Why does milk, cream, yogurt, etc. become rancid if left unrefrigerated, but butter can just freeball in a butter dish for-seemingly-ever and still be safe and edible?

Question

I just don’t get it. I’m sure there’s a very sciency explanation, but it makes no sense to me. I love that it can sit out to be soft and spreadable, but you try that with cream and you’re on the way to a stinky cheese, milk or yogurt you’ve just got a curdled surprise.

So why can butter just sit out and not start to stink or become dangerous to eat?

In: 122

Answer 1

Milk products go bad from an overgrowth of bacteria that naturally occur in basically any environment. Once a product is opened to the air, those bacteria get inside, and begin feeding off the sugars and proteins in the milk. Therefore, the more sugar and protein a product has will make bacteria grow faster, as long as the product isn’t too acidic. Milk is the most famous example because it’s pretty much the perfect environment for those bacteria. Cream doesn’t have as much sugar so it happens a bit slower, and yogurt has lots of sugar and protein, but it is very acidic, and gets outcompeted by the bacteria that already exists in yogurt. Butter *CAN* go bad the same way other dairy products can, because it does contain some milk-solids, and therefore sugar and protein, but it is extremely unlikely. Butter is mostly just the fat that is separated from whole milk, and only about 1% is actual milk. That means butter is about 0.008% sugar and protein, giving bacteria basically nothing to use to fuel themselves. Butter is much more likely to go bad from mold, which doesn’t rely nearly as much on the sugar from the food it grows on.

The other main reason is just a temperature one. Bacteria grow much slower at cold temperatures and much faster at warm ones, so environments where they already can grow at cold temperatures become a feeding frenzy at warm ones. And because bacteria grow exponentially, meaning the more they grow the faster they grow, they can cause spoilage very very quickly. The butter bacteria meanwhile are stuck barely having any nutrients to fuel themselves, and even at a higher temperature there is still so little there they are stuck at first base so to speak.