I made my own starter 3-4 days ago, and it has been growing well; I have fed it twice so far. As far as I know, it is recommended that you feed starter for about 7-0 days before you use it to make bread. My question is, why? If my starter is already growing, it means that the yeast in there is alive and well. How would the bread be different if I made it from the starter I have today, vs the one I will have in a week?
Thanks.
The advice to let the culture mature before you use it is targeted to getting reliable results. A mature sourdough culture has a relatively small number of strains of microorganism, which have outcompeted the others and formed a relatively stable ecology (assuming a regular feeding schedule). At that point the culture is reliable because newly introduced strains can't compete with the established ones. A new sourdough culture may work fine (particularly if you're seeing vigorous activity), or it may be not great, or it may be the best sourdough you've ever tasted. But the bread you make with it today may be significantly different from next week's.
As an example, if you got a bit of commercial bread yeast in there, the bread yeast is going to go crazy. But that yeast strain doesn't do well with low pH, so once the lactobacillus gains a toehold it'll slow down and be replaced by a hardier strain of saccharomyces. Fine bread this week; fine bread next week; but two different breads.
Bottom line: if you like the look of it, no reason not to use it today. Just don't generalize from the results.
The wait is to give the desirable yeast cultures time to fully establish themselves as dominant in the starter. Using a starter that's too young can sometimes mean it doesn't rise as well or that the sourdough flavour is less pronounced because of less yeast than ideal. You could also potentially kill the starter by removing too much of the yeast culture before it's firmly established. Your starter could have taken extremely well and be fine to use already, but it could also lead to a less than optimal loaf. The guideline 7-10 days is to be sure the culture is well established for the vast majority of starters, but individual starters may be ready sooner.
