A Day in the Life of a Sourdough Leaven from Feeding to Maturity

Fermentation is a type of biological process conducted domestically or commercially to preserve foods and beverages, produce alcohol, add nutritional value and improve aroma and flavor. The natural fermentation of flour in water to obtain a leaven for baking, lately scrutinized in the laboratory with the application of metagenomic methods, has been ubiquitous since the dawn of civilization. Commercially, single culture or defined mixtures of microorganisms are used for their predictability, but regularly fed two-domain microorganism cultures are favored in less industrialized and domestic operations. Fungi principally produce the carbon dioxide responsible for leavening. The bacteria produce acid in the bread commonly known as sourdough for its aroma and flavor. A leaven made by fermentation using flour and water can be stored while it is dormant. We studied a mature culture that is fed twenty-fold with water and flour by incubating it for 24 h, sampling it regularly for pH measurements, and plating it. The colonies were suspended for micrography and DNA extraction for PCR and Sanger sequencing. The metagenomic DNAs were analyzed for bacterial and fungal composition. The proportions of the plant and microbial DNA endogenous to the flour decline rapidly, and the predominant bacteria and fungi in mature leaven propagate, without overlap between the respective microbiomes.