First off, the "stew" is already in the bin so this is more theoretical / letting off frustration!
I had beef bones in the crockpot overnight and came up with a beutiful stock, which I put in a stew of beef shin, and venison liver & kidney (neither of which I've tried before), plus some fried onion and mushrooms.
For obvious reasons I cut and rinsed the kidney - but obviously not well enough as a few hours later my house stank of pee!! Eww! Combined with the frothy scum that had formed in the stew...it went in the bin.
Was it just a case of not washing the kidney enough? Is there a technique to it? I'm annoyed at the waste, but luckily I use the nice cheap cuts so it was "only" about £10-worth of ingredients altogether.