The surprising science of the wild yeasts in your sourdough starter
In praise of our fungal friend that we all got to know a lot better during lockdowns.
Humans have been using yeast to make bread for at least 3,000 years. For most of that time we didn’t understand a lot about it, let alone know how to purify and grow it. In fact, until the mid-1800s, few people even thought that yeast was a living creature.
Yeast is a type of fungus that grows pretty much everywhere – in soil, on leaves, plants, flowers, and it even floats around in the air. By happy coincidence, a handful of yeast types will ‘eat’ sugars and starches, turning them into alcohol (something we call fermentation) and belching out carbon dioxide gas.
When some of our ancient ancestors experimented in their prehistoric kitchens by heating cereal grain in water with herbs to make a nourishing drink, they realised that the frothy liquid would turn into a type of beer, courtesy of yeasts that had been growing on the cereal. Mix some of this tangy slurry into a dough, bake it in an oven and you have discovered yeast-risen bread.
Nowadays, a highly productive strain of yeast (Saccharomyces cerevisiae), known as ‘baker’s yeast’ or ‘brewer’s yeast’ is purified, powdered, and sold en masse for making bread and many other baked goods. Much of modern bread’s bland sameness is because the same yeast type is used everywhere.
But that’s not to say you can’t try wild strains, of which there are many. For hundreds of years, bakers would keep a batch of wet, frothy dough aside to use in their next batch of bread. This living slurry of continually reproducing micro-organisms (called a ‘starter’) has a unique blend of microbes that can be kept alive indefinitely by feeding, growing and passing down through generations.
Microbes from a baker’s hands and the air are also infused into the starter, adding to its character and complexity. Acid-producing bacteria similar to those used in yoghurt-making are also typically in the mix, giving sourdough bread its distinctive tartness.
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If you want to bake with wild yeasts (and their bacterial fellows) at home, you could either find a friend who has a sourdough starter and ask them to let you have some, or you make your own by thoroughly mixing equal parts flour (preferably organic) and water into a large clean jar, leaving it partly covered at room temperature and then repeating the process every day for four or five days, adding to the mixture each time until it becomes a bubbly, beery broth; at which point you can take some of it out to use in your bread recipe.
Yeasts need a regular dose of fresh flour as food so they can keep on multiplying, while the water gives their expanding numbers space to grow. Leave it at room temperature and forget to feed it for a few days and it will get progressively more alcoholic (the boozy liquid that pools on the top, called ‘hooch’, can be drained off). Without ‘feeding’, growth will stall, and yeast numbers will dwindle, reducing the rise it gives when used in baking. Yeasts are hardy though, and a forgotten starter can usually be revived with some fresh flour and water.
Most sourdough bread recipes call for 10-20 per cent of the total flour to be starter. Keeping your starter in a fridge slows down its growth, meaning you only need ‘feed’ it with flour and water every three or four days.
The ‘proving’ stage before baking, during which a kneaded dough is left in a warm place to allow the yeast to produce lots of gas bubbles, typically takes longer with wild yeasts than commercial strains. When you put your dough in a preheated oven, the carbon dioxide gas bubbles expand, giving the rise, and the yeast have one final surge in growth until they die and breathe out their last at 60°C. The yeast have come a long way, but they have done a good job.
- This article first appeared in issue 371 of BBC Science Focus Magazine – find out how to subscribe here
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