Unlike breads that rely on commercial yeast and chemical leavening, sourdough bread uses wild yeast and lactic acid bacteria. Sourdough fermentation plays a crucial role in influencing both the flavor and texture of bread and understanding the chemical transformations of this process can help shape the properties of bread.
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Introduction
Sourdough relies on the spontaneous fermentation of lactic acid bacteria and yeasts and has been used for centuries as a natural starter for the production of bread and baked goods.
Adding sourdough can enrich the sensory profile of bread and can increase odor perceptions, such as the flowery, yeasty, and malty odor in the wheat crumb or the roasty aroma in the crust.
Products made with sourdough starters have a longer shelf life than commercially leavened bread and more nutritional and sensory qualities.
Because of the unique microbial composition and functionality, there is great interest in the science behind sourdough. Not only does it have the potential to improve the sensory attributes of baked goods, but it also increases the nutritional attributes of conventional and non-conventional flours.1
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The Science of Fermentation
A key part of the sourdough breadmaking process is fermentation. In this metabolic process, microorganisms like wild yeast (i.e., Saccharomyces exiguus) and lactic acid bacteria (LAB) such as Lactobacillus species break down carbohydrates into simpler compounds.
These microorganisms thrive on the sugars in flour and generate byproducts, such as carbon dioxide, organic acids, and alcohol, in a process that leavens the bread.2
When water and flour are mixed, wild yeast begins to consume the carbohydrates in the flour, producing carbon dioxide, which forms gas pockets in the dough, helping the bread rise.
Unlike commercial yeast, wild yeast ferments more slowly, resulting in a longer fermentation process that promotes the development of more complex flavors.
During fermentation, complex carbohydrates in the flour are broken down into simpler sugars by enzymes such as amylase, which convert starches into maltose and glucose, and carbon dioxide and ethanol are produced.
The slightly acidic environment created by the fermentation process and the LAB activity play a critical role in developing the distinct aroma and taste associated with sourdough bread. In addition, the acids produced also help slow down starch retrogradation, keeping the bread fresher for longer.
Flavor Development
Fermentation also drives the creation of flavor compounds. The flavor profile of sourdough results from a combination of acids, esters, alcohols, and other volatile compounds that develop from the action of wild yeast and bacteria.
For instance, yeast can produce flavor compounds by converting branched-chain amino acids into higher alcohols and their esters (via the Ehrlich pathway), while the aroma of the crust is due to the production of 2-acetyl-1-pyrroline (formed through the Maillard reaction).
Lactic and acetic acid are the main acids involved in the fermentation process. The former is responsible for fresh acidity with a milder, yogurt-like flavor, while the latter is associated with sharp acidity, contributing to a sharper, vinegar-like note.
Moreover, the production of alcohols and esters adds fruity and floral notes to the bread. Ethanol evaporates during baking, but it leaves behind traces of flavor that contribute to the overall complexity of the loaf. In contrast, esters such as ethyl acetate are responsible for fruity aromas.
By controlling fermentation times and conditions, it is possible to adjust the flavor profile of sourdough bread, either enhancing the acidity or developing more subtle, nuanced flavors.
The science behind sourdough
Texture Formation
The texture of sourdough bread – characterized by a chewy crumb and a crisp, golden crust – is a result of both gluten development and the effects of fermentation.
Gluten is a network of proteins found in wheat flour that is essential for creating the structure of the dough. Upon mixing flour and water, gluten proteins form long chains that make the dough elastic.
During fermentation, the CO2 produced by yeast becomes trapped in the dough, causing it to rise and form air pockets, resulting in the airy structure typical of sourdough.
Fermentation also influences the texture of the crumb since the acids produced by LAB weaken the gluten structure, making the dough more extensible. This results in a more open crumb structure with large, irregular holes.
At the same time, the breakdown of carbohydrates increases the dough’s moisture content, leading to a tender, chewy texture. The acids can also lead to a thick, crunchy crust by altering the Maillard reaction responsible for browning during baking.
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Variations and Techniques
The flavor and texture of sourdough can vary significantly depending on the type of starter used and the fermentation conditions. Different sourdough starters can be cultivated using various types of flour, water sources, and other strains of wild yeast and bacteria, leading to unique flavor profiles.
For instance, sourdough addition to common oat baking can provide a distinct flavor. Depending on the starter used, such as L. brevis, L. plantarum, or combinations thereof, it is possible to modify the aroma and flavor.3
Fermentation time and temperature also affect the final bread. Longer fermentation times generally result in a more pronounced sourness, while shorter fermentations yield a milder flavor.
The minimum baking time is reached when 98% of the starch in the dough has fully gelatinized, and this time shortens as the oven temperature rises.
Extended baking times produce a thicker, darker crust with a stronger toasted flavor, while shorter times result in a softer crust and a more subtle taste.
Bakers also use techniques such as retarding the dough (allowing it to ferment in the refrigerator) to slow down fermentation while allowing LAB to continue producing acids and develop more complex flavors.
Conclusion
From the breakdown of carbohydrates to the production of acids and alcohols, the chemistry of sourdough fermentation contributes to defining both the flavor and texture of bread.
Understanding these processes allows bakers to manipulate the quality and sensory profile of bread and, through the selection of specific groups of lactic acid bacteria and yeasts, produce a wide range of sourdough breads with unique characteristics.
References
- Arora, K., Ameur, H., Polo, A., Di Cagno, R., Rizzello, C. G. & Gobbetti, M. (2021). Thirty years of knowledge on sourdough fermentation: A systematic review. Trends in Food Science & Technology, 108, 71-83.https://doi.org/10.1016/j.tifs.2020.12.008. Available: https://www.sciencedirect.com/science/article/pii/S0924224420307202
- Pérez-Alvarado, O., Zepeda-Hernández, A., Garcia-Amezquita, L. E., Requena, T., Vinderola, G. & García-Cayuela, T. (2022). Role of lactic acid bacteria and yeasts in sourdough fermentation during breadmaking: Evaluation of postbiotic-like components and health benefits. Frontiers in Microbiology, 13.10.3389/fmicb.2022.969460. Available: https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.969460
- Cera, S., Tuccillo, F., Knaapila, A., Sim, F., Manngård, J., Niklander, K., Verni, M., Rizzello, C. G., Katina, K. & Coda, R. (2024). Role of tailored sourdough fermentation in the flavor of wholegrain-oat bread. Curr Res Food Sci, 8, 100697.10.1016/j.crfs.2024.100697. https://researchportal.helsinki.fi/en/publications/role-of-tailored-sourdough-fermentation-in-the-flavor-of-wholegra