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Wednesday, 29 July 2015

On this day in Science History: The First Iron Lung was Installed

In 1927, the first iron lung (electric respirator) was installed at Bellevue hospital in New York for the post war polio epidemic. The first iron lung was developed at Harvard University by Phillip Drinker and Louis Agassiz Shaw built with two vacuum cleaners.

The iron lung is a negative pressure machine which surrounds the patient's body except for the head, and alternates a negative atmospheric pressure with the ambient one, resulting in rhythmic expansion of the chest cage (and thus inhalation) in response to the negative extra thoracic pressure. During periods of ambient extrathoracic pressure, the lungs deflate.

Humans, like most animals, breathe by negative pressure breathing: the rib cage expands and the diaphragm contracts, expanding the chest cavity. This causes the pressure in the chest cavity to decrease, and the lungs expand to fill the space. This, in turn, causes the pressure of the air inside the lungs to decrease (it becomes negative, relative to the atmosphere), and air flows into the lungs from the atmosphere: inhalation. When the diaphragm relaxes, the reverse happens and the person exhales. If a person loses part or all of the ability to control the muscles involved, breathing becomes difficult or impossible.

The person using the iron lung is placed into the central chamber, a cylindrical steel drum. A door allowing the head and neck to remain free is then closed, forming a sealed, air-tight compartment enclosing the rest of the person's body. Pumps that control airflow periodically decrease and increase the air pressure within the chamber, and particularly, on the chest. When the pressure is below that within the lungs, the lungs expand and atmospheric pressure pushes air from outside the chamber in via the person's nose and airways to keep the lungs filled; when the pressure goes above that within the lungs, the reverse occurs, and air is expelled. In this manner, the iron lung mimics the physiological action of breathing: by periodically altering intrathoracic pressure, it causes air to flow in and out of the lungs. The iron lung is a form of non-invasive therapy.

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Thursday, 23 July 2015

On this day in Science History: The Last Fragments of Comet Shoemaker-Levy Struck Jupiter

In 1994, the last of the large fragments of the comet Shoemaker-Levy struck Jupiter (Fragment W).

This was a comet that broke apart, colliding with Jupiter and providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its role in reducing space debris in the inner Solar System.

"Shoemaker-Levy 9 on 1994-05-17" by NASA, ESA, and H. Weaver and E. Smith (STScI) - http://hubblesite.org/newscenter/archive/releases/1994/26/image/c/ (direct link). Licensed under Public Domain via Wikimedia Commons 

The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy.  Shoemaker–Levy 9, at the time captured by and orbiting Jupiter, was located on the night of March 24, 1993, in a photograph taken with the 40 cm (16 in) Schmidt telescope at the Palomar Observatory in California. It was the first comet observed to be orbiting a planet, and had probably been captured by the planet around 20 – 30 years earlier. 

Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's Roche limit, and Jupiter's tidal forces had acted to pull apart the comet. The comet was later observed as a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and July 22, 1994, at a speed of approximately 60 km/s (37 mi/s) or 216,000 km/h (134,000 mph). The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.

Observers hoped that the impacts would give them a first glimpse of Jupiter beneath the cloud tops, as lower material was exposed by the comet fragments punching through the upper atmosphere. Spectroscopic studies revealed absorption lines in the Jovian spectrum due to diatomic sulfur (S2) and carbon disulfide (CS2), the first detection of either in Jupiter, and only the second detection of S2 in any astronomical object. Other molecules detected included ammonia (NH3) and hydrogen sulfide (H2S). The amount of sulfur implied by the quantities of these compounds was much greater than the amount that would be expected in a small cometary nucleus, showing that material from within Jupiter was being revealed. Oxygen-bearing molecules such as sulfur dioxide were not detected, to the surprise of astronomers.

As well as these molecules, emission from heavy atoms such as iron, magnesium and silicon was detected, with abundances consistent with what would be found in a cometary nucleus. While substantial water was detected spectroscopically, it was not as much as predicted beforehand, meaning that either the water layer thought to exist below the clouds was thinner than predicted, or that the cometary fragments did not penetrate deeply enough. The relatively low levels of water were later confirmed by Galileo's atmospheric probe, which explored Jupiter's atmosphere directly.

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Wednesday, 15 July 2015

Leaving on a biofueled jet plane

The problem is simple to understand. Molecules of carbon and other greenhouse gases absorb heat. The more greenhouse gases emitted into the atmosphere, the warmer the atmosphere becomes, exacerbating global climate change. Solving the problem is not so simple, especially with regards to aviation -- the source of two-percent of the annual greenhouse gas emissions from human activity. While biofuels have proven to be an effective, renewable, low-carbon alternative to gasoline and diesel, jet fuels pose unique challenges. These challenges have now been met with a new technique developed by researchers at the Energy Biosciences Institute (EBI), a partnership led by the University of California (UC) Berkeley that includes Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of Illinois at Urbana-Champaign, and the BP energy company.

"We've combined chemical catalysis with life-cycle greenhouse gas modeling to create a new process for producing bio-based aviation fuel as well as automotive lubricant base oils," says Alexis Bell, a chemical engineer with joint appointments at Berkeley Lab and UC Berkeley. "The recyclable catalysts we developed are capable of converting sugarcane biomass into a new class of aviation fuel and lubricants with superior cold-flow properties, density and viscosity that could achieve net life-cycle greenhouse gas savings of up to 80-percent." These challenges have now been met with a new technique developed by researchers at the Energy Biosciences Institute (EBI), a partnership led by the University of California (UC) Berkeley that includes Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of Illinois at Urbana-Champaign, and the BP energy company.

Alex Bell, a leading authority on catalysis in biofuels. Credit:Image courtesy of DOE/Lawrence Berkeley National Laboratory
Bell is one of three corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled "Novel pathways for fuels and lubricants from biomass optimized using life-cycle greenhouse gas assessment." Corinne Scown, a research scientist with Berkeley Lab's Energy Analysis and Environmental Impacts Division, and Dean Toste, a chemist with joint appointments at Berkeley Lab and UC Berkeley, are the other two corresponding authors. Additional authors are are Madhesan Balakrishnan, Eric Sacia, Sanil Sreekumar, Gorkem Gunbas and Amit Gokhale.

The concentrations of carbon and other greenhouse gases in Earth's atmosphere are now at their highest levels in the past three million years, primarily as a result of the burning of petroleum and other fossil fuels. Biofuels synthesized from the sugars in plant biomass help mitigate climate change. However, jet fuels have stringent requirements that must be met.

"Jet fuels must be oxygen-free, have the right boiling point distribution and lubricity, and a very low pour point, meaning the fuel can't become gelatinous in the cold temperatures of the stratosphere," Bell says. "Biofuel solutions, such as farnesane, mixed directly with petroleum jet fuel have been tested, but offer only modest greenhouse gas reduction benefits. Ours is the first process to generate true drop-in aviation biofuels."

Scown cites the Intergovernmental Panel on Climate Change (IPCC) on the importance of drop-in aviation biofuels.

"In a 2014 report, the IPCC pointed out that drop-in biofuels are the only viable alternative to conventional jet fuels," she says. "If we want to reduce our dependence on petroleum, air travel is going to require renewable liquid fuels because batteries and fuel cells simply aren't practical."

The process developed at EBI can be used to selectively upgrade alkyl methyl ketones derived from sugarcane biomass into trimer condensates with better than 95-percent yields. These condensates are then hydro-deoxygenated into a new class of cycloalkane compounds that contain a cyclohexane ring and a quaternary carbon atom. These cycloalkane compounds can be tailored for the production of either jet fuel, or automotive lubricant base oils. Lubricant base oils can produce even more greenhouse gas emissions on a per-mass basis than petroleum-derived fuels if even a fraction of the lubricant is repurposed as fuel. The ability of the EBI process to yield jet fuel or lubricants should be a significant advantage for biorefineries.

"Sugarcane biorefineries today produce ethanol, sugar and electricity," says PNAS paper co-author Gokhale, a chemical engineer, who is managing the research project from BP's side. "Expanding the product slate to include aviation fuels and lubricant base oils could allow for operators to manage their market risks better, which is exactly how petrochemical refinery complexes operate today. Rather than optimize for one product, they try to optimize the overall product slate."

Adds Scown, "Another important advantage offered by our process is that it enables refineries to convert a portion of the bagasse, the fibrous residue that remains after juice is extracted from sugarcane stalk, into fuels and other products. The rest of the waste biomass can be combusted to produce process heat and electricity to operate the refinery." This new EBI process for making jet fuel and lubricants could also be used to make diesel and additives for gasoline as Gokhale explains.

"With some minimal modifications to both the catalysts and the reaction schemes we can produce drop-in diesel as well," he says. "We're planning further studies on this."

Although the goal of this study was to develop a strategy for the flexible production of jet fuels and lubricant base oils in a Brazilian sugarcane refinery, the strategy behind the process could also be applied to biomass from other non-food plants and agricultural waste that are fermented by genetically engineered microbes.

"Although there are some additional technical challenges associated with using sugars derived entirely from biomass feedstocks like Miscanthus and switchgrass, there is no fundamental reason why we could not produce similar outputs, albeit in different proportions," Scown says. "We expect that further research will make this option increasingly attractive."

In their PNAS paper the authors acknowledge that the commercial implementation of their proposed process would include financial implications that extend beyond greenhouse gas emission reductions but hold that there still important incentives to encourage investments.

"We've shown in this study that biorefineries can use inexpensive catalysts to produce a suite of hydrocarbon fuels and lubricants," Scown says. "By strategically piecing together biological and thermochemical processes, biorefineries can also operate without any fossil-derived inputs."

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Tuesday, 7 July 2015

The Event that Transformed Earth

Up until 2.4 billion years ago, there was no oxygen in the air. It took something big to change that – perhaps the biggest evolutionary leap of all.

If you could build a time machine and go back to Earth's distant past, you'd get a nasty surprise. You wouldn't be able to breathe the air. Unless you had some breathing apparatus, you would asphyxiate within minutes.

The Great Oxidisation Event (Credit: APIX / Alamy)
For the first half of our planet's history, there was no oxygen in the atmosphere. This life-giving gas only started to appear about 2.4 billion years ago.

This "Great Oxidation Event" was one of the most important things to ever happen on this planet. 

Without it, there could never have been any animals that breathe oxygen: no insects, no fish, and certainly no humans.

For decades, scientists have worked to understand how and why the first oxygen was pumped into the air. They have long suspected that life itself was responsible for creating the air that we breathe.

But not just any life. If the latest findings are to be believed, life itself was undergoing a tremendous transformation just before the Great Oxidation Event. This evolutionary leap forward may be the key to understanding what happened.

Earth was already 2 billion years old at the time of the Great Oxidation Event, having formed 4.5 billion years ago. It was inhabited, but only by single-celled organisms.

It's not clear exactly when life began, but the oldest known fossils of these microorganisms date back 3.5 billion years, so it must have been before that. That means life had been around for at least a billion years before the Great Oxidation Event.

Those simple life-forms are the prime suspects for the Great Oxidation Event. One group in particular stands out: cyanobacteria. Today, these microscopic organisms sometimes form bright blue-green layers on ponds and oceans.

Their ancestors invented a trick that has since spread like wildlife. They evolved a way to take energy from sunlight, and use it to make sugars out of water and carbon dioxide.

This is called photosynthesis, and today it's how all green plants get their food. That tree down your street is pretty much using the same chemical process that the first cyanobacteria used billions of years ago.

It was the cyanobacteria, pumping out unwanted oxygen, that transformed Earth's atmosphere

From the bacteria's point of view, photosynthesis has one irritating downside. It produces oxygen as a waste product. Oxygen is of no use to them, so they release it into the air.

So there's a simple explanation for the Great Oxidation Event. It was the cyanobacteria, pumping out unwanted oxygen, that transformed Earth's atmosphere.

But while this explains how it happened, it doesn't explain why, and it certainly doesn't explain when it happened.

The problem is that cyanobacteria seem to have been around long before the Great Oxidation Event. 

"They're probably among the first organisms we have on this planet," says Bettina Schirrmeister of the University of Bristol in the UK.

We can be confident that there were cyanobacteria by 2.9 billion years ago, because there is evidence of isolated "oxygen oases" at that time. They might date as far back as 3.5 billion years, but it's hard to tell because the fossil record is so patchy.

That means the cyanobacteria were busy pumping out oxygen for at least half a billion years before oxygen started appearing in the air. That doesn't make a lot of sense.

One explanation is that there were a lot of chemicals around – perhaps volcanic gases – that reacted with the oxygen, effectively "mopping it up".

But there's another possibility, says Schirrmeister. Maybe the cyanobacteria changed. "Some evolutionary innovation in cyanobacteria helped them to become more successful and more important," she says.

Some modern cyanobacteria have done something that, by bacterial standards, is remarkable. While the vast majority of bacteria are single cells, they are multicellular.

The individual cyanobacterial cells have joined up into stringy filaments, like the carriages of a train. 

That in itself is unusual for bacteria, but some have gone further.

"Many cyanobacteria are able to produce specialised cells that lose their ability to divide," says Schirrmeister. "This is the first form of specialisation we see." It's a simple version of the many specialised cells that animals have, such as muscle, nerve and blood cells.

Schirrmeister thinks multicellularity could have been a game-changer for Earth's early cyanobacteria. It offers several possible advantages.

On the early Earth, single-celled organisms often lived together in flat layers of gunk called "mats". 

Within each mat there would have been many different species of cyanobacteria, and a host of other things to boot.

A multicellular cyanobacterium would have one clear advantage compared to its single-celled rivals. 

It would find it easier to spread, because its larger surface area would mean it was better at attaching itself to slippery rocks. Such an organism would be "less likely to wash away in the current", says Schirrmeister.

Many modern multicellular cyanobacteria can move around within their mats. "They're not extremely fast but they can move," says Schirrmeister. That suggests the primordial ones could as well.

Moving could have helped them survive. At the time the Earth was being bombarded with harmful ultraviolet radiation from the Sun, and there was no ozone layer to keep it out.

"In modern mats, cyanobacteria will turn around and appear vertical instead of horizontal to protect themselves from excess sunlight," says Schirrmeister. "You have also movement between layers. It might be these multicellular cyanobacteria had the ability to position themselves optimally within the mat."

It's a neat idea. But for it to be true, cyanobacteria must have evolved multicellularity before the Great Oxidation Event.

Schirrmeister has spent the last few years trying to figure out when cyanobacteria first evolved multicellularity.

The clues lie in their genes. By examining genes that all cyanobacteria share, and identifying tiny differences between them, Schirrmeister could figure out how they are all related – essentially drawing up a family tree of cyanobacteria.

With that tree in place, Schirrmeister could then home in on the multicellular cyanobacteria, and estimate roughly when they first became multicellular.

Her first attempt, published in 2011, suggested that most modern cyanobacteria are descended from multicellular ancestors. That suggested multicellularity was ancient, but it was difficult to put a firm date on it.

Schirrmeister refined her methods for a second paper, published in 2013. This suggested that multicellularity evolved not long before the Great Oxidation Event, at a time when cyanobacteria were diversifying rapidly.

But that didn't clinch the argument. Her family tree was only based on one gene, albeit a gene shared by every single species of cyanobacterium. That meant the tree was suspect.

So Schirrmeister has now gone one better.

"This time I worked with 756 genes," says Schirrmeister. "The genes I took are present in all cyanobacteria."

Her estimate of the origin of multicellularity is still rough, but it seems to be around 2.5 billion years ago – before the Great Oxidation Event.

There are several different ways to calculate these family trees, and they all gave the same answer. "No matter how we calibrate our phylogeny, it seems more likely we have multicellularity evolving before the Great Oxidation Event," says Schirrmeister.

The results are published in Palaeontology.

This may not be the end of the story. Even if Schirrmeister's results are confirmed, and cyanobacteria did become multicellular just before the Great Oxidation Event, there are two big questions.

The first is, did multicellularity really offer them the advantages she thinks it did? We don't know, but we could find out: by testing how modern single-celled and multicellular cyanobacteria cope with different situations.

The second question is harder: why did it take so long for cyanobacteria to become multicellular? If it is so advantageous, why did they not evolve it sooner, and trigger an earlier Great Oxidation Event?

"The next step is to find out which genes are responsible for multicellularity in cyanobacteria," says Schirrmeister. "Then I could say why did it take that long, why didn't it evolve earlier." If lots of new genes were required, it becomes understandable that it took the cyanobacteria a long time to evolve it.

Whatever caused the Great Oxidation Event, it's clear that it is one of the most important things to ever happen on this planet.

In the short term, it was probably rather bad news for life.

"Oxygen would have been lethal for many bacteria," says Schirrmeister. "It's hard to prove, because from the fossil record we don't have a lot of deposits from that time… [but] we can assume we had a lot of bacteria dying at that point."

But in the longer term, it allowed a whole new kind of life to evolve. Oxygen is a reactive gas – that's why it starts fires – so when some organisms figured out how to harness it, they suddenly had access to a major new source of energy.

By breathing oxygen, organisms could become much more active, and much larger. Moving beyond the simple multicellularity developed by cyanobacteria, some organisms became far more intricate. 

They became plants and animals, from sponges and worms to fish and, ultimately, humans.

If Schirrmeister is right, those first multicellular cyanobacteria triggered the evolution of complex life, including us, by producing oxygen on a global scale. "It made complex life possible," she says.
Not bad for a bunch of tiny blue-green bacteria.

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Tuesday, 30 June 2015

Search for deadly asteroids must be accelerated to protect Earth, say experts

The search for deadly asteroids that could slam into Earth must be speeded up 100-fold to help protect the future of life on Earth, according to an influential group of scientists, astronauts and rock stars.

The call for action comes as experts around the world take part in Asteroid Day, an event on Tuesday marked by a series of talks and debates aimed at raising awareness of the existential threat posed by hurtling rocks from the heavens.

Lord Rees, the astronomer royal, and Brian May, from the rock group Queen, added their names to the 100X declaration, which calls for a rapid acceleration in human efforts to find and track potentially dangerous asteroids. Other signatories including Peter Gabriel, Richard Dawkins, Brian Cox and Eileen Collins, the first female commander of Nasa’s space shuttle.

“The aim is to ramp up public awareness and the awareness of governments to the fact that we are under threat from a meteor strike,” May told the Guardian. “It’s been made light of, and we’ve seen some great films, like Bruce Willis saving the day, but it is a very serious threat.”

Asteroid Day falls on the anniversary of an asteroid strike in 1908 that saw a 40 metre-wide lump of space rock enter the atmosphere over Tunguska in Siberia at about 33,500 miles per hour. The rock exploded mid-air and released the energy of a large hydrogen bomb, which flattened 2000 sq km of conifer forest.

Were an asteroid of the same size to slam into the atmosphere over London, the blast could destroy much of the capital within the M25. People in cities as far away as Oxford could be burned by the intense heat released in the explosion. In Scotland, the same blast would still have the force to blow peoples’ hats off.

From observations with ground-based telescopes, researchers know that of the million or so asteroids that could one day strike Earth, only about 10,000 are known and tracked. That means we are in the dark about 99% of the asteroids that have the potential to crash into the planet.

“They are clearly a threat and for the first time it is possible for us to do something to reduce that threat,” Lord Rees told the Guardian.

“It is now feasible to do a survey of all the potentially Earth-crossing asteroids above 50m in diameter, and objects like that impact Earth about once per century. One could then check their orbits to see if any are on a collision course with Earth and within 20-30 years have technology to divert any that are on course,” he added.

Huge asteroids several kilometres across are expected to hit Earth every ten million years or so. These can cause destruction on a global scale. A ten kilometre-wide space rock that crashed into what is now Mexico triggered a global catastrophe 68 million years ago which brought the reign of the dinosaurs to an end.

Since most of the Earth’s surface is covered by water, asteroids are more likely to arrive over the oceans. But these can be the worst impact sites for asteroids of about 300 metres wide. If one landed in the mid-Atlantic, it would produce a tsunami wave that could devastate cities on the east coast of the US, and along the coast of Europe.

“We know the rough numbers, we just don’t know when a particular asteroid is going to hit. If we are going to take precautions, we need to know the orbits of all of these bodies,” Rees said.

“The first thing is to do the survey to find out if there are any asteroids which seem to be on course with a high probability of hitting within the next 50 years. If we knew there was one on course to hit the Earth in next 50 years, that would focus minds on the technology.”

One mission, proposed by Nasa, aims to catalogue two thirds of the asteroids and other “near earth objects” that are larger than 140m and come close to Earth’s orbit. The NEOCam mission would use an infra-red camera to garner information on asteroid size, shape, rotation and composition. A private mission called Sentinel, which would put an another infra-red telescope in space, is being led by Ed Lu, a former space shuttle astronaut.

Scientists are actively looking at ways to protect Earth from any asteroids that do turn out to be on a collision course. One strategy is to crash a massive spacecraft into the asteroid and change its trajectory. Another option is a “gravity tractor”. In this scenario, a spacecraft flies alongside an inbound asteroid for long enough that its minuscule gravitational tug diverts the asteroid enough to pass Earth safely. Both could run into problems in a real situation, though: if the nudge does not work as expected, the asteroid may miss one city only to hit another.

The option to lob nuclear warheads at an incoming asteroid is appealing to Hollywood, but less so to many scientists, including May, who has a PhD in astrophysics.

“Blowing it up is probably not the greatest option, because you have a lot of fragments to deal with then, and it becomes rather random, but deflecting it one way or another seems to be an option,” he said.

“It’s absolutely possible there’s something out there of the magnitude that would wipe out a major city of the world, and that’s a very big thing: you’re talking about a human disaster on a vast scale.

“This is about saving us all. All the people on the planet, all the creatures on the planet, everything which we have built up and might be proud of. It’s a kind of insurance if you like,” he said.

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Tuesday, 23 June 2015

Stain Removal - How Does it Work?

We’ve all struggled to get stains out of clothes – but do you understand the science behind this? Most stains are removed by dissolving them with a solvent. But which one do you use? Two factors should help you to decide this:

  • The agent that is causing the stain
  • The material that has been stained

Different solvents will dissolve different stains, however some solvents not only dissolve the stain, but also dissolve the material that is stained as well – something that you don’t want to happen! 

Click to enlarge

Stains can be roughly grouped into a few categories:

Enzymatic stains, such as blood, human sweat and grass stains, are mainly made up of proteins and can therefore be combatted by enzymes in stain remover formulations, such as proteases, lipases and amylases.

Oxidisable stains, like tea, coffee and red wine, which can be broken down by bleaching agents, like hydrogen peroxide.

Greasy stains, which can be attacked by lipase enzymes and surfactants. Compound Chemicals describes these as most commonly being "‘long carbon chain compounds with a charged water-soluble ‘head’ and an oil-soluble ‘tail’ (which) remove oil and grease by forming structures called ‘micelles’ around them.”

Particulate stains, such as soil stains, can be removed by ‘builders’ compounds, which remove positive metal ions from the water and help soften it, in turn removing calcium ions which often bind stains to fabrics.

So, next time you regret that wine spillage or try to take that grass stain out of a football shirt, you’ll know what’s going on behind that brightly coloured stain remover – the science of stains! 

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Tuesday, 16 June 2015

The Real Reason Sweet Tastes Sweet

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.

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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