A pre-ferment is a mixture of flour, water and a relatively small quantity of yeast that is prepared and left to ferment for a period of time, generally between 12 and 24 hours, although some pre-ferments are left for longer. After this period of time, more flour and water is added to prepare the bread dough itself, along with any other ingredients. The dough is then treated in the same way as usual, but no more yeast is added.

Compressed baker's yeast was only developed and made widely available in the late 19th century. Prior to this, the forms of yeast available to bakers were often unpredictable, and relatively weak. Pre-ferments provided a way to ensure that the yeast was active and capable of raising bread before the dough was prepared. During the period that the pre-ferment is left, the yeast cells will reproduce and multiply while metabolising the sugars in the flour. If the mixture can be seen bubbling and increasing in volume, it is a clear indicator that the yeast population is expanding and is healthy.

Although baker's yeast is now widely available and cheap, many bakers continue to use pre-ferments because they can also improve the flavour and texture of bread. The pre-ferment itself is left for much longer than a straight dough; the dough also takes longer to bulk ferment and prove (typically twice to three times as long). The extended fermentation allows a 'friendly' bacteria known as lactobacillus to start producing acids that contribute flavour to the finished bread. It also allows more time for the gluten network to develop, which can contribute to a more open crumb texture.

A pre-ferment is sometimes used with a starter instead of baker's yeast as a way to increase the yeast population and shorten bulk fermentation and proving. It is sometimes referred to in this context as a build.

Pre-ferments are known by different names including sponge, poolish, biga, and old dough. Note that the term should be hyphenated as it is here - preferment has an entirely different meaning!

Want to find out more? Look at the chapter on pre-ferments in the book flour and water.

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