saad,
I agree with you that if your starter has demonstrated its performance regularly and reliably and consistently, then you perhaps can count on it to perform in the same manner the next time. However, it is one thing to say that when you are using your starter in a home setting, where the consequences are not particularly oppressive or burdensome, as opposed to a commercial setting where customers expect you to have bread for them to purchase. In such a situation, I can see how one might use a combination of starter and commercial yeast.
I am not sure I have an exact answer to the hypothetical you posted. However, if you use a small amount of a properly maintained starter and a long fermentation period, for example, along the lines that Marco (pizzanapoletana) advocates, with the amount of starter being 1-5% of the formula water, the primary effect of the starter will be a leavening effect, with relatively small quantities of organic acids that ultimately contribute to crust flavor and aroma. If you use a larger quantity of starter, for example, above the 1-5% range, along with a shorter fermentation time, you will still get organic acid production but I cannot say whether the organic acids will be quantitatively greater than in the first example (we would perhaps need to have actual times). If they exceed the levels of organic acids in the first example, then there should be an increase in dough strength (by tightening up the protein and creating a more elastic gluten matrix) and, ultimately, a greater contribution to the taste and aroma of the finished crust. Whether these two sets of events coincide with equal expansion of the dough is something I cannot say. However, in both cases, the oven spring will be a function of the pH and residual sugars in the dough. More specifically, as the dough matures and fermentation increases, the pH of the dough will decline as the dough becomes richer in organic acids. Generally, you don't want the pH to get too low because a below-average pH generally coincides with low residual sugar levels. A below average pH level and low residual sugar levels at the time of baking translates into a deficient oven spring along with a lack of crust coloration because of insufficient caramelization of sugars and diminished Maillard reactions. Consequently, I believe that it is the relationship of pH and residual sugar levels at the time of baking that is more important than any of the other considerations you mentioned. That means that in your case you will have to determine through experimentation and experience what amount of starter culture and fermentation period to use to be sure that the pH and residual sugar levels at the time of baking are in proper relationship, as reflected by the quality of the oven spring and crust flavors, aroma and color.
In his book, The Taste of Bread, Professor Calvel stresses the importance of properly freshening the starter culture one or more times before using so that the pH levels of the dough don't get too low and result in a deficient oven spring and lack of crust coloration. So, that is something that you should always want to do.
Peter
