The odors of spoiled proteins foods are very objectionable and in some cases, when they are intense enough, even toxic.
Meat and other proteinaceous products can be decomposed by anaerobic bacteria, resulting in putrefaction. Putrefaction of meat is the result of the breakdown of proteins by proteinases.
Some of those obnoxious odors are common enough and the compound responsible for them are ammonia, various sulfides (as in the smell of clam flats), and hydrogen sulfide (the typical smell of rotten eggs). Bacteria that attack food proteins convert these into amino acids, which are broken down again into foul-smelling end products. Digestion of cysteine, for example, yields hydrogen sulfide giving a rotten egg smell to food.
Digestion of tryptophan yields indole and skatole, which give food a fecal odor.
Odor of spoiled food proteins