Sourdough, before and after commercial yeast

Every leavened bread made before isolated commercial yeast was available was raised by a mixed population of wild yeasts and lactic acid bacteria, whether or not anybody thought of it as a maintained culture. A piece of yesterday’s dough put into today’s is a starter, and that practice is as old as leavened bread.

What changed in the nineteenth century was that yeast became a product. Once a baker could buy a known quantity of a single organism in a stable state, fermentation became predictable, fast and schedulable — and the maintained culture became a cost rather than a necessity. Commercial bakeries dropped it first and most completely.

So the modern framing of sourdough as an artisan alternative inverts the history. The alternative is the packet.

The word describes the taste, and the taste is a side effect. Lactic acid bacteria in a maintained culture produce organic acids alongside the carbon dioxide the yeasts produce; the acids lower the dough’s pH, which changes gluten behaviour, slows staling and contributes flavour.

It is worth being clear that not all naturally leavened bread is sour. The acid balance depends on hydration, temperature, flour and schedule, and a culture kept warm and fed often produces a bread most people would not call sour at all. Bakers who prefer the term 'naturally leavened' are making that point.

In rye baking the acidity is not a flavour preference at all — it is what restrains the grain’s own amylase during the bake. That is a structural requirement, and it is why the sour rye traditions of northern and central Europe look the way they do.

A lactic acid bacterium first characterised from San Francisco sourdough carries the city’s name, and this is routinely reported as proof that the city’s bread is unique and that its distinctiveness comes from something in the local air or water.

What is established is that the organism was isolated and named there. What is not established is that it is confined to there — related organisms have been identified in cultures elsewhere — nor that any local environmental factor accounts for a difference a controlled comparison has demonstrated. BakeHQ has read no such comparison.

The honest summary is that a maintained culture’s character comes overwhelmingly from the flour it is fed, the temperature it is kept at and the schedule it is kept on. Those travel with the baker. This record treats the local-uniqueness claim as a traditional account.