Why your berry storage hacks aren’t working

Berries last longer when they’re cold

Fresh produce is alive and, like us, takes in oxygen and expels carbon dioxide (this is entirely separate from photosynthesis). Cold temperatures slow berries’ respiration, which extends their lives (once harvested they essentially have the resources for a certain number of breaths before they start to fall apart). Getting berries home from the store or farmers market and into your fridge fast is often the best thing you can do to extend their life. Psychologically, I think this probably feels less satisfying than a special storage method, but prompt refridgeration really should be your priority. 

The most delicate berries “breathe” at a fast rate, three times faster than an orange, for example, which is part of what makes them so perishable, explained Elizabeth Mitcham, emeritus director of the Postharvest Technology Center at the University of California. 

Some storage hacks urge you to wash and dry berries when you get home from the store. But all that time washing and drying means more time breathing faster at room temperature. And while the store-bought berry clamshells have vents for air circulation, if you leave berries at room temperature for a while and then put them in a closed glass container, the fridge’s cold air will struggle to reach them. 

“So if you’re leaving them out to dry, they’re warming up,” said Mark Ritenour, a postharvest biologist at the University of Florida. “And once you put them in the jar, they’re hard to get cool again.”

They’re more susceptible to mold (and bacteria) when they’re wet…

Mold loves moisture, so keeping your berries dry will keep them good for longer. The little pad at the bottom of some plastic clamshells is designed to absorb loose drops of water or juice. 

And this is another reason why experts discourage washing berries before storing them. If they aren’t perfectly dry, which is hard to achieve given raspberries’ cup-like shape and strawberries’ tricky under-leaf areas, mold will have an easier time digging in. While it’s a good idea to wash fruit right before you eat it, the action of handling the fruit and the movement of water around it can also distribute microbes, enabling them to cover more of the fruit. Lingering drops of water are no big deal if you’re about to eat the berries, but if the water sits for a while, microbes will accept the invitation. 

….But humidity keeps them plump

You want to keep berries dry, but humid air will keep them deliciously plump and juicy (kind of frustrating, right?). Fresh produce, like all plants, releases moisture. Dry air will pull more moisture from your berries, shrinking and shriveling them. Berries shed much less water in air that’s already heavy with moisture. So, any storage hack that involves a sealed container will trap moisture, keeping them plumper longer and giving you the impression that something’s working. But a tight seal can invite other problems, which we’ll address in the “tightly sealed jars” section below.  

No matter what you do, berries will inevitably have invisible-to-the-naked-eye microbes on them 

Berries can pick up bacteria and fungal spores from splashes of water in the field, from bumping up against contaminated fellow berries during harvest and packaging and even from handling in your own kitchen (yet another reason to avoid washing berries before storing them). While you should wash berries right before eating them, it’s practically impossible to remove all hitchhiking bacteria and fungal spores from fresh produce. Some residual microscopic life is simply part of eating fresh, uncooked fruits and vegetables and the slight risk is worth the benefit of eating fresh fruit. Still those tiny berry-residing creatures certainly contribute to the roulette wheel of bringing berries home. 

A stray microbe on a berry won’t immediately cause a problem. It will usually hang out until the berry is weak enough, either from an injury or the softening that naturally occurs as a berry ages, for the microorganism to easily access the soft, juicy tissue beneath the skin. Once it gets to work you’ll see softening, leaking juice or mold fuzz.

One more reason why a pre-storage rinse is unlikely to help: some mold spores are impossible to wash off–they essentially embed themselves in berries. 

“It could have been there from the time the flower developed when the fruit first formed,” Mitcham said. “And it’s just sitting there waiting for the fruit to ripen and the temperature to be optimal. And then it starts to grow. So a rinse isn’t going to kill that. You’re really just making things worse.”

Handling and jostling berries can damage them and spread bacteria

Strawberries are so delicate that harvesters hand pick and place them directly into the clamshell you purchase them in. If you’re spending time moving them from one container to another, you’re providing more opportunities for them to trade microbes and get bruises or cuts, which open the door to microbial growth. 

Every berry has a different life story and lifespan

One thing that frustrates me about storage hacks is they assume that every berry is a blank slate when it enters your kitchen. It’s not! As mentioned above, it may or may not have been exposed to mold spores that are about to blossom into fuzz. In the off season, berries likely traveled from afar, so are older when you buy them. A pallet of berries that someone forgot on a loading dock might have lost some of their shelf life as they warmed up. Same goes for berries on a truck or in a grocery store with faulty temperature management. 

Also, while your raspberries might simply be labeled “raspberries,” there are many cultivars with slight differences (growers pick the ones that will do best in their regions). Some cultivars have genetic quirks that might impact how they fare.  

Delicate berries have thin skin

Strawberries, raspberries and blackberries have thin skin that easily loses water and is easily penetrated by invading microbes. Blueberries, which typically last longer, have thicker skin. 

Mitcham noted that thin skin can also absorb water, not in a useful way that might hydrate the berry, but rather in a way that can flood the tiny passageways berries use for gas exchange. Unable to get oxygen, flooded cells will switch to anaerobic respiration, whose byproducts are foul flavors and chemicals that will break down cell membranes, causing the berries to collapse and making them more vulnerable to microbes. 

If you store berries in tightly sealed jars, they might appear good for longer, but they’ll ferment faster and could create conditions conducive to the growth of dangerous bacteria

Hacks that urge putting berries in sealed containers or jars are certainly onto something. A sealed jar will trap moisture, making the berries appear plump for longer. As the berries breathe in oxygen and expel carbon dioxide, oxygen in the jar will diminish and carbon dioxide levels will rise. Berries respire slower in a low-oxygen environment, which extends their life, and mold grows slower when there’s more carbon dioxide around. The produce industry deliberately stores many types of produce in low-oxygen environments to increase shelf life.  But, here’s the big difference between commercial produce managers and the berries you just poured into a jar with a tight-fitting lid: produce managers make sure the fruits and veggies still get sips of oxygen–that prevents them from switching to anaerobic respiration, which creates off flavors and destroys the berries. 

“Imagine there are many of us together in a room where there is no air entering or leaving. What is going to happen? The amount of oxygen is going to deplete and at the end we won’t be able to respire and we will die,” said Eva Almenar, a professor who specializes in produce packaging at the Michigan State University School of Packaging. This, she said, is what you’re doing to produce stored in tightly sealed jars. 

Possibly, Almenar said, people who swear by storing berries in jars don’t realize their berries’ quality suffers. 

“They could be eating fermented fruit and they don’t notice it,” she said. 

Of course, if you’re storing berries in jars, maybe you’re opening the jar every so often. That could replenish the oxygen. Buuuuuut there are so many variables here. How big is the jar? How full is the jar? How often are you opening it? Is water condensing inside the jar, paving the way for microbes? It would be impossible or at least highly impractical for a consumer to get this right. 

You may be wondering why it’s a bad idea to put your berries in sealed jar, but totally OK for the grocery store to sell sealed packages of produce. Almenar explained that those sealed containers are filled with calibrated ratios of gasses and are made with plastics that allow limited amounts of carbon dioxide to escape and oxygen to enter. You may notice that the produce looks good while it’s in the sealed container, but declines at a faster clip once you open it. That’s because it’s losing the benefit of the modified atmosphere.

But forget fermented flavors for a minute; the top reason to skip any berry storage method that instructs you to leave berries in a tightly sealed jar is that it could invite some gnarly food safety problems.

If the berries manage to deplete the oxygen in the jar, you’d have an anaerobic environment with ample nutrients for hungry bacteria. One particularly scary type of bacteria, Clostridium botulinum, only grows in anaerobic environments and produces a deadly toxin. Now, this bacteria doesn’t do well in acidic conditions, so it may not grow well in the company of berries, which tend to be acidic. But is it worth messing with? I don’t think so, especially since we don’t want to get in the habit of putting fresh produce of any kind into tightly sealed jars (here’s a paper about C. botulinum growing on mushrooms stored at room temperature in packaging that didn’t let oxygen in fast enough).

How to handle berries so they last longer

So given their many needs, what’s the best way to store berries? For me, the easiest path is the best – straight from grocery tote to fridge. I eat them quickly and accept that sometimes you get a problem batch. With the disclaimer that no trick is a slam dunk, the following best practices can help.

• Keep berries cool and limit time outside of the fridge. Get them home and into the fridge as fast as you can. Keeping them cool is really the best thing you can do to extend their life. 
• Put them in the crisper drawer (or otherwise make sure they have high humidity, air flow and a way to absorb moisture). Berries do best in as humid an environment as possible, as long as there’s enough air flow for them to breathe. Ideally, they’re also staying dry. It’s not perfect, but the crisper drawer can help. 
• Leave them in the original clamshell or transfer to a vented container.  If you’re willing to risk complications from transferring berries between containers, put them into a slightly opened (to allow airflow while trapping humidity) container lined with a cotton napkin (to absorb loose condensation or juice). Or you could try one of these storage containers, designed by a team including University of Florida scientists, which come with adjustable vents and instructions on calibrating them for specific types of produce. 
• Remember that blueberries are longer lasting berries. If you can’t make it to the store often and want to stock up with fewer worries about berries going bad, eat the most fragile berries, aka strawberries, raspberries and blackberries, first. Blueberries, with their thicker skin and slower respiration rate, will last longer, assuming they start out in good shape.
• Freeze what you won’t get to in time. Berries also freeze well. Just rinse and then immediately freeze. Some folks advise freezing them on a sheet and then putting them into a container to avoid clumping. That makes sense, but I usually just rinse, then quickly pour them into a container and pop it in the freezer and later work them apart with my fingers if I need to. If you’re freezing them, you’re stopping their metabolic processes and pausing microbial growth, so there are no concerns about lingering water or transferring microbes. And washing before freezing will remove more microbes than washing after freezing.  All that said, it’s best to eat them frozen or in a cooked dish. When the water in their cells freezes, it will break the cells down and they’ll be collapsed and mushy when you defrost. I like to eat frozen berries while they’re still frozen or blended into smoothies. Sometimes by the strollerful.