Wed Nedarin 52

If someone says "I make a Neder on meat or wine that I'll eat" if some fall into a stew, we prohibit the stew if you can taste it in the stew.

Ran asks: we see later that a Neder is a Davar Sheyesh Lo Matirin (it has a way to permit it by Hataras Nedarim.) We know the rule that if it has Matirim it' not Batul even in a thousand. If so, why don't we forbid it if you cannot taste it?

His Rebbis answered: since Davar Sheyesh Lo Matirim is only Batel if it's mixed in the same type of food. Here it's not applicable since it's mixed into a different food. Therefore they ask on the Rif that forbids bread baked with a small amount of milk (that doesn't give taste) with meat. Since it has a Matir to eat it without meat, he needs to. So the Ran's Rebbis asks: we already said that only if it's mixed with the same type of food is it not Batul. However, the milk is not the same type of food as the bread and should be Batul.

However, the Ran defends the Rif. he explains the reason that Davar Sheyesh Lo Matirin needs to be mixed with its own type of foo based on a Gemara. R' Yehudah holds That anything mixed with its own type is not Batul. We see this from Yom Kippur that the Kohain Gadol mixed the bull's and goat's blood together, and yet the Torah calls it a mixture of bull's and goat's blood. Although the bull's blood is more, we say the goat's blood is not Batul. The Rabanan explain that there is different since they're the same category, Kosher Olos. This is no proof to a mixture of Kosher and non-Kosher.

So the Ran theorizes that the Rabanan consider that being the same amount Kosher is more of a reason to say they're equal so that they're not Batul together, than the fact they're the same type of food. Therefore, if one is Kosher and one is not, we consider them so different that we say they are Batul despite being the same food. A Davar Sheyesh lo Matirin is somewhat similar. Although it's not permitted now, since it will be permitted, so we consider it somewhat similar. So if it has another aspect in common, that it's the same type of food, so we say it's also not Batul. However, if it's completely Kosher, we consider them completely similar that it's not Batul even if the foods are totally different types. Since the milk and the bread are both completely permitted, we say that the milk cannot be Batul in the bread and therefore cannot be eaten with milk.