Milk is actually a colloid, which means it’s a mixture of fat and protein particles suspended in a liquid. It consists of about 85 to 90 percent water. In its pure form, the proteins in milk repel each other and spread evenly throughout the liquid, much like how Harry Potter and Lord Voldemort despise each other. However, when milk is left out in temperatures above 40 degrees Fahrenheit, it becomes a breeding ground for bacteria.
Although most milk is pasteurized, which helps eliminate bacteria, it doesn’t completely eradicate them. Bacteria can still contaminate milk once it’s opened. The bacteria feed on the milk’s lactic acid, altering its pH. This change in pH not only causes the milk to taste sour but also causes the protein particles to clump together, forming curds. This is what we refer to as spoiled milk.
Once milk is opened, it will spoil within five to seven days, whether it’s left out or refrigerated. Fresh, unopened milk will also spoil within five to ten days. Ultra-pasteurized milk, on the other hand, can last between 30 to 90 days when unopened, but it will eventually spoil as well. Leaving your milk out simply speeds up the spoilage process, which is inevitable.
It’s important to note that rotten milk doesn’t always have an off-putting smell, so you need to be aware of how quickly you’re expediting the spoilage. Any perishable food left out at temperatures above 40 degrees Fahrenheit for longer than two hours becomes unsafe to consume. If you’ve left your milk sitting out at or above 90 degrees Fahrenheit for more than one hour, it’s definitely not safe to drink.
However, don’t just throw away sour milk! As long as it was pasteurized, you can still use it for baking purposes, similar to using buttermilk. So, if you accidentally leave your milk out for too long, you now have the perfect excuse to make some delicious pancakes or biscuits.
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