Prefermentation and delayed fermentation
As we have learned, the most important factor in making great tasting bread is the long, slow fermentation of the dough. This builds flavour partly because enzymes in the flour break the starches in the flour into simpler sugars, and partly because as well as carbon dioxide and alcohol, there are other complex flavour compounds which are by-products of the yeast fermentation process. A quick, warm fermentation will allow time for the yeast to produce enough carbon dioxide to raise the dough, but not enough time for the enzymes to work or for the development of the other flavour compounds. If we ferment our dough for too long, however, the gluten becomes too weak to hold the shape of the loaf. So artisan bakers generally use a preferment (pre-ferment, not prefer-ment!) as part of their dough. This allows the bread to gain flavour from the longer fermentation, but still maintain a strong gluten network for a good rise. There are a number of different names for the preferment: biga, poolish, chef, levain, sponge, pâte fermentée - however, they all follow the same principle.
A preferment is a mixture of flour, water, and yeast that is allowed to ferment for a period of at least 12 hours, or sometimes up to several days, before being mixed with the rest of the flour and other ingredients for the final dough.
A simple sponge can be made by taking a standard bread recipe and mixing half the flour, water, and yeast together and leaving in a covered bowl for 12-24 hours before adding the remaining ingredients, rising, shaping, proving and baking. A poolish generally uses more of the total water, making a preferment with a soupy consistency. A chef is a small piece of fermented dough - often made with a sourdough/wild yeast starter - that must be refreshed (i.e. have more flour and water added) several times before the final mixing and the rest of the baking process. The refreshed chef is called a levain, and it will be left to ferment again after each "build", allowing more complex flavours to develop in the final bread.
Pâte fermentée, is otherwise known as old dough, and is exactly that. The first time you make a particular bread recipe, you can increase the quantities by 10 - 20%, and before shaping and proving, remove the corresponding amount of fermented dough. If you are going to bake the same recipe again the next day as professional bakers tend to do, you can keep this extra piece in a plastic bag in the fridge, otherwise freeze it. If you leave the dough too long without freezing, the alcohol produced by fermentation breaks down into acetic acid, just as an open bottle of wine turns into vinegar, and this gives the bread an unpleasant sour taste. (This should not be confused with the lactic acid sourness of sourdough bread, which is produced by lactobacilli, not the breakdown of alcohol.) This aged dough can then be defrosted, broken into small pieces and added with the dry ingredients before kneading next time you make the same type of bread. Again, you should remove the corresponding weight of fermented dough before shaping and proving, to be kept for the next time you make that type of bread, and so on.