The following information comes from King Arthur Flour
"A Short Course in Cooking With & Keeping the Elusive
Wild Yeast".
What is a Sourdough Starter? "A sourdough starter is a
wild yeast living in a batter of flour and liquid.
Yeasts are microscopic fungi related distantly to
mushrooms. There are many varieties of these tiny
organisms around us everywhere. Wild yeasts are rugged
individualists which can withstand the most extreme of
circumstances. Some will make delicious loaves of
bread; others will create yogurt and cheese out of
milk; still others will turn the juices of grains and
fruit into beer and wine." "Active dry yeast, the kind
we can buy in packets at our grocer's, is a
domesticated descendant of these wild relatives, one
which has been grown for flavor, speed of growth and
predictability. But domestic yeasts are much more
fragile and can't be grown at home without eventually
reverting to their original wild state."
"If you can imagine a world without any packets of
active dry yeast, you can imagine how important your
sourdough starter would be to you. Without it, you
would be doomed to some pretty awful eating. It is no
wonder that sourdough starters were treasured, fought
over, and carried to all ends of the earth. To the
early prospectors, it was such a valued possession
(almost more than the gold they were seeking), that
they slept with it on frigid winter nights to keep it
from freezing. (Ironically, freezing won't kill a
sourdough starter although too much heat will.)"
Fermentation (or the Microscopic Magic of Yeast): "As
we mentioned above, yeast is a microscopic fungus. As
it feeds on the natural sugars in grain, it multiplies
and gives off carbon dioxide (just as we do when we
breathe). This invisible activity of yeast is called
fermentation. When you make bread with wheat, by
kneading the long elastic strands of wheat protein
(called gluten) into an elastic mesh, you create traps
for these carbon dioxide bubbles causing the dough to
expand as if it contained a million tiny balloons."
