Once the starter has strengthened to the point where it is consistently rising to double or triple its size within one day (or ideally, faster) and you’ve been discarding half (or more) at the top of the rise to mix in fresh flour and water, you may be ready to bake with it.
You can test this by feeding your starter and letting it rise to double or triple in size. Take a spoonful of it and drop that spoonful of starter into a full glass of water. If it floats, you’re ready to move onto the next step (because it’s full of air bubbles, which is a sign of good yeast activity).
All we’re doing in this next step is scaling up the amount of dough we’re fermenting. There isn’t enough active yeast in the starter to cause a large amount of dough to ferment in a reasonable amount of time, so we work up to it gradually with this middle step: the levain. Here’s we’re essentially just making a larger starter. I’m preparing to bake 2 loaves, so I’m mixing a pretty large starter. Even if you’re only baking one loaf, having a larger starter is a good idea as having more active yeast going into the bulk fermentation (the step after the levain) will help with a faster fermentation time.
To mix the levain, I combine most of my starter (you need to keep some of it for future bakes, so be sure not to use ALL of your starter!) with 5 heaping tablespoons of flour and enough water to achieve that thick doughey consistency – like thick peanut butter. I aim for a thick texture because a soupy consistency is more difficult to monitor for changes. A “stiff” dough will rise more obviously because it has more structural strength and does a better job of trapping all the gasses released during fermentation. A really runny dough will bubble at the surface, but it might not rise as much (because the gasses can get out through the top).
Once mixed, wrap the levain in a clear bag so you can keep in the moisture and watch it rise. The rise will take only a few hours if your environment is, and it could take closer to 24 hours if you’re baking in a cold environment. It’s fall right now, so ours will probably rise in about 12 hours.
Once the levain has risen fully, we’re ready to move on to the next step, which is mixing a big ball of dough to ferment and then bake – called bulk fermentation.