|Leavening is the increase in volume of a mixture of water and flour. The yeast is the agent that makes the dough rise.
The natural yeast is composed of microorganisms working in combination: the yeasts.
Obviously the proportions must be adequate and it is said that a yeast is stable when over time the amount of yeasts and lactobacilli remains constant. It takes years to achieve this result.
Why are there those who add brewer's yeast to a sourdough yeast? To accelerate the leavening process. The sourdough is slow in its growth and development. If you accelerate the time, the consequences are spilling over the characteristics of the final product, because all that beneficial part induced by the operation of the bacteria is lost inexorably.
That's why between a product made using exclusively sourdough yeast or a product made using partly brewer's yeast there is a huge difference.