How is it possible to buy apples all year round? Veronique Greenwood investigates the chemical alchemy that puts ripening on hold.
Apples are an autumn speciality, their scent rolling off farmers' market carts and inspiring pie bakers and cider makers to delicious heights. But they're available to eat all year as well, of course. There almost isn't a moment, these days, when you can't get an apple: sweet, crisp, and not at all seeming like it's spent months off the tree.
While some countries get their apples from abroad in the off-season – the UK relies on 70% imports – domestic farming still dominates in other major markets. In the US, for example, fewer than one in 10 apples come from overseas, according to a 2016 report by the US Department of Agriculture (USDA). So how do farmers maintain a supply when there’s nothing to harvest? It's thanks to an innovation with some interesting chemistry: a type of storage that stops ripening in its tracks.
After apples are picked, they are still alive – they continue to carry out the chemical processes of a living plant, more or less, as they take in oxygen, create energy, and get closer and closer to ripeness. The chemical reactions that lead to ripening in apples are in large part driven by a hormone called ethylene, which changes the expression of myriad genes in the fruit. Fruit that responds to ethylene – not just apples but bananas, mangoes, papayas, some pears, and others – grows more sensitive to the hormone as ripening gets closer.
The ripening reactions happen more quickly when temperatures are higher, so keeping the fruit cold, in a cellar or even a cave, will buy you some time, says James Mattheis, the research director at the USDA ARS Tree Fruit Research Laboratory in Wenatchee, Washington. And sealing the fruit in a confined space and letting it deplete its oxygen supply will also slow things down, since oxygen is necessary for the reactions to proceed. These strategies have been used in fruit storage for ages.
However, in the mid-20th Century, researchers began to experiment with something a little more radical: directly manipulating the composition of the air in storage rooms. If the air is formulated so that it has very little oxygen, they learned, ethylene will have less of an effect on the fruit, and the natural respiration process slows down. Out in the world, the atmosphere we breathe every day is about 21% oxygen. In a modern controlled atmosphere (CA) storage room, as these facilities are called, it ranges from 3% all the way down to 0.5%.
These nearly oxygen-less rooms, which are also temperature and humidity-controlled, can be gigantic. “Where I live in Washington state, there is capacity for half to three-quarters of the fruit produced here,” Mattheis says. That's storage for two to three billion pounds of apples, all told. “That's a huge amount of physical space,” he continues. “You can have three or four basketball games going on in one of these rooms – full court.”
These suspended animation chambers for apples occasionally make the news when someone thinks they'll be alright if they just hold their breath while on a quick visit inside. In 2013, two workers on the Earl of Selbourne's apple farm on Blackmoor Estate in Hampshire in the UK were found dead in the controlled atmosphere storage room, and in 2015 the manager who sent them in without proper breathing apparatus was convicted of manslaughter. “[Deaths have] happened in most areas where CA has been used,” Mattheis says. “Apples can still be alive after being in there but we can't.”
While removing the oxygen extends apples' lives for months on its own, a molecule called 1-methylcyclopropene has also made its way into storage chambers over the last two decades. This molecule, a synthetic creation patented in 1997, fits neatly in the receptors that ethylene attaches to. It's like those plastic covers for electrical outlets, slotting into the holes and blocking the access of the prongs or, in this case, the hormone. When that happens, it doesn't matter how much ethylene is in the environment, the apples will hold steady at their current stage of ripeness. Together, low oxygen and 1-MCP can keep apples in stasis for more than six months.
So, when the apples are ready to be sold, can farmers re-start ripening by removing them from the facility? Actually, Mattheis says, starting the process again isn't as easy or consistently possible as starting it.
“For the most part,” however, “people would rather have apples that are not ripe,” he says. The apples that come out of storage remain essentially as they were when they came off the tree, and never develop the smells that characterise the tree-ripened fruit. But people buy them all the same – and certainly prefer them to over-ripe, mealy apples where the ethylene response has gone on too long. Something to think about when you pick up a few at the supermarket: the delicate dance of hormones, oxygen, and other substances that brings you an apple nearly every day of the year, although not quite the same apple it would be if it had stayed on the tree.