The biga is a traditional Italian preferment. It's a less hydrated mixture and so it's much stiffer than most of the other preferments. I get asked a lot how it is that the biga is a stiff preferment when Italy is known for its really well hydrated breads like ciabtta. My answer is that Ciabatta is one among many different kinds of breads, and you don't use the same preferments for each one.
A biga is a less hydrated preferment that uses about half as much water as flour (maybe 60 percent water sometimes), in addition to the small amount of yeast. I generally use a stand mixer to mix a biga, because it's supposed to have a more dough-like consistency. A biga should be left at covered at room temperature for about an hour and then refrigerated overnight. It needs to be allowed to warm up at room temperature for about an hour before being used, and the biga should have a nicely domed top to tell you it's ready. If it looks a little deflated that's OK, but too much deflation is not good.
This type of preferment is more hospitable to the kind of bacteria which produce acetic acid, so it will have a more astringent flavor and smell.
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