Sourdough Science: Wild Yeast, Lactic Acid Bacteria and the Biology of Bread Fermentation

A mature sourdough starter contains two primary microbial groups: wild yeasts and lactic acid bacteria (LAB), typically in a ratio of roughly 1:100 (yeasts to LAB by cell count). The dominant wild yeasts in most starters are Kazachstania humilis (formerly Candida humilis) and Saccharomyces cerevisiae, though the exact species composition varies by flour type, geographic origin and feeding regime. Unlike commercial baker's yeast (pure-culture Saccharomyces cerevisiae), wild yeasts are more acid-tolerant and can coexist with LAB without being inhibited by the acidic environment. The LAB population includes obligate homofermentative species (producing only lactic acid from glucose — primarily Lactobacillus species) and obligate heterofermentative species (producing lactic acid, acetic acid, carbon dioxide, and ethanol — including Fructilactobacillus sanfranciscensis, now renamed Fructilactobacillus sanfranciscensis, previously F. sanfranciscensis). The ratio of homofermentative to heterofermentative bacteria determines the acid profile of the bread: homofermentative strains produce more lactic acid (milder, yoghurt-like sourness); heterofermentative strains produce more acetic acid (sharper, vinegar-like). Lower hydration starters and cold fermentation temperatures favour heterofermentative bacteria and acetic acid production. Higher hydration and warmer temperatures favour homofermentative bacteria and lactic acid. This is the biological basis of the baker's control over sourness.

Gluten is not a single protein — it is a network formed when two flour proteins (glutenin and gliadin) are hydrated and physically manipulated, forming disulphide cross-links and hydrogen bonds into an elastic, extensible matrix. In commercial bread making, gluten development is primarily achieved through mechanical kneading — physical energy that aligns and entangles the protein chains. In sourdough, fermentation itself is a significant contributor to gluten development through several mechanisms. First, carbon dioxide produced by yeast metabolism inflates tiny gas cells throughout the dough. The pressure of these bubbles stretches the surrounding gluten network, achieving mechanical extension without external manipulation. Second, protease enzymes present in flour (and produced by LAB) partially hydrolyse gluten proteins — making the dough more extensible (able to stretch without tearing) and improving its ability to retain gas. Third, the acidity produced by LAB affects gluten's electrical charge, influencing how tightly it cross-links. Moderate acidity strengthens gluten by increasing disulphide bond formation; excessive acidity weakens it by denaturing proteins. This is why an over-fermented sourdough becomes slack and sticky — the gluten has been partially hydrolysed beyond optimal levels. The dough tears rather than stretching when shaped.

Autolyse — the technique of mixing flour and water briefly and resting before adding starter and salt — is one of the most impactful, least physically demanding improvements a sourdough baker can make. Developed by French baking scientist Raymond Calvel in the 1970s, autolyse exploits the same enzymatic activity that fermentation eventually performs, but faster and in a controlled way. During autolyse, protease enzymes in the flour begin cleaving bonds in glutenin and gliadin proteins, making them more extensible. Simultaneously, flour starches fully hydrate, and amylase enzymes begin producing fermentable sugars from damaged starch. When yeast and LAB are subsequently added, these sugars are immediately available for metabolism, accelerating fermentation activity. The practical benefit is dramatically reduced kneading time: a dough that would require 10–15 minutes of intensive kneading to develop adequate gluten structure needs perhaps 3–5 minutes after a 30–60 minute autolyse. The gluten is better developed (more extensible, better gas retention) and the dough is less likely to tear during shaping. Autolyse should be done without salt (which tightens gluten and slows hydration) and without starter (whose acid would begin modifying gluten too aggressively before the flour is hydrated).

Traditional bread making develops gluten primarily through continuous kneading — applying mechanical energy to align protein chains and encourage cross-linking. Sourdough, particularly at high hydration (70 %+ water), uses an alternative approach: repeated stretch-and-fold cycles during bulk fermentation. A stretch-and-fold cycle consists of grasping one side of the dough, stretching it up and over the mass, then rotating 90 degrees and repeating four times (creating a 'packet'). Performed at 30-minute intervals during the first 2–3 hours of bulk fermentation, four to six sets of stretch-and-fold provide gluten development equivalent to moderate kneading — without the tearing and heat generation that vigorous kneading produces in wet doughs. The biological rationale: stretching aligns glutenin chains in the direction of extension, encouraging the formation of disulphide cross-links in that orientation. The rest period between sets allows the newly formed bonds to stabilise and the dough to relax before the next set. Lamination (stretching the dough on an oiled surface into a very thin sheet before folding) provides the most intensive single set for developing gluten structure, often used as the final strengthening step before shaping. Coil folds — lifting the centre of the dough to allow the sides to fold under — are used in very wet doughs where direct stretching would tear the gluten before it has developed sufficiently.

The three variables the sourdough baker has most control over are dough hydration, fermentation temperature and fermentation duration. Hydration (expressed as a percentage of flour weight in water — 75 % hydration means 750 g water per 1000 g flour) determines crumb openness, handling difficulty and microbial activity rates. Higher hydration produces more open crumb (larger, irregular holes) but is far more difficult to shape and requires excellent gluten development to support gas retention. Lower hydration (65–70 %) produces a tighter, more even crumb, is easier to shape and is more forgiving of timing variations. Temperature directly controls microbial metabolic rate: at 26–28 °C (typical room temperature in a warm kitchen), a well-fed starter will peak in 4–6 hours and a bulk fermentation will complete in 4–6 hours. At 4 °C (refrigerator), both are slowed dramatically — 12–48 hours or more. Cold fermentation (retarding the shaped loaf in the refrigerator overnight) is the most powerful single technique for flavour development. At low temperatures, yeast activity slows more dramatically than LAB activity — meaning acid production continues while gas production slows. The extended low-temperature fermentation allows enzymatic and microbial processes to develop a far wider range of aromatic compounds (esters, aldehydes, alcohols, organic acids) than a rapid room-temperature fermentation can produce. The cold proof also produces a firmer, colder dough that scores more cleanly and achieves better oven spring.

The nutritional differences between genuine sourdough and commercial yeasted bread are mechanistically well-supported. The organic acids produced by LAB — particularly lactic acid — lower the glycaemic index (GI) of sourdough bread in several ways. Acid inhibits alpha-amylase enzymes in the small intestine that break down starch into glucose, slowing absorption. The lower pH also changes starch structure (reducing rapidly-digestible starch while increasing resistant starch), and the gelling properties of partially hydrolysed proteins create a physical barrier to digestive enzymes. Multiple studies have documented GI reductions of 25–40 % in sourdough bread compared to matched commercial bread. Additionally, the long fermentation partially breaks down phytic acid (phytate), an anti-nutrient in wheat bran that chelates minerals like zinc, iron and magnesium and reduces their bioavailability. Phytase enzymes — both endogenous in flour and produced by LAB — hydrolyse phytate during fermentation, improving mineral absorption from the final bread. Commercial bread, which ferments for 1–2 hours maximum, does not achieve meaningful phytate reduction. Sourdough fermented for 8+ hours can reduce phytate content by 50–80 %.