Sometimes Your Milk Expires Before The Labeled Date. Here's Why
Expiration and best-by dates can be confusing. We all know that eating foods that have spoiled is not a good idea, but there are many instances when whether something has actually expired is more of a question mark. When your milk shows classic signs of going off — it smells sour or clumps up in your hot tea before the date on the container — it could be that something's gone wrong with how you stored your milk.
Milk should be kept at temperatures below 40 degrees Fahrenheit for the longest possible shelf life. If you find that your milk has soured before the best-by date on the carton, the milk has most likely been exposed to higher temperatures that have allowed naturally occurring bacteria to multiply. That may have happened at the store before you brought it home or possibly after due to how you've kept the milk once it got to your kitchen.
How to curtail milk spoilage
We all know to store milk in the refrigerator, but there might be times you've left the container out on the counter while cooking — a short time at higher temperatures can give bacteria a jumpstart to sour your milk. Also, you might find that your refrigerator is not consistently keeping to a constant temperature of 40 degrees Fahrenheit or below, which is the recommended safe storage temperature. Keep an eye on the digital temperature indicator in your fridge if it has one, or you can buy an inexpensive thermometer at most grocery stores to keep on a shelf to be sure all your food is safe, not just the milk.
You might be surprised to learn that not all areas of your refrigerator stay at the same temperature. The door shelves, where we conveniently store those things that we use frequently (like milk!), tend to maintain a warmer temperature because opening and closing draw in more of that warmer ambient air. That might not be a problem if you use up your milk quickly, but if it's spoiling, consider keeping your milk on an interior shelf instead. If you happen to notice your milk is sour before it's completely gone bad, there's a silver lining! You can use it in recipes that call for sour milk in place of buttermilk so that you don't completely lose your grocery investment.