Pre-ferment

Pain poolish - a pre-ferment

A pre-ferment (pâte fermentée), also called a sponge, a starter, or the mother dough is a fermentation starter used in bread baking. It usually consists of a simple mixture of flour, water, and a leavening agent (typically yeast), 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 and a longer fermentation in the breadmaking process gives greater complexity of flavor, through yeast- and bacterial-action, enhancing the flavour and keeping time of the finished product. The starter ingredients are mixed in a container at least three times bigger than the ingredients, to allow room for the starter to grow. The starter is left sitting at room temperature, up to 72 hours, before being added to the dough. Starters typically last three to five days, but this time can be extended through refrigeration and providing more water and flour until 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 (typically made with a one-part-flour-to-one-part-water ratio by weight), while biga can be wet or dry. The common, but undocumented, origin given for the poolish is that it was first used by Polish bakers around 1840, hence its name, and as a method was brought to France in the beginning of 1920s. "Poolish" however is an old English version of "Polish", whereas the term seems to be most used in France (where "polonais" is the word for "Polish"). There is not currently any credible explanation for the origin of the term.

Wikibooks Cookbook has a recipe/module on Sourdough Starter

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