Sourdough Microbiology

Sourdough ecology is defined by cross-feeding and mutual protection. LAB metabolize damaged starch and pentoses (from wheat bran arabinoxylan) that yeast cannot efficiently ferment, producing acid and CO2 without competing for glucose and maltose — primary yeast substrates. LAB-produced lactic and acetic acids lower pH to ~3.5–4.5, inhibiting pathogens and mold while the acid-tolerant yeast and LAB flourish. Bacteriocins produced by LAB (e.g. nisin-like compounds from Lactococcus species) further suppress competing bacteria. K. humilis dominates commercial sourdough starters because it thrives in high-acid, high-CO2 anaerobic environments where standard S. cerevisiae is suppressed. Organic acid ratios are key: at low hydration and cooler temperatures, heterofermentative LAB (L. brevis, Fructilactobacillus sanfranciscensis) favor acetic acid production via the phosphoketolase pathway (when forced to use alternate electron acceptors); at high hydration and warm temperatures, homofermentative pathways and heterofermentative pathways that produce more lactic acid dominate. The ratio of lactic to acetic acid (LA:AA) governs perceived sourness — a high ratio produces mild, dairy sourness; a low ratio (more acetic) produces sharp tang. Microbial succession in a new starter: early (days 1–3) enterobacteria dominate, producing off-smells; LAB begin to acidify (days 3–6) and suppress enteric bacteria; by day 7–14, a stable yeast-LAB community establishes. 'Terroir' in sourdough refers to the reality that microbial populations vary by flour provenance, local water chemistry, and baker's hands — studies have found that starters from different geographic locations maintain distinct microbial communities even when fed identically for months.

• Understanding the ecology explains why starter behavior changes seasonally and why adjustments to hydration, temperature, and feeding schedule can correct sluggish or overly acidic starters.
• LA:AA ratio is the primary tool for controlling sourness profile — bakers can shift flavor between mild tangy and sharply sour by adjusting temperature and hydration alone.
• Sourdough fermentation pre-digests phytic acid (an antinutrient) and partially hydrolyzes gluten proteins, making the bread more digestible and improving mineral bioavailability.
• The acid environment and bacteriocins in sourdough inhibit mold growth, giving naturally leavened bread a much longer shelf life than commercial-yeast bread.
• A healthy starter is a renewable, indefinitely maintainable culture — once established, it never needs to be repurchased.

1. 1Maintain a starter at 100% hydration (equal parts flour and water by weight) for a liquid levain with clean lactic sourness; stiff starters (60–65% hydration) favor acetic acid production and sharper flavor.
2. 2Peak activity ('ripe' or 'ready') occurs when the starter has doubled and begins to dome, typically 4–8 hours after feeding at 24–26 °C; use it at peak for maximum gas production.
3. 3To increase acetic character: lower hydration, use cooler temperatures, and extend the fermentation time between feedings.
4. 4To increase lactic character: higher hydration, warm temperatures (28–30 °C), and more frequent feedings.
5. 5Whole grain flours (rye especially) provide more microbial diversity and fermentable pentoses, accelerating starter activity and adding complexity.
6. 6A sluggish or dormant starter can often be revived by: feeding with whole rye flour, raising temperature to 26–28 °C, increasing feeding frequency for 3–5 days.
7. 7Never use chlorinated water directly — let tap water stand overnight or use filtered water; chlorine inhibits LAB more severely than yeast.
8. 8The 'float test' (a spoonful of starter floats in water) indicates sufficient CO2 production for leavening — though it is not perfectly reliable for all hydration levels.

• A ripe starter smells pleasantly tangy — yogurt-like lactic notes, with some yeasty aroma; acetic-dominant starters smell sharper and more vinegary.
• A starter at peak activity has doubled or more and domes slightly at the top; bubbles are visible throughout the mass.
• Overripe starter collapses and develops a strong alcohol or acetone smell — yeast have exhausted sugars and shifted to producing off-compounds.
• A healthy starter has a web of small, even bubbles in cross-section when a spoon is dragged through it.
• Gray or pink discoloration of the surface layer indicates contamination or oxidation of flour proteins; usually harmless if only superficial.

• Discarding the starter during the off-smell phase of establishment (days 1–3) — early enterobacteria are natural and will be outcompeted by LAB as pH drops.
• Storing at too-cold a temperature (below 4 °C for extended periods) — this kills off a disproportionate share of the LAB community, leaving yeast-heavy starters that leaven but produce little acid.
• Using only white flour — rye, spelt, or whole wheat additions accelerate activity and complexity; a white-only starter can be sluggish and bland.
• Judging readiness by time rather than doubling behavior — a starter is ready when it peaks, not by the clock.
• Adding commercial yeast to 'help' a new starter — this floods the ecosystem with S. cerevisiae, delays the natural establishment of acid-producing LAB, and produces a starter that behaves like commercial yeast rather than sourdough.
• Using a liquid starter for a retarded overnight dough without adjusting the feeding ratio — a 1:1:1 starter will over-ferment in the refrigerator; use 1:5:5 or 1:10:10 for overnight builds.

• Lactic Acid Fermentation

  LAB are half the sourdough ecosystem and produce the acids that define flavor and preservation.

• Yeast Fermentation & Gas Production

  Wild yeast in the starter produce the CO2 and ethanol responsible for leavening and some flavor.

• Dough Temperature Control (DDT)

  Sourdough is more temperature-sensitive than commercial-yeast doughs because both yeast and LAB populations respond differently to temperature shifts.

• Gluten Development

  Sourdough acid lowers dough pH, which influences gluten extensibility and proteolysis during the long bulk fermentation.

1. 1.Tartine Bread — Chad Robertson (Chronicle Books, 2010)
2. 2.The Rye Baker — Stanley Ginsberg (W. W. Norton, 2016)
3. 3.Sourdough Microbiology — Gobbetti & Gänzle, eds. (Springer, 3rd ed., 2023)
4. 4.On Food and Cooking — Harold McGee (Scribner, 2004), Chapter 6

Confidence: high