Sponge and dough

The sponge and dough method is a two-step bread making process: in the first step a "sponge" is made and allowed to ferment for a period of time, and in the second step the "sponge" is added to the "final dough's" ingredients, creating the "total formula". In this usage, synonyms for "sponge" are "yeast starter" or "yeast pre-ferment". In French baking the "sponge and dough" method is known as "levain-levure". The method is reminiscent of the sourdough or "levain" methods; however, the sponge is made from all fresh ingredients prior to being used in the final dough.
Method.
A sponge ferment is usually a sticky process that uses part of the flour, part or all of the water, and part or all of the yeast of a total- or straight-dough formula. Highly liquid sponges of batter consistencies are mixed with a whip, spoon, or fork. Lower hydration, stiffer sponges are lightly mixed or kneaded just until the dough begins to develop. The sponge is allowed to rest and ferment for a period of time in an environment of a desired temperature and humidity. When the sponge's fermentation time has elapsed or it has reached a desired volumetric growth characteristic, the "final dough's" ingredients are added. The "gluten is developed" in the mixing or kneading process, and it may then be processed through further "work" and "rest" cycles before being "proofed" then baked.
The sum of the sponge and final dough's ingredients represents the total formula. A generic 65% pre-fermented flour sponge-and-dough formula using bakers' percentages follows:
The sponge's fermentation time depends on its temperature and that of the surrounding area, the ingredients used, and the percentage amount of yeast. It ferments in a humid environment at , where it may "rise" and expand to 4-5 times its original volume, when it "falls" it has reached 66-70% of its allotted time.
One significant decision the baker must make when designing such a formula, or adapting a "direct" or "straight-dough" formula or recipe, is to decide the sponge-to-dough flour ratio. While the relative amounts of ingredients used may vary, the method remains the same.
Purpose.
The sponge method is used for 3 different reasons: taste, texture and chemistry.
The flavour that is created is dependent on the ingredients used and the fermenting yeast. Just like sourdough, the longer the ferment, the greater the taste difference.
Sponge doughs were used before bread improvers were invented. Texture is partly a byproduct of the chemistry going on in the fermentation, which does several important things such as activate the different enzymes (protease and amylase) needed to leaven bread. Modern grain-harvesting practices have reduced the naturally occurring enzymes that grains had in former times, a result of no-longer-used grain-storage processes, so today small amounts of enzymes are routinely added to flour by manufacturers, often in the form of malted barley or sprouted grain.
Proteases, dependent on their time of action and concentration levels, soften the gluten in the dough, hydrolyzing peptide bonds, increasing dough "extensibility" which allows the protein matrix to stretch out as the mix expands, thus leading to increased baked volumes and better structure.
Products.
Many bread recipes call for a sponge method, especially traditional French breads. Some examples of breads that use the sponge method: