Sourdough Science Part I - The Starter


Deep in the recesses of my kitchen cabinets, there’s a glass jar that contains flour, water and millions and millions of tiny organisms. It emits a subtle vinegary flavor as gaseous bubbles slowly rise to the top and burst across the pale beige surface.  


Meet my sourdough starter.


This recipe for bread is ancient, dating as far back as 5,000 years to create a dough that is pleasantly tangy and sweet. Humans have been mixing flours and water while waiting for it to ferment for thousands of years, though the science and microbiology at work were not understood until relatively recently. In 1856, French scientist Louis Pasteur discovered that microbes, or microscopic living organisms, were behind the fermentation process.


Here’s where things get interesting. Sourdough starter relies on ambient microbes, called wild yeast for the fermentation process. Think of it as starting a zoo with wild animals who happen to wander into your enclosure. These bacteria are sensitive to factors such as temperature and humidity, and while the species will not vary much from place to place, the specific ratio of bacteria will vary depending on climate.  Because of this, the sourdough starter from different geographic areas will taste slightly differently, reflecting the specific strains present in your particular area. Perhaps you’ve heard of the famous San Francisco sourdough, after which the bacteria  L. sanfranciscensis is named.


Wild yeast, or saccharomyces exiguus, is a different strain of yeast than what you find in pre-packaged yeast, which is a monoculture of S. cerevisiae. The yeast doesn’t really affect the taste of the sourdough, but wild yeast is much heartier than commercial yeast, and better suited to the highly acidic environment of sourdough starter.


L. sanfranciscensis is not the only actor present in sourdough starter - most will also contain a combination of wild yeasts and lactic acid bacteria such as Lactobacillus brevis, L. reuteri, L. johnsonii, orL. Acidophilus, and L. plantarum. These little guys feed on amylase, an enzyme naturally present in flour, and the yeasts act as a leavening agent while producing alcohol and carbon dioxide as a by-product. Hence, the bubbles. The lactobacillus, similar to the microorganisms found in cheese and yogurt,  also feed on sugars present in your starter and produce two different kinds of acid as a by product.


These little guys churn out the lactic acid that gives sourdough its lovely, rich mellow flavor, and acetic acid, which is responsible for the tangier element of sourdough’s flavor profile. This acetic environment protects your starter from picking up harmful bacteria that might make you sick. Thanks, guys!


As the sugars in sourdough starter are converted to acid, the pH drops down to around 3.8, similar to that of mayonnaise, cherries or cider. These little guys lived in pretty cramped corners in my small jar and are forced to survive through a symbiotic relationship. Resources are scarce, and yeasts such as C. milleri might  rely on fructose, glucose, or galactose as food sources, leaving L. sanfranciscensis bacteria to indulge on maltose as their source of food. The ecological niche that is my kitchen cabinet requires cooperation and coexistence and does not allow for much competition between microbes.


So, how’s your starter faring? The gasses produced by the yeast will rise to the top, and this is how you know your cultures are healthy and thriving.