All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The Prophet said: "Any kind of intoxicant is considered alcohol and every kind of alcoholic drinks (intoxicants) are forbidden." [Muslim] Therefore, if this yeast does not contain any kind of alcoholic ingredients at the origin but it becomes intoxicating when it is left for a period of time whereby it becomes brewed, then it is permissible before it becomes brewed, but after it becomes brewed it is forbidden and impure and not permissible to use in cooking bread or any other food. Yeast in this situation is like any other food, if it is brewed and became intoxicating, as there is no difference between yeast and other food in such a case.
Allaah knows best.