What is a poolish?
In the breadmaking world there are two pre-ferment varieties: sponges based on baker’s yeast, and the starters of sourdough based on wild yeasts and lactic-acid bacteria.
Biga (Italian) and poolish (French) are terms used for sponges made with domestic baker’s yeast. Poolish is a fairly wet sponge (typically one-to-one, this is, made with a one-part-flour-to-one-part-water ratio by weight), whereas biga is usually drier. Bigas can be held longer at their peak than wetter sponges, while a poolish is one known technique to increase a dough’s extensibility.
A sourdough starter is likely the oldest, being reliant on organisms present in the grain and local environment. In general, these starters have fairly complex microbiological makeups, the most notable including wild yeasts, lactobacillus, and acetobacteria. They are often maintained over long periods of time. The Boudin Bakery in San Francisco for example, has used the same starter dough for over 150 years. A roughly synonymous term in French baking is levain.