Have you ever bitten into a warm, fresh piece of bread and wondered why it had a sweet and tangy taste? The answer is sourdough starter! This interesting fermented dough differs greatly from normal yeast that you buy at the grocery store, and has been a coveted baking ingredient for centuries. Sourdough starter was even brought to North America on the Mayflower!
Sourdough bread has become largely popular in the last few years, especially with the large amount of time spent at home during lockdown, due to the Covid-19 pandemic. Many people have created their own sourdough starters, and even given them names! Although I do not have a name for my starter, I enjoy sharing my sourdough creations with my family. After all, food it a great way to socialize with others!
As a baker myself, I have found sourdough starter to be a very unique way to spice up different yeasty recipes. I use my sourdough starter for much more than just bread---sourdough discard adds an incredibly fluffy texture to pancakes, and can even add a delectably tangy taste to pizza dough! Also, sourdough starter exhibits the incredibly intriguing process of fermentation, but not the type of fermentation that occurs when you workout.
Sourdough Starter and Sourdough Bread
What is sourdough starter?
Sourdough starter is synthesized from a combination of flour and water, which ferments over time. Flour contains wild yeast, so when the flour is combined with water, the mixture captures the yeast and brings it to life. In order to keep the yeast alive and thriving, the starter is fed everyday by creating a levain. A levain is a mixture of flour and water with a little bit of the sourdough starter. It is important to not add too much starter to the levain. This ensures that the yeast all have enough to eat, and are not competing for food.
Although sourdough starter contains wild yeast, there are many different types of yeast sold in commercial stores, such as instant yeast, active dry yeast, and fresh yeast. Each type of yeast is marketed for a different purpose. For example, active dry yeast is composed of dehydrated yeast granules, which are dormant, similar to a seed produced by a plant! On the contrary, instant yeast is composed of smaller dehydrated yeast granules, which means that each granule has a larger surface area, and therefore blooms faster. Fresh yeast is most similar to sourdough starter because it has not been dehydrated, and is rather a dried mixture of flour, water, and yeast, which are the exact ingredients of sourdough starter.
What are the benefits of eating sourdough bread?
Unlike many other types of bread, the natural yeast in sourdough starter makes it a healthier option. The probiotics in the starter promote gut health, which means that the bread promotes better digestion. According to EatingWell.com, sourdough bread promotes a better digestion of gluten, due to the process of fermentation in sourdough starter, which contains specific enzymes that aid in the process of breaking down gluten. Sourdough bread is also a good source of vitamin B12, along with folate, which is an important vitamin for pregnant women.
How does sourdough starter work?
Fermentation is a form of anaerobic respiration, which means that it occurs in the absence of oxygen, therefore differing itself form aerobic respiration. The yeast within the starter feeds on flour by using enzymes that convert the flour into sugars. During cellular respiration, the sugars go through glycolysis, which produces some ATP and pyruvate. The pyruvate then goes through oxidative phosphorylation, where it is converted into Acetyl-CoA. The Acetyl-CoA then moves to the Krebs Cycle, where some ATP is produced, and then goes on to the final stage, the electron transport chain. This metabolic process produces molecules of carbon dioxide, which are held within the gluten strands of the dough, and cause the dough to rise. This carbon dioxide production is the reason for the holes and bubbles in bread.
The tangy taste behind sourdough bread is a result of fermentation. Yeast goes through alcoholic fermentation, which largely contributes to the taste of the final product. Fermentation occurs when pyruvate, the product of glycolysis, releases carbon dioxide, which then produces acetaldehyde. The acetaldehyde is then phosphorylated from the hydrogen ions from NADH, creating ethanol.
Although yeast uses alcoholic fermentation, a different type of fermentation is greatly utilized by humans. This process is known as lactic acid fermentation. Lactic acid fermentation differs from alcoholic fermentation in that it produces lactate from glucose and other six-carbon sugars. Lactic acid fermentation occurs when cells run out of stored energy, such as ATP or glucose. The body begins lactic acid fermentation in glycolysis, where glucose is converted into pyruvate. The pyruvate then goes through lactic acid fermentation, where NADH is phosphorylated to produce lactic acid, which is then used as an energy source.
The metabolic process of yeast fermentation
When making sourdough bread, it is important to not overproof the bread (let it rise for too long). Over proofing will cause the bread to collapse in the oven because the yeast has eaten through the gluten strands that give structure to the bread. Over proofing will also lead to a stronger fermented taste because more ethanol has been produced. An over fermented dough has a very sour taste, which is not very appetizing.
Once the sourdough bread has been put in the oven, the yeast increases its metabolic process, which produces more carbon dioxide and causes the bread to rise even more. Once the bread becomes too hot, the yeast dies off, leaving behind large pockets of carbon dioxide.
Unlike other breads, it is recommended to allow sourdough bread to completely cool before digging in. The high water content within the dough greatly contributes to the cooking of the bread, which creates an airy bread. When the bread is cut into while hot, the cooking process is cut short, resulting in a gummy texture.
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