The Science

Courtesy of Dr Hazel

What is leaven and how does it work?

Technically speaking, a leaven is any substance that can be used to lighten a dough or batter, but on this blog we’re talking about yeast, and specifically about “wild” yeasts found in the air.

Yeasts are a type of microscopic fungi that live off sugar, which they convert to ethanol and carbon dioxide through a process called fermentation. In brewing, the important bit is the ethanol, but in breadmaking we’re more interested in the carbon dioxide, which gets trapped as bubbles within the dough which then ‘sets’ as the bread is baked, leaving behind the holes left by the gas bubbles.

A ‘natural leaven’ (or ‘starter’) is made by mixing flour and water and waiting. Flour contains lots of wild yeasts and lots of starches – long chains of sugar molecules all joined together. When it’s mixed with water, naturally-occurring enzymes in the flour start breaking those long starches into smaller sugars, which the yeast can feed off. The yeast converts the sugars into ethanol and carbon dioxide and it gets the chance to reproduce.

Other bacteria present in the flour start growing as well, particularly a type called lactobacilli, which convert the alcohol made by the yeast into lactic acid and acetic acid (vinegar). The mixture of ethanol and acid is what gives the leaven that distinctive smell and the bubbles you can see on the top of it are full of carbon dioxide. You end up with a mixture of different yeasts and bacteria that all work together to create a distinctive flavour. By adding different flours, which contain slightly different sugars and starches, you end up with leavens that taste distinctly different from each other. Even leavens started in different parts of the country can taste very different because of the different yeasts and bacteria present.

After a while, the yeast starts running out of food. It’s used up most of the sugars present in the flour and rather like Audrey II in Little Shop of Horrors, it needs feeding, so you need to add more flour and water. As before, the flour is the food – enzymes break the starches into sugars and these get eaten by the yeast, which gives out carbon dioxide and ethanol in return. Bear in mind that if you just keep adding more food, it will just grow bigger and bigger, so eventually you need to separate it, either by taking some out to make bread or by dividing it up.

Why do you keep it in the fridge?

Yeast works faster the warmer the room is. If you leave it at room temperature, it’ll go like the clappers, fermenting away and eating its way through its food at lightning quick speed. Before you know it, it’ll be flaccid and spent and (most importantly for breadmaking) it won’t be producing any more carbon dioxide. Keeping it in the fridge means it turns over more slowly and you don’t need to refresh it every day. Taking it out and refreshing it the day before you want to make bread gives it time to work up a sweat and be just at the peak of carbon dioxide production by the time you mix it with your dough.

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