Meet Isoamyl Acetate, the Banana-Flavored Molecule You Love to Hate

Some of the most fascinating conversations I’ve had with chefs, brewers, and distillers have been thanks to their ability to match individual flavor notes to specific chemicals, which makes sense when you think of the human mouth and nose as a chemical analysis laboratory. That framework has also helped me out my favorite flavor molecule, isoamyl acetate, which smells and tastes like bananas... just not the ones at your local supermarket.

As a kid, I loved banana-flavored stuff. I drank banana Nesquik, picked out the banana Runts and preferred yellow Laffy Taffy. Laugh all you want—I can’t begin to summarize the grief I’ve gotten on this subject—but it was honestly a real advantage at Halloween: All my friends were more than happy to trade my bottom-tier chocolate bars for their top-level neon-yellow candies. Even today, I love a banana Moon Pie or a handful of yellow, brown-spotted banana Jelly Bellys.

Strangely, though, I’ve always been a little "meh" on actual bananas. That’s because today’s bananas aren’t the bananas those candies were made to evoke—bananas that were decimated by an early-1900s fungal plague. Bananas that had isoamyl acetate.

A Star Banana's Fruity Esters

Bananas were first introduced en masse to the US in the mid-1800s, and got a big cultural boost from the Centennial Exhibition of 1876 in Philadelphia, where bananas sold for 10 cents apiece. The most popular variety back then was called Gros Michel (or "Big Mike," which is what that means in French), and when banana candy–makers started developing artificial flavors, that was the model they used. Gros Michel bananas are rounder, squatter, and, crucially, richer in isoamyl acetate than the cultivars on supermarket shelves today.

Isoamyl acetate is an ester, one of a huge group of organic molecules that’s a little difficult to explain in a non-science-y way. "In ‘simple’ terms, it will have a carbon atom somewhere in the center linked to two oxygen atoms, one with a single bond, and one with a double bond," laughs Dr. Corinne Cluis, R&D director for Lallemand Biofuels and Distilled Spirits and an expert in yeast biochemistry. It’s an extremely wide and varied group: Triglycerides, one of the molecules animals use to store energy in fat, are esters. They’re also used in making plastics (like polyester) and explosives.

But the tasty-for-people esters are the ones created by fruits and flowers. Plants evolved to produce isoamyl acetate and its highly scented cousins in order to attract insects and other pollinators. "There’s a very low detection threshold," Cluis says, explaining that humans can pick out the chemical at a concentration of just a handful of parts per billion.

Besides bananas and banana candies, isoamyl acetate is a common note you’ll find in beer (it really jumps out in Coors Light for me), rum (especially the funky Jamaican-style rums often called "high-ester"), and other alcoholic beverages. You’ve probably heard that the yeast that makes both booze and bread eats sugar to produce alcohol and carbon dioxide, but that’s not the whole story. Yeast can also consume amino acids, and when it does, esters are the result. (Isoamyl acetate is produced when yeast breaks down leucine, specifically.) The exact process of fermentation and distillation determines just how much of each ester ends up in the final product, which is why different beers, wines, and spirits offer a wide variety of fruity and floral notes.

"We think that yeast has evolved to produce esters to convert chemicals that are toxic to the cells into nontoxic ones," Cluis says. The fact that the fragrant molecules also attract pollinators is just a side benefit. "Attracting insects allows the yeast to go to another fruit or flower where it can reproduce, and could be a competitive advantage in the wild." (Cluis’s own favorite ester is ethyl butyrate, which smells "a bit like pineapple.")

Downfall of The Gros Michel

So what happened to the Gros Michel banana? A nasty little bug called fusarium oxysporum. A strain of the same fungus that causes fusarium wilt in backyard tomatoes and cucumbers, fusarium was first reported in Gros Michel banana plants in Australia in the late 1800s. From there, it spread throughout the Caribbean and Latin America and, by the 1950s, Gros Michel bananas were virtually wiped out. (In the 1923 novelty song "Yes, We Have No Bananas", fusarium is probably the reason why.)

"Most bananas have very little genetic variability because they are mainly clonally propagated through ‘suckers’ that grow from the bottom of the plant." says Dr. Leena Tripathi, a principal scientist for the International Institute of Tropical Agriculture and expert in banana botany and disease. That means every single plant on a typical commercial banana farm is genetically identical to every other one; if one is infected, the disease will spread like wildfire across them all.

After the Gros Michel disaster, banana growers turned to a different variety, called the Cavendish, which is resistant to fusarium and has a thick skin that makes it easier to ship long distances. It also produces less isoamyl acetate and tastes less like the banana candies of old—bad news for rare birds like me. Today, the export market for bananas is about 90 percent Cavendish, Tripathi says. Bananas are America’s third-best-selling fruit, and if you buy one at the supermarket, you’re almost entirely sure to find Cavendish and only Cavendish.

Preventing The Next Banana Apocalypse

Unfortunately, that didn’t solve the genetic diversity problem, and there’s some bad news. At the end of the last century, a new strain of fusarium emerged, called TR4, and it can infect Cavendish bananas. Despite efforts to contain it, it spread through Southeast Asia and parts of Australia. By 2010, TR4 had popped up in the Middle East and Mozambique, and last year it was found in Colombia, the world’s largest exporter of bananas. "Farmers are really worried," Tripathi says. "In Africa it’s only in Mozambique, but Africa has very porous borders. We don’t know what other types of bananas will be susceptible. Farmers, particularly in Kenya, Tanzania, Uganda, they are worried about this. The countries closer to Colombia should be worried as well."

Tripathi and her colleagues are at work on the problem, though. Her Nairobi, Kenya-based team is using CRISPR and other gene-editing techniques to create resistance to a variety of banana diseases. "Banana is more like a fruit crop in most of the world, but in Africa it’s a staple food like rice," she says. "East Africa, Rwanda, Burundi—these are the places where banana is most important." The continent grows almost a third of the world’s bananas, in hundreds of varieties, but they’re mostly grown by small farmers for local consumption.

Currently, Tripathi is working on a bacterial banana disease called xanthomonas wilt, which has been spreading throughout Africa in the last two decades. "Once it’s in the field, it wipes out the whole plantation within a few months. There are no resistant varieties; it affects all the varieties of bananas grown," Tripathi said. "To solve the problem, we transferred resistance genes from a green pepper into the banana. It worked in the lab and greenhouse, and it’s in confined-field trials now."

Resurrecting a Classic

One day, Tripathi hopes to bring back the Gros Michel and return isoamyl acetate to its rightful spot in the banana bin. "I love Gros Michel. Right now, we are working on control of fusarium wilt using gene editing. My dream is to bring it back."

For now, if you want to get a taste of the banana that inspired banana candy in the US, it’s a little difficult, but not impossible. Gros Michels can be found in backyard gardens in warmer parts of the country, including Florida and Southern California, but you generally need a hook-up to find them. If you’re in the right climate, you can even buy your own plant.

In the meantime, there’s still bright-yellow candy packed with isoamyl acetate for you to enjoy.