This makes enough for my baguette recipe, but of course it can be increased if need be for some other application. The amount of yeast employed in a poolish is tiny relative to the flour and water. So tiny, in fact, that for a poolish sponge this small, we’ll need to dissolve some instant yeast in water, then administer the solution in spoonfuls. Start by combining:
2 ounces water
1/8 teaspoon instant yeast
…in a small bowl.
Stir until the yeast is completely dissolved:
Fetch a slightly larger bowl, and put in it:
4 ounces all-purpose flour
2 teaspoons of the yeast solution
4 ounces of water:
And stir with a fork until it’s all combined into a batter.
Cover with plastic wrap and leave it at room temperature for 12-16 hours until it looks about like so:
Nice and bubbly, yes? That’s what you want. And there you have it: one poolish. This will keep well in the fridge for two days.
Note that if you don’t use it, or make more than you need, you can convert leftover poolish into a regular “barm” or sourdough-type starter, simply by leaving it a room temperature and feeding it on a regular basis for about five days until it gets into a steady, predictable feed-expand-collapse rhythm (the starter should peak within about four hours of a feeding, then settle down). At that point you can simply store it in the fridge as you would a barm starter and use it when you like.