What foods/ingredients are prohibited from being eaten by those who keep passover?
The tradition of eliminating "Chametz" during Passover is explained as follows:
"Chametz (חמץ, "leavening") is made from one of five types of grains combined with water and left to stand for more than eighteen minutes. The consumption, keeping, and owning of chametz is forbidden during Passover.
Yeast and fermentation are not themselves forbidden as seen for example by wine, which is required, rather than merely permitted. According to Halakha, the ownership of such chametz is also proscribed.
Chametz does not include baking soda, baking powder or like products. Although these are defined in English as leavening agents, they leaven by chemical reaction, not by biological fermentation. Thus, bagels, waffles and pancakes made with baking soda and matzo meal are considered permissible, while bagels made with
sourdough and pancakes and waffles made with yeast are prohibited.
Observant Jews spend the weeks before Passover in a flurry of thorough housecleaning, to remove every morsel of chametz from every part of the home. Jewish law requires the elimination of olive-sized or larger quantities of leavening from one's possession, but most housekeeping goes beyond this. Even the cracks of kitchen counters are thoroughly scrubbed, for example, to remove any traces of flour and yeast, however small. Any item or implement that has handled chametz is generally put away and not used during Passover."
Read more: http://en.wikipedia.org/wiki/Passover
MATZO BALL SOUP