As we have learned (daf, or page 97), if a small amount of non-kosher food fell into a large amount of kosher food, the mixture will remain kosher if the ratio is 60:1 such that the taste of the non-kosher food is nullified. This same rule applies to meat and milk. Thus, if a small amount of milk falls into a large amount of meat, if the ratio of meat to milk is at least 60:1 the milk is considered nullified and the mixture is kosher.
The Mishnah on today’s daf distinguishes between two different cases where a small amount of milk becomes mixed into a pot of meat. According to the Mishnah, if the milk fell on a single piece of meat, we gauge the possibility of nullification solely on the relationship between the milk and that piece of meat. If, however, the milk fell into the pot and then the pot was mixed up, the question will be whether the entire mixture has a 60:1 ratio of meat to milk.
Tosafot explain that the first case of the Mishnah must be talking about a situation where the piece of meat is entirely outside of the mixture of food in the pot, which is why we focus on it as an individual case when determining whether or not nullification takes place. Rashi, however, suggests that the piece of meat was partially in the gravy.
Based on Rashi’s explanation, the Ta”z argues that unless that piece of meat is immediately removed, the entire pot may become prohibited. His reasoning is straightforward: Once the piece of meat on the top becomes forbidden as a mixture of meat and milk, the ratio of 60:1 needed to permit the rest of the pot will not be measured against the drop of milk, but against the entire piece. The Drisha rejects this argument. He points out that one way or another the pot of food will remain permitted. If we view the milk as affecting the entire pot (since the meat that it fell on is partially in the mixture) then the volume of meat in the entire pot will be used to nullify the drop of milk. And if we view the drop of milk as affecting only the single piece of meat upon which it fell, then only that single piece will become forbidden – unless someone mixes up the pot.