Until relatively recently, humans ate little fruit. It was often a morsel to savor if one happened to find oneself near the right bush at the right time. Even after the advent of agriculture, fruit remained scarce. “In the Genesis story, it made sense for Eve to tempt Adam with an apple precisely because . . . fruit was a seasonal treat,” writes Dartmouth College researcher Susanne Freidberg in her book Fresh: A Perishable History.

At different times and in different places, people even mistrusted fruit. In medieval Europe, apples, in particular, were suspected to be poisonous. Even where planting orchards became commonplace, their fruit was not necessarily eaten fresh. Early American colonists, for example, preferred to turn it into cider, dry it, or fatten livestock with it, writes Freidberg. As one farmer she quotes put it: “As soon as our hogs are done with the peaches, we turn them into our apple orchards.”

Consuming fresh fruit only gained broad appeal in the United States and Europe during the 19th century, and then even more in the early 20th century, particularly after vitamins were discovered and found to be vital for human health. Following the Gold Rush, California reinvented itself as a producer of oranges, grapes, melons, and other fruit; new railways and refrigerated boxcars carried this produce across the country.

But refrigeration alone proved insufficient for keeping fruit appetizing. “Delayed and misrouted shipments were frequent and ruinous. Even without scheduling problems, crates that started the journey containing a few bruised or nicked fruits often arrived a fuzzy, rotten mess,” writes Freidberg. And producers soon discovered excessive cold storage degraded a fruit’s texture. Apples would still look pretty but became as mealy as if they had been stuffed with cotton, Freidberg notes. They needed a way to control the ripening process itself.

The solution would come from an unexpected source: streetlamps. In the 1800s, cities across Europe and the United States had started to install lanterns that were fueled by a coal-derived gas piped to homes, businesses, and streetlights.

Curious things tended to happen near such gas lamps: trees would suddenly turn yellow and shed leaves, even in midsummer. In Hamburg, Germany, 159 linden and elm trees were felled in 1851 after they unexpectedly died. In a Philadelphia greenhouse, several thousand exotic plants withered overnight.

It didn’t take long to figure out these happenings were tied to leaks in the gas pipes, and scientists zeroed in on one component: ethylene. Colorless and almost odorless, ethylene nevertheless seemed to affect plants in powerful ways.

Around 1900, a graduate student named Dimitry Neljubow at the Botanical Institute of St. Petersburg in Russia showed that pea seedlings in the laboratory grew stunted and horizontally instead of upward when exposed to ethylene. When the gas was removed, the seedlings recovered.

Once researchers started looking, they found the effects of ethylene everywhere. In Florida, citrus packers used kerosene stoves to warm oranges and lemons to turn them from green to yellow and orange. But it wasn’t the heat that caused this change—it was the ethylene in the ovens’ smoke. Ethylene also blanched celery, making it milder and less bitter, something that growers had traditionally achieved by arduously covering the plants in the field with boards or paper. Using the old approach, it could take up to 10 days for the green chlorophyll to fade from the stalks; exposing the plants to ethylene produced the same effect in half the time.

Concurrently, scientists pondered the effects fruits and vegetables seemed to have on one another when stored in close proximity. “A stream of air which has passed over an apple would appear to be harmless to other forms of life,” said William B. Hardy, director of the Low Temperature Research Station, in a 1932 address to the British Association of Refrigeration. “The appearance is wrong—the air contains some subtle emanations which profoundly influence other vegetable forms.”

Soon British biologist Richard Gane made the discovery that connected these phenomena. Gane collected the air around apples to show that it contained traceable amounts of ethylene, which, he deduced, the fruit had produced on its own.

Ethylene, it turned out, was a plant hormone. Plants release it when they are stressed but also to regulate aspects of their development and seasonal changes. It’s ethylene that prompts a tree to change color in the fall, then drop its leaves. Many plants also release ethylene to ripen their fruit.

“A plant spends so much of its energy and chemistry defending itself,” producing toxic compounds because it can’t move, says James Giovannoni, a plant molecular biologist at Cornell University. This includes, at least initially, its fruit. Unripe tomatoes, for example, contain alkaloids that can make you sick, says Giovannoni. “They’re bred out of a lot of the tomatoes we produce commercially, which is why you can have fried green tomatoes.”

The defensive strategy, however, abruptly changes when a piece of fruit ripens. It turns from sour, hard, and noxious to something that says, “Come and get me. I’m attractive. I smell good, I taste good, I’m nutritious.” This is so animals eat the fruit and disperse the seeds, says Giovannoni. Ethylene is the key molecule controlling all of these processes, the plant biologist notes. “Which is kind of amazing.”

Once a plant’s fruit has grown to its full size, the plant releases ethylene, which triggers a cascade of biochemical changes. Starches that have accumulated in the fruit convert into sugars. Green chlorophyll withdraws, and colorful pigments take over, turning the fruit red, yellow, or blue. Toxins degrade. Enzymes loosen the cell walls, softening the fruit.

Initially, it proved hard to study these mechanisms in detail. Plants react to ethylene in concentrations as low as two parts per million. A proportion that small is comparable to just one inch in an 8-mile journey, and the analytical instruments of the 1930s struggled to detect substances at such minute levels. To isolate this emanation, Gane had to filter the air around 60 pounds of apples for a month before he had collected enough ethylene, just 0.85 grams, to prove its existence.

In the 1950s the emergence of gas chromatography, an analytical method that efficiently separates and detects volatile compounds in a sample, finally allowed a more thorough study of ethylene effects on plants. Banana traders were among the first fruit producers to adopt the technique. Bananas, technically an herb, generally thrive in tropical climates. Thanks to refrigerated ships, traders could transport bananas to stores in the United States and Europe provided they had been harvested early enough. But if just one banana turned yellow early and released ethylene, a whole container of fruit might become mushy before they could be sold.

These days, bananas are harvested when they’re still as green as grass. After what is often weeks of travel, they arrive at specialized fruit-ripening companies that transfer them to insulated rooms where they’re exposed to controlled blasts of ethylene.

“Sometimes you’ll see a whole pile of bananas in the supermarket that is green because they’ve just been gassed, and the next day they’re turning yellow,” says Giovannoni. The artificial ripening is done so precisely that retailers can order a specific grade of ripeness—say, 3.5 on a scale from 1 to 7—and receive batches that have been uniformly ripened to that level.

With other fruit, withholding ethylene becomes more important. Apples are put into cold storage after harvest, and growers scrub the ethylene the fruit emits from the air by running it through a filter lined with activated charcoal or potassium permanganate—chemicals that react with and thus capture the ethylene. Alternatively, growers may spray the fruit with compounds that keep it from reacting to the gas. Doing so can delay the apples’ full ripening by months. “All the apples you buy are harvested between August and maybe October, at the latest,” Giovannoni says. “Basically, you put them into hibernation . . . and they just kind of go to sleep.”

Controlled ripening is a big reason you find kiwi or mango in supermarkets most of the year and often far from where they grew. Suppliers can harvest these fruits unripe, ship and store them, then blast them with ethylene before putting them up for sale.

“A well-timed dose of ethylene ensures uniformity—a prerequisite for any food or drink that aspires to the status of a commodity, let alone a brand,” writes journalist Nicola Twilley in Frostbite: How Refrigeration Changed Our Food, Our Planet, and Ourselves. The Hass avocado, for example, was first cultivated by Rudolph Hass, a mail carrier and amateur horticulturist in Southern California, who patented the variety in 1935. With its creamy consistency and a nubby skin sturdy enough for shipping and storing, it quickly became a favorite with growers and consumers. But it was its availability in ethylene-ripened, “ready-to-eat” form that helped make this fruit a staple throughout North America.

The system of controlling ripening with ethylene isn’t perfect. The gas is flammable and has caused explosions in ripening rooms, sometimes killing workers. Improved equipment has mitigated such dangers, especially in the United States, but the chemical’s relatively high cost drives growers in some developing countries toward cheaper alternative chemicals, such as calcium carbide or ethephon, that mimic ethylene’s effect but are moderately to acutely poisonous.

And even though ethylene gives food sellers a great deal of control, fruits and vegetables still top the charts in food waste statistics. “Supermarkets are tossing them all the time,” says Giovannoni. “The estimates go from a few percent to almost 100%, depending on the particular crop and situation.” And the problem starts well before retail. A 2019 study in California concluded that close to 44% of strawberries never even make it off the farm.

Increasingly, these are losses the world can ill afford. Globally, food waste—including the energy for growing, harvesting, and transport—contributes an estimated 10% of greenhouse gas emissions. To minimize production losses, researchers continue to fiddle with methods of ethylene management. Some kiwis today are fumigated with ozone to reduce their ethylene production. Likewise, limes are dunked into hot water for the same reason, while cherry tomatoes are exposed to UV light to limit their reaction to the hormone.

Yet even as researchers refine these techniques, they confront a more vexing obstacle: certain fruits remain unresponsive to ethylene treatment.

One of the most significant laggards in the fruit revolution has been the strawberry.

Back when Americans first developed a taste for the berry, it was smaller than today’s cranberries. Founding Father Thomas Jefferson reported that it took 100 strawberries from his garden in Shadwell, Virginia, to fill “half a pint.” In London, a strawberry dish serving seven people at a wedding is said to have cost six shillings in 1680, or about six times a laborer’s daily wage. Today, this would be close to $350 at the U.S. federal minimum wage.

Crossbreeding with the Chilean variety made strawberries larger. But because they have a thin skin and contain particularly high amounts of water, they blemish easily and spoil quickly. They also can’t be artificially ripened.

Ethylene will just make strawberries age and soften quicker, says Irwin Donis-Gonzalez, a postharvest engineer at the University of California, Davis. The same holds true for other berries, such as raspberries, as well as pineapples, cherries, grapes, and certain types of melons.

Broadly speaking, strawberries and cherries ripen differently from bananas and apples in one crucial way: bananas and apples accumulate starches as they grow. Once fully developed, these starches convert to sugars as the fruit changes color and softens. But in a strawberry, the plant accumulates sugars in its fruit directly. First this happens slowly, then increasingly rapidly, but with no detour via starches. The process stops as soon as the fruit is plucked from the vine.

Biologists call fruits that increase ethylene production and intake during the ripening process “climacteric”; fruits that do not ramp up ethylene production, and thus are less responsive to artificial exposure to the gas, are called “non-climacteric,” although the distinction isn’t always clear-cut, with some non-climacteric fruits—or even just cultivars of a specific fruit—reacting to ethylene in limited ways.

So far, researchers haven’t found anything that would allow them to control ripening in non-climacteric fruits as reliably as ethylene does in climacteric fruit. Instead, non-climacteric fruit must be picked at an optimal stage of ripeness, then hustled to consumers as quickly as possible.

“Strawberries are taken care of like babies,” says Donis-Gonzalez. Ideally, they are picked in the early morning or even during the night, when it’s cool, and packaged directly into plastic clam shells for selling. Stacked into flats, the berries are sent to facilities where they are chilled to slightly above freezing.

“We’re slowing down their metabolism,” says Donis-Gonzalez. “A strawberry is alive, it’s like an insect or animal: absorbing oxygen, producing heat, consuming its sugars.” Some distributors even inject carbon dioxide into the packages, which inhibits the berry’s “breathing.”

But judging the best moment for harvest is a tightrope. Pick too early, and you risk sour, unsellable produce; too late, and it may spoil before reaching consumers. Historically, strawberry growers have relied on counting the days since bloom or judged berry color by eye, maybe helped by tasting a sample.

This challenge isn’t unique to strawberries. Across the fruit industry, determining ripeness has long relied on ad hoc, complicated, or roundabout methods that blend sensory evaluation with laboratory-based approaches. Growers have floated fruit in water to judge its sweetness—the more sugar it contains, the more volume it displaces. They have run chemical analyses of juice samples to quantify starches and sugary components such as fructose. They even tried offbeat methods, such as having testers chew apples in front of a microphone so that the sound of their munching can be acoustically analyzed to infer how “juicy-crisp” the sample was. But finding an ideal solution has been elusive; these methods are time-consuming, require complicated laboratory equipment, or have proven to be unreliable. Importantly, most also require sacrificing any fruit that is tested.

This is changing as researchers increasingly develop technologies that objectively evaluate ripeness by looking directly under a fruit’s skin without destroying it. If you shoot a beam of light at a strawberry, for example, different wavelengths interact in different ways with the sugars and other chemical compounds under the berry’s skin. By analyzing which wavelengths are reflected back in what measure, scientists can create a “spectral fingerprint” that indicates ripeness.

In firmer fruits, such as apples, researchers are also using an approach that borrows from an old technique: tapping a fruit to judge its ripeness by sound. As fruits ripen, their internal structure changes—cells break down, water content shifts, and firmness decreases. With the acoustic vibration technique, experts gently strike a fruit with a small weighted ball while sensitive microphones listen for the distinctive sound patterns that signal maturity.

There are also electronic “noses,” sophisticated sensors that detect and analyze the chemical compounds fruits release as they ripen. Strawberries, for example, emit more than 360 kinds of volatile substances such as esters and alcohols; these high-tech noses have already been deployed to monitor stored strawberries for early signs of spoilage.

As growers increasingly look to automate their production, machine-driven fruit-ripeness detection could become a common sight in fields, mounted on drones or robots to monitor crops and time their harvest. A grower in Florida has pioneered a strawberry harvester that comes equipped with cameras that scan the plants, artificial intelligence to analyze what it “sees,” and soft rubber claws to grab fruit. The size of a school bus, the harvester deploys 16 individual robots that can pick a bush clean of ripe strawberries in about eight seconds and keep working for 20 hours a day.

Researchers are coming at strawberries from another front, too: genetics.

As a fruit matures, it produces enzymes that break down the cellulose and pectin in its cell walls, compounds that keep its membranes rigid while unripe. As a result, the fruit softens. But what if scientists could uncouple this process from the other aspects of ripening, such as the increase in flavor compounds and sugars?

Beginning in the mid-1980s, a U.S. company called Calgene tried to do this with the tomato. Geneticists knocked out one of the tomato’s genes that had been responsible for producing a wall-softening enzyme called polygalacturonase. This meant that the company’s tomato—called Flavr Savr—could be left longer on the vine to develop a better taste yet stay firm enough for shipping.

Although the Flavr Savr hit the market in 1994 to great fanfare, it ultimately fizzled.

“In theory, it was a great idea,” says Harry Klee, a plant geneticist and expert in tomato ripening at the University of Florida. In practice, however, it turned out that targeting one enzyme wasn’t enough.

“Softening is much more complicated than people initially thought. It is controlled by a dozen or more enzymes that are responsible for taking the cell wall apart,” says Klee. With the project becoming more difficult while public opinion turned against GMOs, the Flavr Savr tomato was soon abandoned.

Now, however, the strategy is being resurrected for the strawberry. Genetic manipulation of commercial strawberry cultivars is particularly challenging because they are octoploids, meaning they carry eight sets of chromosomes in each cell (compared to two sets in humans). Since a breeder might select for many different traits—such as a creamy texture and sweetness on top of firmness—and since each trait could be controlled by several genes, things quickly get complicated.

But in 2019, an international team of scientists managed for the first time to sequence the genome of a cultivated octoploid strawberry. Breeders are now increasingly using molecular-assisted breeding techniques, which allow them to run DNA tests on seedlings to predict traits without waiting for plants to mature, speeding up the timeline to develop strawberries that are both sweeter and sturdier than what we find in stores today.

Some researchers are also employing techniques, such as CRISPR, to directly modify specific genes within the fruit’s genome. In March 2023, a team of scientists reported using CRISPR to edit out a gene responsible for polygalacturonase production in strawberries, mimicking the earlier project in tomatoes. Doing so increased the strawberries’ firmness by 33% to 70%, the scientists stated in a paper published by the journal Horticulture Research. “The results obtained suggest that editing genes encoding cell wall pectinases could be an excellent way to improve the fruit shelf life of elite strawberry genotypes,” they concluded.

It might be years before genome-edited strawberries enter the commercial market. For now, experts recommend that you keep the strawberries you buy chilled as much as possible. Peaches, pears, and other climacteric fruit might benefit from sitting on the counter for a few days while they develop sugars and flavor, but strawberries, those holdouts in the revolution of controlled fruit ripening, should always go in the fridge.

Postharvest expert Donis-Gonzalez shudders whenever he comes across stores that stack the popular fruit outside refrigerated displays.

“You’re basically killing that strawberry.”

Support for this article is provided by The Pew Center for Arts & Heritage as part of the Science History Institute’s latest exhibition, Lunchtime: The History of Science on the School Food Tray.