Sourdough bread, one of the oldest types of bread dating back to 3700 BC, has seen a resurgence in popularity. This renewed interest stems not only from its unique flavor and texture but also from a growing appreciation for its potential health benefits. Unlike commercially produced breads that rely on baker’s yeast (Saccharomyces cerevisiae), sourdough bread undergoes a natural fermentation process driven by a complex combination of wild yeasts and bacteria. This intricate microbial ecosystem, housed within the sourdough starter, is responsible for the distinctive characteristics of sourdough bread.
Here we’ll scratch the surface of the science behind sourdough. Like all recipes, they are flexible and part of the fun is exploring, testing, a learning!
The Fermentation Process
Sourdough fermentation is a captivating process that transforms a simple mixture of flour and water into a living culture teeming with microorganisms and exemplifies the science of sourdough. The process begins with the creation of a sourdough starter, which serves as the foundation for sourdough bread. To make a starter, flour and water are combined and left at room temperature over the course of a few days. The flour naturally contains wild yeasts and bacteria, which, along with microorganisms from the surrounding environment, begin to ferment the mixture.
During fermentation, these microorganisms, primarily yeasts and lactic acid bacteria (LAB), metabolize the sugars present in the flour. This metabolic activity leads to the production of various substances, including carbon dioxide, ethanol, and organic acids such as lactic acid and acetic acid. Carbon dioxide is responsible for the characteristic rise of sourdough bread, as it gets trapped within the gluten network of the dough, creating air pockets. The acids, particularly lactic acid, contribute to the signature sour flavor of sourdough bread. Notably, if the starter is not fed enough, it can lead to the production of acetone.
Fermentation can be broken down in to 3 parts:
- Making a strong and active starter and levain: This initial stage involves the cultivation of a stable and active sourdough starter. The flour and water mixture is regularly “fed” with fresh flour and water to sustain the microbial population. Over time, a balanced ecosystem of yeasts and LAB develops, with the specific composition varying depending on factors such as the type of flour, the environment, and the feeding schedule.
- Ensuring proper bulk fermentation: This stage occurs after the starter is added to the bread dough. During bulk fermentation, the microorganisms continue to ferment the sugars in the dough, producing carbon dioxide and acids. This process contributes to the development of flavor, aroma, and texture in the final bread.
- Final rise and retarding: According to expert bakers, bulk fermentation starts as soon as you add the levain to a dough and ends when you divide or shape the dough. This stage is usually the longest period of time. During which there is extensive breakdown of carbohydrates and proteins, potentially leading to improved digestibility and enhanced nutritional value. Temperature also plays a crucial role in fermentation.
After the initial fermentation, the dough is transferred to a clean, lightly floured surface and kneaded or folded by hand. This kneading process involves stretching and folding the dough, which helps develop the gluten network and influences the final texture of the bread . The dough is then left to proof and ferment for 3 to 4 hours at room temperature.
Microorganisms in Sourdough Starter
The sourdough starter is a dynamic microbial community, with lactic acid bacteria (LAB), yeasts, and acetic acid bacteria (AAB) being the key players. These microorganisms work synergistically to ferment the dough and contribute to the unique qualities of sourdough bread.
Lactic Acid Bacteria (LAB)
LAB are a diverse group of bacteria that produce lactic acid as a byproduct of their metabolism. In sourdough starters, LAB play a crucial role in several aspects:
- Acidification: LAB produce lactic acid, which lowers the pH of the dough. This acidic environment inhibits the growth of undesirable microorganisms, contributing to the preservation and extended shelf life of sourdough bread.
- Flavor Development: LAB contribute to the complex sour flavor profile of sourdough bread. Different species of LAB produce varying levels of lactic acid and other organic acids, resulting in a wide range of flavor nuances.
- Texture: LAB also influence the texture of sourdough bread. They produce enzymes that break down gluten proteins, which can result in a more extensible dough and a softer crumb.
Some common LAB found in sourdough starters include:
- Lactobacillus sanfranciscensis
- Lactobacillus plantarum
- Lactobacillus pontis
- Lactobacillus reuteri
Yeasts
Yeasts are single-celled fungi that play a vital role in leavening bread dough. In sourdough starters, wild yeasts work in conjunction with LAB to produce carbon dioxide, which causes the dough to rise. Yeasts also contribute to the flavor and aroma of sourdough bread.
Some common yeasts found in sourdough starters include:
- Saccharomyces cerevisiae
- Saccharomyces exiguus
- Candida milleri
Acetic Acid Bacteria (AAB)
AAB are another important group of microorganisms found in sourdough starters. While less abundant than LAB and yeasts, AAB contribute to the overall flavor and aroma profile of sourdough bread. They produce acetic acid, which adds a subtle vinegary note to the bread.
The specific combination of LAB, yeasts, and AAB in a sourdough starter can vary depending on various factors, including the type of flour used, the geographic location, and the baker’s practices . This microbial diversity contributes to the unique flavor and characteristics of each sourdough starter and the bread it produces.
Sourdough Bread vs. Other Breads
Sourdough bread differs from other types of bread in several key aspects, offering valuable insights into the science of sourdough:
Leavening Agent
Sourdough bread relies on a sourdough starter containing wild yeasts and LAB for leavening, while other breads typically use commercial baker’s yeast (Saccharomyces cerevisiae).
Fermentation Time
Sourdough bread generally undergoes a longer fermentation process compared to other breads. This extended fermentation allows for more extensive breakdown of carbohydrates and proteins, potentially leading to improved digestibility and enhanced nutritional value.
Flavor and Texture
Sourdough bread has a distinctive tangy flavor due to the presence of lactic acid and other organic acids produced by LAB. It also often has a chewier texture and a more open crumb structure compared to other breads.
Nutritional Profile
Sourdough bread may offer some nutritional advantages over other breads. The fermentation process can increase the bioavailability of certain nutrients, such as minerals and antioxidants. Sourdough bread may also have a lower glycemic index, meaning it causes a slower rise in blood sugar levels compared to other breads .
Storage
Sourdough bread can be stored at room temperature for several days, thanks to its natural acidity.
Versatility
While not traditional, it is possible to make sourdough French baguettes, showcasing the versatility of sourdough and its potential applications in different bread varieties.
| Feature | Sourdough Bread | Other Breads |
| Leavening Agent | Sourdough starter (wild yeasts and LAB) | Commercial baker’s yeast |
| Fermentation Time | Longer (several hours to days) | Shorter (typically 1-2 hours) |
| Flavor | Tangy, sour | Varies depending on type |
| Texture | Chewy, open crumb | Varies depending on type |
| Nutritional Profile | May have higher bioavailability of nutrients, lower glycemic index | Varies depending on type |
| Shelf Life | Longer due to natural acidity | Shorter |
| Crumb Structure | Often more open | Varies depending on type |
| Presence of Additives | Typically no additives | May contain preservatives, sweeteners, and other additives |
And we’ve only scratched the surface on the science of sourdough!
Sourdough bread is more than just a delicious and versatile food; it is a testament to the power of natural fermentation and the intricate interplay of microorganisms. The sourdough starter, a living culture of wild yeasts and bacteria, is the heart of sourdough bread making. There is so much to explore!