To understand what happens to milk when we’ve forgotten it on our kitchen counter, we’ve got to understand “what” milk is. “Milk is the creamy white key to the yumminess of my favorite cereal,” you’re probably thinking. Or, maybe: it’s a fundamental ingredient to the cappuccinos that happen to be the single most reliable reason that we get out of bed in the morning.
Milk, as it turns out, is a colloid: just a bunch of fat and protein particles hanging out together in a liquid. It’s about 85 to 90 percent water, and in their pure form, milk’s proteins do not like each other, kind of like Harry Potter hated Lord Voldemort. They repel each other and evenly spread out throughout the liquid. When you leave milk out in temperatures over 40 degrees Fahrenheit, however, it invites the growth of bacteria. Most milk nowadays 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 to coagulate together into curds. Welcome to the world of spoiled milk.
Opened milk will spoil within five to seven days, whether you leave it out on the counter or not. Even if you leave it unopened, fresh milk will spoil between five to 10 days. Ultra-pasteurized milk lasts between 30 to 90 days unopened but still spoils eventually. The long and short of it is that if you leave your milk out, you’re hurrying up an inevitable process. And because rotten milk won’t always smell off to people, you’ve got to know how fast you’re speeding it along. Any perishables left out at temperatures of more than 40 degrees Fahrenheit for over two hours. 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. But 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 for baking, now you’ve got the perfect excuse to make pancakes or biscuits.