Biga is a type of pre-ferment used in Italianbaking. Many popular Italian breads, such as ciabatta, are made using a biga. Using a biga adds complexity to the bread's flavour and is often used in breads which need a light, open texture with holes. Apart from adding to flavour and texture, a biga also helps to preserve bread by making it less perishable. Biga techniques were developed after the advent of baker's yeast as bakers in Italy moved away from the use of sourdough and needed to recover some of the flavour which was given up in this move.
Bigas are usually dry and thick compared to the Frenchpoolish or a sourdough starter. This thickness is believed to give a Biga its characteristic slightly nutty taste. Biga is usually made fresh every day, using flour, 50% to 60% water and about 0.8 to 1.5% fresh yeast it is then allowed to ferment from 12 to 16 hours to fully develop its flavour.
After fermenting overnight, biga is then added to the bread dough in place of, or in addition to, regular baker's yeast, depending on the recipe, and the bread dough is mixed, kneaded, raised, shaped, proofed, and baked like any other yeast dough.
There are a few bread books whose authors specify a much higher hydration to the biga. Franco Galli, in his "Il Fornaio Baking Book," specifies a biga that is about 100% hydration, which takes it into the level of a French poolish. In general, however, a biga is a pre-ferment of around 60% and the poolish is a pre-ferment of around 100%.