What Is Sourdough? Wild Yeast, Bacteria and a Slower Loaf

A mature starter is not one organism but a stable ecosystem, and it is dominated by two groups working together. The yeasts, usually species of Kazachstania and Saccharomyces rather than the single commercial species, ferment sugars and produce the carbon dioxide that lifts the dough. The lactic acid bacteria, mostly of the group formerly called Lactobacillus, outnumber the yeasts by something like a hundred to one and produce the acids that give the bread its flavour and its name. The two coexist because they are not competing for the same food and because the acidity the bacteria create suppresses almost everything else, including the moulds and spoilage organisms that would otherwise take over, which is why a starter fed regularly is remarkably hard to kill and why it is safe to keep a jar of fermenting flour on the counter for years. Two acids do most of the work: lactic acid, which is mild and yoghurt-like, and acetic acid, which is sharp and vinegary, and the balance between them is what a baker adjusts.

The procedure is unpromising and reliable. Mix roughly equal weights of flour and water, leave it covered at room temperature, and each day discard most of it and feed the remainder with fresh flour and water. The first flush of activity after two or three days is usually the wrong organisms, a bacterium that produces gas and an unpleasant smell, and it subsides; the community that stabilises after one to two weeks is the one that lasts. The variables a baker controls afterwards are few but effective:

• •Temperature: warm conditions around 26 to 28 degrees favour lactic acid and a milder taste, while cooler conditions push towards acetic acid and a sharper one
• •Hydration: a wetter starter goes mild, a stiffer one goes sour, for the same reason
• •Flour: wholemeal and rye carry far more micro-organisms and minerals and ferment faster than white flour
• •Feeding frequency: a starter fed often is mild and vigorous, one left hungry becomes acidic and sluggish
• •Storage: refrigeration slows everything and lets a starter be fed weekly rather than daily, and one that has been neglected for months usually recovers after two or three feeds

Against a loaf risen with packaged yeast in two hours, a sourdough is a different product rather than a nostalgic version of the same one. The fermentation runs for eight to twenty-four hours, and that time is what produces the differences. Flavour comes from the acids and from the hundreds of aromatic compounds generated slowly by bacterial metabolism, none of which a fast rise has time to make. The crumb is more open and the crust thicker and darker. The bread keeps for days rather than hours, because the acidity retards both staling and mould. And the acidity changes the chemistry of the flour itself: enzymes active at low pH break down phytic acid, which otherwise binds iron, zinc and magnesium and makes them unavailable, so the minerals in a long-fermented loaf are absorbed better. Some gluten proteins are also partly broken down, which is the basis of the frequent claim about digestibility, discussed below.

Three claims circulate, and they are not equally true. The mineral availability improvement is well supported and is the most solid nutritional advantage. The glycaemic response is genuinely lower than for a fast-risen white loaf in several controlled studies, with the organic acids slowing gastric emptying and starch digestion, though the effect varies with flour and method. The claim that it is safe for people with coeliac disease is false and dangerous: long fermentation degrades some gluten peptides, and small trials of specially fermented bread with selected bacterial strains have shown promise under laboratory conditions, but an ordinary loaf retains far more gluten than the threshold for coeliac safety. For the much larger group who report discomfort with bread without having coeliac disease, the more likely explanation is the fermentable carbohydrates known as fructans, which the long fermentation does substantially reduce, and that is a real effect with a real mechanism.

The word is not protected in most countries, and a loaf may be labelled with it while being made with commercial yeast, a short rise and an additive that supplies the flavour, sometimes with a small quantity of genuine starter added for the label. The practice has been named sourfaux by campaigners, and attempts to legislate a definition have so far succeeded only in a few places, notably France, where a decree specifies what may be sold under the traditional term. The practical tests for a buyer are the ingredient list, which should contain flour, water and salt and nothing else, the price, since twenty hours of proving space costs money, and the keeping quality, since a loaf that is stale in a day was not fermented long. Making it at home takes about twenty minutes of actual work spread over two days, which is the real reason the industrial version exists.

A starter is a stable community of wild yeasts and lactic acid bacteria, the bacteria outnumbering the yeasts around a hundred to one and producing the acids that give flavour, keeping quality and protection from spoilage. Temperature, hydration, flour and feeding schedule shift the balance between mild lactic and sharp acetic acid. The long fermentation improves mineral absorption, lowers the blood sugar response and reduces the fructans that upset many people, but does not make bread safe for coeliac disease, and the label is unregulated in most countries.