What Is a Starter? Keeping a Culture Alive to Raise Your Bread
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A jar of flour and water left out will populate itself with yeasts and bacteria that raise bread and sour it. Maintaining that mixture is keeping a small ecosystem alive, and it behaves according to rules worth understanding.
The community in the jar
A mature starter is a stable community of wild yeasts and lactic acid bacteria living in a flour and water paste, and the two groups do different jobs. The yeasts ferment sugars and release carbon dioxide, which is what raises the dough, and they are generally not the same species used in commercial baking yeast. The bacteria produce lactic and acetic acid, which is what makes the bread sour, and they outnumber the yeasts substantially, with ratios of around a hundred bacteria to each yeast cell being typical. The acid they produce lowers the pH enough to exclude most other organisms, which is why a well-maintained starter resists spoiling and can be kept for years or decades. The two groups tolerate each other's conditions, which is why this particular combination is what establishes rather than something else.
What the maintenance does
Feeding a starter is managing a population and each part of the routine has a purpose:
- •Discarding part before feeding keeps the population from outgrowing its food and accumulating acid
- •Adding fresh flour supplies sugars and the starch the enzymes work on
- •The ratio of old starter to new flour sets how long the rise takes
- •Warmth speeds everything and shifts the balance towards lactic acid and a milder flavour
- •Cold slows everything and shifts it towards acetic acid and a sharper flavour
- •Refrigeration lets a starter be fed weekly rather than daily, at some cost to vigour
Where the organisms come from
The common claim that the organisms come from the air and therefore reflect the location has been tested and is largely wrong. Sequencing studies of starters from many places found that the organisms come overwhelmingly from the flour itself, with the baker's hands contributing as well, and that the community that establishes depends far more on the flour, the maintenance routine and the temperature than on geography. A widely reported study in which bakers from many countries made starters from identical flour under identical instructions found the resulting communities were similar to each other and unlike the bakers' own established starters, which undercuts the romantic account fairly decisively. What a long-maintained starter does acquire is a community adapted to the specific conditions it has been kept in, which is a real effect and not the same thing as absorbing a local character from the air.
Reading the jar
Judging whether a starter is ready to use is a practical skill and the indicators are worth listing. Rise and fall is the main one, since a healthy starter roughly doubles after feeding and then subsides, and the peak is when it is most active. Bubbles throughout rather than only on the surface indicate the gas is being produced within the mass. Smell shifts from sharp and vinegary when overdue for feeding to mildly yoghurt-like when recently fed, and a smell of acetone or nail varnish means it is very hungry rather than dead. A dark liquid forming on top is alcohol and indicates the same thing, and it can be stirred in or poured off. A starter that has been neglected for weeks usually recovers over several feedings, and genuine death is rare, with mould of any colour on the surface being the one condition that means starting again.
What the acid does to the bread
The souring is not only a flavour and the other effects are substantial. Acid strengthens the gluten network up to a point, which improves the structure of a loaf that has risen slowly. It slows staling, so a naturally leavened loaf keeps noticeably longer than one made with commercial yeast. It inhibits mould and spoilage organisms, which was the original practical reason for the technique. It breaks down some of the compounds in flour that bind minerals, improving how much iron and zinc the body can absorb. It reduces some of the proteins implicated in gluten sensitivity, which has been measured and which does not make the bread safe for people with coeliac disease. And the long fermentation develops flavour compounds that a fast rise does not produce, which is what most bakers are actually after.
The takeaway
A mature starter is a stable community of wild yeasts producing gas and lactic bacteria producing acid, with bacteria outnumbering yeasts by around a hundred to one. The acid excludes other organisms, which is why a jar can be kept for decades. The organisms come mainly from the flour rather than the air, which sequencing studies have established fairly decisively.