Why Don't Penguins' Feet Freeze?: And 114 Other Questions - Mick O'Hare (2009)

Chapter 4. Food and drink

Banana armour

The skin of bananas in the fridge turns brown faster than those in a room, but the fruit is still edible. I thought the browning was oxidation, but if so why does it happen faster in the cold?

Alun Walters

Cardiff, UK

I wouldn’t recommend putting bananas in the fridge to keep them fresh. Like all living organisms bananas adjust the composition of their cell membranes to give the right degree of membrane fluidity for the temperature at which they normally live. They do this by varying the amount of unsaturated fatty acids in the membrane lipids: the colder the banana, the greater the level of unsaturated fatty acid and the more fluid the membrane at a given temperature. If you chill the fruit too much, areas of the membrane simply become too viscous and the cell membranes lose their ability to keep the different cellular compartments separate. Enzymes and substrates which are normally kept apart therefore mix.

Over-ripe fruit kept out of the fridge go brown by the same mechanism but, in this case, membrane breakdown occurs as a part of the general senescence of the tissue. In chilled commercial storage injury is, in fact, a big problem with tropical fruits, whereas temperate fruits like apples and pears can happily be stored at temperatures near freezing. I wonder, therefore, whether bananas stored in the fridge really taste as good as those left out? Incidentally, since tomatoes are a semitropical fruit I wouldn’t suggest you keep them in the fridge either.

Alistair MacDougall

Institute of Food Research

Norwich, Norfolk, UK

While many fruits are stabilised by refrigeration, most tropical and subtropical fruits (bananas in particular) exhibit chill injury. The ideal temperature for bananas is 13.3 °C. Below 10 °C spoilage is accelerated by the release of enzymes and the skin can blacken overnight as the banana fruit and skin softens. The enzymes leak from cellular storage sites and the leakage is caused by increased membrane permeability. This is mediated by ethylene gas which controls ripening and response to chill injury and such events as attack by parasites.

The two enzymes which break down the main polymers responsible for plant structure are cellulase and pectinesterase. These break down cellulose and pectin respectively. The breakdown of starch by amylase type enzymes is also involved in softening banana fruit tissue.

The blackening of the skin is caused by the release of another enzyme, polyphenyl oxidase (PPO). This is an oxygen-dependent enzyme which polymerises naturally occurring phenols in the banana skin into polyphenols similar in structure to melanin formed in suntanned human skin.

PPO is also inhibited by acid and this is why lemon juice is used to prevent browning in apples. Bananas are low in acidity and this may be one reason why they darken so quickly. Finally, blackening of the skin can be slowed by coating the banana with wax to exclude oxygen.

M. V. Wareing

Braintree, Essex, UK

Further to your previous answers: yes, the browning is an oxidation reaction. Yes, it is initiated by cooling. But no, the low temperature itself does not speed up the oxidation reaction in bananas.

Bananas like hot climates, and their cell membranes are damaged in the fridge. Membrane damage lets the phenolic amines such as dopamine, which are normally present inside the vacuoles of banana skin cells, leak out and encounter oxidising enzymes (polyphenol oxidases) elsewhere in the cell. The dopamine can then be oxidised by atmospheric oxygen to form brown polymers, which may act as a defensive barrier. Once started by the cold-induced membrane damage, the browning reaction is promoted by warming.

For an extreme demonstration, put a banana skin in the freezer for a few hours. It stays creamy white because, although the membranes are shattered by freezing, oxidases cannot work at such low temperatures. Now let it thaw overnight at room temperature: the skin will go pitch black as the dopamine is oxidised. A control banana skin kept at room temperature overnight stays whitish because the vacuolar membranes remain intact.

Stephen Fry

University of Edinburgh, UK

White-water drinking

Why do anisette-based drinks, such as Pernod or Sambucca, turn white when water is added to them?

Alexander Hellemans

Amsterdam, The Netherlands

Anisette-based drinks rely on aromatic compounds called terpenes for their flavour. These terpenes are soluble in alcohol, but not in water. The 40 per cent or so alcohol in the drink is enough to keep the terpenes dissolved, but when the drink is mixed with water they are forced out of solution to give a milky-looking suspension.

Absinthe, a similar drink which is based on wormwood and is now banned in some countries because of its toxicity, gives a more impressive green suspension. Terpenes are responsible for a lot of the harsher plant scents and flavours including lemon grass and thyme.

Thomas Lumley

Newtown, New South Wales, Australia

Transparent rock

How do you get transparent ice? Ice from my freezer always contains bubbles. I’ve used filtered and boiled water but the ice is never like that seen in ads for Scotch.

Philip Susman

Monash University, Victoria, Australia

Ice made in domestic freezers is inevitably cloudy because of the dissolved air that tap water contains (around 0.003 per cent by weight). As the water in the ice trays drops below freezing point, crystals form around the edges of the compartments. These are pure ice and they contain very little air because the solubility of air in ice is very low and the liquid left behind can still hold it in solution.

Once the concentration of air in the liquid reaches 0. 0038 per cent by weight and the temperature has dropped to -0.0024 °C, the liquid can contain no more air and a new reaction begins. As the water freezes the air is forced out of solution. The natural state of air at the temperature and pressure involved here is a gas, so it forms bubbles in the ice.

Commercial ice machines produce attractive, clear ice by passing a constant stream of water past freezing metal fingers, or over freezing metal trays. This freezes out a fraction of the water while the rest of it is discarded before the concentration of air gets too high. When the ice is thick enough the metal fingers or tray are warmed to release their crystal-clear ice that is good enough to film.

Alas, without an ice machine, the questioner may have to make do with cloudy ice cubes.

Andrew Smith

Newcastle-upon-Tyne, UK

Water has its highest density at around 4 °C. Below that the water gets less dense as it approaches freezing point.

Air bubbles form in the ice when the cooling of the water is too rapid, which causes one part of the water to be at a different temperature to other parts. Ice is usually formed at the top of the water first because the warmer and denser water sinks beneath the ice layer that begins to form there.

Additionally, the top layer is usually the part that is in contact with the cold environment. This is similar to what happens in a frozen lake. The various expansion rates of different parts of the water will inevitably create air bubbles that cannot escape because of the ice sheet above.

To avoid ice bubbles, the trick is to cool the water very slowly so that there is no large temperature gradient which can cause widely different expansion. Cooling it slowly also allows air to have sufficient time to move through the liquid and evaporate before it is trapped by the solid ice.

Han Ying Loke

Edinburgh, UK

Your failure to achieve clear ice even after boiling the water may depend on what precautions you took in addition to filtration and boiling. For instance, gas can still be dissolved if water is poured after boiling has finished and hard water may also need deionising to rid it of gas.

It also helps to exclude air while the water is cooling, perhaps with clingfilm. Try to achieve zone freezing by cooling the water slowly from the top down, maybe in a polystyrene container with only clingfilm over the top. Do not use a vacuum flask because the glass is far too fragile.

Although the amount of gas that freezes out is unaffected by the method of freezing, zone freezing starts by giving a nice, thick chunk of clear ice. As freezing progresses murky ice appears and then you can stop the process.

Jon Richfield

Dennesig, South Africa

Water contains dissolved gases. When it freezes, the gas is forced out of it to form bubbles that are then trapped in the ice, making it look opaque.

In order to get transparent ice you should use warm rather than cold water, since warm water contains less dissolved gas. Also, try to reduce the power of your freezer in order to allow time for the gases to diffuse out just before freezing. I have tried this and it works very well.

Gabriel Souza

Cambridge, UK

I’m afraid that your correspondent has been seduced by a professional photographer who uses hand-carved perspex ‘ice’ cubes in adverts for Scotch because they don’t melt under the studio lights. If he looks very carefully, he might also see the tiny glass bubbles on the meniscus of other drinks - these won’t disappear at the wrong moment either.

Martin Haswell

Bristol, UK

Over the top

Sparkling wine or beer poured into a dry glass froths up. If the glass is wet this does not happen. Pour some sparkling wine into a glass so that it froths up to the rim, let the bubbles subside and you can then pour the rest of the wine quickly, knowing it will not froth over the top. Why?

H. Sydney Curtis

Hawthorne, Queensland, Australia

Beer, sparkling wine and other fizzy drinks are liquids which are supersaturated with gas. Although thermodynamics favours the gas bubbling out of the dissolved state, bubble formation is unlikely since bubbles must start small.

The pressure of these tiny bubbles can reach about 30 atmospheres in a bubble only 0.1 micrometres in diameter. Because the solubility of gases increases with increasing pressure (Henry’s law), the gas is forced back into solution as quickly as it comes out.

Bubbles can form around dust particles, surface irregularities and scratches. These nucleation sites are hydrophobic and allow gas pockets to grow without first forming tiny bubbles. Once the gas pocket reaches a critical size, it bulges out and rounds up into a properly convex bubble whose radius of curvature is sufficiently large to prevent self-collapse …

D. P. Maitland

Department of Pure and Applied Biology

University of Leeds, West Yorkshire, UK

In addition, there is a cascade effect. If the quantity of bubbles reaches a certain critical number per unit volume, this in itself constitutes a physical disturbance and results in the release of yet more bubbles.

Nucleation may be precipitated by a variety of imperfections. Minute crystals of salts (such as calcium sulphate) may remain if the glass has been left to dry by evaporation after being washed in hard water. Tiny cotton fibres may be left behind if the glass has been dried with a tea cloth. Dust particles may have settled on the glass if it has been left standing upright for any length of time. And tiny scratches will be present on the inside surface of all but brand-new glasses.

Once the inside of the glass is wet, any salt crystals will have dissolved and any cotton fibres will no longer function as centres of nucleation. Most of the dust particles and all of the scratches will, of course, still be there. However, these will have been coated with liquid and the fresh carbonated liquid can only reach them very slowly, by diffusion. Bubbles will still be produced, but at a rate that is too slow for the cascade effect to come into play. As a result, the drink will not froth over.

Allan Deeds

Daventry, Northamptonshire, UK

To demonstrate the above, take a glass and thoroughly coat the inside with an oil, which is a more efficient surface covering agent than water. Then add a less expensive carbonated drink such as lemonade. The effervescence will be nil or minimal. Add a few million centres of nucleation from a large spoonful of granulated sugar and the effervescence will be volcanic.

Ronald Blenkinsop

Westcliff-on-Sea, Essex, UK

Thanks to modern production techniques, today’s glasses are of such good quality that some manufacturers build in deliberate imperfections, especially in beer glasses, in order to generate enough bubbles to maintain the head on the top of your tipple.

Tony Flury

Ipswich, Suffolk, UK

Slice crisis

What is the irritant that causes eyes to water when slicing onions?

Is there any way to prevent this?

Stephen Mitchell

Redruth, Cornwall, UK

Onions and garlic both contain derivatives of sulphur-containing amino acids. When an onion is sliced, one of these compounds, S-1-propenylcysteine-sulphoxide, is decomposed by an enzyme to form the volatile propanthial S-oxide, which is the irritant or lacrimator.

Upon contact with water - in this case your eyes - the irritant hydrolyses to propanol, sulphuric acid and hydrogen sulphide. Tearfully, the eyes try to dilute the acid. However, it is these same sulphur compounds that form the nice aroma when onions are being cooked.

To prevent watering eyes, I would suggest one of the following: stop using onions (but you would lose the tasty aroma); wear goggles (you would look slightly silly); slice the onion under water (you will wash some of the aroma out); before slicing the onion, wash it, and keep it wet.

Bernd Eggen

Exeter, Devon, UK

To attempt to reduce the severity of watering eyes you must allow the maximum possible time for the irritant to disperse before it comes into contact with the eyes. The most obvious way of achieving this is to stand as far away from the onion as your arms will allow. It also helps if you are not standing over the onion, but back from it.

Another way of reducing the amount of irritant reaching your eyes is to breathe through your mouth. This means that instead of creating a current of air which flows up to your nose and onwards to the eyes, carrying the irritant with it, the air is either directed into the lungs when breathing in or forced away from the face when breathing out.

In order to ensure that you breathe through your mouth, hold a metal spoon lightly between your teeth. There will be space for the air to enter and escape, and while our mouths are open we breathe preferentially through them, rather than our noses. I find that holding the spoon upside down works best, although I don’t know what scientific reason there could be for this.

C. Burke

Farnham, Surrey, UK

I have found that wearing contact lenses prevents eye irritation when chopping onions.

Elaine Duffin

Keighley, West Yorkshire, UK

A slice of lemon should be placed under the top lip while slicing. One does not look particularly attractive but it does prevent the eyes from watering.

Sheila Russell

Staines, Middlesex, UK

I suggest the old tip of holding a sugar cube between your teeth to absorb the irritant. It does work, as do sulphur matches, though very few people use these now.

Michel Thuriaux

Geneva, Switzerland

Hold a piece of bread - say a quarter of a slice - between the lips as you slice onions. This was taught to my family in Tanzania in the early 1960s by our cook, Victor Mapunda, from Malawi.

John Nurwick

London, UK

Question of class

We are told to let red wine breathe before drinking it to improve the flavour. At the risk of appearing a philistine, wouldn’t it be quicker to pour it into a cocktail shaker, shake for 10 seconds and let the bubbles subside?

Chris Jack

London, UK

Wine is left to breathe to allow the volatile and aroma-bearing substances to start evaporating, so that we may enjoy the bouquet. Shaking a drink is completely different. An agitated drink incorporates gas, letting oxygen reach as much liquid as possible. This oxidised liquid provides a very different taste.

For some drinks this taste may be pleasant. However, if you oxidise wine you obtain vinegar, which, I suspect, is not the flavour you wish to taste. Therefore, there is a genuine reason for drinks being ‘shaken, not stirred’ or vice-versa, depending on what you have in your glass.

Paul Mavros

Aristotle University

Thessalonika, Greece

The reasons usually given for decanting red wines have changed during the past few years. This is because of two developments: one in wine-making technology and the other in wine tastes.

The original reason for decanting was to separate the wine from organic particulates formed by precipitation, and aggregation from tartaric acid, tannin compounds, original microparticulates present in the pressed grape juice and proteinaceous material that is formed during maturation of the wine.

Because these particulates are small to minuscule in size and of a density not much higher than the wine itself, Stokes law predicts that they will sink back to the bottom only extremely slowly should they be suspended by careless motion of the bottle.

This is the reason for those magnificent mechanical decanting machine which allow precisely controlled tilting of the bottle to reduce suspension of particulates.

A very different reason for decanting lies in aerating the wine to hasten the release of the secondary elements of its nose. While traditional old wines may actually lose some of their olfactory elements through intense aeration and become stale quickly, decanting for aeration parallels the development of taste in younger wines or wines elevated in oak casks, with associated different weighting of primary and secondary smells.

In Italy, where many progressive vintners have been experimenting with new assemblages and methods of elevation, decanting often means pouring the contents of a bottle straight down into a decanter, generating lots of chaotic turbulence with an intense mixing of air and wine.

In the hands of a self-confident wine waiter this process can look flamboyantly spectacular. As a logical development of this reason for decanting, some modern Italian glass decanters have a flattened shape that allows for the maximum air-wine interface, giving further aeration.

Oliver Straub

Basle, Switzerland

It is generally recognised that red wine should be drunk at ambient temperature and, since it is often stored in a relatively cold room or location (near the floor), the most important aspect of the so-called breathing process is to raise the wine’s temperature.

However, the ambient temperature in the UK is often a little low and red wine is usually best if drunk at about 30 °C. Placing a bottle of red wine in a microwave oven for 50-60 seconds (depending on the season) on high power will produce the required effect without having to resort to allowing the wine to breathe before consumption, but do not forget to remove the foil capsule and the cork. The alternative concept of shaking the wine in a cocktail shaker will result in the formation of various oxidation products including vinegar, which will have a negative effect on the flavour.

M. V. Wareing

Braintree, Essex, UK

Only chemists drink red wine at a temperature of 30 °C. Our wine experts suggest a temperature of around 17 °C - Ed.

One or two?

Expert advice says that you should use freshly drawn water every time you make a pot of tea or coffee. Why is this? What is wrong with water that has been boiled twice? Can anyone tell the difference?

Ivor Williams

Okehampton, Devon, UK

The reason that freshly boiled water is more effective for making tea than water boiled twice is that the fresh water has a higher oxygen content. This should result in a tastier cuppa because more tea will be extracted from the tea leaves.

This can be easily demonstrated by placing a measured amount of tea leaves in two glass tumblers and adding freshly boiled water to one and repeatedly boiled water to the other. Examination of both tumblers after three minutes will reveal a much stronger brew from the freshly boiled water.

J. R. Stafford

Marks & Spencer

London, UK

I was told as a child that the reason for using freshly drawn water to make tea was because the dissolved oxygen made the tea taste better. Water which has been standing or, worse, had been boiled contained less dissolved oxygen. The British Standard 6008, which describes in great detail how to make a cup of tea, says that the water must be freshly boiling but does not say anything about it being freshly drawn. It also says that the milk should be put in the cup first to avoid scalding it.

As this British Standard is identical to International Standard ISO 3103, the supplementary question is why can’t I get a decent cup of tea abroad?

N. C. Friswell

Horsham, West Sussex, UK

The traditional explanation for making tea with freshly boiled water is that prolonged boiling drives off the dissolved oxygen, making the tea taste ‘flat’. My own experiments with water simmered for an hour against freshly boiled water produced little perceptible difference, even though high-quality leaf tea was used and brewed for five minutes.

I would be surprised if the difference was of the slightest practical importance for tea made by dunking a tea bag, especially if the water had merely been boiled twice.

David Edge

Hatton, Derbyshire, UK

I see that at least one reader remains unconvinced on the need to use freshly boiled water for tea.

Once, during an emergency overseas, we were instructed to boil all drinking water for several minutes. It didn’t seem to affect the tea. However, we decided that it would be a good idea to use a domestic pressure cooker to raise the water temperature to beyond boiling point to sterilise the water thoroughly. This was fine when used for drinking or cooking, but when we tried using it for making tea the result was absolutely dreadful.

On the other hand, I have drunk tea at an altitude of 2100 metres where, of course, the boiling point is lower than 100 °C, but I noticed no difference in the taste. Nor did my tea-planter hosts make any comment on the point.

Pressure-cooked water apart, I think the length of time the tea is allowed to infuse is a more critical factor.

A. C. Rothney

East Grinstead, Surrey, UK

A. C. Rothney may be surprised to hear that his/her shiny pressure cooker probably caused his/her nasty tea. Dissolved aluminium in the water, not the higher temperature to which the water had been subjected, is the reason the tea tasted awful. In the days when most kettles were made of aluminium they carried instructions to prepare the new kettle by repeatedly boiling fresh water and then discarding it. Only then should the first pot of tea be made with fresh water. During these repeated boilings, a patina of dull oxide built up inside the kettle and prevented the water dissolving the pure aluminium.

Lorna English

London, UK

The preference for fresh water when making tea has little to do with oxygen but is related to dissolved metal salts (mainly calcium and magnesium bicarbonates, sulphates and chlorides) which are present as impurities in tap water and which affect the colour and taste of tea.

The effect of metal salts on the colour of tea can be demonstrated by comparing a brew made with freshly boiled pure water (deionised or melted freezer frost) with tea made with freshly boiled tap water. The salts in tap water give a darker brew, which is cloudier as a result of precipitated insoluble salts such as tannates.

Boiling tap water destabilises the bicarbonates (so-called temporary hardness) which precipitate out as insoluble carbonates on cooling (this is why a kettle furs up with time). In hard-water areas, where more dissolved salts are present, repeated boiling and cooling will remove sufficient calcium and magnesium salts, although boiling for a long time without cooling has less effect.

There are three reasons why repeatedly boiled and cooled water can produce a less palatable tea. First, some of the precipitated carbonate remains in suspension, even after reboiling, as a white scum (particularly noticeable in new plastic kettles) and this taste is more marked than bicarbonates dissolved in water - especially when the scum interacts with the tea.

Secondly, the salts in the water which are not destabilised by boiling (so-called permanent water hardness) are gradually concentrated by evaporation, producing unpleasant flavours.

Finally, traces of metals, such as iron and copper, can accumulate in repeatedly boiled water and these can interact with oxygen and reducing agents in the tea (phenols) by complex redox reactions to produce further effects on flavour.

M. V. Wareing

Braintree, Essex, UK

As a caffeine addict, I suffer severe headaches if I go more than a day without my cups of tea. To conserve fuel on hikes lasting a number of days, I tried leaving a tea bag in a bottle of cold water for a few hours. It worked. Not only did it give me my fix of caffeine, but it tasted like tea, albeit cold tea. I haven’t yet tried making such a cold infusion, then heating it in a microwave, but it should prove quite drinkable.

Syd Curtis

Hawthorne, Queensland, Australia

The truth runs counter to A. C. Rothney’s ideas.

My father was a tea taster and faultless at detecting whether we had boiled the water for too long. How did he do it?

Hard waters (and most waters do have some mineral salts in solution causing hardening) brew slower than soft or alkaline waters. If you boil hard water for considerably longer than the standard half a minute or so, more of the dissolved salts deposit themselves on the inside of the kettle. The emerging water is then softer than expected and softer than the tea taster balanced the tea blend for. It will brew quicker and with a darker colour than usual.

Tea manufacturers ensure constant performance by balancing their blend differently for sale in different water areas of the country, even where the brand label is the same. Hard water can be artificially softened with a pinch of bicarbonate of soda, but the dramatic darkening of the colour and change of flavour are unacceptable to most people - including tea tasters.

Bernard Howlett

Loughton, Essex, UK

Twister

Here in Zimbabwe we buy milk in plastic packets. Most people cut a tiny piece off the edge of the packet to pour the milk. I have noticed that when the milk leaves the packet under pressure it exits in a corkscrew or a spiral fashion. Of course, other liquids would behave in a similar fashion. What forces operate to allow an unchannelled liquid to follow this path? I have noticed that the smaller the opening in the packet the greater the amount of twisting in the path followed by the milk.

David White

Chinhoyi, Zimbabwe

The corkscrew effect you notice is just the bottom end of the whirlpool that is occurring inside the carton as the milk exits. The force that causes it is usually called the Coriolis force. This is responsible for all whirlpooly stuff you might find. Milk cartons and bottles give you the same effect, but it is less noticeable because of the shape of the cross section of their openings.

When the milk leaves the contents under pressure from squeezing the sachet while pouring, you effectively increase the liquid’s speed. This increases the Coriolis force - which is proportional to the speed of an object in a rotating inertial frame, as well as the frame’s angular velocity and the distance from the object to the axis of rotation. This gives a tighter corkscrew. In effect, milk under pressure screws up.

John Lenton

Cordoba, Argentina

The twisting of the stream of milk coming out of the package has more to do with the shape of the hole (usually a long, thin one), the difference in pressure on the milk from one side of the exit hole to the other, and the force of the surface tension between the milk and the side of the container. It has nothing to do with the Coriolis force as suggested by your correspondent.

The Coriolis force is a real phenomenon. Because the Earth rotates, a fluid that flows along the Earth’s surface feels a Coriolis acceleration perpendicular to its velocity. In the northern hemisphere, Coriolis acceleration makes low-pressure storm systems (hurricanes) spin anticlockwise. But in the southern hemisphere storm systems (typhoons) spin clockwise because the direction of the Coriolis acceleration is reversed.

This large-scale meteorological effect leads to the speculation that the small-scale bathtub vortex that you see when you pull the plug from the drain spins one way north of the equator and the other way south of the equator. This is incorrect. The Coriolis force is far too small to have an effect on the direction of bathtub whirlpools or twisting milk coming out of cartons.

The force can be seen in a tub of water only under controlled experimental conditions, including a symmetrical low-friction tub, tight control of thermal currents, and letting the water stand for a long time (a day or more) so the residual fluid motion from filling has ceased.

Raymond Hall

by email, no address supplied

Your correspondent’s answer to the question is not entirely correct. While it is correct to say that the corkscrew is the end of the whirlpool occurring within the packet, he is wrong to suggest that the cause of the whirlpool is the Coriolis effect.

Instead, the ‘ice-skater’ effect is responsible. Any small wobbles you have given the milk packet will set the fluid moving inside in one direction or another. As the fluid moves out through the small hole, its angular momentum is conserved. That means that, as it moves into a smaller diameter stream, it spins faster, just as ice-skaters spin faster when they pulls their arms in closer. This is also why the corkscrew effect is enhanced with smaller openings.

Sonya Legg

California, US

Aim and pour

When I open a carton of milk I have to pour the liquid quickly from the opening so that it goes into my glass. If I tip the carton too slowly, the milk runs down the underside of the carton and pours onto my foot or the floor. Orange juice and other liquids do the same. Why do they stick to the carton when poured slowly?

Tom Khan

Bradford, West Yorkshire, UK

When a carton of liquid is tipped during pouring, the free surface of the liquid in the container is raised relative to the opening. This creates a pressure difference between the free surface and the opening, which forces fluid from the carton. In addition to this pressure force, there are also surface tension forces acting on the fluid that tend to draw the fluid towards the surfaces of the container. At high pouring speeds, the pressure force is much greater than the surface tension forces, and the fluid will leave the carton in an orderly fashion, following a predictably curved (parabolic) path towards a glass below.

However, at low pouring speeds, a point is reached where the surface tension forces are sufficient to divert the path of the fluid jet so that it fails to leave the opening cleanly and becomes attached to the top face of the carton (assuming a flat-topped carton). Once attached to a surface, a jet of liquid will tend to remain attached to that surface due to these surface tension forces and a phenomenon known as the Coanda effect. This occurs when a fluid jet on a convex surface (such as a water jet from a tap curving round the back of a spoon) generates internal pressure forces that effectively suck the jet towards the surface.

The combined result of surface tension and the Coanda effect enable an errant flow of fluid to negotiate the bend from the top face of the carton round to the carton’s side, thus ensuring maximum transport of fluid from the carton to your shoes.

Experiments have shown that when cartons are full, the ‘glugging’ that occurs as air is sucked in to replace the lost fluid causes the fluid jet to oscillate, leading to periodic surface attachment of the jet (and wet shoes) even at relatively high pouring speeds.

Bill Crowther

Aerospace Division

University of Manchester, UK

The Coanda effect or ‘wall attachment’ is named after the Romanian Henri Coanda (1886-1972) who invented a jet aircraft propelled by two combustion chambers, one on either side of the fuselage pointing backwards, and situated towards the front of the aircraft. To his horror, on take-off the jets of flame, instead of remaining straight, clung to the sides of the fuselage all the way to the tail. At least his name has now been immortalised, thanks to this effect.

Some 30 years ago this wall attachment phenomenon was used in machine control systems known as fluidics, where a small jet of fluid was used to persuade the main flow to leave the ‘wall’ to which it was attached and divert to another course. It then became attached to this.

John Worthington

Stourbridge, West Midlands, UK

A picture of the Coanda, the first true jet aircraft to be built, in 1910, can be found at www.allstar.fiu.edu/aero/coanda.htm. The next answer describes a simple demonstration of the effect - Ed.

The effect is seen as a general tendency for fluid flows to wrap around surfaces. An entertaining experiment consists of taking a vertical cylinder (a washing-up liquid bottle or a wine bottle) and placing a lighted candle on the far side. When you blow against the bottle, the candle is blown out, because the current of air wraps around it.

Richard Hann

Ipswich, Suffolk, UK

Double trouble

I recently purchased a box of eggs, each of which was guaranteed to have two yolks. And the claim was correct. How does the supplier ensure that each egg has two yolks?

John Crocker

Solihull, West Midlands, UK

These special eggs are a natural phenomenon over which we have no control. Double yolk eggs are larger than those laid by the majority of the flock and are set aside to be tested individually. Demand for double yolkers far outstrips supply and we need to be very sure that they do in fact contain two yolks. Therefore, each egg is checked by holding it against a bright light. During this process (still known as candling from the days when a candle provided the source of light) the number of yolks will be clearly visible as shadows.

Graham Muir
Stonegate Farmers Limited
Hailsham, Sussex, UK

Try it at home - you’ll be surprised how much of the inside of an egg you can see - Ed.

Frying problem

When I view the surface of cooking oil in a pan by reflected light, a pattern of honeycomb-like shapes appears as the pan is heated by a gas flame. The unit size of the pattern is smallest where the oil is thinnest. Why is this?

Rex Watson

Broadstone, Dorset, UK

The honeycomb cells observed in heated cooking oil are known as Rayleigh-Bénard convection cells. At low temperature differences between the bottom and the top of the oil, the heat is dissipated through ordinary thermal transport (collision of individual molecules) and no macroscopic motion can be observed. At greater temperature differences, convection (a collective phenomenon involving many molecules) is a more effective means to transport the thermal energy. The heated cooking oil on the bottom is less dense and wants to rise. The top of the oil cools down by contact with the air and sinks again. This motion becomes circular and creates rolls of liquid, which self-organise into a honeycomb pattern which can be easily observed.

Quite a bit of research has been carried out on this phenomenon - which anyone can create in the kitchen - and we now have an explanation as to why the pattern of the convection cells is honeycomb. The form of the convection rolls depends on the shape of the container in which the liquid is heated. Hexagonal patterns seem to develop easily in round pans. Other containers may lead to long, rectangular rolls, with a square cross-section.

As the liquid moves in a circular fashion (up, across, down and back across), the unit size of the pattern depends linearly on the thickness of the liquid. It is interesting that many parameters such as the unit size of the convection cell are determined, whereas the direction of the circular motion is undetermined at the onset of convection. Once a rotation direction (clockwise or anticlockwise) is established, it remains stable.

Bernd Eggen

University of Exeter

Devon, UK

Twenty seconds or so after heat is first applied, the really interesting phase of convection begins suddenly. When the temperature gradient within the oil layer has built up to a certain critical value, each of the many scattered convection currents present in the oil finds that it conserves energy better if it shares its region of descending flow with the down-flow regions of its immediate neighbours. This stops any contra-flow problems. This cooperative repositioning of the centres of convection forms a regular pattern of closely packed convection cells. Their honeycomb-like appearance occurs to allow each cell to have the maximum area consistent with sharing its cell walls with its neighbours.

Because of this cell cooperation, convection proceeds vigorously and the rising hot oil can be seen to form a small fountain at the centre of each cell. The force that maintains this pattern, in the face of mechanical and thermal disturbance, is the flow of heat energy up through the oil layer. In the same way, a biological system needs energy throughout - in this case food - to maintain its integrity.

A substantial increase in the temperature gradient leads to the break-up of the cell pattern, which may pass through several phases of greater complexity before degenerating into chaos.

Roger Kersey

Nutley, East Sussex, UK

It can be shown analytically that the most efficient flow pattern in a large expanse of fluid transferring heat from bottom to top is hexagonal, with cells about the same width as the depth of the fluid. The hot fluid moves up the centre, cools at the surface and then drops down the perimeter of the hexagon. Similar patterns can be seen on all scales from millimetre-sized experiments to patterns on the surface of the sun.

Gary Oddie

Cranfield, Bedfordshire, UK

The readers above have already provided answers to this question. However, as the writer below points out, the previous explanations using the Rayleigh convection model were not wholly correct, for the Rayleigh model only applies if the frying liquid is of sufficient depth - Ed.

The behaviour of hot oil in a pan is a classic example of Bénard convection, the unstable motion of fluid on a heated flat plate which takes the form of regular hexagonal cells of circulating fluid. It is well known that Lord Rayleigh developed a theory to explain this instability. What is not so well known is that his model was wrong.

Rayleigh considered a horizontal layer of liquid with flat surfaces heated from below, and assumed that the instability took the form of parallel, contra-rotating rolls driven by buoyancy forces due to variations in the fluid density. Then, by heuristic arguments, he deduced a size for hexagonal cells close - fortuitously - to that observed by Bénard. He also predicted the minimum temperature gradient across the layer for the onset of this motion but this was about 100 times greater than the gradient needed to initiate the cellular flow in Bénard’s experiments.

Other researchers extended Rayleigh’s analysis in various ways. When the flat upper surface condition was later relaxed it could be seen that the surface is elevated above rising fluid between adjacent rolls while it is depressed above descending fluid. This is precisely the opposite of what Bénard observed. When Bénard’s experiment was repeated it was found that the cells could also be produced when the heating plate was cooled, whereas according to Rayleigh’s ideas the fluid should remain at rest. The instability has also been observed for a layer of liquid beneath a plate being heated from above and in space, where gravity and hence buoyancy forces are zero.

In the late 1950s a new model for Bénard convection was developed in which variations of surface tension caused by temperature variations on the surface of the liquid drove the motion. This model also predicted a depressed surface above rising fluid. In reality both Bénard and Rayleigh effects must be present. Conditions determine which predominates. Buoyancy forces drive the motion when there is no free surface or the liquid layer is thicker than about 10 milli metres; otherwise surface tension governs the flow.

Whichever driving force dominates, it must be sufficient to overcome the effects of viscous drag (which tends to inhibit motion) and diffusion of heat within the fluid (which tends to reduce the temperature gradients) before it can initiate the unstable flow. For buoyancy-driven flows the onset of instability is governed by the Rayleigh number:

buoyancy forces/(viscous drag rate of heat transfer) while for flows driven by surface tension the corresponding variable is the Marangoni number, in which surface tension forces replace buoyancy forces.

For thin layers the unstable flow takes the form of a regular array of hexagonal cells regardless of the shape of the container. For thicker layers the basic unstable flow is a series of rolls parallel to the container’s sides with the direction of flow adjacent to its rim and determined by its temperature relative to its base. These rolls degenerate into polygonal (but not necessarily hexagonal) cells when the temperature gradient is increased.

Richard Holroyd

Cambridge, UK

Stale tale

Why does a biscuit that is left in the open overnight become soft by the morning but a baguette left out for the same length of time become so hard that one could knock someone out with it?

Lorna Hall

Bullion, France

Biscuits contain much more sugar and salt than baguettes. The finely divided sugar and salt are hygroscopic and soak up moisture from the atmosphere - the osmotic pressure in a sweet biscuit is quite high. The dense texture of a biscuit helps maintain the moisture by capillary effects.

The baguette, on the other hand, contains little salt or sugar, and has a very open structure. The flour doesn’t care if there’s moisture around it or not. So, because of their different make-up, one attracts water, the other doesn’t. Try a series of different biscuits, varying from very sweet, dense ones to light, fluffy sponge biscuits. The ‘overnight sogginess index’ increases with density and sugar/salt content. I find that if I put both traditional Italian biscotti (not very sweet and fairly open-textured) and dense, sweet ginger biscuits in a sealed container, the biscotti go rock hard and the ginger biscuits end up very soft.

Chris Vernon

Kwinana, Australia

A baguette dries out while a white sugar biscuit becomes soft because of the hygroscopicity of the white sugar in the biscuit. I researched this last year when entering a competition at the age of 13. We were asked to produce a project about whether cookery was a science.

The water vapour in the air is attracted to the sugar and this makes the biscuit softer. Baguettes however, have no sugar in them and therefore have nothing to attract the water vapour, which evaporates to leave the baguette hard.

When we performed the experiment we used three types of biscuit: one made from caster sugar, another from honey, with the last being the control which had no sweetener. The control lost 2.17 grams of water after being left outside overnight, and the honey lost 2.03 grams, but the caster sugar biscuit gained 1.23 grams. The honey biscuit lost water because the atmosphere had a lower concentration of water than the biscuit.

Tom Winch

Ely, Cambridgeshire, UK

Starch consists of about 20 per cent amylose and 80 per cent amylopectin. The key to bread becoming stale is amylose retrogradation. Naturally, loss of moisture is involved or it wouldn’t dry out. However, bread can be prevented from losing moisture and still go stale. The linear amylopectin molecules in the starch grains, which are separated by moisture in fresh bread, move closer together and become more ordered as the bread becomes stale making it stiffer.

The process is temperature dependent, with the rate fastest at just above freezing and slow below freezing. Studies show that bread stored at 7 °C (average fridge temperature) becomes stale at the same rate as bread stored at 30 °C. So putting bread in the fridge does not keep it fresher for longer.

Allie Taylor

London, UK

The feature referred to in the question has a parallel in legal terms. Here there is a difference between cakes and biscuits for VAT purposes. This is important because cakes are subject to VAT, while bread is not. Now we have a new definition: a biscuit is something which goes soft when left out, whereas a cake goes hard. What the implications are for VAT on baguettes, I wouldn’t like to imagine.

Richard Butlin

London, UK

Strings attached

Why does grilled cheese go stringy?

John Mitchell

Wishaw, Strathclyde, UK

The uncooked cheese contains long-chain protein molecules more or less curled up in a fatty, watery mess. When you heat cheese, the fats and proteins melt and if you fiddle with the fluid, the chains can get dragged into strings. Grab a bit of the molten cheese and pull, and you get a filament, in much the same way that you can draw and twist cotton wool into yarn.

You can do similar things with polythene from plastic bags by heating or stretching the plastic to curl or stretch the long-chain molecules. When the molecules are curled up, the plastic is softish and waxy. When they are stretched into fibres, the result is elastic and strong in the direction of the stretch, although it splits easily between the chains lying along the fibre.

Jon Richfield

Dennesig, South Africa

As the cheese melts, the long-chain protein molecules bind together to form fibres in the liquid mass of melted cheese. I believe that this characteristic can actually be used to measure the protein content of a cheese sample directly. A string of cheese is pulled away from the sample, and the distance to which the fibre will extend away from its attachment point on the main piece of cheese is measured against some reference sample of known protein content.

Mike Perkin

By email, no address supplied

Micro madness

A colleague of mine is in the habit of heating bottled water for his tea in a mug in a microwave oven. When the water is up to temperature he removes the mug.

On several occasions, the water has started to bubble violently after he has added a tea bag. On one occasion, the boiling started when he was removing the mug. It was so violent that it blew 90 per cent of the water from the mug - which is obviously quite dangerous. What is happening?

Murray Chapman

By email, no address supplied

A portion of the water in the cup is becoming superheated - the liquid temperature is actually slightly above the boiling point, where it would normally form a gas. In this case, the boiling is hindered by a lack of nucleation sites needed to form the bubbles.

This never occurs when boiling a kettle, for example, because the presence of the rough surface of the element, as well as the convective stirring from rising hot water, are sufficient to produce proper boiling. Turbulence in liquids is known to provide enhanced nucleation in other cases: when you pour a cola drink, for example.

In your colleague’s case, the addition of a tea bag (and, in the other case, simple movement) sufficed to allow bubble formation. Even with a large proportion of the water superheated, only a little will convert to steam, as the amount of latent heat required for this phase change is very large. I imagine that by keeping the cup still and microwaving for a long time, you could blow the entire contents of the cup into the interior of the microwave as soon as you introduced any nucleation sites. It is this sometimes explosive rate of steam production that means you should take great care when using a microwave oven.

Richard Barton

Guildford, Surrey, UK

Superheated liquid can boil explosively if something is added, as in the examples given by your previous correspondent, or if the vessel is moved. I have seen a spectacular explosion of a bottle of liquid which had just been removed from a microwave in a laboratory - glass and hot liquid were thrown across the room. This can be avoided by leaving any liquid that has been heated in a microwave to stand for at least a minute before touching it or opening the door. This allows for slight cooling and for the heat to become more evenly distributed. I recommend that everyone does this when heating liquids in a microwave, even to make a cup of tea.

Diane Warne

Cambridge, UK

Green ham common

What causes the greenish iridescent sheen that I often notice on bacon and ham? Is it harmful, and why does it vanish when the product is heated? Does this occur on any other foodstuffs?

Georgina Godby

Cambridge, UK

You are likely to find such a sheen on foods containing traces of fat in water. When it is cool this mix separates out microscopically into a film, like oil on a wet road.

In some types of cold meats, such as sliced silverside of beef or some hams, you may see a handsome opalescence. The beauty of an opal results from light being refracted and diffracted by arrays of microscopic beads of glassy material in a matrix of a different refractive index. In the meat, the effect is caused by microscopic spheres of fat dispersed in watery muscle tissue. Heat up the meat and you destroy the droplets and change the optical character of the matrix so that the effect is spoilt.

Jon Richfield

Dennesig, South Africa

The green colour that is sometimes observed on bacon and ham is the result of the action of nonpathogenic bacteria which break down the oxygen transport protein myoglobin to produce porphyrin derivatives. These derivatives are large heterocyclic compounds which can have greenish colours.

Stephanie Burton

Department of Biochemistry and Microbiology

Rhodes University

Grahamstown, South Africa

My father, working alone in the Australian bush in the 1920s and 1930s, ate meat either fresh, soon after it was killed, or after it had been hung in a tree long enough for it to turn a brilliant green. The meat was put into a bag to keep the flies off it.

He claimed that the green colour showed that the meat was no longer dangerous to consume, and it certainly never killed him. However, there is little doubt that it did change the flavour considerably.

Jan Morton

West Launceston, Tasmania, Australia

Iridescence is caused by light striking a surface and being scattered. The scattered waves interfere to produce a spectrum of colours which changes depending on the position of the observer. However, if you see a bright green colour rather than a mere iridescent sheen then your meat may be only for the hardy stomachs of those who tramp the Australian bush - Ed.

Floaters

What is the force that drives an isolated and floating piece of wheat or rice breakfast cereal through the milk to the side of the bowl where it aggregates with its companions?

John Chapman

Perth, Western Australia

The force is due to an imbalance in the pull from the surface tension of the liquid around the sides of the floating piece of cereal. A simple experiment explains what is happening.

You need tap water and two polystyrene cups plus two small pieces taken from a third cup (two 1-centimetre diameter circles will do nicely). Fill the first cup up to within 1 centimetre of the rim, fill the second cup to the top and then carefully add more to the second cup until the water is up over the top of the cup but not spilling over, that is until the water is held in a convex bulge above the top of the cup by surface tension.

Now place the small circles of polystyrene in the middle of each. The piece floating in the partially filled cup will, with a little prompting move to the side of the cup and be held there. By contrast, the piece floating on the convex bulge of the water in the second cup will remain naear the centre. Furthermore, if you push the piece to the edge of the cup, say with the tip of a pencil, the edge repels the small piece towards the centre with considerable force.

This is all caused by the surface tension of the water. In the partially filled cup the water surface curves up to meet the polystyrene. This occurs because water molecules are more attracted to polystyrene than to each other. The water forms the convex bulge at the top of the second cup because the surface tension constrains the liquid surface to the smallest area possible, which similarly accounts for the spherical shape of liquid drops.

The water also curves up to meet the small circle of polystyrene on all sides. Where the water meets the polystyrene of the small circle, the surface tension pulls on each contact point in a down and outward direction provided by the angle of contact with the water. When the circle is in the middle of the cup the pull on the circle on one side is directly balanced by the pull on the opposite side, because the water curves up to meet the circle equally at all points.

However, if the piece is moved towards the side of the partially filled cup, the upward curve of the water surface near the cup side reduces the curve of the surface in contact with the circle. This increases the outward pull on the side of the circle nearest to the cup edge, resulting in a net force towards the side of the cup.

The effect also accounts for the clumping together of cereal pieces on the surface of milk in your bowl and similar behaviour of leaves and twigs on ponds and lakes.

Ray Hall

Warrenville, Illinois, US

Maybe it is a defensive strategy, they huddle together like bison, to protect each other from the predator (you). Or maybe it’s just the surface tension in the milk.

Per Thulin

By email, no address supplied

The fact that rice and wheat - or any grain for that matter - can gravitate towards its companions in this fashion, depends upon their being able to ‘sense’ their way towards the common centre of mass. This ability is known as the Grain of Common Sense.

Research has shown that when human beings are dropped into a large bowl of milk this flocking or aggregation potential is entirely lacking, thus proving that they don’t have a single grain of common sense at all.

Martin Millen

Kidlington, Oxfordshire, UK

Vulcanised eggs

Most substances melt when heated, so why does my scrambled egg turn from liquid to solid as I cook it?

David Phillips

Warwick, UK

Not all changes between solid and liquid are to do with melting or cooling, including congealing scrambled eggs and polymerisation of plastics.

Yolk and albumen - egg white - get their textures from globular proteins dissolved in them. The globules form because the chain-like protein molecules curl up into balls. Electric charges at particular positions on the chains hold the proteins in the shapes suited to their functions. Charges on the outside of the globules attract water molecules, thereby repelling other proteins and stopping them from clumping together.

The balls are not permanent structures, and the charges do not fasten the proteins very tightly. Rattle them violently, by heating for example, and they unravel, exposing their inner charges. This is called denaturation, because the changed proteins are unsuited to their biological functions. Opposite charges in neighbouring molecules can now meet and stick the proteins together, congealing them into huge tangles. But your digestive enzymes can break down such tangles more easily than the undenatured proteins - so bon appétit!

Jon Richfield

Somerset West, South Africa

When you heat a solid, such as ice, you transfer energy to the molecules, allowing them to break the chemical bonds that hold them in a solid state. In the liquid state, they have enough energy to move around, but not quite enough to separate completely from other molecules and form a gas.

When you heat a raw egg, an entirely different process takes place. The egg is made up of individual proteins floating in water, the proteins consisting of long-chain molecules twisted and held in a roughly spherical shape by chemical bonds. As the egg is heated, these bonds break and the molecules unravel, bonding with other molecules to form a network that traps the water and turns the egg solid. Heating the egg further causes even more bonds to be formed, so the egg becomes less watery and more rubbery.

Nicholas Smith

Hollybush, Cwmbran, UK

Eggs are mainly made of proteins dissolved in water, the most abundant of which is albumen, constituting most of the egg white. Proteins are made up of a variety of 20 different amino acids, which form polymer chains folded densely in a unique and relatively stable 3D structure.

On heating, the egg dehydrates and the protein chains unfold and denature. The heat causes sulphur-hydrogen groups on the amino acid cysteine to oxidise and form covalent bonds between neighbouring molecules. These strong, stable bonds are called disulphide bridges, and this cross-linking causes the chains to form networks, so the egg hardens. Disulphide bridges also contribute to the high tensile strength of fingernails and the shape of hair. When hair is ‘permed’, the disulphide bridges are broken by a reducing agent. The hair is then styled into the desired shape and an oxidising agent is used to reintroduce the covalent bonds and maintain the new shape.

Ignatius Pang

Enfield, New South Wales, Australia

A matter of taste

How does temperature affect the taste of food and drink? For example, white wine, tap water, Cointreau, lager and even chocolate taste much better cold. On the other hand, tea, coffee and brandy, as well as most cooked meals, taste much better warm or hot. English beer and red wine are better at room or cellar temperature. Why?

Andrew Newell

Cape Town, South Africa

What we normally refer to as ‘taste’ is more correctly termed flavour, which is made up of taste, irritation and aroma. Taste per se consists only of the five sensations that can be detected by the tongue: salt, sweet, sour, bitter and umami. These are not affected by temperature and nor is irritation from, for instance, chilli peppers. But aroma, which is sensed in the nose, is strongly affected by food temperature because it depends on the release of volatile oils. The higher the temperature, the more volatiles are released, and the stronger the aroma and thus the total flavour sensation.

The flavour of foods that have little aroma is enhanced by heating, whereas foods with strong aromas may become overpowering at high temperatures. Red wines, for instance, tend to be drunk at room temperature with meals that have strong flavours, so achieving a balance in which food and drink complement each other, rather than cancelling each other out. White wines, on the other hand, are often drunk cold with fish or weakly flavoured foods. When imbibed at room temperature on its own, however, white wine gives a perfectly pleasant flavour sensation, and one suspects it is just convention for white wine to be served chilled.

Another important effect of temperature on meals is its influence on the viscosity of starch-thickened sauces, which drops at higher temperatures because starches react to heat. The texture of food is very important to people. A meal covered in a cold, starch-thickened sauce is pretty unappealing, while a non-starch-thickened sauce such as mayonnaise covering the same ingredients in a sandwich would be a very different prospect.

There is also a large element of convention and cultural preference involved. We prefer our gazpacho cold but our minestrone piping hot. Beer is served at room temperature in the UK but chilled almost everywhere else. Some people prefer whisky on the rocks, others - especially in Scotland - find ice an abomination. Hot coffee and iced coffee are equally acceptable to most people, and choice depends mainly on ambient temperature. It’s all about circumstance, accompanying flavours, and how we are used to having our food and drink served.

Jon F. Prinz

Wageningen, The Netherlands

Lager doubt

Two advertisements for lager that ran on British TV presented a paradox. The first, for American brand Budweiser, suggests that the key to good lager is fast shipment from brewery to bottle to drinker. It says fresh lager tastes better. The second, for Dutch beer Grolsch, makes exactly the opposite claim. It stresses the importance of a long conditioning period to improve flavour before the beer is bottled. Which will produce a better beer and why?

Mick McCarthy

Northwood, Middlesex, UK

As a keen home brewer I feel qualified to answer the question on the ageing of lagers.

All true lagers are aged before consumption. Lager in fact comes from the German word meaning to store. After fermentation, the beer undergoes a storage - or lagering - process at low temperature to allow the beer to mature and take on the distinctive clean taste for which lagers are famous. Lagering takes from one week to more than six months, depending on the style. I suspect that both Budweiser and Grolsch undergo this process.

In general, European lagers tend to be more complex than American lagers, which are usually lighter and less intricate in style. Because a complex beer will gain more from lengthy lagering, European lagers tend to be matured for longer than American ones.

After lagering the beer is bottled. Once bottled the beer can spoil easily through exposure to light, oxygen or high temperatures. Fast shipping and sale minimises the chance of beer spoilage. So, in short, both claims are correct. A lager needs to be matured to develop the correct flavours, and fast shipping, once matured, is important.

As for which brand is best, that is a matter of personal taste.

Dave Martin

Hornsby Heights, New South Wales, Australia

Both advertisements are correct and the two claims are unconnected.

After fermentation a beer needs first to be matured and aged at a cool temperature, usually between 4 and 7 °C. During this period the residual yeast in the beer carries on metabolising and, because the beer has become nutrient-poor during brewing, reabsorbs compounds that had previously been excreted. The most notable of these is diacetyl, which imparts a butterscotch taste to the beer. Meanwhile the yeast content of the brew steadily falls as the yeast sediments.

Next, the beer is chilled to -1 °C or below. This promotes protein coagulation and precipitation, which increases the physical shelf life - or the time the beer takes to go hazy. At the end of all this the beer is filtered and bottled.

From here on it’s all downhill. Bottling is traumatic to beer. It is filtered, pumped, packaged and pasteurised. Some contamination with oxygen is unavoidable, and this immediately gets to work on the compounds in the beer, starting a process of deterioration.

In conclusion, mature it slowly and at length to get a good flavour and then get it into the drinker as fast as possible before it deteriorates. A reasonably good taster can distinguish between a week-old and a month-old bottle from the same batch.

David Cefai

San Gwann, Malta

Beer is ‘raw’ immediately after fermentation, and any harsh sugars that are present, such as the Belgian candy used in some beers, burn the nose, while the hops taste like freshly cut grass. The conditioning, or lagering, period is a very slow fermentation during which these raw flavours mellow and the subtler flavours increase in complexity.

At some point the beer reaches its peak of flavour and starts losing taste. A pale ale might peak between one and three months after fermentation, while a high-gravity imperial stout could still be developing years later. Many beer experts think that US-style Budweiser is a very light taste to begin with and that because their breweries have strong quality control over every step of their process, they can reduce the need for longer maturation and clarification periods without affecting the taste too much. European lagers, on the other hand, have longer lagering periods because they are far more complex in taste.

After pasteurisation, beer is essentially defenceless against degradation. Any temperature swings between the brewery and consumption spoil the taste. Even worse, compounds known as alpha acids from the hops are light-sensitive - photons break down the isohumulones in the liquid, creating 3-methyl-2-butene-1-thiol, which gives the beer a skunky smell and taste. And yes, it really is the same compound found in skunk spray. Brown bottles slow this process, but clear and green bottles provide almost no protection. Some brewers use chemically modified hop compounds that are resistant to skunking, but even so it is best to use an opaque container, and a steel cask beats anything else.

So both adverts are correct. You need a conditioning period for the flavour of the beer to peak, however long that may be. But once you reach that peak you would ideally drink it immediately, especially if it is pasteurised.

Ron Dippold

Brewer

San Diego, California, US

The time it takes to brew a beer and how quickly it is shipped to consumers are really two different aspects of the overall brewing, packaging and distribution process. So, the two claims are not opposite, but complementary in ensuring a good-quality beer.

Anheuser-Busch brews Budweiser for just the right amount of time to give the beer its unique clean, crisp taste. While we assume all quality brewers understand the time needed to brew and mature their beer, we make the additional effort to bring a freshly packaged beer to the consumer. We even suggest when it should be consumed to ensure it has the freshest taste: within the first 110 days. Our ‘Born On’ date provides this information and recommendation.

We know consumers are looking for the best-tasting beer available, and the fact is, fresh beer tastes better, hence our policy.

Alan Henderson

Production Director, Stag Brewery

Anheuser-Busch Companies, UK