Reference for Chullin 145:7

חתכה אלא לרבינא מאי חתכה

says: The whole of it must be immersed! - You can even say that [the teaching of our Mishnah] is in accordance with the view of the Sages, for a mass of foodstuffs is always to be regarded as separated into parts and [the parts] as touching each other.

Jastrow