Are Biga, Poolish, and Sponge Interchangeable?
A preferment is a portion of the dough that’s made in advance of the final dough; it’s already ripe (ready to use) when it’s mixed into the final dough. When a preferment is added to a dough, it not only accelerates fermentation but is itself a partially developed dough, which reduces mixing time. Preferments can be divided into two categories: those started with commercial yeast and those started with wild yeast and bacteria, which we call a levain.
As we researched various baking books throughout this project, we noticed that different authors use different terms for exactly the same preferment made with commercial yeast. What one called a biga, another referred to as a poolish, pâte fermentée (prefermented dough), or sponge. And we also were puzzled at the wide range of hydration levels specified for these preferments among baking books. Although many referred to a 100% hydration preferment as a poolish, and stiffer ones as sponges and bigas, there seemed to be no universally accepted hydration levels for each variety. The terminological mess raises an interesting question: what happens if you use a liquid preferment in a recipe that calls for a stiff one?
The answer surprised us: as long as you adjust the hydration of the dough to compensate for any water added or withheld in the preferment, the bread turns out the same no matter whether you make it from a 100% hydration polish or a biga containing half as much water. In other words, the much-fought-over distinctions among these different kinds of preferments don’t really serve much purpose. You can use whatever preferment you want in any bread recipe and achieve essentially identical results—just pay attention to the net hydration of the dough.
Our experiment was simple. We baked loaves of our French Lean Bread recipe with equal amounts of six different kinds of preferments, all made with commercial yeast but ranging in hydration from 50% with the water:flour ratio of the dough ranging from 50%–100%, at 10% intervals. As we prepared the dough, we adjusted the amount of water added so that the total water:flour ratio of the dough worked out to 69% hydration.
The loaves that emerged from the oven were nearly identical in appearance. Any variations in taste were exceedingly subtle at best. Although the end product is essentially the same, the convenience of the preferments is not. Sponges, bigas, and other stiff varieties are hard to incorporate evenly into a dough. We much prefer working with a loose, 100% hydration poolish, which dissolves readily into the water for the final dough.
The amount of yeast in the preferment can also affect the bread, of course, but that is a function of both how much yeast you mix in and how long you let the yeast grow. With a 100% hydration poolish, you can expect similar results from a 3-hour preferment containing 0.45% yeast to one made with 0.25% yeast but fermented for 8 hours.
In our recipes, we specify how to make the preferments, and we do use the common terms to indicate the level of hydration involved. Poolish has a loose consistency and is typically made with equal parts water and flour with a small percentage of commercial yeast. Biga, which is more like a dough than a batter, has a lower hydration than poolish and sponge, which is mostly used for enriched doughs, contains milk, eggs, butter, and/or sugar. But it’s good to keep in mind that a biga is really a stiff poolish, and a sponge is really an enriched biga. The next time you find a recipe that calls for a 12-hour poolish but don’t have that much time, just bump up the yeast percentage. Or make mixing easier on yourself by using a 100% hydration poolish in place of a stiff preferment. Just don’t forget to then adjust the amount of water you add to the final dough accordingly.