This a question for anyone here with a organic chemistry background, or at least a thorough scientific bent. Any leather that's been cared for has had oil applied to it over time, but leather never smells rancid (assuming something hasn't been actively done to cause it to do so). What is it about the application of the oil to leather that prevents it from smelling rancid? Is it a mechanical interaction wherein the aromatics are simply unable to escape? This seems unlikely since leather has a c