What’s up with things that alter our sense of taste? Why does orange juice taste revolting if you drink it too soon after brushing your teeth? Or how about sipping white wine after eating artichokes? For some people, that makes the wine taste weirdly sweet. Even more bizarre are the berries of the so-called miracle fruit, Synsepalum dulcificum, which make even sour lemons taste sweet.
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Some scientists are even trying to exploit such taste oddities to make healthier foods that trick the brain into thinking it’s getting something more delicious than it really is. Linda Bartoshuk, an experimental psychologist at the University of Florida, and her team have been investigating naturally occurring volatile compounds in fruit that make foods taste sweeter or saltier by influencing how the sense of smell and the sense of taste interact in the brain. “Those interactions are much more interesting than we ever dreamed, and they may lead to new ways to add sweet and salty taste to foods without actually adding sugar or salt,” Bartoshuk said.
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Miracle fruit
In tropical West Africa, berries of the shrub Synsepalum dulcificum (photo above) were traditionally used to sweeten palm wine and make stale bread more palatable. More recently they’ve been used in Queens at “flavor tripping parties” where people do vinegar shots and say things like “Doughnut glaze, hot doughnut glaze!” after someone puts Tabasco sauce on their tongue.
The secret to this freaky fruit is a chemical called miraculin. It’s a protein with sugars attached to it. Normally these sugars don’t activate the sweet receptors on the tongue; they are like a key that doesn’t quite fit the lock. That’s why miracle berries on their own don’t taste particularly sweet. But in the presence of vinegar, lemon juice, or other acids, the molecule changes shape in a way that allows it to fit the sweet receptors and trigger the perception of sweetness. (It’s also possible the receptors themselves change shape—scientists haven’t yet worked out all the details). “It adds an intense sweet taste and in turn suppresses the perception of sour in the brain,” Bartoshuk says. The effect can last an hour or two—plenty of time to eat enough strange things to give yourself a serious bellyache.
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Artichokes
Artichokes are notoriously hard to pair with wine. You’d think they’d go well with whites, but for many people they make wine (or other beverages) taste unusually sweet. Whether that’s good or bad depends on your perspective.
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In a 1972 paper in Science, Bartoshuk identified the compound responsible for this effect. It’s called cynarin, and unlike miraculin, it doesn’t require acid to work its magic. It makes even water taste sweet. Again, the cellular-molecular details are a bit murky, but Bartoshuk says what seems to happen is that cynarin temporarily inhibits sweet receptors. Then, when you wash off the cynarin by taking a drink, the receptors bounce back and fire, sending a signal to the brain that they’ve detected something sweet.
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Orange juice and toothpaste
Scrubbing your teeth before breakfast is a bad idea for lots of reasons, but if your breakfast includes orange juice it’s even worse. The normally delicious sweet-tart juice will taste bitter and awful.
The chief culprit is probably sodium lauryl sulfate, a detergent added to many toothpastes to increase foaming and make your mouth feel clean. “The detergent tends to reduce your ability to taste sweet, and whenever you encounter any type of acid, there’s a bitter taste that’s very unpleasant,” Bartoshuk said. The most likely explanation at the molecular level is that the detergent alters the responsiveness of taste receptors by disrupting the fatty membranes that enclose each cell (much as dish detergent would break up the oily layer atop a sink filled with dirty dishwater).
Pine nuts
What do you do when one of your favorite ingredients suddenly turns against you? For some people, that’s what happened with the emergence of “pine mouth,” a mysterious syndrome in which eating pine nuts causes an unpleasant bitter or metallic taste that starts a day or two later and can last for a week or more.
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One species in particular, Pinus armandii, a white pine from China, has been linked to “pine mouth.” According to one report, a poor pine nut harvest in 2010 resulted in Chinese imports accounting for up to 80 percent of pine nuts sold in the U.S. that year, coinciding with an uptick in reports of pine mouth.
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The culprit could also be a contaminant or something used to process the nuts, Pelchat adds.
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So far, scientists can only guess at the mechanism, and some of those guesses are kind of creepy. One idea, suggested by Bartoshuk, is that the metallic taste is a phantom caused by nerve damage, essentially a taste equivalent of the phantom limb experience of some amputees. The nerves that carry taste signals to the brain inhibit one another, she explains. Damaging one nerve releases inhibition on the others and can cause phantom taste sensations. Indeed, she notes, a metallic taste is a common side effect of damage to the chorda tympani nerve. This nerve passes through the middle ear on its way from the tongue to the brain, and it can be damaged by surgery or infections of the middle ear. So far, though, no neurotoxic agent has been identified in pine nuts.
Another idea, proposed by Gregory Möller, a Professor of Environmental Chemistry and Toxicology at the University of Idaho, is that the metallic taste arises, at least in part, not in people’s mouths, but in their guts. The small intestines have bitter receptors similar to those on the taste buds, and some pine nuts may contain compounds that either stimulate those receptors directly or by prompting the production of bile,
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We smell things in the air around us by sniffing with the nose. But when we eat, there’s an additional pathway involved. “When you put food in your mouth and chew it up, the aroma from the food is forced up behind your palate and into your nasal cavity from the back,” Bartoshuk explains. “That’s called retronasal olfaction.” She thinks retronasal olfaction has a bigger impact on our perception of taste than the regular old route through the nose.
Her group has been studying this effect in tomatoes and strawberries. They’ve found, for example, that people perceive tomatoes of one variety, Matina, as twice as sweet as another, Yellow Jelly Bean, even though the sugar content of Matinas is actually lower. The reason, Bartoshuk says, is the mix of volatile compounds in each strain. She says her group has identified more than 80 volatile compounds that alter the perception of sweetness and saltiness via retronasal olfaction. Most have only modest effects on their own, but they have much stronger effects when combined. Bartoshuk thinks it should be possible to exploit those effects to create healthy foods that taste better.
Our brains evolved to crave sweet, salty and fatty foods because our ancestors needed to gobble up energy and nutrient-rich foods when they found them, Bartoshuk says. That’s why it’s so hard to change people’s behavior with education alone. ‘If we want people to get healthier, we can either keep trying to educate them, or we can find ways to make food taste the way evolution makes us want it to taste,” she said.
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