To get the full picture on this matter, the thread that I think you should read is Chau's thread, including the posts referenced therein, starting at http://www.pizzamaking.com/forum/index.php/topic,12824.0.html
. In that thread, Chau starts to talk about trying to make a commercial starter and perpetuating it through subsequent feedings at Reply 43 at http://www.pizzamaking.com/forum/index.php/topic,12824.msg128775.html#msg128775
and continuing, with some discussion of related matters in between, to Reply 76 at http://www.pizzamaking.com/forum/index.php/topic,12824.msg131448.html#msg131448
. I believe during the span of those posts that Chau used commercially leavened starters that were held both in the refrigerator and at room temperature. Chau can correct me if I misinterpreted what he said but I believe that he concluded at least two things: that commercially leavened starters did not produce a tangy or sourdough flavor in the finished crust, and it was highly questionable that a commercially leavened starter could be perpetuated for long before being overtaken by wild yeast. In fact, in Reply 76 referenced above, Chau said that the cake yeast leavened starter that was fermented at room temperature was so acidic and masked all other flavors that he decided to end the experiment.
It has always been my belief and contention that it is not possible to perpetuate a starter based entirely on commercial yeast. Rather, I believe that such a starter is eventually taken over by wild yeast.
FWIW, this is what I believe happens when a commercial yeast is used to form a starter that is subjected to future feedings of water and flour, but with no further additions of more commercial yeast.
Once flour, water and commercial yeast (including cake yeast) are combined, there is an initial aerobic phase during which the yeast uses the available oxygen in the dough for reproduction purposes. The oxygen is a prerequisite of cell reproduction. The aerobic phase is followed by an anaerobic phase during which the starter dough ferments (as I understand it, there can also be some further reproduction of the yeast during fermentation, at least in the early stages). The fermentation of the starter dough takes place in stages. In the first stage, the yeast ferments simple sugars that are naturally present in the flour in very small amounts (0.5%). These simple sugars (glucose and fructose) start the fermentation process during which carbon dioxide and ethyl alcohol are produced. In the following stage, certain complex sugars present in the dough (at 1%) are hydrolyzed to simple sugars (glucose, fructose and maltose) to further feed the yeast. The bulk of the sugars needed to feed the yeast come in a final stage from very complex sugars, comprising mostly starch in the flour, that are converted to simple sugars (glucose) by the action of amylase enzymes (alpha- and beta amylase enzymes) on the damaged starch component of the starch (damages starch occurs during milling and in some cases with poor wheat crops). Many of the above conversions require certain enzymes that are included in the yeast cells themselves.
The rate at which the above actions take place depends on the amount of yeast used to form the starter dough, the hydration of the starter dough, and the temperature at which the starter dough is held, either under refrigeration, at room temperature, or maybe a combination of both protocols. In general, the higher the amount of yeast, the higher the hydration value, and the higher the fermentatation temperature, and all else being equal, the faster the fermentation process.
After the initial preparation of the commercially leavened starter, it can be fed with regular feedings of flour and water. With the introduction of more oxygen into the starter dough as addtional flour and water are mixed in with the initial starter dough, there can be further yeast cell reproduction and further conversion of natural sugars into forms of simple sugars needed to feed the yeast, as discussed above. How long this process can continue will depend mostly on the acid levels in the starter dough. The acids in the starter dough are formed as byproducts of the fermentation process (there are many different acids formed although they are individually in rather small amounts). As it so happens, commercial yeast has a high tolerance to pH, even at extremes, but the pH of the starter dough eventually reaches a stage where the commercial yeast ceases to act. According to the article at theartisan.net website at http://www.theartisan.net/The_Artisan_Yeast_Treatise_Section_Two.htm#Yeast GrowthThat
(which also discusses the above matters but in much greater detail), the point at which the commercial yeast ceases to act is a pH value of 2 or a pH value of 8. It is perhaps at these values where wild yeast, which can tolerate a wider range of pH values than commercial yeast, can step in and, eventually, take over the commercially leavened starter dough. When this happens will depend on the starting amount of commercial yeast, its hydration value (including when fed regular feedings), the number and timing of feedings, and the fermentation protocol and temperatures. As I understand it, the wild yeast and possibly the bacteria use the dead commercial yeast cells as nutrients. Also, what you end up with as a starter will be governed by the species of wild yeast. It might be a great starter but, then again, it might not.