My mother in law asked me if she could throw in the remains of apples she had used to make juice. To make the juice she had used cream of tartar (no sugar) and poured boiling water over them and let them sit for a couple of days. Can the discarded apples be thrown into the compost or will the cream of tartar interfere with the breakdown...

I'm pretty sure, that it would not have any
effect on the composting process..
Especially if it's a product that is considered
edible when cooking..
Shouldn't have any problem in the compost..

I would!