Natural yeast is the happy and peaceful coexistence of wild yeast and lacto-fermenting bacteria, named lactobacillus. Yeasts are mainly responsible for production of gas (carbon dioxide, CO2), thus leavening the dough, while lactobacillus produce lactic acid (but also acetic acid and CO2), contributing to the sour flavor. So, technically, the natural yeast is a balanced and stable culture of a variety of microorganisms that can be cultivated by regular feeding with flour and water.
Where are these microorganisms coming from? They are naturally occurring in flour, especially organic and whole wheat, and bakers capture and cultivate them. When flour and water are mixed, amylase, an enzyme naturally present in flour, breaks down starch into simpler sugars that can be used by yeasts. Lactobacillus can process more complex sugars and create byproducts that can be further metabolized by yeasts. The pH of a healthy culture is ~3.5-4.5 due to lactobacillus production of lactic acid that inhibits other bacteria but not yeasts.
Baking products made with natural yeast have richer depth of flavor, as well as improved digestibility and durability, thanks to the extensive processing of starches by both yeasts and lactobacillus.
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- 2019 – Xu et a., “Effect of Mixed Cultures of Yeast and Lactobacilli on the Quality of Wheat Sourdough Bread” Frontiers in Microbiology https://doi.org/10.3389/fmicb.2019.02113
- 2005 Vuyst & Neyens “The sourdoughmicroflora:biodiversity andmetabolicinteractions” Trends in Food Science & Technology
- 1994 Gobbetti et al., “The sourdough microflora. Interactions between lactic acid bacteria and yeasts: metabolism of carbohydrates” https://link.springer.com/article/10.1007/BF00939035