What is Sourdough?
A sourdough starter, or culture, is a mix of flour and water that maintains a mix of yeast and bacteria. The yeast provides lots of rise and some flavor, the bacteria provides lots of flavor and some lift. The bacteria makes the starter more acidic to protect it from other organisms taking over the starter. The yeast is a strain of yeast that can survive the acidity. Baker's yeast is not one of those yeasts.
Sourdough starter is used to make baked goods. All manner of baked goods. Sourdough has been in use since at least 6,500 BCE, and was universally used until the mid to late 1800's when commercial bakers yeast became available, reliable and cost effective. As a result, almost any bread you can think of has been a "sourdough bread".
It might be less misleading to use the term "natural leavens" instead of sourdough because many people associate sourdough with the American old west. However, sourdough is the term we are most used to, and, in fact, sourdough cultures are used all over the world, and there are some strong similarities in the way they are handled. The basic similarity is that every time the baker uses the culture, it is refreshed (or fed), and part of the culture is saved for the next baking session. In this way, the culture is maintained and propagated.
Over time, the culture will tend to become a mixture of one or two of a dozen or so wild yeast strains, and one or two of twenty or so different lactobacillus strains.
When we first got involved with sourdough, the research suggested there were only 3 different yeasts and 5 strains of bacteria that could form a stable culture. The number keeps growing, though the fundamentals of sourdough remain unchanged.
The yeast and lactobacillus bacteria form a symbiosis that is interesting, and makes the culture quite stable. Dr. Michael Gaentzle, a German researcher, has sourdough cultures that have not changed in over 50 years of use. In the classic San Francisco sourdough, the yeast doesn't eat maltose, one of the starches in flour. However, the lactobacillus bacteria do. In all sourdough cultures, the lactobacillus makes the culture quite acidic, and that acidity acts as an antibiotic, keeping stray bacteria and yeast out.
Few yeast strains can survive in a sourdough culture. Yeast that can survive the acidity produced by the lactobacillus will thrive in the culture. Also, the lactobacillus eat the dead yeast cells, which helps keep the culture from getting nasty.
Commercial bakers yeast can't survive the acidity in a sourdough culture, so we don't use commercial bakers yeast in our starters. It has been shown that commercial bakers yeast cannot survive more than two refreshment cycles in a sourdough starter. As a result, we don't use it in our starters, or in our sourdough bread.
For years we felt that using commercial bakers yeast in sourdough indicated the baker didn't know how to use sourdough, or didn't trust their starter. Overall, we still believe that, but there are as many ways of handling sourdough as there are bakers handling sourdough. Many commercial bakers do use commercial yeast in making sourdough starters in order to meet rigorous production cycles, but this is not really necessary, and I don't approve of the practice. The standard practice in Germany has been to allow a starter to ferment to the point where it is so acidic the yeast is largely killed off. They start a fresh starter weekly from laboratory pure cultures. The very acidic starter is used to acidify rye breads, with commercial bakers yeast used to provide rise. While this isn't our preferred way to raise dough, it reminds us that there are many ways of handling sourdough.)
A common way of starting a sourdough culture is to use grapes, cabbage leaves, or other fruits or vegetables in the theory that they have yeast on their skins. While it is true that they do have yeast on their skins, the yeast on the skins aren't appropriate types. They will cause the culture to get started in a hurry, but in the end, the yeast on the skins of the fruit or vegetable matter will die off, much as commercial bakers yeast will. We suggest just enjoying the fruits or vegetables, and starting a culture without resorting to the use of the foreign flora.
It is worth again noting that different bakers do handle their starters differently. This doesn't mean that one baker is wrong, or another right, just that there are many ways of handling starters, depending on the types of breads you want to bake, and what you've been taught. The techniques discussed in this web page work for me, and I am sure they will work for you. However, they are not the only way of handling sourdough.