Late on the night of October 28, 2005, and early into the next morning, New York City’s 311 service hotline was flooded with calls reporting a strange odor wafting across Manhattan. Large swaths of the island smelled like maple syrup and nobody knew why.
Was it terrorism? A stunt from the Eggo people? A sneak-attack by sentient maple trees?
NYPD, NYFD, and NYC’s emergency management and environmental protection agencies launched an investigation that determined the smell was harmless, but it failed to identify a source. The odor disappeared, and life went on. Until it popped up again one day in March 2006. And again that November. And again a year after that. Then-hit TV show 30 Rock even made a joke about the mysterious smell.
It took almost four years of sporadic aromatic events to finally solve the mystery: The maple syrup smell came from a factory in New Jersey processing fenugreek. Often called by its Hindi name, methi, fenugreek’s seeds and leaves can be found in stews and spice mixes throughout most of the varied regional cuisines of India and Pakistan.
But even an experienced Indian chef has trouble describing exactly what methi tastes like on its own. "My advice is, don’t taste it. It’s bitter!" laughs Pawan Mahendro, owner and chef of Badmaash, which he opened in Los Angeles in 2013 with his two sons. Mahendro grew up in Punjab, in northern India, where methi seeds often go into pickles. "My grandmother would do a lot of pickling, and we would always play with the methi seeds. One time she said, ‘bite this.’ I chewed on some and they were so bitter I never wanted to taste them again!" he says. "That was my first memory of methi."
Since then, Mahendro’s spent 50 years in the restaurant business—in India, Canada, and the US—and changed his mind about methi. At one Toronto restaurant, he was in charge of making house-cured salmon for brunch and experimented with lots of different ingredients. One combination of lemon, dill, and methi was especially popular. "It turned out to be very flavorful," he remembers. "People started asking, ‘What is different about this?’ But nobody could pick out why."
Maple syrup, though? Kinda. "When I roast and powder methi, it has a sweetish kind of flavor but there’s a bitter ending to it," Mahendro says. "Maybe like a strong, dark caramel, yes."
The molecule responsible for both methi and the Manhattan Maple Whodunit is sotolon (sometimes spelled with an "e" at the end—"sotolone"), which is present in fenugreek in large quantities and pops up in all sorts of other unexpected places. "I’ve used it in banana, pumpkin, elderflower, strawberry, and peach flavors," says Kim Juelg, a principal flavorist for Givaudan, the world’s largest maker of flavorings and scents. After 25 years working her way up through the ranks of the company and training in tasting and chemistry, she’s now in charge of formulating savory, sweet, and beverage flavors for brands you’ve definitely heard of, but which she’s not allowed to name. (If you see "natural flavors" or "artificial flavors" on an ingredients list, there’s a good chance Givaudan made them.)
Juelg describes sotolon as a "sweet, brown" molecule that you’d likely find in chemical facsimiles of things like molasses, caramel, and, yes, maple syrup. Compared to other ingredients flavorists have at hand, sotolon is quite expensive, so it’s typically used in conjunction with other, more affordable chemicals. Your pumpkin spice beverages, and granola bars, and candies, and candles, are made with a mix of molecules that may include sotolon, too.
Sotolon is a lactone, which has a very specific definition related to molecular structure that’s way too complicated to talk about here. But for flavor purposes, lactones tend to be oily, and don’t dissolve well in water. That means their scents linger: When Juelg uses sotolon at work, it "sticks" to her skin and clothes much longer than, say, banana-flavored isoamyl acetate, which evaporates and washes away quite readily. "I have left work, gone to the grocery on my way home, and while in line have heard people say, ‘Do you smell pancakes?’" she remembers. "Even after showering, you’re gonna smell it for a few days."
Sotolon’s texture also makes it useful for the elusive component of flavor called mouthfeel. It "tastes" kind of thick, if that makes any sense. "The oiliness of lactones just lays there and sticks on your tongue," Juelg says. "We use them a lot for ‘fleshiness’ in strawberry or coffee. Stuff you’re trying to give a fuller mouthfeel without being sweet." Unlike molecules that evaporate quickly and you taste on the "front" of the palate, you taste these "mid-to-finish," she says.
Sotolon’s "heavy" stickiness and slow evaporation is how it was able to blow across the Hudson, but the reason so many people noticed the maple smell is that it’s especially potent. People can taste it at concentrations of .02 parts per million, which is 2000 times as potent as vanillin (as the name suggests, a major component of vanilla flavor), another "sweet, brown" molecule Juelg works with frequently.
With an important role in both curries and fake caramel, sotolon atomically bridges the divide between sweet and savory. And some of the other natural sources of the chemical do the same. You can find lots of sotolon in candy cap mushrooms, which chefs often turn into ice cream or caramels, as well as in the oxidized minerality of sherry and other barrel-aged wines and spirits, and in the toasty sweetness of cigar tobacco. Homebrewers sometimes add fenugreek to their beer to produce a subtle maple flavor without adding sugar. And there’s even a connection to funny-smelling pee: Sufferers of a rare genetic disorder called maple syrup urine disease can’t process certain amino acids properly, leading, through a series of chemical steps, to sotolon—and its distinctive odor—in their excretions. (The disease is typically diagnosed in infants whose parents smell, well, maple syrup urine, and it can be fatal but it's fairly easily treated by regulating amino acids in the diet.)
So the next time you’re wandering the streets and get an overwhelming whiff of maple syrup (assuming you’re not in Vermont in early spring), look for a spice factory or an Indian restaurant—no need to call the police.
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