I usually buy brazil nuts in pieces. I was wondering whether the bitter flavor was a sign of the polyunsaturated fats gone rancid or maybe it's something else? Would soaking or toasting get rid of the bitterness? Polyunsat fats make up about 1/3, while sat fats make up just under a quarter of the lipid profile.
Try to get them whole if possible, they'll oxidize faster when they're broken up.
Bitterness might be mold, or something else. Usually you can smell them when they're rancid, it'll smell like old "vegetable" oil.