As any of you who have made wine (or other alcoholic drinks) know, yeast is essential in the fermentation of alcoholic beverage. So how can we drink wine on Pesach, more than this it's an essential part of the seder? The presence of fermented yeast is surely chametz, something we can't even have in our possession on Pesach.
So far 2 answers have been offered.
This site says:
Chametz is "leaven" -- any food that's made of grain and water that have been allowed to ferment and "rise."
1) the yeast in liquid is not considered chametz as it is not a baked, flour/water combination
2) yeast is similar to bacteria and is present in unfermented grapes so no grainlike products are required