- Old milk will give off an unpleasant odor.
- The best way to check to see if your carton of milk is bad is to smell it, according to food safety expert Theodore Labuza.
- Be wary of the date listed on the carton of milk, as it is just a guess to the lifespan of the product, per Labuza.
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Got milk? Perhaps you remember the slogan from the popular '90s dairy campaign. But, if you've actually got milk, do you know how to tell when it goes bad?
There are a few obvious signs to look for when trying to decide if your carton of milk is no longer good. Theodore Labuza, a food safety expert and professor at the University of Minnesota's Department of Food Science and Nutrition, has been studying the shelf life of food and beverages for nearly 50 years. Here's are the five signs your carton of milk is bad, according to Labuza.
The milk has surpassed the sell-by date
The date listed on your carton of milk isn't necessarily an expiration date. It may also be a sell-by date or best-used-by date. The timeframe should be viewed as a barometer of how long the milk may possibly be good for. This is why it's important to be familiar with the other signs that may indicate a carton of milk is no longer drinkable.
"The companies who put dates on milk or other products are just guessing," said Labuza. "When they put a date on the package of food or carton of milk, that's a guess. They don't know what temperature your refrigerator is at. Everybody's refrigerator isn't set to the same single temperature."
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The milk hasn't been consistently refrigerated at the right temperature
Coincidentally, refrigeration temperature has a lot to do with the sell-by date listed on the milk carton, per Labuza.
"The biggest problem with the shelf life of a product is that the date does not take into account that the refrigerators cycle in temperature," Labuza told INSIDER. "Everybody's refrigerator is not the same temperature. The higher in temperature your refrigerator is set to, the shorter the shelf life of the carton of milk becomes."
Labuza recommends 32 degrees Fahrenheit as a safe bet to ensure the longevity of your milk. Don't worry, he said it's not going to freeze.
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"Once the refrigerator reaches the lower set temperature, it shuts off," said Labuza, who keeps his refrigerator set to run between 31 and 33 degrees Fahrenheit. "At some point, the temperature of the refrigerator goes up to the upper limit and that kick starts the refrigerator into cooling back down to the lower temperature."
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The milk has a foul odor
Give your carton of milk the sniff test. If your milk doesn't smell like milk, it's likely expired. Milk that's gone bad exudes a foul odor — and it will be very obvious upon taking a whiff. Labuza said this is the best procedure for determining if your milk has gone bad.
"The odor is very obnoxious. If you drank it, it would attack your nasal sensors and tell you this is bad. Your nose is a very very sensitive tool and it gives you something that says the milk is no good at that time," said Labuza, who said he has worked with NASA and the military on food safety.
"If you drank it, you'd probably throw up. It's not food poisoning but it's food objection. The organisms in there produce various kinds of compounds that have an objectional odor. It definitely smells like bad milk."
There are changes to the texture and consistency
When you unscrew the lid on a carton of milk, the white liquid should appear smooth and free of any particles. If you open up a gallon of milk and notice clumps and curdling, it's time to toss it out.
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Why does this happen? As Labuza explains, it all has to do with enzymes in the milk reacting with one another.
"If the milk is bad, it will curdle. It's spoiling because the microbes produce certain enzymes," said Labuza. Milk is mostly composed of fat, protein, and sugar. "The acid produced by the organisms will cause the milk to precipitate out and curdle."
It's lost its pearly white color
Unless you're drinking chocolate milk, the color of plain old milk should always be white. If you crack open a carton of milk and notice anything different, that's a sign that the dairy product is no longer good.
"The other enzymes impact the protein in the milk, so it breaks down the protein into amino acids and that will have different bad flavors," said Labuza. "The same reaction that produces browning on a piece of toast — the Maillard reaction— that's a reaction between the protein molecules and the sugar in the milk. That causes a browning color and flavor in the milk."
