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