Discarding bones and fat when cooked with shemitah ingredients

Answer

Short Answer

The rule of thumb for kedushat shevi'it is that whatever is edible has kedushah, and it is supposed to be used in a regular fashion. However, whatever is not edible is not holy and can be discarded regularly.

The status of the bones and fat actually depends on you: If you generally suck marrow from bones (or eat bones; this is more applicable to chicken bones and not to steak bones) or eat the fat, only the parts you generally eat have kedushah, while the parts you don't, do not. But if you don't eat bone parts or fat and consider them waste, then they are not edible and thus not holy and may be discarded.

Expanded Answer

🍖 We learn many of the halachot pertaining to shemitah food from other holy foods like korbanot (sacrificial meat). The bones of korbanot are not holy, even if small bits of meat are stuck to them (!), and can be discarded regularly (Kessef Mishneh, Hilchot Ma'aseh Hakorbanot 10:10).

From here we learn that we can be lenient with bones that assumed flavor from kedushat shevi'it ingredients. (See Shabbat Ha'aretz 5:3 §2).

On this topic, see Shevet Halevi (VII §180:2), who writes that since bones cooked with shemitah ingredients are not generally used as food for humans or animals, they are devoid of kedushat shevi'it.

However, Rabbi Mordechai Eliahu writes that if the bones can be eaten by humans (by sucking the marrow, etc.), they do have kedushat shevi'it. If not, then they are completely devoid of kedushat shevi'it.

Bottom line: Again, whatever you consider edible has kedushah, while what you consider inedible does not.

If you aren't sure or you want to play it safe, you can always discard the scraps in a double bag/leave outside of the refrigerator overnight, which certainly renders the scraps inedible, and then discard. In any case, though, you need not refrain from using holy ingredients.

To learn more on this topic, see The Consumer's Guide to Shemitah, Chapter 19, also available here.