Why Did My Milk Spoil Before The Expiration Date?

If you've opened up the refrigerator to grab milk for a bowl of cereal or a recipe, knowing full well it's still well within its expiration date, only to find that it has soured, you know how frustrating that can be. The smell of spoiled milk is enough to send even those with stomachs of steel reeling. 

But if the printed date on milk means best by, why did it spoil before then? Chef Rhonda Stewart, associate professor at Johnson & Wales University’s Charlotte campus tells Southern Living the answer lies in how the milk has been stored.

"Time and temperature abuse is the most common issue for any food spoilage," she says. "Milk should be stored below 40 degrees Fahrenheit and should not be out of refrigeration for more than two hours."

Here's what to know about why your milk spoiled before the expiration date, how to prevent it, and what consumers should know about dates posted on dairy products.

Why Did My Milk Spoil Before The Expiration Date? 

Stewart says that although most commercially purchased milk is pasteurized to prevent bacterial growth, improper storage can result in quicker spoilage. 

"Warmer temperatures promote bacterial growth, causing the milk to go bad," she says.

To prevent milk from spoiling before the expiration date, Stewart recommends checking your refrigerator to make sure that it is working properly, minimizing the time that the milk is out of the refrigerator, and keeping it sealed as long as possible. 

"Be sure to put the top back on the carton when storing your milk as this prevents it from absorbing odors from the other foods stored in your refrigerator," she says.

What Does The Date On Milk Mean?

According to U.S. Dairy, states have different labeling regulations when it comes to "sell by," "best by," and "expiration date," so the variations of terminology can be confusing for consumers.

"Dairy products are usually posted with a 'sell by' date which tells the grocery store the last day that it should be on their shelves,” Stewart explains. “The ‘expiration date’ or ‘best by date’ indicates the last day that the manufacturer guarantees the quality of the product.”

How Long Is Milk Good Past a Best By Date?

Because the freshness of milk is dependent upon time and temperature, the reverse can also occur: Your milk still seems OK after the best by date listed on the carton. So, is it bad to drink expired milk? Stewart says not necessarily. Depending on the type of milk, the extended window of safety varies.

According to the U.S. Department of Agriculture (USDA), lactose-free and nonfat milk are considered good to consume up to 10 days after expiration, low-fat milk (1 or 2%) is safe for seven days, and whole milk can be safe for five to seven days after the expiration date.

“It is best to smell the milk and look at its texture before consumption,” Steward advises.

How Do You Know When Milk Has Gone Bad?

Because the date cannot always be a clear indicator of when milk has gone bad, it’s important to be able to tell when milk expires. Stewart says that when milk spoils, there are several indicators, the first typically being a sour smell.

"Its appearance can look discolored, lumpy, or separated, and it will have a sour or pungent taste," Stewart explains.

Tips For Keeping Milk Fresh

Stewart says there are a few things you can do to prolong the shelf life of your milk.

• Move to the back: Milk should be stored in the back of the refrigerator where it is the coolest. 
• Skip the door: “Avoid storing it in the door or in front of the refrigerator as those areas have the most temperature fluctuations,” she says.
• Keep a lid on it: The longer the milk container stays sealed, the longer it will stay fresh. 
• Stay mindful of time and temperature abuse: Don't take out of the fridge more than you have to.
• Put it back: "Promptly return the milk to the refrigerator as soon as you have poured the required amount," she says.