Yeast is but one of several microorganisms in sourdough starter. It
adds carbon dioxide to the mixture as well as a bit of ethyl alcohol.
There are thousands of types of yeast and, of those, only 250 will
produce fermentation, and only 24 of those are "good" yeasts with
respect to food/beverage production.
Wild yeasts can be cultured from fruit, grain, flour, and the air
itself. Most of the flavor if sourdough, however, is generated by
other microorganisms, primarily acetobacter and lactobacillus. These
microorganisms are specific to a particular geographic area and will
be overtaken, in time, by organisms from another area if the starter
is relocated. This is the primary reason that the flavor of sourdough
products is peculiar to a given locality and can't be truly
reproduced elsewhere.
I have successfully made starters from a variety of substrates
including both wild and commercial yeast. The commonality is that
starters need time to develop their flavors. The strongest yeast will
prevail and the starter will ultimately have that strain as its
predominant yeast. I have maintained two starters, used alternately,
that have excellent flavor and aroma in the finished product. One is
old and was made from airborne yeast the other was made more recently
(five years ago) using a very small amount of commercial yeast,
around 1.5 grains. (1 gram = 15.4 grains) There is no appreciable
difference in flavor. Both will develop sour flavors if left to
ferment for a reasonable time. Both are true sourdough starters.
As for refrigeration of a starter I am not in a position to bake
daily so, for me, refrigeration is a necessary tool. Yes, the old
herders/shepherds, etc. did not have refrigerators but they generally
traveled by foot, horseback, or wagon and they cooked over wood fires
and in Dutch Ovens. I am not willing to go back to the not so good
old days. New technology helps me adjust the activity of my starters
and doughs to my schedule rather than making me a servant of their timing.
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