What Really Happens To Milk When You Accidentally Leave It Out

To understand what happens to milk when you've forgotten it on your kitchen counter, you've got to really understand "who" milk is. "Milk is the creamy white key to the happiness of my Honey Nut Cheerios," you're probably thinking. Or, maybe: it's a fundamental ingredient to the cappuccinos that happen to be the single most reliable reason that I get out of bed in the morning. You're not wrong. You're not right, either. 

Milk, as it turns out, is a colloid: just a bunch of fat and protein particles hanging out together in a liquid (Science Notes). It's about 85 to 90 percent water (via Science World), and in their unspoiled form, milk's proteins dislike each other, kind of like Harry Potter disliked Lord Voldemort. They repel each other and spread out evenly throughout the milk. When you leave milk out in temperatures over 40 degrees Fahrenheit, however, you invite the growth of bacteria (via Undeniably Dairy.

Most milk today is pasteurized, but that doesn't completely eliminate bacteria, which can also recontaminate milk once it's opened. The bacteria start eating up the milk's lactic acid, thereby decreasing the milk's PH. Decreasing the PH not only makes the milk taste sour but it forces the milk's protein particles into coagulating together into curds. If you're wondering whether that's good, just picture what happens when Harry Potter and Lord Voldemort have a get-together. Welcome to the world of spoiled milk. 

Did you forget your milk on the counter? Here's what to do

Opened milk will spoil within five to seven days, whether you leave it out on the counter or not (via Eat By Date). Even if you leave it unopened, fresh milk will spoil between five to 10 days depending on whether it's whole (in which case you're looking at around five) skim (in which case you're looking at around seven) or skim and lactose-free (in which case you're looking at around 10). Ultra-pasteurized milk lasts between 30 to 90 days unopened but still spoils eventually (via Best Food Facts).

The long and short of it is that if you leave your milk out, you're hurrying up an inevitable process. And because bad milk won't always smell bad to people, you've got to know how fast you're speeding it along. That's where Registered Dietician, Sarah Downs, comes in. She told Best Food Facts that you should never consume any perishables left out at temperatures of more than 40 degrees Fahrenheit for over two hours. Undeniably Dairy suggests that if you've left your milk sit out for more than one hour at temperatures at or over 90 degrees Fahrenheit, it's not safe to drink.

Don't throw out sour milk! As long as it was pasteurized, if you've left your milk out too long, you can use it like you would use buttermilk to bake (via NPR). You're welcome – you've got the perfect excuse to make pancakes or biscuits.