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Sourdough Starter- -Background
(Source: On Food and Cooking - The Science and Lore of the Kitchen, Harold McGee, p.313.)
Sourness in bread was long considered a serious defect, usually the result of poor yeast or
letting the fermentation stage go on too long. In poorer areas of northern Europe, where rye bread was
the rule, sourness was more acceptable.
The term sourdough brings to mind the hard-bitten pioneer whose sharing of the bread starter
was a true act of friendship and a lifesaver for many. It also brings to mind the sourdough bread made
famous by the gold miners in California and Alaska. They did not have access to fresh supplies of yeast
and had to revert to the ancient method of using a piece of leftover dough to start each new fermentation.
Eventually, the mother dough or starter would come to be dominated by the microflora characteristics of
the local area: contamination by airborne spores would dilute the initial yeast population. Breads with
different mothers would have slightly different compositions. This may be why it is reputedly difficult to
keep sourdough starters from deteriorating, or changing in microbial content, when they are brought to
other parts of the country. Chicago's atmosphere is just not the same as San Francisco's.
The predominant yeast in sourdough is not the usual baker's yeast, Saccharomyces cerevisiae
but rather Saccharomyces exigus (exigus means small or scanty; cerevisias means "of beer") which
thrives in very acidic environments and cannot metabolize maltose (a form of sugar) at all. The acid
responsible for the sour taste-about 75% is lactic acid, the rest acetic-is produced by a group of bacteria
that has an absolute requirement for maltose and so would be unlikely to grow in a dough populated by
baker's yeast. This results in the unique flavor of San Francisco sourdough.
Starters that have maintained for decades appear to be very resistant to contamination, and it
is thought that some sort of antibiotic action, analogous to that of the Penicillium molds in cheese, may be
involved.
Sourdough made with baker's yeast has a different flavor but is much easier for the average
homemaker to make. The following recipe for a sourdough starter uses such a yeast, and is the one
used for the lab experience the students will use.