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My wife and I keep a stick of butter in the cupboard rather than the refrigerator. Even with just the two of us, it rarely lasts very long, but I'd say it may go as long as a week without refrigeration before it's all gone. There have been times when I have told friends about this and they express concern due to the fact that butter is a dairy product. They cannot comprehend not refrigerating it at all times.

What allows butter to stay perfectly tasty and edible, even after a week outside of the refrigerator, where milk, sour cream, or cheese would be foul in a day or less?

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Related: to get maximum time of keeping butter at room temperature, see: cooking.stackexchange.com/q/36 –  Dinah Sep 12 '10 at 19:10

2 Answers 2

The reason the fat goes rancid slowly is that it is mostly saturated. Saturated fats are much less susceptible to oxidation (and therefore rancidity) than polyunsaturated ones. Same reason that pure lard is shelf stable.

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Actually, the question should be "why do other dairy products spoil so fast?"

If I remember correctly, the spoilage of milk is caused primarily by lactose, which breaks down into lactic acid over time due to the presence of Lactobacillus bacteria, which thrive in that environment.

Butter is mostly fat, and fat does not go bad (it does eventually, but not in the same way or at the same rate - see below) - lactose and sugars together, on the other hand, go bad quite fast. That is also why cream lasts longer than whole milk, and whole milk lasts longer than skim milk. It is also the reason why lactose-free milk seems to last forever - there's nothing to break down, nothing for the bacteria to feed on.

When butter does go bad, it's generally not even for the same reason - it's simply due to oxidation which causes rancidity, similar to a typical vegetable oil.

In short, butter lasts so long with or without refrigeration because proportionally speaking, it doesn't have as much of the lactose that causes other dairy products to go bad.

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Of course, if you get bread crumbs in the butter, then you're liable to get regular old mold growing on your butter... –  Eclipse Sep 8 '10 at 2:57

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