What is sourdough?
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, and part of the culture is saved for the next baking session. In this way, the culture is maintained and propagated.
This avoids answering the question at the top of the screen, "what is sourdough anyway?" A sourdough culture is a mixture of wild yeast and lactobacillus bacteria living in a mixture of flour and water. Over time, the culture will tend to become a mixture of one of a dozen or so wild yeast strains, and one 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 bread. Using commercial bakers yeast in sourdough indicates the baker doesn't know how to use sourdough, or doesn't trust his starter. 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.
A common way of starting a sourdough culture is to use grapes, cabbage leaves, or other fruits of 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 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.