Sourdough’s tang comes from organic acids produced by the symbiotic community of microbes in the starter: lactic acid bacteria and wild yeasts. These organisms metabolize flour carbohydrates and produce a mix of lactic acid, acetic acid, ethanol and aroma compounds. The relative amounts of these acids determine whether a loaf tastes gently sour or sharply tangy. That balance is not fixed; it shifts with starter composition, fermentation conditions, and ingredients.
Microbial chemistry behind the tang
Two families of metabolites dominate flavor. Lactic acid gives a mild, rounded sourness and contributes to a soft crumb through its effects on gluten and starch. Acetic acid produces a sharper, vinegar-like tang and stronger aroma notes. Lactic acid bacteria such as Lactobacillus sanfranciscensis, named for its association with sourdough in San Francisco, commonly appear alongside Saccharomyces and non-Saccharomyces yeasts in starters; the bacteria are primarily responsible for acid production. Research by Marco Gobbetti at the University of Bari documents how diverse lactic acid bacteria in sourdough produce acids and enzymes that reshape dough chemistry and nutritional quality. Robert Hutkins at the University of Nebraska-Lincoln reviews how this microbial ecology determines metabolite profiles and therefore flavor.
Fermentation parameters steer which acid predominates. Higher hydration and warmer temperatures tend to favor lactic acid production because faster bacterial growth channels metabolism toward homolactic or heterofermentative pathways that yield more lactic acid. Lower hydration, longer fermentation, and cooler or more aerated conditions favor acetic acid, since certain heterofermentative lactobacilli switch to pathways producing more acetic acid under stress or limited sugar availability. Jens Walter at the University of Alberta and other researchers have shown that manipulating temperature, feeding intervals, and oxygen exposure reliably shifts acid ratios and aroma outcomes. The result is a toolbox for bakers to tune tang without changing ingredients.
Cultural, environmental and practical consequences
The microbial composition also reflects local ingredients and bakery practices, creating regional character or “terroir.” San Francisco’s historic tang is linked to strains of Lactobacillus sanfranciscensis and long, cool fermentations; other regions produce different sensory profiles because local flours, ambient microbes, and water influence starter communities. This cultural dimension means traditional recipes and feeding routines are as important as chemistry in shaping flavor.
Beyond taste, acidity affects preservation and nutrition. Higher acetic acid levels inhibit mold and spoilage, extending shelf life, while lactic acid bacteria can reduce phytic acid in whole grains, improving mineral bioavailability, as demonstrated by Marco Gobbetti’s work. For bakers, that creates trade-offs: a long, cool retardation may increase tang and shelf life but change crumb structure, while warmer, shorter fermentation produces a milder loaf with different texture. Understanding these microbial and process drivers lets bakers and food scientists deliberately shape sourdough flavor while respecting cultural and environmental influences that make each starter unique.