6. How do wild and commercial yeast differ?

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