Sourdough Chemistry

The sourdough method for making bread and bread-like food is the oldest way of preparing a dough—5,000 years old or older. Sourdough is used as a leavening agent and improves digestibility, aroma, taste, shelf life, and the cut of baked goods, and makes rye dough suitable for baking in the first place.

First, a starter is prepared. This is done by mixing flour and water. The natural microbes in the flour begin to grow and are regularly fed with more flour and water. At first, almost any microbe can grow on this rich energy source, including spoilage agents, so not every approach succeeds. If the desired microorganisms succeed in developing further, the dough becomes noticeably sour. This leaves only lactic acid bacteria and some acid-tolerant yeasts.

When baking, part of the sourdough is used to inoculate the bread dough (about 30 wt% of the flour used), the other part is used to produce new sourdough. This makes it easier to cultivate the right microorganisms. As long as this starter culture is regularly fed flour and water, it will remain active.

Sourdough AdventCalendar ChemistryViews

Wild Yeast vs Baker's Yeast

During sourdough fermentation, many cereal enzymes, particularly phytases, proteases, and pentosanases, are activated through acidification and contribute to biochemical changes. Many fragrant and flavorful substances are developed.

There is a great variety of microbes in different sourdoughs. They come from the flour but also from the kitchen. Typical types of lactic acid bacteria are Lactobacillus plantarum (homofermentative) and Lactobacillus brevis (heterofermentative). Homofermentative means that they only produce lactic acid, heterofermentative means that they produce a combination of lactic and acetic acid.

Grains differ in the sugar mixture that they make available to the sourdough microbes. Rye based starters, for example, attract more heterofermentative lactic acid bacteria, whereas wheat flour favors homofermentative lactic acid bacteria.

References

Klaus Roth, Our Daily Bread — Part 2, ChemViews Mag. 2013. https://doi.org/10.1002/chemv.201300034
J. B. Wood, BREAD | Sourdough Bread in Encyclopedia of Food Sciences and Nutrition (Second Edition), Elsevier B.V., The Netherlands 2003

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