What are the biochemical processes occurring when food spoils? Let's assume for a minute that microbes themselves and their _direct_ toxic products (i.e. endotoxins) aren't toxic to humans. Let's also discount any innate immune responses the body mounts against the invading microbe (i.e. inflammation and production of cytokines). What happens to food molecules (mechanistically) as it spoils and what deleterious effects do these "spoil products" have on the body if ingested? I'm looking for compounds that can result from the spontaneous breakdown of food or the byproducts of microbial metabolism (that is NOT a "direct" toxin) that is harmful to the body. For example, do the proteins in food break down into some toxic nitrogenous substance?

During putrefaction of animal tissue, lysine is decarboxylated into cadaverine and arginine is decarboxylated into putrescine. These compounds are deemed to be toxic.

A serving of meat contains 8 g of protein, corresponding to 640 mg lysine and a little bit less of arginine. Let's go straight and say that a spoiled meat serving contains 640 mg cadaverine and a little bit less of putrescine.

In rats, the acute oral toxicity for both polyamines is around 2000 mg/kg, let'assume that this is valid for humans also. According to these rough calculations, to have an acute toxic effect, a 70kg man that is resistant to the direct toxic effects of microbes, should eat 140 grams of cadaverine, corresponding to 218 smelly rotten meat servings.

[composition and toxicity data taken from wikipedia]