Tractate Tahorot - Chapter 1 - Mishnah 4

The hide, meat juice, sediment, dried-up meat, bones, sinews, horns and hooves join together [to make up the minimum quantity in order] to convey food-uncleanness, but not to [make up the minimum quantity in order to] convey nevelah-uncleanness. Similarly, if a man slaughtered an unclean animal for a Gentile and it still has convulsions, it can convey food-uncleanness, but it conveys nevelah-uncleanness only after it is dead, or its head has been chopped off. [Scripture] has [thus] made more cases that convey food-uncleanness than those that convey nevelah-uncleanness.