Why Can't Elephants Jump?: And 101 Other Tantalising Science Questions - New Scientist (2010)

Chapter 1. Food and drink


 Super food

Is there a single foodstuff that could provide all the nutrients that a human needs to stay reasonably healthy indefinitely?

Andy Taplin

Cambridge, UK

Any single substance such as water or fat? No. Any single tissue such as muscle or potato? No. But if we allow free drinking water and air to breathe – even though those are also nutrients – then we can relax our rules. Even so, drinking milk while eating corn would count as two foodstuffs, and who knows how many foodstuffs pizza contains.

Not surprisingly, no strict monodiet can rival any healthily balanced diet, but there are two classes of foodstuffs that in appropriate quantities can maintain a reasonable level of health. One such class is baby food. Examples include eggs, milk, certain seeds, and so on. None is a perfect option, but some are adequate.

Alternatively one might cheat by counting essentially whole animals: oysters or fish such as whitebait or sardines might supply the necessary nutrient uptake. Animals sufficiently closely related to humans might also do, if eaten in the correct form and quantity. Farming families in the semidesert Karoo region of South Africa apparently ate mainly sheep or cattle.

For the most perfectly balanced human monodiet, however, other humans would be the logical food of choice. Not sure there would be many takers though.

Jon Richfield

Somerset West, South Africa

Despite various claims made over the years for spinach, baked beans or bananas, the answer is ‘no’. To remain healthy over the course of a natural lifetime, a human needs a balanced diet, with a combination of carbohydrates and protein and a proper range of vitamins. The balance may vary with age, from individual to individual, and even between societies in differing environments, but balance is the key to healthy eating.

Probably the most homogenous diet of any human community is that of the Inuit in Arctic North America, which traditionally consists of 90 per cent meat and fish and effectively no carbohydrates. The explorer Vilhjalmur Stefansson not only established that Inuit hunters often lived for between six and nine months of the year on a wholly carnivorous diet, but also claimed to have sustained himself by eating just meat and fish on his expeditions.

In a series of controlled experiments under the auspices of The Journal of the American Medical Association, Stefansson and a number of his colleagues reproduced the dietary regime they had followed in the Arctic, without any apparent ill effects and without, much to the supervising doctors’ surprise, developing scurvy. However, subsequent long-term studies of health factors in the Inuit community have established a strong correlation between the carnivorous diet and early deaths among Inuit men from heart attacks and other cardiac problems.

The bottom line is to remember what your mother told you: always eat your greens.

Hadrian Jeffs

Norwich, Norfolk, UK

Humans have been suggested as the ideal diet for other humans, but eating human flesh would not satisfy all our dietary needs, no matter how healthy the diner or victim, for a very simple reason: not every nutrient, mineral and vitamin in the body remains available to the next step in the food chain. Some substances are ‘used up’ and others are built into indigestible tissues and structures.

Cooking can increase the digestibility of many foodstuffs for a human, but things like hair, bones and teeth cannot be prepared to make them amenable to our digestion. However, we need the minerals, amino acids and other nutrients in them to make these substances for ourselves.

The human body is a tenacious machine and will continue to survive on a very poor diet for quite a while. Many people living in harsh climates have their own dietary supplements in local foods and ‘delicacies’ that they may not even realise are making up a shortfall. The same is true of those who live in a self-imposed harsh regime, such as true vegetarians. One can live on such diets and remain healthy as long as there is a balanced variety of nutrients, or by taking artificial supplements such as vitamin B12.

Some natural foodstuffs provide a reasonable balance of necessary nutrients, but the only known and proven foodstuffs that truly provide everything that a human body would need, in a single wrapper, are manufactured. Survival rations and trail foods are many and varied, but remain unpopular because they are generally dry and unpalatable.

Foods made to cover all the needs of large and physiologically similar mammals, such as dogs, pigs and other omnivores, could probably sustain us very well too, although we may have to eat more of the stuff than we’d like to get sufficient supplies of any human-specific nutrients.

Nat Guthrie

By email, no address supplied


 Stirring stuff

What is the significance of James Bond’s famous phrase ‘shaken, not stirred’? Is there really a difference in the taste of a shaken vodka martini, as opposed to a stirred one? And if so, why?

Mark Langford

Stockport, Cheshire, UK

The dispute over the difference between a shaken or stirred martini has run for some years in the Last Word. An earlier book in this series, How to Fossilise Your Hamster, drew on the following three answers to explain the differences. However, more information has come to light – Ed.

Supposedly, when a martini is shaken, not stirred, it ‘bruises’ the spirit in the martini. To seasoned martini drinkers this changes the taste.

Padraic O’Neile

Newcastle, New South Wales, Australia

Because a martini is to be drunk within seconds of preparation rather than minutes, there is a difference. The tiny bubbles caused by the shaking mean that a well-shaken martini is cloudy. This will also have an effect on the texture of the drink – it is less oily than the stirred version – hence the taste. The long-standing assumption that the spirit is bruised by the process is nonsense because vodka does not have a vascular system.

Peter Brooks

Bristol, UK

James Bond may have appreciated the softening and ripening effect of partial oxidation of the aldehydes in vermouth – akin to letting red wine breathe before drinking. In a refined and homogeneous substrate such as vodka martini, a good shake can speed the process.

Alan Calverd

Bishops Stortford, Hertfordshire, UK

We have since learned, however, that other chemical reactions may be taking place – Ed.

Biochemists at the University of Western Ontario in Canada have suggested that the change in flavour is not caused by the oxidation of aldehydes, but because shaking martinis can break down hydrogen peroxide present in the drink. Stirred martinis have double the amount of hydrogen peroxide that shaken ones contain. This significantly affects the flavour.

Peter McNally

Vancouver, British Columbia, Canada

The reason that shaken martinis are cloudy is not so much down to bubble formation from the cocktail-shaking process but rather that the crushed ice used in the shaker deposits tiny crystals into the poured drink. These make the drink more cloudy and then slowly melt, allowing it to clear.

Frank Melly

New York City, US

This clearly called for more research. Was it bubbles or ice that caused the cloudiness in a shaken martini? And could either account for any difference in taste? First up we needed a good martini recipe. This one came from cocktail mixologist Eric Keitt, who works the bar at Oceanaire in Washington DC:

Double vodka and a few drops of dry vermouth

Pour into a cocktail shaker with crushed ice

Shake until the hand holding the shaker is very cold

Strain into a martini glass

Add an olive or a twist of lime zest

Eric tells us: ‘The application of vermouth should be a few – and I mean a few – drops, maybe two or three. Vermouth will release the aromatics of the vodka, making for a more enjoyable drink.’ Eric is a fan of stirring the drink for the reasons given below, but in this instance we had no choice but to shake.

Three martinis were prepared. The first was shaken with crushed ice. The drink was very cloudy and took a long time to clear but, as far as we could ascertain, the cloudiness was caused only by tiny bubbles from the shaking plus the condensation on the chilled martini glass. No ice crystals were present unless they were microscopic.

The second was a room-temperature martini, shaken without ice. Bubbles formed in this when it was poured but quickly dissipated, much faster than in the iced martini.

The third martini was made in an attempt to replicate the chilled conditions of the iced martini but without adding ice to the shaker. The martini and its shaker were wrapped in a drinks chiller until ice cold and the same temperature as the first martini. Then it was shaken. When poured it was cloudy for much longer than the room-temperature martini but not for as long as the iced martini.

From this we ascertained that the ice does have some effect on the clouding process, as do cold conditions. Iced martinis do produce the cloudiest drink, but no ice spicules appeared present in the drink, contrary to the suggestion above by Frank Melly. Chilled martinis without ice produced a cloudy drink too, but for a shorter time than iced martinis. The room-temperature martini cleared the fastest. So nothing conclusive here yet, except that temperature plays a role of some kind – more experimentation is needed. Is there any reader out there who can examine the shaken martini microscopically to rule out (or, indeed, confirmthe presence of ice crystals?

But there’s more. Putting cloudiness to one side for the moment, Anna Collins seems to have answered the original question of what makes a shaken martini taste different to a stirred one, her suggestion apparently being confirmed in a blind tasting – Ed.

The reason Bond orders his martinis shaken is that the ice helps to dissipate any residual oil left over in the manufacture of vodka from potatoes – the base vegetable for many vodkas at the time Ian Fleming’s original novels were written. With the rise of higher-quality grain vodkas the process is now unnecessary. In fact, shaking the martini with ice dilutes it too much for many fans of the drink. Stirring chills the martini without losing its essential strength.

Anna Collins

Washington DC, US

Anna Collins is correct, according to our blind trial. We bought two bottles of vodka, one grain based, the other potato based. First we tasted the vodkas. In the blind trial all six people in our sample said the potato vodka was oily, the grain vodka wasn’t. Then we made two vodka martinis using the potato vodka. One was stirred with ice, the other shaken with ice. The difference was quite distinct and in a blind tasting every one of the six drinkers correctly identified the shaken martini as being much less oily. But the martini had to be consumed quickly. If left to settle for about 5 minutes or so, the shaken martini became oily again.

Peter Simmons

London, UK

Maybe that’s the last word on vodka martinis. Although knowing our readers’ propensity to keep unearthing new evidence, we suspect not – Ed.


 Stick that!

Having discovered the joys of the ‘appletini’ (vodka mixed with apple juice, cider or apple liqueur), I have a question. The garnish is a slice of apple and a Maraschino or glacé cherry on a cocktail stick. If the cherry is at the bottom of the stick it floats in the appletini, but with the apple slice at the bottom it sinks. Why? Surely the buoyancy of the two items combined is an absolute and their orientation should make no difference. I shall leave it to the imagination as to the number of ’tinis consumed before this anomaly became a burning topic of conversation.

Richard Batho

St Saviour, Jersey

Ethanol acts as a wetting agent, so in an alcoholic drink the submerged slice of apple holds too little air to float the nonbuoyant, sugary cherry. The assembly will thus sink, though if you add soda, enough bubbles might attach to the apple to make the whole thing float again.

Human sloshing complicates insights after the fourth glass of appletini, but buoyancy is a more complicated affair than density considerations might suggest. For example, a boat that is seaworthy might sink if capsized.

Try dropping a clean, dry pin or razor blade gently onto a glass of still, pure water. Dropped endwise the object sinks; the metal is too dense. But surface tension will support the item if you gently drop it flat onto the fluid, especially if the metal is lightly waxed or oiled.

The behaviour of your appletini garnish is similar in some ways. The waxy skin and the broad shape of an unpeeled slice of apple on the surface of the drink can resist both the wetting and the shipping of fluid over the edge of the slice.

The stimulation of considering this question’s complications should mitigate the brain-addling aspects of appletini, though, sadly, not to the extent of fully reversing them.

Antony David

London, UK

Surface tension effects probably account for the observations. If the buoyancy of the cocktail stick assembly is nearly neutral, a flat slice of apple, when uppermost and level with the cocktail surface, may provide a sufficiently long perimeter for surface tension to hold the assembly up. The spherical cherry in the same position has little or no perimeter on which surface tension can act. The coating on the cherry may also reduce surface tension.

Try replacing the apple slice with a small ball of apple and see if the stick now sinks even with the apple uppermost. If this experiment fails, the cocktail may have aged, so drink it and try again with a new one…

Paul Gladwell

Northwich, Cheshire, UK


 Hot to trot

Mustard and chillies are both hot, but the burning sensation from a chilli stays in the mouth for ages while the sensation from hot mustard disappears in a few seconds. Why is this?

Dominic Lopez-Real

By email, no address supplied

The chemical mainly responsible for the burning spice in chilli peppers is capsaicin, a complicated organic compound that binds to receptors in your mouth and throat, producing the desired (or dreaded) sensation.

Capsaicin is an oil, almost completely insoluble in water. This is why you need a fat-containing substance like milk to wash it away – watery saliva doesn’t do the trick.

On the other hand, the compound responsible for the hotness of mustard (as well as horseradish and wasabi) is called allyl isothiocyanate. This chemical is slightly watersoluble, and can be more readily washed away into the stomach by saliva.

Further, the chemical in mustard is more volatile than capsaicin so it evaporates more readily, allowing its fumes to enter the nasal passages (explaining why the burning sensation from mustard is often felt in the nose). These fumes can be easily removed by breathing deeply, a useful strategy if the sensation becomes overwhelming.

Zachary Vernon

Toronto, Canada

The hotness of mustard comes from allyl isothiocyanate, which is formed when myrosinase and sinigrin (in mustard seeds) react together in water. It dissolves well in most organic compounds, and to an extent in water, and is also volatile, so will quickly disperse.

On the other hand, capsaicin, the hot ingredient of chillies, is not very water-soluble. So its heat tends to stay. It is soluble, however, in alcohol, which raises the question: which came first, the lager or the vindaloo?

Matt Billingham



 Fungal goodness

We are constantly being exhorted to eat five servings of fruit or vegetables a day, cut down on red meat, eat more fish and so on. But very little of this advice mentions that other kingdom of gastronomic delights, fungi. What nutritional value does your average edible fungus have?

Rachel Cave

Galway, Ireland

Until recently the village of Bourré in central France, where I live, was a major production centre for mushrooms. Now all we have left is an artisanal operation as a tourist attraction.

However, the two kinds of mushrooms that were the mainstays of the industry are still produced here: Agaricus bisporus, or champignon de Paris, which in English tends to be simply called a ‘mushroom’; and Lentinula edodes, the shiitake mushroom. The former has slightly more than 3 grams of protein per 100 grams, and a range of trace minerals including calcium, iron, magnesium, phosphorus, potassium, zinc, copper and manganese. It also contains vitamin C and several B vitamins. Shiitake mushrooms contain rather more zinc but are lower in protein and vitamin C.

As a vegetarian I find mushrooms invaluable as a way of providing some variation in the texture of my food. They tend to go well with many sauces usually designed for meat. Nutritionally, they compare reasonably well with other foods from non-animal sources. And cooked properly, in olive oil with garlic and thyme, for example, they taste great.

Steve McGiffen

Bourré, Loir-et-Cher, France

Fungi, mostly represented by mushrooms and truffles, are edible, nutritionally rich organisms. Mushrooms are an excellent source of proteins, minerals and dietary fibre, with only small quantities of fats, cholesterol and fatty acids. They are also a source of three essential B vitamins – riboflavin, niacin and pantothenic acid – along with other vitamin groups. All told, mushrooms make an exceptionally good foodstuff for people with diabetes or high cholesterol.

Several Basidiomycetes species have been reported to contain phytochemicals that might be beneficial in the fields of immunology and cardiology, while preliminary results on the effect of lectin from the common edible mushroom, Agaricus bisporus, have shown some potential for treating psoriasis.

Saikat Basu

Lethbridge, Alberta, Canada


 Cool colour

Why is frozen milk yellow?

Mickey Wright

Armley, West Yorkshire, UK

The yellow colour of frozen milk comes from the vitamin riboflavin, which actually got its name from its colour – flavus is the Latin for yellow.

Riboflavin is dissolved in the watery portion of milk, which is also filled with minute particles of protein and droplets of butterfat. In fresh milk, all the suspended particles and droplets scatter any light that strikes them evenly, so that the milk appears opaque and white – milky, in other words.

However, as the milk freezes and most of its water crystallises into ice before other substances, the normally dilute riboflavin becomes concentrated in the remaining liquid water. This means these areas start to turn yellow and, as the clear water-ice crystals form, we are able to see it.

Harold McGee

San Francisco, California, US

Harold McGee is the author of On Food & Cooking: The science and lore of the kitchen (Fireside, 1997) – Ed.


 Down in one

Why is it so much easier to drink a whole pint of beer or orange squash, say, in one go than it is to down a pint of water?

Geoff Lane

Bury, Lancashire, UK

Is it fair to suggest that the questioner’s preference for downing a pint of beer rather than a pint of water is dependent on individual taste?

I have never been able to down a whole pint of beer, much to my dismay – and not for want of trying! But I am able to drink a whole pint of water or more in one go, even if I do feel quite sick afterwards. I much prefer the taste of water to beer and I find it difficult to consume any fizzy liquids in large quantities. I assumed that this was the case for most people, but obviously not.

Jayne Staines

Eastbourne, East Sussex, UK

Is this just hearsay or is it the result of a well-designed experiment? I suspect the former.

Even so, a possible explanation could be that orange squash and beer have strong and pleasant flavours while water is bland, and that there is more motivation in a pub to persist with beer or orange squash than with water.

But surely it depends on context: if I were seriously dehydrated after some time in a desert, say, I would certainly prefer a pint of water to a pint of beer. In a pub, where one is not dehydrated, beer would be my preference.

So we need to establish whether the questioner’s proposition is really true (the clincher would be for non-drinkers to compare beer with another carbonated drink, such as cola) and whether it is true in a variety of contexts. If there is experimental evidence, then we can examine it and see if it was well designed and worth believing. Then we would need to find out why. This is the correct scientific response.

Mike O’Mahony

Professor and sensory scientist

Department of Food Science and Technology

University of California, Davis, US


 Veggie violence

Occasionally, when reheating broccoli and sweet potato in the microwave, what sounds like violent electrical arcing occurs if the two are in contact with each other. This results in blackened sections. What is going on?

Stephen Plevier

Hadfield, Victoria, Australia

One may think of microwaves in the oven as light shining through translucent food, with the light that gets absorbed being turned into heat. This is a good description of what happens when the food is a continuous mass measuring more than about 6 centimetres in all directions.

However, if the food mass is smaller and oddly shaped, the interaction between the wavelength of the radiation (usually around 12.24 centimetres) and the shape becomes dominant. Microwaves then behave more like radio waves, and some slender, spiky or complex-shaped vegetables become electrical conductors. These conductors act like radio aerials along which electric charges surge back and forth, typically at about 2.5 billion times a second. Wherever these ‘aerials’ form small gaps or fine contacts, electrons leap across, creating hot spots or sparks that scorch the food. This is why conductors such as forks, plates decorated with metal leaf, or unsuitably shaped foods cause arcing.

If your broccoli is in firm contact with the lumps of sweet potato or thoroughly wet, the fluid clogs the gaps and masks any fine tips, preventing harm, but wherever the electrons can arc across fine contacts or gaps, you get sparks or charring.

Brian Allen-Smith

Philadelphia, Pennsylvania, US


 Weighty flier

As part of Christmas dinner this year I cooked a tasty goose. I was astounded at the amount of fat that poured off it during cooking. Why do geese need so much fat?

Debora MacKenzie

Brussels, Belgium

I would argue that geese don’t need this fat, but rather that their intensive rearing has provided them with an excess of calories which they have laid down as fat, just as occurs in many other animals – including humans and their pets if they overeat and under-exercise.

Wild geese have very lean carcasses because their diet typically comprises grasses which contain small amounts of energy, on which they graze for long periods each day and which they often have to fly long distances to find.

Farm-reared geese, on the other hand, though they may actually be free-range, will have access to high-energy concentrated diets similar to those of broiler chickens. They expend very little energy each day on their basic functions and so will store the majority of calories as fat once they have achieved their mature size.

Ian Jeffcoate

University of Glasgow Veterinary School

Glasgow, UK

In the wild, geese are aquatic birds, with many species being migratory. They consequently need both a substantial energy reserve to sustain them during the long flights of their migration, and good insulation to protect them from the cold and wet. Fat covers both necessities quite nicely.

Goose fat, therefore, is by no means simply dead weight. An adult greylag (the species from which almost all breeds of domesticated goose descend) can weigh up to 5.5 kilograms. With a wingspan of over 160 centimetres, that makes for a relatively low wing loading – the ratio of mass to wing area.

In aviation terms, geese could be considered the ‘longhaul wide-bodies’ of the bird world, and a fuel load that can sustain them over thousands of kilometres is vital rather than a burden.

Of course, the ratio of goose fat to body weight is much lower in a wild bird than in the domesticated fowl typically eaten for Christmas dinner. For a number of reasons, domestic geese have had their body fat augmented by a combination of selective breeding and diet.

Prior to the introduction of the railways, from a farmer’s perspective the goose had quite a big advantage over the turkey – it was a lot easier to transport. Goose farmers based in East Anglia in the UK, for example, would simply walk their geese to London’s Smithfield market. They knew that, unlike turkeys, a goose could not roost overnight up a tree from which it would be almost impossible to retrieve the following morning.

Furthermore, geese could sustain themselves on this 160-kilometre waddle by grazing on grass, drawing on their reserves of fat for additional sustenance.

From a cook’s perspective, the goose’s advantage over the turkey was that it required no additional basting, its own supply of body fat being sufficient if spooned over the bird periodically while roasting. Indeed, so much fat comes off a full-grown bird that the excess can be used to baste other meat cooked at the same time, a favourite culinary trick of Charles Dickens.

Through his writing, Dickens helped make turkey an essential part of the British Christmas dinner. Ironically, he preferred the taste of goose, and to further his enjoyment of the bird as part of his seasonal lunch he would roast a whole ox heart in a pan placed underneath the trivet on which the goose was cooking, so that the heart’s bland but succulent meat would soak up the goose’s flavour, along with its fat as it dripped from above.

Hadrian Jeffs

Norwich, Norfolk, UK


 Spice attack

An earlier book in this series told us why garlic makes your breath and body smell, but I want to know why the spice methi, or fresh fenugreek, has a similar, possibly stronger, effect.

Nikki Bedi

BBC Radio Asian Network, UK

Depending on their biochemical nature, volatile components of foods or their metabolic products enter the blood and exit via lungs, urine, sweat, saliva or sebum, more than most people notice. As a result, families or communities with distinctive cuisines have distinct body scents.

There are many examples beyond obvious ones such as asparagus and onion-like foods. Stewed mutton and beef give a recognisable odour to one’s urine. No doubt any selfrespecting dog could identify other meats.

Many nitrogen compounds are particularly likely to be excreted in urine or sweat. I rather like the yeasty smell of thiamine, but my wife hates it, as does a friend who once had to have daily thiamine injections. His skin would reek before the doctor even finished the injection. Some people can even guess which cheese you have eaten in the last few days; presumably the smell gets into your sebum.

Fenugreek contains a range of sulphur and nitrogenrich aroma molecules that the body modifies and excretes in breath and sweat, but the main burnt-sugar smell comes from the lactone sotolon, whose smell we can detect even in minute concentrations.

Antony David

London, UK


 Con gas, sin gas

Is fizzy water lighter than still?

Asked by listeners on BBC Three Counties Radio, UK

Fizzy or carbonated water is heavier – that is, denser – than non-carbonated water if the carbon dioxide is in solution rather than forming bubbles. For a given weight of water and dissolved carbon dioxide, the volume can be calculated by adding the volume of the water alone (about 1 millilitre per gram of water) to that of the gas (about 0.8 millilitres for each gram of carbon dioxide). So a solution of, say, 2 grams of carbon dioxide in 998 grams of water would have a mass of 1 kilogram, a volume of 999.6 millilitres and a density of 1.0004 grams per millilitre (at 4 °C, the temperature at which water is at its most dense). There is more information on this at the University of California’s eScholarship website (bit.ly/3kxoIU). However, if the water is fizzing, then it will be lighter – less dense – than still water. It is the bubbles that reduce the density: 2 grams of carbon dioxide gas would have a volume of about 1 litre.

Lake Nyos in Cameroon has carbon dioxide seeping into it from below. Usually the carbon dioxide-laden water stays at the bottom of the lake because it is denser, but sometimes it begins to form bubbles which cause it to rise. This sets in train a process that brings up a massive amount of carbon dioxide, which bubbles into the air.

In 1986, 1,700 people were killed by carbon dioxide that escaped from the lake and smothered the surrounding valley (New Scientist, 24 March 2001, p. 36).

Eric Kvaalen

La Courneuve, France

Fizzy water can be lighter than still water, but it depends on what you define as still water. For example, is the questioner asking about pure distilled water, tap water or mineral water? All these have different densities. Tap water varies a great deal: it is hard and chalky in the south of England, but soft in both Scotland and northern England, for example.

If you assume that still water means pure distilled water, then at atmospheric pressure carbonated water will be denser, but only very slightly so, thanks to the extra weight of the gas dissolved in it. Of course, this assumes that the carbonated water contains no bubbles. If it is fizzing then the bubbles lower the density to below that of still water.

However, if you are considering still mineral water or any water containing dissolved salts, such as some tap water supplies or seawater, then it is difficult to determine the density without proper testing.

For example, seawater has a density of about 1.025 kilograms per litre – greater than that of fresh water. So still mineral water, depending upon where it is from, can be either heavier or lighter than carbonated water.


By email, no address supplied


 Citric surprise

Some friends and I were drinking from a jug of water that contained wedges of both lime and lemon. All the lemon wedges were floating, but all the lime wedges had sunk to the bottom of the jug. There were enough pieces of both for us to infer this was not just coincidence, and all of us were pretty certain that we’d seen lime slices floating before. Can anyone offer an explanation?

Bladon Mooney

Leicester, UK

Head for this great link to a page on the Steve Spangler Science site (bit.ly/aMMLze), where the answer is tested – Ed.

This is down to the interplay of two factors: air and solutes. The cells in fruit tissue typically have quite high concentrations of solutes, mainly organic acids for citrus and sugars for apples. In some types this can amount to as much as 18 per cent of the total weight. The more concentrated the solutes, the denser the cells will be and the more likely it is that the fruit will sink.

In plant tissue there are also air spaces between the cells, so the tissue is less dense than the cells are. If the air spaces are large enough, the tissue will float even if the cells alone would be dense enough to sink. Air spaces can range from as little as 1.5 per cent of the tissue’s volume in the case of a potato to more than 20 per cent for some leaves.

The effect of air spaces tends to outweigh cell sugar content, which is why a potato sinks while an apple with 15 to 20 per cent air spaces by volume will float, despite the high sugar content of its cells.

A variety of factors affect the volume of the air spaces and concentration of solutes in different fruit, including growing conditions, ripeness and storage conditions. With citrus fruit, a major factor is the peel. The inner white layer – the albedo or pith – is low in solutes and notably high in air spaces, while the edible segments are high in sugar/acid content, and low in air spaces. Peel a mandarin and put pieces of peel and segments in a bowl of water, and you will find the segments sink while the peel floats. Some lemons have a thick peel and are resolute floaters, while holding lemons in storage for a time reduces the thickness of the albedo, making the fruit more likely to sink.

So, more than anything, what determines whether your citrus slice floats or sinks in your gin and tonic is the thickness of the albedo.

Rod Bieleski

Retired plant physiologist

Devonport, New Zealand


 Pepper pot

When a pepper, or capsicum, is cut open there is a space inside, but there are no gaps in the pepper where air could get through. What is the composition of gases in this space, and how did they get there? If a green pepper contains chloroplasts would there be more oxygen and less carbon dioxide in a green pepper than in a red, yellow or orange one?

Rosa Clements

Harrogate, North Yorkshire, UK

The questioner is not entirely correct in suggesting that ‘there are no gaps in the pepper where air could get through’. Like most other plant surfaces the surface of a pepper or capsicum has stomata. These are orifices which are controlled by a pair of special cells, the guard cells, to open or shut as the plant requires.

They communicate with an extensive network of air spaces within the tissues, without which the gas exchange required for both photosynthesis and respiration could not take place.

The source of the air is, therefore, the atmosphere, via the stomata and the intercellular air spaces in the wall of the fruit. All capsicum fruit are initially green, with functional chloroplasts, so it is possible that there could be some enrichment in oxygen from photosynthesis at this stage, but not very much, because without gas exchange with the outside air there would be no source of carbon dioxide for further photosynthesis.

When a capsicum ripens to a red or yellow colour the chloroplasts cease to function and turn into chromoplasts containing fibrous deposits of carotenoids and protein. At this stage photosynthesis has ceased and the internal gas will be unlikely to differ very much from the outside air other than in water vapour content.

Guy Cox

Sydney, Australia

As the pepper develops, the gases from the atmosphere diffuse into the capsicum’s growing cavity. The composition of the internal gases will depend on the respiration rate of the pepper as well as the resistance to gas diffusion. Generally, the more immature the pepper, the higher the respiration rate of the tissues.

We decided to test the internal gas composition of different coloured peppers in our laboratory using gas chromatography. The mean percentage levels of oxygen and carbon dioxide respectively were: green (19.85, 0.068), yellow (18.45, 1.08), red (18.36, 1.15).

It is possible that the higher oxygen/lower carbon dioxide levels in the green fruit were due to photosynthesis because the lab bench was in bright sunshine during all the measurements.

Normally, however, internal light levels are much too low to support any significant photosynthesis in harvested produce.

Julia Aked and Allen Hilton

Silsoe, Bedfordshire, UK


 Ordering a meal

When we are really hungry the quickest way for our body to gain energy is surely found in the kinds of simple sugars available in desserts. So why is it that we usually eat savoury foods followed by sugary puddings and not the other way around? I have noticed that on aircraft, where all the food arrives together on a tray, people often eat the courses in a quite different order from usual.

Toni Lluddmecy

London, UK

Meals comprising a sequence of courses are arguably unique to civilised humanity. Most animals eat what they can when they can. Given a choice, they proceed from their favourite items to the necessary evils. The more prized the food, the higher the probability of losing it through procrastination and the greater the penalties. Availability is more important than quick energy, so natural selection favours an eye for the main chance, in every species from microbes to hunter-gatherers. Even today, children will go for the goodies first when they can.

Only once humans had achieved security, productivity and the leisure time for multiple-course meals did they formulate the principle of ‘never a sweet before the meat’. Long before people discovered that flooding the blood with sugar is an unhealthy habit, they had learned that sweet starters spoil the appetite. This is useful if one needs to discourage guests from overeating, but spoils a good feed and forfeits that pleasant anticipation of treats to follow. Sugary desserts enhance the sense of comfortable repletion and are less harmful at the end of a meal, because diluting the sugar in a gutful of chyme buffers the surge of sugar into the blood.

Stuart van Dyck

The Hague, The Netherlands

It’s only in relatively modern European cuisine that the sugar is reserved for the last course of the meal. In most of the world, sugar is added to meat dishes, such as in oriental sweet-and-sour recipes or in Mexican mole.

European cuisine did the same until the 17th century, working under a theory of nutrition – inherited from ancient Greece – that sugar was the perfect food. Many meat dishes were sweetened until a new theory developed which saw sugar as harmful and relegated it to a small course served after the main meal, when appetites were lower.

We can still see a remnant of the older cuisine in condiments like steak sauce and ketchup, with their high sugar content.

For more history of this change in European eating habits, search out Rachel Laudan’s article ‘Birth of the modern diet’ in the August 2000 issue of Scientific American. Elsewhere, Laudan has pointed out that many supposedly traditional ethnic dishes are less than 100 years old, and created to please tourists.

Alan Chattaway

Surrey, British Columbia, Canada