The problem of fats going rancid is a comparativlely simple bit of organic chemistry The carbon atom has a valence of four, meaning it can combine with four other atoms. In fats there are long chains of carbon atoms: -CH2-CH2-CH2- but there can be double bonds between carbon atoms -CH=CH=CH= . The double bonds are readily attacked by oxygen, making relatively short chain fatty acids that smell and taste bad. That we call "rancid".