If milk is left out of the fridge for an extended period of time it can become a food-safety issue. According to the US Food and Drug Administration, refrigerated foods, including milk, should never be out of the fridge at room temperature for longer than two hours. That window closes quickly if the ambient temperature rises. That's part of the reason the Dairy Council of California recommends going straight home after grocery shopping and immediately putting your milk in the fridge. If you do accidentally leave milk out on the counter for a couple of hours, you won't necessarily get sick from drinking it-but you're not helping your chances.
So refrigerate your milk as soon as you get home, and leave it in the coldest part of the fridge when you're not using it. If nothing else, you'll limit your chances of smelling rotten milk, which, in my opinion, is a win in and of itself. Nowadays, it is usually heated to 71 C and then rapidly cooled to kill off bacteria and other microbes.
Bacteria tend to get a lot of bad press. But lactic acid bacteria occur naturally on plants like grasses and can easily end up in milk. They ferment lactose at room temperature and turn it into lactic acid. More acid means more hydrogen ions, a lower pH and a sour taste.
Lower pH change the structure of proteins like casein, which gives milk its whiteness, so it curdles into clumps called curds. The leftover yellowy liquid is whey. Apparently, little Miss Muffet was just having unpasteurized milk that had been left out for a while. Microbes like Lactococcus lactis nominated the Wisconsin "State Microbe" get added back to the milk after pasteurization to make dairy products like yogurt and cheese.
Editions Quartz. More from Quartz About Quartz. Follow Quartz. These are some of our most ambitious editorial projects. By Mun Keat Looi. Published July 27, This article is more than 2 years old. Sign me up. Update your browser for the best experience.
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