Chametz on Pesach differs from other prohibited foods in that other forbidden foods can be "neutralized" in a mixture. If forbidden food becomes mixed with kosher food, then the mixture is permissible for consumption if there is at least a 60:1 proportion of kosher food to forbidden food. Chametz, however, is forbidden for consumption even "Be'ma'shehu," meaning, even in the slightest proportion. If a slight morsel or crumb of Chametz falls into other food, then the food may not be eaten even if that morsel comprises just one-thousandth of the product.
The Shulchan Aruch (447:4) rules (listen to audio for precise citation) that this applies only if the Chametz mixed with the other food on Pesach. If the Chametz fell into the other food before Pesach, when the principle of Bittul (the "neutralization" of forbidden food by a 60:1 proportion) is in effect, then the Chametz is considered neutralized and does not "reawaken" when Pesach sets in. Some other authorities maintain that the Chametz is indeed "Chozer Ve'nei'or" – it "reawakens" – when Pesach begins, but the Shulchan Aruch ruled that once the Chametz was neutralized through Bittul before Pesach, it remains "asleep" even on Pesach, and the mixture is permissible.
We find practical application of this rule in a number of customs. Some people have the practice not to purchase milk on Pesach, out of concern that some nutrients containing Chametz may have been added to the milk. They therefore purchase all their milk for Pesach before the holiday, since any Chametz in the milk would be subject to Bittul and the milk would be permissible during Pesach. Similarly, some people in Israel do not drink water drawn from the Kinneret on Pesach, because secular Jews have picnics near the Kinneret on Pesach and throw Chametz items like beer bottles and sandwiches into the lake. Since Chametz is forbidden even in the slightest proportion, all the water in the lake becomes forbidden. Some people therefore purchase all their drinking water before Pesach, or fill urns with water before Pesach, when Bittul applies and hence renders the water permissible for consumption during Pesach.
Summary: Although the presence of Chametz in even the slightest proportion renders a food forbidden for consumption on Pesach, if the Chametz mixed with the other food before Pesach, the product is permissible for consumption if in a 60:1 ratio.
