smell similar to sweaty feet, rotten cheese or vomit. When butter goes rancid, short-chain fatty acid butyric acid is liberated from the glyceride by hydrolysis, leading to this kind of unpleasant odor. Human sweat can have this odor because of Corynebacterium that metabolizes animal fats excreted on skin. It can also be created from amino acids in proteins (valine, leucine, and isoleucine). Cows can have this smell if they are on high-grain or high-starch diets vs hay diets. According to a Swiss study published in "Chemical Senses"in 2008, men might have more of butyric acid and similar chemicals in their sweat, while women have more sulfur giving them smells similar to onions or grapefruit.
Rancid butter odor can be associated with tyrosinosis (rare genetic metabolic disorder characterized by a deficiency of enzymes such as fumaril acetoacetate hydrolase which prevents the breakdown of tyrosine which then builds up in the liver. Another condition characterized by this odor is Hypermethioninemia or Baber's syndrome: a very rare condition characterized by the association of congenital liver cirrhosis with Fanconi syndrome.
