The real reason sweet tastes sweet
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(Credit: Getty Images)
You might think that the sweet taste of fruit is all down to those natural sugars. Think again, says Veronique Greenwood.

We tend to think of sugar as the supreme ruler of the sensation of sweetness. If an orange tastes sweet, it's because of the sugars it contains hit the sweet receptors in your taste buds. The same, it’s fair to say, should ring true for any other fruit, from blueberries to tomatoes.

But Linda Bartoshuk, a University of Florida taste scientist interviewed for this column before, and her colleagues think there is a different explanation. They've found that the chemicals responsible for a large chunk of the perception of sweetness in fruit are ones you smell – not the ones you taste.

Now, this is a different phenomenon than the old trick of plugging your nose while you eat a jelly bean and finding you can't identify its flavour. If you haven't done this, try it – it's a marvellous glimpse into how much of flavour isn't about the tongue. At first all you can taste is sweet, but when you open your nose, the sensation of strawberry or root beer or whatever the specific flavour is washes over you.

I knew that the issue existed, but I didn't think anything hot had been done on it, and I was right - Linda Bartoshuk

In the case of Bartoshuk and company's recent work, however, it isn't the complex overtones of flavour they are talking about. This is more fundamental. It's the sweetness itself.

Bartoshuk says that the idea that volatile compounds emanating from fruit could be linked to sweetness was being discussed in the 1970s. But the effects of individual volatiles were very small, and the amounts of each chemical in the fruit were small as well. “I knew that the issue existed, but I didn't think anything hot had been done on it, and I was right,” Bartoshuk says. A few years ago, however, while she and colleagues were working on a study attempting to dissect exactly which molecules are responsible for what you experience while eating a tomato, she found something surprising.

(Credit: Science Photo Library)

(Credit: Science Photo Library)

The team had analysed the make-up of 152 heirloom varieties of tomato, recording the levels of glucose, fructose, fruit acids, and 28 volatiles. At the same time, over the course of three years, they organised 13 panels of taste-testers to sample more than 66 of these varieties, rating each according to how much they liked it, its sweetness, its sourness, and other taste characteristics.

Bartoshuk still remembers the moment when she was sitting in her office with this mountain of data one afternoon and ran a test, out of curiosity, to see which compounds contributed most to sweetness. She was expecting the answer to be sugar, and it certainly was key, but “I about fell out of my chair,” she says. Also significantly contributing were seven volatiles.

Sweet mystery

Moreover, the volatiles seemed to account for why panellists had reported some tomato varieties to taste sweeter than others that had far more sugar. The team tested a variety called Yellow Jelly Bean, for instance, and another called Matina. The Yellow Jelly Bean has 4.5g of glucose and fructose in 100 millilitres of fruit and rated about a 13 on a scale used for perceived sweetness. The Matina has just under 4g but rated a whopping 25. The major biochemical difference between the two was that the Matina had at least twice as much of each of the seven volatiles as the Yellow Jelly Bean did. When the team isolated those volatiles from a tomato and added them to sugar water, its perceived sweetness jumped.

They've also investigated blueberries and strawberries, among other fruits. Strawberries have much less sugar than blueberries but are consistently rated much sweeter. Bartoshuk and colleagues suggest that this is because strawberries have so many more volatiles – something like 30 – than blueberries, which have “maybe three”, Bartoshuk estimates. They found that adding strawberry volatiles to sugar water boosted perceived sweetness even more than the tomato volatiles did, and adding volatiles from both together doubled it.

(Credit: Getty Images)

(Credit: Getty Images)

And it wasn't that an aroma of strawberries, or cherry tomatoes, was wafting up off the water. The volatiles weren't concentrated enough to float up and hit the nose. (Which is a good thing – one of the volatiles in tomatoes is isovaleric acid, which, on its own, smells like stinky cheese.) The more sugar there is, the less the volatiles contribute to sweetness. But the effect gets stronger, somehow, when greater numbers of volatiles are involved: even volatiles that aren't present in large amounts still seem to contribute to the sensation.

Could you make fresh lemonade with less sugar if you tossed in a cocktail of volatiles?

What is going here? Researchers are still investigating how and why the brain is blending this information. It's known that the signals coming from smell receptors activated by volatiles from the back of the mouth are shunted to the same part of the brain that handles taste, rather than being bundled with signals from the nose itself. Bartoshuk says. Though she is not a neuroscientist herself, she suggests that “in the brain, when you have volatiles affecting some of the same cells as taste, it integrates the message. And part of the integrating, for certain volatiles and certain tastes, is enhancement”. 

While researchers continue to investigate the causes of this strange effect, we can daydream about the possibilities. Could you make fresh lemonade with less sugar if you tossed in a cocktail of volatiles? Possibly, Bartoshuk says, if you added many of them. She is also curious about the idea of breeding a fruit that's as sweet as it can possibly be. Could plant breeders analyse volatiles and select for strains that maximise this volatile effect? Bartoshuk thinks so.

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