Butter

[BLING]

why does butter go rancid?

[jbeputnam]

It is a product of milk, and, as milk goes sour, so does butter. Most dairy products do the same.

[Piljim]

Butter goes rancid because of fat oxidation. Sorry if this sounds patronising - I don't know of your scientific knowledge, but oxidation is a chemical reaction that fats and oils are particularly vulnerable to. This reaction can be caused by many things - exposure to light and presence of Copper ions are two principle things (the latter sounds an unlikely source to most people but can take place if the wrong production equipment is used). Fat oxidation is a chain reaction so a small exposure to such an agent can have a big effect, and if you mix rancid with good butter, chances are the whole lot will go bad in a short amount of time.