Well ...its likely that the pH will change during the fermentation
process by a certain value( but not much,i.e. in fractional/decimal
units))..Measuring titratable acidity is one of the quality control
method used in industrial baking and is partly based on that premise..
But in the presence of other acids ( e.g. acetic acid)the change in pH
is considerable.
Now regarding yeast nutrient, Ascorbic acids does not act the same way
as the ammonium , ,phosphates and other mineral ions which invigorate
the yeast cell.What ever other growth factors the yeast needs it just
synthesis itself as long as the required nutrient precursors are
present, but vitamin C is not one of them.
It is a fact that (even now) a lot of bakers still believed in that
idea that the improving effect of vitamic C is due to its effect on
yeast metabolism which is not true. Yes the dough matures faster in its
presence( vitamin C)( in the same way that higher levels of ethylene
accelerates fruit ripening) but its effect is on the gluten proteins
and never on the yeast cells.
You can make bread without vitaminc but that needs extended
fermentation process as dough matures by slow biochemical means
through enzymatic and chemical oxidations process. If vitamin C is
purposely added then that will change the reaction rate( dough
maturation) by the process in which ascorbic acid is reduced
enzymatically to dehydroascorbic acid which had the active oxidizing
effect on the flour protein structure in particular the amino acid
linkages which affect the protein conformation resulting it to tighten
creating what is known as dough strengthening effect that is beneficial
to the dough performance.
So what ever is the type of breadmaking process being applied vitamin C
does not take part in the yeast metabolism equation; rather on the
flour proteins itself.
Roy