Preferment terminology is confusing, but its worthwhile to learn what people mean by it, since you’ll almost certainly encounter it. You might remain confused if you watch a few videos about preferments because the word is used in some strange ways. For example, starter is a type of preferment, but you’ll frequently hear of people using starter to make a preferment. What they are generally talking about is a Levain–adding a little starter to a larger amount of flour and water in advance of making dough for bread. This differs from simply feeding a starter since the entire preferment is added to the remaining flour, water and salt to make the final dough. Sometimes the preferment will have a different ratio of water to flour than the starter (which is generally 100 hydration–equal weights of flour and water) which results in an actively fermenting amount of Levain that is closer to the ratio of the final dough so it can easily be uniformly distributed in the final dough as an active and growing culture. Liquid Levain is typically 100 percent hydrated, meaning the weight of the water equals the weight of the flour. Stiff Levain is less water (less hydrated). When liquid Levain is added to more flour and water to make dough the baker needs to do a little math to get the final hydration level they intend. See the post and video on “Baker Math” for more detail on how that is done
A starter is just a solution of flour and water that has been exposed to wild yeast and bacteria in the air that results in a beneficial mixture called SCOBY: a Symbiotic Culture Of Bacteria and Yeast. We have a post and video on making starter here <here>. The residual sugars in the flour feed the yeast and it produces carbon dioxide gas and ethanol. The bacteria eats the ethanol and some of the gluten and produces lactic acid and acetic acid that give sourdough its characteristic flavor. Its fundamentally the same sort of culture that produces yogurt, wine, beer, Kambucha, and all sorts of fermented products, but starter is generally wild, not cultured in a laboratory. Since the bacteria also eats gluten, and the acid produced shortens the strands if it’s given too much time the dough will become soft and won’t be able to retain the CO2 bubbles that make it lighter. On the plus side, people with mild gluten intolerance may be able to eat sourdough bread. The difference between yeasted breads and sourdough is that commercial yeast is only yeast–no SCOBY.
Continuing the confusion, there are also yeast-only preferments like poolish for making baguettes. And there are stiff or liquid preferments, where liquid is like pancake batter and stiff is more like bread dough. Starter-based preferments come in both style: Liguid Levain is just runny starter and stiff levain is, well, stiff like dough. Poolish or Sponge are liquid yeasted preferments and Biga is stiff.
The primary benefit of a preferment is that the dough has a head start, both in rising since the preferment will be very active, and in flavor, since the bacteria has more time to work it’s flavor magic. The dough will retain a useful amount of gluten for a longer period since the flour the preferment is added to will have all of it’s gluten even if the preferment has become soft.