How to evaluate greens past their “best by” date

Quick summary: If your clamshell or bag of fresh greens is a little past its “best by” date, but still looks fresh and dry, the greens are probably fine. If, however, you’re seeing slimy or odorous leaves, then it’s time to compost them. The shelf lives of leafy greens like spinach, lettuce and arugula are variable and can change based on how they’re handled. While we expect them to go bad sometime after their “best by” date some might still look great for a little while after the date, and some might decline before the date (in this case they’ve likely been mishandled). Read on for more about why fresh greens, especially salad mixes, spoil quickly and how you can keep them at their best for longer. 

Can you eat baby spinach after the “best by” date? What about baby kale, arugula, spring greens or other greens?

Clamshells and bags of greens can be tricky to assess, and a “best by” date can add to the confusion. Basically, as long as the greens still look, smell and feel fresh, you’re probably fine. Rather than view the “best by” date as a bright line, consider it a soft deadline for making sure to use your spinach before it’s too late. (Is your spinach at the cusp of breaking down? Here’s how to salvage it.)

Generally, two issues cause the demise of your greens:

• Physical processes – Greens are alive and “breathing,” but once separated from the plant they can’t access everything they need to sustain themselves indefinitely; their cells eventually collapse and the leaves will break down. In dry environments, leafy greens go limp and shrink as they lose moisture.
• Spoilage by microorganisms – Naturally present bacteria enjoy salad as much as we do. Once they find an opening, either from an injury or because of the breakdown process described above, spoilage bacteria get to work. They can cause sliminess and odors. 

Once your leafy veggies are going mushy, these processes have advanced beyond the point where you’d want to eat the greens.

Do your greens look odd in other ways? Check out our Greens Library.

Storage at 40 degrees Fahrenheit or lower will extend the shelf life of greens

Both natural breakdown and microbial growth are slowed by cool temperatures, which is why you should set your fridge to 40 degrees Fahrenheit or less–a temperature proven to significantly slow microbial growth. Myriad factors that are hard to control or predict, particularly storage temperatures throughout the supply chain, can also speed up and slow down the process. 

In fact, if your fridge is too warm (or if your greens were stored in a too-warm place before you bought them, like on the truck, loading dock or store shelf), they could start to go soggy before the printed date arrives. 

Cold temperatures slow the greens’ respiration. Slower respiration means they use up their nutrients slower and thus age slower, too. Low temperatures also slow the activity of microorganisms like spoilage or pathogenic bacteria. (But don’t let your fridge dip below freezing—if the water in the greens freezes that will also lead to injuries that can make the greens more vulnerable to spoilage once they warm up.)

On greens, spoilage bacteria usually grow faster than pathogenic bacteria

Microbial growth is certainly off-putting and means the greens will not provide an enjoyable eating experience. However, while it’s possible for human pathogens to be feasting on rotting greens, the bacteria that typically spoil fresh greens are different from the bacteria that could make us sick. I spoke to several scientists, whose expertise ranged from shelf life to produce microbiology, and they all said the same thing: bacteria that spoil food, especially raw foods like greens, typically grow faster than bacteria that sicken people. The greens will probably look too unappealing to eat before they contain a disease-causing load of bacteria. 

“Usually the shelf life of the product is inherently limited, to the point where if there is pathogen growth, it’s usually not very much if at all, because the product deteriorates so fast because of spoilage microorganisms,” said Doug Marshall, chief scientific officer at Eurofins Microbiology Laboratories, which helps food companies determine their products’ shelf lives.

In rare instances where greens are implicated in foodborne illness outbreaks, the problem is usually that pathogens make it through the washing process, not that pathogens grow to infectious levels during storage, Kathy Glass, associate director of the Food Research Institute at the University of Wisconsin, explained. That contamination is invisible to the naked eye and can be present on greens that look perfectly fresh. 

So, if you get a foodborne illness from salad greens, it’s most likely bad luck. But Faith Critzer, a food science professor at the University of Georgia, says not to assume that eating a clamshell of greens in an early state of sliminess isn’t taking a risk. When the greens start to break down it gets that much easier for disease causing bacteria to grow. 

“If you’re ever in doubt, especially when it comes to things looking as if they are spoiled, it doesn’t mean you can play this numbers game in your head and be like, ‘Oh, there’s more spoilage organisms, there’s probably not many foodborne pathogens,’” Critzer said. “Because the foodborne pathogens could also be there.”

Greens are cleaned, dried and cooled to extend shelf life

Growers do all they can to keep greens clean and long-lasting in our fridges. That includes washing, drying (any extra moisture can boost microbial growth), and packaging the greens in bags or clamshells. They’re also kept cold.

Washing doesn’t remove all the naturally occurring microbes on greens

While washing is an important step between the farm and your salad bowl, it doesn’t remove all microorganisms. And no one expects it to do so.

“It is possible to reduce the number of pathogens on produce by washing in sanitized water, but it is not currently possible to eliminate them,” the authors of a chapter on food safety wrote in Postharvest Technology of Horticultural Crops. 

Critzer pointed out that some of the bacteria that remain are ones that we’re glad to eat in other contexts—lactic acid bacteria, for example, which turns milk into yogurt. On products like yogurt, you might even see lactic acid bacteria marketed as a “probiotic.” 

“Some of them aren’t necessarily helpful for you, but they don’t hurt you either,” she said.

On the other hand, it is possible for pathogens, like salmonella, E. coli and listeria to survive the washing and drying steps. Unless your fridge is too warm, salmonella and E. coli are unlikely to grow in the refrigerator. Listeria is the rare pathogen that can grow and become more dangerous under refrigeration, though, especially on raw produce, it will probably be outcompeted by spoilage microorganisms. 

“So there’s so many more of these spoilage organisms on these products,” Critzer said. “And they’re on there not because they’ve been held in insanitary conditions or filth or anything along those lines. It’s because there are microorganisms in our environment. We would expect our spoilage organisms to kind of pop up and you know kind of make the food unpalatable pretty quickly. And they do that. They do their job very well.” 

Younger “baby” greens and pre-chopped greens have shorter shelf lives

Any factor that speeds respiration makes greens (and any fresh produce) go bad faster. Some greens, like spinach, have especially high rates of respiration. And young leaves (e.g. baby spinach, baby kale, etc.) respire faster than leaves harvested when they’re more mature. Additionally, when greens are chopped for salad mixes, the wounds from chopping cause the leaf pieces to respire even faster than intact leaves; and their shelf life decreases considerably. For example, shredded lettuce respires two to three times faster than a head of lettuce, according to the Postharvest Technology of Horticultural Crops. The book also notes that lettuce chopped with a sharper knife will last longer than that chopped with a dull knife (the dull knife would create more jagged edges and more cut surface area).

Ultimately, when it comes to leafy greens, the best way to maximize safety and minimize waste is to use them quickly and only buy what you need. Make sure your fridge is 40 degrees or less, remember that smaller leaves have shorter shelf lives, and steer clear of any slimy, leaky or mushy greens.