yeast leavened bread was discovered and gained in popularity and spread to Greece and then the Roman Empire. During these early times, yeast was either obtained by saving parts of the leavened dough to add to fresh dough during the next dough preparation or from yeast in the beer making process (Egyptians already appreciated a good draught!).
The next milestone was in 19th century Europe. At this time, most yeast for bread baking was obtained from beer brewing. New methods were developed to extract yeast and improve industrial manufacturing processes of baker’s yeast. Today’s commercial baker’s yeast has been selected for specific strains that grow fast and create a lot of carbon dioxide which is what makes bread rise. Using the handful of yeast strains that work best for bakers ensures consistency in the baked products, which is so important when producing large volumes of baked goods. And we home bakers can buy yeast at the store and make bread in a matter of hours.
Now, we know more about yeast, let’s go back to sourdough:
Sourdough bakers sometimes talk about the wild yeast sourdough they use to make their artisan bread. These sourdough starters are created by letting the local wild yeast settle on a flour-water mix where they can multiply. Once the yeast starts to grow a baker simply continues to feed this new sourdough with water and flour to make sure the yeast doesn’t die. Yes, yeast lives and needs to be fed regularly. Sourdough contains other beneficial organisms: bacteria, which also occur wild in the air and on your vegetables. These are similar to the beneficial bacteria in your yogurt or that are used for fermenting kombucha, sauerkraut, or kimchi. What they do in the sourdough is create the sour taste.
Different strains of yeast and bacteria can be found in different places. San Francisco sourdough is known for its unique sourdough taste. The yeast strains and bacteria in San Francisco are different from, e.g. those in Ventura and therefore create different flavor profiles and different tasting bread. Once a sourdough starter is established, yeast and bacteria in the dough are considered to have formed a stable micro-environment and are resistant to contamination with local microbes, so taking a starter from one place to another should keep its original mix of organisms in place. In other words, San Francisco Sourdough bread doesn’t need to be baked in San Francisco to have its unique taste.
So in sourdough bread, yeast rises the dough by creating air bubbles, bacteria are responsible for the sour taste. Yeast grows relatively fast, especially at warm temperatures (80-90F), bacteria need more time to grow and develop the unique sour taste. This is why sourdough bread is allowed to ferment for 24 to 48 hours. And it can do this, especially at cool temperatures (e.g. in the fridge) because the concentration of yeast in sourdough is not as high compared to baker’s yeast, which is essentially pure yeast cells selected for fast growth.
To summarize:
Commercial yeast contain concentrated yeast cells from strains that are selected for certain desired properties to help the baker create consistent, well risen bread, and fast.Sourdough is a combination of yeast and bacteria that creates complex flavors which needs more time. Sourdough, besides its great taste, has other other benefits in the bread making process and health benefits for us, but that is not for this post (if you are interested read this article).