In Food Science, Dave McCowan from the University Of Chicago’s Department Of Physics answers our confounding questions about the mysterious world of food.
Go to your kitchen and lean in close to the fruit bowl. Put your ear to an apple and listen. Can you hear the tiny screams? Can you hear the last gasps escaping, the crying out to nearby fruit that the end is neigh? “Change brothers!” the apples seem to say. “Repent your acidic ways and turn toward the light of sweetness!”
Maybe your ripening produce isn’t calling out with words per se, but many fruits do emit something that serves as a doomsday message to others nearby: ethylene gas. This simple molecule acts as a plant hormone to turn different growth processes on or off, but its most important duty is to stimulate ripening. And, importantly, when one piece of fruit begins to ripen, the ethylene it emits is a signal to the others that it’s time to turn, too. (Plants whose fruit ripens together attract more animals, which then eat the fruit and poop out the seeds. It’s the circle of life.)
Luckily, we can borrow nature’s trick when we’re stuck with unripe fruit from the grocer. Those green bananas and flavorless pears can be quickly coaxed to delicious maturity with a few spurts of ethylene—from their friends. Let’s look a little closer.
Humans have been using tricks to artificially ripen fruit for millennia. In ancient Egypt, for example, foragers found that figs from the sycamore tree wouldn’t ripen on their own. If a patient fellow climbed the tree and slashed a few figs open, though, the fruit matured and the tree could be harvested just a few days later. Egyptians didn’t know why the trick worked—damaged or distressed plants increase ethylene production to stimulate repair—but they were proud to have figured out such an important agricultural advantage. Visitors from Greece brought back stories about the cut figs, and the technique is depicted in paintings and reliefs around the kingdom. Archeologists have even found dried figs with distinctive cuts in tombs.
Fruit science remained mostly tradition and hand-me-down advice like this through the end of the 19th century. The first real breakthrough came in 1901 when a Russian scientist noticed changes in plant growth after a gas leak, but it was 30 more years before anyone figured out that fruits produced ethylene on their own. Today, a full understanding of the science means that supply chains have been optimized by picking and shipping some fruits unripe, and treating them with carefully calibrated ethylene doses closer to the customer.
If you want to practice ripeness manipulation at home, it helps to first understand that most fruit falls into one of two categories: climacteric and non-climacteric.
Non-climacteric fruit ripen only while attached to the plant. They don’t produce or respond to ethylene once they’ve been picked, so there is nothing to be done if they are harvested too soon. Though these fruits sometimes soften or continue to change color—which can make them more appealing—those changes are signs of the onset of decay, not of maturation. They don’t get sweeter.
Examples of non-climacteric fruits include berries, pineapple, cherries, and grapes. Citruses are also in this category—the edible insides stop developing when the fruit is picked—but their skins do have the ability to keep changing. Ethylene breaks down chlorophyll, changing the color of a fruit from photosynthesis green to brighter, more attractive hues. Lemons and oranges follow this arc, but often still show some spotting even when fully ripe; a dose of ethylene gas can remedy that before sale. On the flip side, limes are almost always harvested under-ripe to preserve their signature verdure. Fully ripe limes are yellow like lemons, and far less tart than what you find in stores.
Climacteric fruits, on the other hand, react happily to ethylene gas—whether their own or not. Fruits like pears, mangos, kiwis, tomatoes and apricots call all be picked early and ripened when needed, but the kings of post-harvest ripening are bananas and apples. These two fruits give off so much gas that they can be used to kick start the ripening process in just about any other climacteric fruit. Or, if you aren’t careful, can push a bowl of perfect fruit over the edge into spoiled territory. The adage that “one bad apple will spoil the bunch” is cliché, but also very true.
The avocado is a climacteric fruit, too, but with a twist. This quirky fellow doesn’t ripen at all on the tree. As the avocado grows, the plant puts all its energy into developing the oil within the fruit, and it’s not until an avocado has been cut from the branch that any ethylene production begins.
What can you do if you’re stuck with a pile of unripe fruit?
Sometimes, proximity is enough. A ripening banana next to an unripe cantaloupe, for example, will help the slowpoke along. Or, if you don’t have access to starter fruit, you can at least make more efficient use of the natural ethylene emissions by closing up an unripe specimen in a paper bag. As the gas is trapped around the fruit, ripening accelerates. Slightly warmer temperatures—like spots in the kitchen near heating vents or above a warm fridge—can speed up the process, too.
Why use a paper bag over an air-tight plastic one? Ripening fruits also expel carbon dioxide, and as the concentration of CO2 goes up, ethylene production goes down. A porous paper bag provides a good compromise between trapping power and loss. Even so, avoid overcrowding the bag, and be sure to open it every day or two to refresh the air if you’re keeping fruit that long. Or, skip the bag all together. Burying fruit in a bowl of rice can have a similar effect, allowing sufficient ethylene build-up while also venting carbon dioxide.
Even more specialized techniques exist if you’re looking—Food52, for example, suggests terrycloth towels as the perfect wrappers for peaches and plums, and Epicurious has a whole host of ideas for bananas, including quick-ripening in the oven at 250 degrees for 15 minutes—but in the end, all techniques are just forms of ethylene control.
Can you play god with your fruit? Yes, and it tastes so good.