There are several reasons.
It builds more microbes.
- By refreshing your microbes again, it creates the next generation and so your leaven is young, fresh, vigorous and full of life, but not too acidic.
- It means you can have one pot of starter and make all kinds of leaven at the same time. In this sense, it is flexible. So I often make a muesli leaven, or a Khorasan leaven, or a pumpkin leaven. It means I can play easily.
- Some recipes call for a younger leaven. One that is not fully developed, which might be too soon to return your starter back the fridge. Again it is about flexibility. As home bakers, we generally bake once a week; if we are enthusiastic bakers then perhaps twice. So as your starter ages, it acidifies, and refreshing your starter every 8 hours is not practical – trust me I have tried it and it is exhausting. Even at 14 hours old, I consider it too acidic to make bread with. It is also important to understand that you are effectively farming yeast. That means breeding them. It’s a bit like getting the most workers possible to build a pyramid, the more you have the easier it is to get the job done. The yeast’s job is to create carbon dioxide, so making a leaven from a freshly refreshed starter is the easiest way to to ramp up the numbers of yeast. This way they are young, vigorous and enthusiastic. It is however as important as it is also a means of controlling the acidity. The other factor to consider here is the foundation of your dough. You may not want the same flavour from your starter in your dough. Making a leaven gives you the opportunity to control the flavour through changing the flour in the leaven. To make great sourdough, we have to create that robust colony again and for a domestic baker, the most practical way to do this is by making it a three step process. You make a leaven using 10% of your starter mixed with equal quantities of water to flour at about 28 degrees C and mixing with the flour you want to use for the bread. And you do this 3 – 8 hours before you bake, depending on the method you are baking with.
