No one has ever done a definitive study, but there are two schools of thought on how the bread rises. The other school of thought says that a bacterial process, similar to lacto-bacillus, is multiplying in the culture. A study at the University of California at Davis was done to determine exactly how San Francisco sourdough breads rose. They found that the addition of yogurt to the bread dough, with its lacto-bacillus cultures, gave the best result. We only use the flour and water as explained above, and it works very well.
Yeast is airborne throughout a bakery, and sticks to work surfaces, aprons and bakery equipment.
If a bakery uses yeast in anything, it is in everything they make. That's why we have never used yeast, and never will. Our bread rises as a result of the starter method we use. They are not suitable for Passover because they are leavened. Many people believe that yeast can aggravate allergies.