>Which also makes me think that yes this culture does contain *some*
commercial yeast

A natural culture can never contain commercial yeast ,but strains
related to the same genus can exist in natural starters and this what
confuses some investigators who that insist that bakers yeast ( or
they immediately recognize as S. cereviseae) cannot survive in the
acidic conditions of the starters. That is if the kept on thinking
that saccharomyces cerevisease is just bakers yeast and nothing else.
But there are more than 40 types of Saccharomyces and bakes yeast is
just one, another one in wine, for spirits and for beer etc., more on
the wild yeast area which are tolerant to adverse conditions than
the cultured ones used in normal baking and beverages and was one
time re classified under the name saccharomyces cerevisieae!
Therefore the mildness of some starters is caused by the presence of
these species not necessarily the Candida milliere associated with San
Francisco., or the population density of other yeast surpass the
candida milliere.
Now these other saccharomyces are not that acid tolerant like the C.
milliere, but exist only in less acidic conditions.
Therefore this seems to coincide with the French investigators
findings( in my previous posts) that subspecies of Saccharomyces
cereviseae can dominate in milder starters which is common in French
sourdoughs and classified under the same name due to similar
microbiological characteristics.. Other species of yeast were also
found not belonging t the Saccharomyces.
But there are some strains that Candida millere is the dominant yeast
and its usually in the acidic type of French levain which coincides
with organisms found in the San Francisco sourdough and other
related more acidic culture characteristics.

Be reminded that yeast classification is ever evolving and that
confuses layman and even scientific professionals in other fields who
want to get a consistent classification in this particular microbe when
they encounter this in their research work. This reason may have
affected earlier investigators conclusion that all strains of
saccharomyces cereviseae cannot survive in the acidic nature of
sourdough cultures which is erroneous as what later investigator founds
out elsewhere.

>A number of microbioligists have tested starters and found that bakers
yeast will not survive more than two refreshments in a starter. The
acidity is high enough >that bakers yeast can not survive.
Again I reiterate that....
Generally bakers yeast cannot survive extreme acidic conditions; but
there are yeast strains belonging to the same species that are hardy
enough and incidentally they are classified as saccharomyces
cerevisiae. Which I mentioned above.
Besides bakers who added yeast in the sourdough place it on the dough
preparation side which has milder pH conditions and never in the
starter side( which is more acidic).

I think this what should the people of Carl Sourdough Starters to look
into , and investigate the nature of their culture flora and establish
the population density of different yeasts species that can coexist n
their culture like what the French investigators did with their Levain
( as they cannot understand previously )why in many parts of France
the natural sourdough is milder while in certain areas its more acidic
..And bakers and consumers there noticed that their bread sometimes
fluctuates in flavor and taste when its known to be made wholly from
natural starters that had been maintained for generations.
And not just assume that all sourdough cultures is populated by only
two microbes:
The candida milliere and the lactobacilli L brevis variety Lindniere(
Aka L. Sanfranciscensis).
It must not be forgotten that French flour had moderate ash content
that is similar with the American clear flours and seldom use lower ash
flours for breadmaking but only for pastry anc cake baking.

Sourdough enthusiasts should not have their eyes glazed by the stature
of the earlier investigator like Kline, Sugihara, Gantzle etc who
only focus their attention on the usual tandem of L San Francisco and C
milliere. And forget (or not believe ) other equally competent but not
popular researchers findings (whose works were never published in
English language) and discovered other critters that can exist also in
natural sourdough cultures.
Besides it must not be forgotten that aside from the Saccharomyces
there are other specie like the hansenula, torupsis and pichia that can
exist also in natural starters and that can also affect the variability
of sourdough tastes.
Roy