The world of baking is not all black and white (commercial yeast or sourdough), there is actually a wide zone of grey represented by the indirect methods of baking.
Which is the difference between a direct and indirect method?
Direct method: all the ingredients are mixed together with the regular yeast or the sourdough, and the dough is let leaven.
Indirect method: a pre-leavened dough (biga or poolish) is prepared with part of the flour and water called in the recipe. Regular yeast is then added and the dough is let leaven.
Which are the indirect methods?
Basically, biga and poolish. The difference between the two lies in the degree of hydration of the dough.
Biga was invented in Italy and has been widely used in the preparation of ciabatta-type breads. Biga is a leavened dough made with flour and water, in a ratio of 2:1, and regular yeast. This proportion can vary and call for a higher amount of flour. Generally speaking, biga tends to be a bit dry, although the long leavening time (12-16h) allow rehydration of the dough.
Poolish seems to be of Polish origin, from here, the name, I guess. It has been widely employed in the production of French baguettes. Poolish has a high degree of hydration and is basically a batter, with a ratio flour:water of 1:1. Time and conditions of leavening might vary. I prefer to leave the poolish in the fridge for about 12h to limit the action of the yeasts.
Which are the advantages of an indirect method?
Indirect methods were invented after the introduction of the regular yeast that consists of the sole strain of Saccaromyces cerevisiae. Commercial yeast was warmly received by bakers, as it allowed a fast and standardized leavening. Unfortunately, the final product didn’t show the same aromatic complexity and durability of the one produced with sourdough. The indirect method of baking was so invented. Indirect methods bring some advantages of both commercial yeast and sourdough.
The long leavening times of biga and poolish allow the flour enzymes to degrade proteins and complex sugars, which results in a more digestible product.
Furthermore, as we do not live in a sterile environment, both poolish and biga collect bacteria and yeasts from the surrounding. Their fermentation produces molecules (e.g. lactic and acetic acid) that enriche the aromatic bouquet of the final product.
Not to mention, the presence of compounds as lactic and acetic acid limits the development of molds and, consequently, makes the final product more durable.
Obviously, neither biga nor poolish can bring all the advantages of a mature sourdough. They are, however, a good compromise towards the achievement of a more unique product.
Biga or poolish, which one is better?
It depends, of course :)
The different degree of hydration of the two doughs affects the type of fermentation. With low-hydrated dough (biga), the acetic fermentation is favoured, while with a high-degree hydration (poolish), the lactic one prevails.
Now, we just have to test the poolish! :D
S.P. Cauvain, Sour Dough Technology, Encyclopedia of Food Grains, Second Edition, 2016, Elsevier Ltd
T.R. Moore, Bread, AIB International, 2016, Manhattan, KS, USA