הלכה על חולין 215:6

And they were also stringent in this matter according to some of the commentators to [deduce] a novel prohibition in this, more than in other prohibitions of food (Chullin 108a). [It is] in that with the matter of meat with milk, if milk is mixed with meat, and there is not sixty parts in the piece of meat corresponding to the milk, we see both of them as one piece of prohibited food. And if this piece fell into a stew of meat or a stew of milk, we measure against all of [the piece]. And this is what, they, may their memory be blessed, said (Chullin 108a) "The piece itself becomes a carcass." And the reason is because their mixture is what makes them prohibited. And hence, after they have mixed, behold, they are like a piece of carcass. And with other prohibitions, it is not such; as [with] prohibited food that is mixed with a piece of permissible food and there is not sixty parts in the permissible food to negate the prohibited food, and afterwards they fall into a stew, we only measure according to the measurement of the prohibited food that fell into it, and [the permissible portion of the piece] itself helps to [count against] the prohibited food. [This is] because that piece does not become a carcass, and [so] the permissible food in it is found to be like the rest of the permissible food in the stew - and [so] it assists [in counting against] the prohibited food. However if it is recognizable, the piece itself is forbidden forever, according to the opinion of some of the commentators.