Sourdough bread gets its sour taste from two organic acids produced by bacteria living in the starter: lactic acid and acetic acid. These acids are natural byproducts of fermentation, created as bacteria feed on sugars in the flour. The balance between these two acids, along with how long and at what temperature the dough ferments, determines exactly how sour your loaf turns out.
The Bacteria Behind the Sourness
A sourdough starter is a living ecosystem of wild yeast and lactic acid bacteria. While the yeast produces the carbon dioxide that makes the bread rise, it’s the bacteria that generate the sour flavor. In a mature starter, bacteria outnumber yeast by roughly ten to one.
The most common species found in traditional sourdough starters is Lactobacillus sanfranciscensis (named after San Francisco, where it was first identified). A study of 19 Italian sourdoughs found it made up about 28% of all bacterial isolates. Other common species include Lactobacillus plantarum and Lactobacillus paralimentarius. The exact mix of bacteria varies depending on the flour you use, the temperature of your kitchen, and how you maintain your starter, but they all share the same basic job: breaking down sugars and producing acid.
Lactic Acid vs. Acetic Acid
The two acids responsible for sourness contribute different flavors. Lactic acid produces a smooth, mild tanginess, similar to what you taste in yogurt. Acetic acid delivers a sharper, more vinegary bite. Every sourdough loaf contains both, but the ratio between them shapes the overall flavor.
A well-balanced sourdough typically has a lactic-to-acetic acid ratio between 4:1 and 10:1. When that ratio sits closer to 4:1 or 5:1, the bread has a more complex sour flavor with noticeable sharpness. When lactic acid dominates at higher ratios, the sourness is gentler and more mellow. This ratio isn’t fixed. You can push it in either direction by changing your fermentation conditions.
How Bacteria Turn Flour Into Acid
Flour contains starch, and enzymes in the flour break that starch down into simpler sugars like maltose. The bacteria in your starter feed on these sugars through two different metabolic pathways. Some bacteria (called homofermentative) produce almost exclusively lactic acid. Others (called heterofermentative) produce both lactic acid and acetic acid, along with carbon dioxide and small amounts of alcohol.
Lactobacillus sanfranciscensis is heterofermentative, which is one reason it plays such a central role in sourdough flavor. It breaks down maltose using a specialized enzyme called maltose phosphorylase, an energy-efficient process that gives it a competitive advantage over other microbes. Meanwhile, the wild yeast in the starter tends to prefer glucose and other simple sugars, so the two organisms avoid direct competition for food. This is why the bacteria-yeast partnership is so stable in sourdough starters, sometimes persisting for decades.
Temperature Controls the Type of Sour
Temperature is the single biggest lever you have over sourness. Warmer fermentation (around 80 to 85°F) favors lactic acid production, giving you a milder, yogurt-like tang. Cooler fermentation (around 50 to 65°F, like inside a refrigerator) shifts the balance toward acetic acid, producing a sharper, more pronounced sourness.
This is why cold proofing, where you place shaped dough in the fridge overnight or longer, tends to make bread taste more sour. The bacteria slow down at cold temperatures but don’t stop entirely, and the cooler environment favors the metabolic pathway that produces acetic acid. Bakers who have pushed cold proofing to extreme lengths (three days or more) report noticeably stronger sour flavors, even when the bread becomes overproofed and loses some of its structure.
Longer Fermentation, More Acid
Time works alongside temperature. The longer your dough ferments, the more acid the bacteria produce, and the more sour the final bread. A sourdough that ferments for 4 to 6 hours at room temperature will be milder than one that ferments for 12 to 18 hours. The finished pH of sourdough bread typically falls between 3.5 and 4.3, which is significantly more acidic than commercial yeast bread (which usually lands around 5.0 to 5.5).
There’s a practical limit, though. As acidity increases, the acids start breaking down the gluten network that gives bread its structure. Over-fermented dough becomes slack, sticky, and difficult to shape. The bread may taste very sour but have a dense, gummy crumb. Finding the sweet spot between flavor development and structural integrity is a core skill in sourdough baking.
Flour Type Makes a Difference
Whole grain flours produce more acidic doughs than white flour. Whole wheat and rye contain the bran and germ, which carry more minerals, sugars, and enzymes that fuel bacterial activity. A starter fed with whole grain flour acidifies faster and reaches a lower pH than one fed with white flour.
This extra acidity is a double-edged sword. It boosts sour flavor but can also degrade gluten more aggressively, making the dough harder to work with. Many experienced bakers maintain their starter on white flour to keep acidity balanced and the starter’s activity predictable, then incorporate whole grains into the final dough for flavor. Rye flour is particularly effective at driving sourness because it contains high levels of the sugars and minerals that bacteria thrive on.
How to Make Your Bread More or Less Sour
If you want a more sour loaf, you have several options that all work through the same basic chemistry: giving bacteria more time or shifting conditions toward acetic acid production.
- Use a longer cold proof. Shape your dough and refrigerate it for 24 to 48 hours before baking.
- Use a riper starter. Let your starter go longer between feedings so it’s more acidic when you mix your dough.
- Add whole grain flour. Substituting even 20 to 30% of your white flour with whole wheat or rye will increase acidity.
- Use less starter. A smaller amount of starter means a longer, slower fermentation, which gives bacteria more time to produce acid.
For a milder loaf, do the opposite. Use your starter at its peak (not past it), ferment at warmer temperatures, keep fermentation times shorter, and stick with white flour. A young, recently fed starter produces less acid, and warm bulk fermentation favors the gentler lactic acid over the sharper acetic acid.
The interplay between these variables is what makes sourdough endlessly adjustable. Two bakers using the same recipe can produce loaves with very different levels of sourness just by changing the temperature of their kitchen or the timing of their bake.