Thank you for the explanation. does that mean all of the water plus all the yeast is used? salt plus the rest of the flour comes later?
The definition of biga is the Giorilli biga as defined by Piergiorgio Giorilli, the inventor of this preferment. It is 100% flour, 44-45% water, 1% fresh yeast (dry yeast conversion is typically specified as 0.5-0.6%).
Now, if you're using 100% biga, then all the flour is in the biga, and you only add water to arrive at your desired final hydration, salt, oil and malt if using.
If you're using 80% biga, then 80% of the flour is in the biga. This is typically strong flour such as Caputo Nuvola Super or Polselli Super. For the refreshement, you add 20% of a weaker flour, salt, malt and oil (if using).
In some cases, you add a bit of yeast to the refreshment, for example, if you want to go straight to balls and bake in a few hours. But the biga contains plenty of yeast so there is no actual need to add more unless you want to accelerate your time to use.
