The yeasts role in a sourdough starter is to leaven the bread (i.e. produce gas). Commercial yeast is very good at this job since that is all it was selected to do. Common bakers yeast that most normal people have access to is slightly acid sensitive and most sourdough yeasts are moderately acid resistant. Commercially on a bakery level you can obtain yeasts that are acid resistant and a host of other desirable properties (freeze tolerance, sugar tolerance etc.).
In a laboratory environment a common medium for a laboratory form of bakers yeast is Yeast nitrogen base whose pH is 5.4! Most sourdoughs have a pH at the end of the fermentation of around 3.5 - 4.2. Since the scale is logarithmic this is relatively large difference.
The acids produced by lactobacilli definitely slow the yeast down (be they commercial or sourdough). The natural yeast are obviously more tolerant of acid. You could overcome the acid sensitivity by adding more yeast or proofing longer. This is not to say I advocate doing it - I am merely pointing out it can be done. You have to be judicious in how much yeast you add since too much will cause the bread to be overwhelmingly yeasty in flavour.
Another aspect of leavening sourdough breads is that the gluten is attacked under acid conditions through the action of several acid proteases. Thus the ability of the individual cells of the gluten net to hold gas is compromised. If you let your dough develop to such a point it will obviously rise very feebly no matter what your source of leavening is - wild or commercial since any gas produced will simply leak away.
One of the pleasures of sourdough is understanding the rhythms of both the yeast and lactobacilli and holding them both at just the right level - optimal acidity, optimal flavour (I suspect when most people here say they want their bread more sour what they actually mean to say is more flavour full - a very sour bread can be excruciatingly unappetizing) and optimal leavening. This is achieved by manipulating the starter to maximize the number of organisms, varying the "wetness" of both starter and dough and controlling time and temperature of all stages.
I should point out that if you do play with commercial yeast there is a very good chance that you will pollute your starter and you obviously do not want to add it to the starter i.e. should you use it you definitely need to develop a procedure to maintain the starter uncontaminated.
Commercial bakeries oftentimes use yeast as a leavening in a sourdough not because they do not know better but because they require very predictable rises - they may have hundreds of pounds of different breads developing at different rates and have to hit the oven in fairly tight windows. A commercial leavening in this context can be controlled far easier. Obviously an equal number of bakeries develop the bread naturally but this requires more skill, time and ultimately for the baker $.
To address the original point of this thread though: a starter made from commercial yeast performing better than an established starter (I believe Russian from Sourdough International). If I remember correctly, the poster mentioned they had obtained the starter second hand. Based on my experience with home started vs purchased starters I suspect that the starter you obtained is probably far from the original sold by SI. I have had the most consistent results with legitimate "established" starters.
I should point out however that I have noticed a deterioration in some starters over time - I have not figured out the root of the problem since I was not careful enough to pinpoint exactly when the change occurred but I have found a starter that I loved evolving into a dud. Obviously this means contamination/loss of a favorable lactobacillus. I was originally very careful when I bought the starter and would boil the water used to feed the starter (and let it cool!) & once it was established decided it could fend for itself. In hindsight I think this may have been an error in judgement: the boiling apart from getting rid of any other unfriendly beasts probably also got rid of chlorine etc. I suspect that this could have been one of the things that did my lactobacilli in. Flour obviously has organisms that you cannot get rid of and this is potentially another source of contamination: lactobacilli have several bacteriophages and produce bacteriocins that could have killed my treasured lactobacilli (the reason I think I have lost lactobacilli complexity is because the bread rises fine but the flavour is middling). The starters from SI have predictably activity peaks & the Russian is very fast, you could use this as a test to see if what you have is still legitimate. I can vouch for the fact that the Russian, Austrian and Bahrain rise as described in their literature. Also since the Russian rises so fast you may be tempted to bake the bread before the lactobacilli have had a chance to do their magic. Among the above three starters I like the flavours of the Austrian the best.
-Roland
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This page was last updated on Tue, Aug 3, 2010 at 7:52:51 PM.