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.
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.
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.
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.
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| How sweet can a tomato be? |
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.
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.
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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