Gray patches on basil

“It would be better to toss it out if there’s severe chilling injury,” said Arlan James Rodeo, an assistant Professor at the University of the Philippines Los Baños who studies how basil retains its flavor and aroma after harvest. “The damage caused by chilling injury can lead to greater susceptibility to decay. This would mean it is also more prone to having harmful bacteria, fungi and other pathogens.”

Chill-injured basil, he said, has also lost some of the compounds that make it taste delicious. But, fortunately, the early stages of chilling injury are less dire.

So, if that’s you staring, vexed, at a stack of basil with a grayish blush, try this: first, make sure the basil isn’t black and there’s nothing fuzzy, slimy or stinky lurking. No? Great. Are you a healthy adult comfortable with a marginal increase in risk of something you don’t want to eat being on the leaf (keeping in mind that all uncooked produce carries a slight risk)? Yes? Ok, then sample the leaves that are going shadowy. If you don’t like the taste, substitute dried basil or head back to the store. If it tastes fine, then go ahead and use it, just with the knowledge that it’s not at its peak. 

Why does basil turn gray?

Once harvested, basil, like other greens and herbs, needs to be kept relatively cool to slow microbial growth and physiological decomposition. However, if an environment is too cold, like, say, the temperature of most fridges, basil will succumb to chilling injury. The cold disrupts membranes in basil’s cells, which causes them to leak substances that mingle with other chemicals leading to color-changing reactions within the leaves. Gray, brown and eventually blackened leaves are the result. If you’ve ever put a banana, another tropical-origin food, in the fridge and watched it turn inky black, you’ve seen another version of chilling injury. (See what chilling injury can do to eggplants, potatoes, and avocados.)

Too cold temperatures can also cause basil to look dull and can cause its leaves to drop from its stems. While the leaky cells brought about by cold temperatures won’t instantly make basil unsafe, they will weaken the herb and make it more susceptible to microbes; its shelf life will be shortened and its risk of hosting a pathogen will go up. You’ll know microbes are doing their thing if the basil gets slimy, looks very wet and wilted or develops fuzz.  A gray fluffy mold called Botrytis cinerea is common on basil stored in humid conditions. 

As Rodeo pointed out, those cold temperatures also cause the flavor compounds that make basil delicious to flee at a faster rate. Known as “volatiles,” those flavor compounds start out in tiny, impossible-for-us-to-see hairs on the surface of the leaves, he explained.

“Since these organic compounds generally have low boiling point, they easily volatilize into the air when the leaves are handled or exposed to stresses,” he said. “Based on our study, the loss of volatile compounds is faster at lower temperatures (chilling temperature) compared to say, 15 degrees Celsius.” (That’s about 60 degrees Fahrenheit.)

Fascinatingly, basil’s signature mix of flavors has some crossover with foods you might never associate with basil. Here’s how Rodeo explains it:

“Since basil is an aromatic herb, most of what we experience from eating it is due to the presence of these volatile compounds. For example, the volatile eucalyptol confers ‘Genovese’ basil a fresh and minty aroma/flavor while eugenol gives it the characteristic clove flavor. That same flavor (clove) is also found associated with alpha-humulene, known to be abundant in hops which gives beer its bitter flavor.”

Home fridges still aren’t ideal for basil

Our fridges need to be kept at 40 degrees Fahrenheit or less because those chilly temperatures prevent or slow pathogenic growth. Beyond their effect on microbes, cold temperatures also help preserve fresh fruits and vegetables because they slow their respiration (fresh produce is alive and breathing!) which means they use up their nutrient stashes slower and stay fresh longer. The ideal storage temperature for basil is about 15 degrees higher than what’s needed to keep our refrigerated food safe.

Basil’s ideal storage temperature, 54 degrees Fahrenheit, slows its internal processes and stalls microbial growth enough to keep it fresher for longer, but isn’t so cold it damages the herb. But since basil is often shipped and displayed alongside more cold-tolerant herbs, it is often kept too cold before it arrives in your home. So the graying damage can be done before you even get to the checkout line. And the graying effect isn’t necessarily immediate; it often shows up a little while after the initial cold exposure.

How should you store basil to prevent it from discoloring?

No matter what you do, basil won’t last long. Your best bet is to buy it as close as possible to when you need to use it. Or, grow it fresh yourself (#goals). Most home kitchens, or even supermarkets, aren’t equipped with spaces at the precise temperature and humidity that will keep cut basil fresh and beautifully green for as long as might be technically possible.  

Still, even though the fridge’s cold can damage basil, I’ve found that grocery store clamshells of the herb do best when I simply leave them in the original container in a drawer in my fridge. You may have heard about basil-storage “hacks” like a vase on the countertop or a vase set up in the fridge. For me, these vase setups make for cute, inspiring photos, but rarely work (and I’ve literally done tests where I divided a package up and stored each in a different way). The reasons basil often still does best in its original clamshell container in the fridge include:

• Slower respiration at cold temperatures.  The chill of the fridge, while ultimately damaging, will cause basil in the fridge to respire slower, which can have a shelf-life-extending effect. 
• Less oxygen, more carbon dioxide in the container. While they still allow for air circulation, grocery store clamshells are likely filled with a balance of gases (less oxygen, more carbon dioxide than air) that slows the basil’s respiration. Rodeo noted that the right balance of carbon dioxide and oxygen in one of those containers can delay the impact of chilling injury for a few days. You lose the benefit once you open the container, however.
• Trapped humidity. Basil likes high humidity. Basil clam shell containers can help trap moisture so the sprigs don’t wilt. Some basil-vase-storage proponents suggest accounting for this by putting a plastic bag over the top of the bouquet of basil. Personally, however, I’d rather not use a single-use baggie or spend time fussing with it.

Putting basil in a vase with water will also, obviously, introduce abundant water into the equation, which in the short term can keep the leaves hydrated, but eventually could nurture bacterial or fungal growth. 

Of course, no matter how you handle your basil, its shelf life will be limited: “Fresh cuttings of sweet basil have been reported to remain marketable from only 2 to 14 days or an average of 6.8 – 7.3 days” at room temperature, according to a paper in Frontiers in Plant Science. Of course, the paper also points out that definitions of the end of “shelf life” vary: “Shelf life here is defined as the length of time until the first obvious signs of deterioration appear on the stored leaves either due to fungal growth or chilling injury… An example of a more ‘lenient’ description of shelf life is when the limit of marketability is defined as black stains appearing on more than 30% of the leaf area and the limit of edibility is defined as black stains appearing on 30-50% of the leaf area.”

Are all herbs sensitive to cold?

While basil leaves can blacken in as little as a day at 32 degrees Fahrenheit, most fresh herbs will last the longest when stored just above freezing, so the fridge will definitely extend their life.  Still, shiso and some oregano species are sensitive to fridge temperatures, Roberto Lopez, an extension professor at Michigan State University, wrote in a newsletter for E-gro.org. 

While all basil is sensitive to cold temperatures, some varieties tolerate the cold better than others. And even within the basil plant, different leaves have different chilling sensitivities. The oldest leaves and the tiny leaves at the tip of each sprig tend to be the most vulnerable. Young leaves that have not yet fully expanded tend to tolerate the cold best, according to a paper in the Israel Journal of Plant Sciences. 

Reducing the odds of gray, brown or black patches on basil on the farm

Many factors can influence how susceptible basil is to chilling injury. So much so that the advice given to farmers can sound like steps for a magical spell. For example, people used to think basil was best harvested in the morning, when essential oils are strongest, according to a guide to the herb from the Herb Society of America. But, these days researchers recommend harvesting basil in the afternoon or evening for better shelf life and resistance to the cold. After harvest, exposing the basil to high temperatures for a short period of time can, counterintuitively, help protect it against future cold, as can exposing it to lower than normal temperatures that are still above the threshold for chilling injury.

These guidelines, of course, have a rational basis. Later in the day, the plants have a higher starch content, which researchers have found makes them better able to tolerate colder temperatures. Briefly exposing them to high temperatures after harvest jolts the basil’s inner workings, including by altering enzymatic activity and cell membranes so they’re less likely to weaken in the cold, studies have found. Exposing them to cool temperatures gives them a running start to accumulate cold-protective soluble solids and starch and work up their other defenses against chilling injury. 

Basil growers have still more factors to balance: The basil’s age at harvest, its genetics, the light and humidity in the spaces it’s stored after harvest, and still other factors can influence the likelihood of chilling injury.