Pre-ferment

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A pre-ferment (pâte fermentée), also called a "sponge" or a "bread starter", is a fermentation starter used in bread baking. It usually consists of a simple mixture of flour, water, and a leavening agent (typically yeast or yogurt), and is added to bread dough before the kneading and baking process as a substitute for yeast. Though they have declined in popularity as direct additions of yeast in bread recipes have streamlined the process on a commercial level, pre-ferments of various forms are widely used in artisanal bread recipes.

Using a pre-ferment imparts several advantages in the breadmaking process -- the act of giving a long fermentation to parts of the dough allows greater complexity of flavor through yeast and bacterial action, as well as enhancing the keeping qualities of the finished product. The starter ingredients are mixed in a container at least three times bigger than the ingredients, to allow plenty of room for the starter to grow. The starter is left sitting at room temperature for anywhere from ten hours to three days before being added to the dough. Starters typically last three to five days, but this time can be extended through refrigeration by providing more water and flour when it is ready to be used.

There are several kinds of pre-ferment commonly used in bread baking:

• Sourdough starter is likely the oldest, being entirely reliant on wild yeasts present in the grain and local environment. Sourdough starters are maintained over long periods of time. The Boudin Bakery in San Francisco for example, has used the same starter dough for over 150 years. These starters generally have fairly complex microbiological makeups, most notably including wild yeasts, lactobacillus, and acetobacteria. A roughly synonymous term in French baking is levain.
• Old dough (pâte fermentée) sponges can be made with any sort of yeast, and essentially consist of a piece of dough reserved from a previous batch of bread, with more flour and water added in to feed the remaining yeast.
• Biga and poolish are terms used in Italian and French baking, respectively, for starters made with domestic baker's yeast. Poolish is usually a fairly wet starter, while biga can be wet or dry. Poolish was first used by Polish bakers around 1840, hence its name, and as a method was brought to France in the beginning of 1920s.