One of the fundamental principles established in the Shulchan Aruch regarding the holiday of Pesach concerns the unique severity of leaven. Generally, in other areas of dietary law, such as the mixing of milk and meat, a prohibited substance is neutralized if it is surrounded by sixty times its volume of permitted food. For instance, if a drop of milk falls into a pot of meat, the mixture remains permissible as long as there is sixty times more meat than milk. However, Maran explains that the laws of leaven on the holiday are far more stringent. During the festival itself, leaven is never neutralized; even a single morsel or a tiny crumb in a massive quantity of kosher food renders the entire mixture forbidden.
There is, however, a significant distinction regarding when this leaven was introduced. This strict rule applies only during the days of the holiday itself. If a minute amount of leaven was mixed into a product before the holiday began and was neutralized by sixty times its volume at that time, the law follows the standard rules of all other prohibitions. Therefore, technically speaking, products purchased before the holiday that underwent this neutralization are permissible, provided they have reliable kosher supervision. While some authorities argue that the leaven "reactivates" once the holiday begins because the threshold for prohibition becomes much smaller, Maran holds that once a substance is neutralized, it remains so. It does not "wake up" to become forbidden again when the sun sets on the eve of the holiday.
This principle has practical applications in various scenarios. For example, some people are careful not to purchase milk during the holiday because of concerns that the cows may have been fed leaven or that certain nutrients might contain trace amounts of it. To avoid the stringency of minute amounts, they purchase all their milk before the holiday begins, relying on the fact that any potential leaven would have been neutralized beforehand. Similarly, in Israel, questions often arise regarding the water from the Kinneret. Because people may have picnics by the water and inadvertently discard leaven into the lake, one might worry that the entire water supply becomes prohibited even by a minute amount. Consequently, some individuals fill their water urns before the holiday starts, ensuring that any trace of leaven was already neutralized and will not become an issue once the holiday commences.
A second important principle to consider is the concept of imparting a spoiled taste. This law addresses situations where food is cooked in a vessel that was previously used for leaven. For instance, a woman once sought guidance because she had mistakenly cooked rice for the holiday in a pot normally used for noodles. She was distressed, fearing the food was forbidden and that her husband would be upset. The rabbi inquired about the history of the pot and learned it had not been used for at least twenty-four hours.
The rabbi ruled that the food was permissible to eat after the fact. The reasoning is that after twenty-four hours, any flavor absorbed in the walls of the pot is no longer considered "positive" or "fresh"; it is classified as a spoiled taste. While one should never intentionally cook in a leavened pot during the holiday, if it happens accidentally and the pot was clean and unused for twenty-four hours, the resulting food is not forbidden. This distinction ensures that while we maintain the highest standards of caution, we also recognize the specific legal parameters that allow for leniency in cases of honest mistakes.