Milk has a sell-by date usually printed near the top of the container.
It's a rough start to the day when that first sip of milk reveals it has gone sour. Not only can sour milk taste funny or make your cereal crunchy in an unwanted way, it can also smell bad. Milk goes sour when its natural bacteria uses sugar and lactic acid is formed.
Bacteria
Milk contains bacteria that are not initially bad, as well as lactose, a form of sugar. After a period of time, the bacteria consume lactose to produce even more bacteria. A resulting product is lactic acid, which in turn leads to a sour taste. The milk is no longer smooth and now curdles, making it unpleasant to drink or use.
Sour Milk Uses
While sour milk should be disposed of, it has several uses. Sour milk leads to other dairy products, such as cheese and yogurt. However, these products can't be made by letting your milk go sour at home. Cheese and yogurt are made with a special kind of bacteria, one that won't turn your stomach and make you say, "Yuck!"
Sell-By Date
Milk comes with a sell-by date, indicating when it's best to buy the product. This doesn't guarantee the milk will go sour on that date, but is a good indicator when it will lose its good, smooth taste. For best results, buy milk with a sell-by date that's as far in the future as possible and consume it by that date. Milk is often stocked in grocery store refrigerators with the older milk positioned in the front, so look through the available milk to find a sell-by date that's further in the future.
Storage
Milk should always be stored in a refrigerator when not in use. Get in the practice of putting milk back in refrigerator right after pouring it into a glass. Don't leave it sitting out for long periods. Milk will sour at a faster rate the more it is taken out of the refrigerator. To help determine if milk has gone sour, smell it or take a small sip. Stay mindful of the sell-by date, and don't hold onto milk long after that date.
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