The only leavened products (chametz) forbidden on Passover are
leavened products made from the following five grains: Wheat,
Oats, Barley, Rye, and Spelt (have I spelt that right?).
[Maimonides, Yad HaChazaka ch. 5] Yeast is typically cultured from
fruit fungus, often grapes, and is permitted on passover.
Reuven Subar
Ohr Somayach Computer Center Jerusalem
<subar@ohr.israel.net> www.jer1.co.il/orgs/ohr
Ph: 972-2-810-315 Fx: 972-2-812-890