Chulin 8:5-6

Chulin 8:5

(The Mishna now turns to the subject of milk found in the stomach of a slaughtered calf.) If the stomach belongs to a non-Jew and/or is of a neveila, the milk is prohibited. Let’s say that one curdled milk using rennet from a kosher animal’s stomach; if the rennet imparts flavor, the resulting cheese is prohibited. If a kosher animal sucked milk from a treifa, the milk in its stomach is prohibited. If a treifa sucked milk from a kosher animal, the milk in its stomach is permitted because it’s just collected there.

Chulin 8:6

In some ways, forbidden fats (cheilev) are more stringent than blood; in other ways, blood is more stringent than forbidden fats. Cheilev is more stringent in that it is subject to the laws of misappropriation (me’ilah) and one can be liable to piggul (improper intention), nosar (leftover sacrifices) and ritual impurity because of it, which is not the case with blood. Blood is more stringent in that it applies to domesticated animals, wild animals and birds, both kosher and non-kosher, while cheilev only applies to kosher domesticated animals.