Bluto strikes back
My Italian recipe book says that I should cut cooked spinach with a stainless steel knife to avoid discoloration. Which would become discoloured if I don’t – the knife or the spinach? And what would be the chemistry at work?
Hans Hamich
Hull, East Yorkshire, UK
The reason you should you always use a stainless steel knife to cut your spinach is intriguing and is a major stumbling block to the fortification of food with iron. Remember that a lack of iron is the world’s most prevalent nutritional deficiency.
Both iron blade and spinach will become discoloured because of the reaction between the polyphenols in the spinach, and the iron blade. If you want to see a dramatic illustration of this, make yourself a cup of tea and add a few crystals of a soluble iron salt such a ferrous sulphate (don’t drink it).
The black discoloration you see is caused by the reaction between polyphenols, called tannins, in the tea, and the iron. The resulting black compound is highly insoluble. The implications for iron absorption in the body are huge because iron in this form is virtually unavailable for absorption. So whatever the source of strength in Popeye’s spinach, it is not the iron.
Polyphenols are found in many vegetables and, together with phytates, are the reason why many people who subsist on cereal and vegetable diets have iron deficiency. Fortifying such diets with iron salts creates two problems. Firstly the iron is not absorbed, and secondly the coloured iron-polyphenols make the food look unattractive.
Patrick MacPhail
Department of Medicine
University of the Witwatersrand
Johannesburg, South Africa
Beer orders
I occasionally add lemonade or ginger beer to a glass of beer to make a shandy. If the beer is poured first and the lemonade or ginger beer are added afterwards, the contents will fizz and even overflow. Pouring the soft drink first and then topping it up with beer avoids the problem. Why is this?
Bryan Harris
Nairobi, Kenya
If you pour the liquids into separate glasses you will notice that only the beer forms a head. That is because it contains surfactants, proteins and other long-chain molecules that help liquid films to form and stabilise bubbles. Lemonade bubbles, on the other hand, burst much too quickly to form a head.
Lemonade that is poured into beer sinks turbulently, stimulating vigorous bubble formation. The beer on top is barely diluted at first, so bubbles surfacing through it quickly form a beery froth. Beer poured into lemonade also rushes to the bottom, but in this case the bubbles surface in lemonade, and therefore quickly pop as usual. By the time enough beer to support a froth has reached the top, most of the fizzy fuss is over and so no head forms.
A possible extra cause of shandy foam’s weakness is that liquid films are terribly sensitive to unequal strengths in their surfaces. Mix practically any two different kinds of foam, and the bubbles that are present start breaking much faster. As an experiment, pour a glass of beer with a good head, then add droplets of lemonade, dishwashing liquid, gin, grains of salt, squirts of citrus zest and so on, and see which of them causes the most drastic erosion of the froth.
Jon Richfield
Somerset West, South Africa
Spectral images
When condensation forms on a clean bathroom mirror, you can draw pictures in it. When the condensation evaporates, the pictures disappear. But when it forms again, they reappear. Why?
Glyn Williams
Derby, UK
When water vapour condenses on a dry mirror, it does so as separate droplets, a process known as dropwise condensation. The numerous drops effectively screen the mirror so that it appears opaque.
When you draw on the surface with your finger, the droplets coalesce into a thin film of transparent water, so the mirror becomes reflective again in these areas. When the mirror warms up, or the air humidity falls, the droplets evaporate. The image disappears because the surrounding droplets no longer contrast with it.
The film of water evaporates more slowly than the droplets because of its lower surface area. If it does not have time to completely evaporate, any condensation occurring soon afterwards will be dropwise where there were droplets before, and in a film where some of the film remains. This latter process is known as filmwise condensation. The image then reappears on the glass.
If the mirror dries completely, the pattern should not normally reappear when further condensation occurs, though it might if the surface has been contaminated where you drew the image. Drawing a finger across the mirror when making the image may leave traces of sweat on the surface which would, due to its salt content, help to promote filmwise condensation.
Dropwise condensation is known to chemical engineers to be more efficient at transferring heat than filmwise condensation, but in practice it is much more difficult to promote, because as the droplets enlarge, they touch each other and coalesce, so the process tends to become filmwise. On the other hand, dropwise condensation is easy to prevent.
Wiping the mirror with a cloth or a tissue wetted with a small amount of detergent such as shampoo leaves an invisible film on the surface. This reduces the surface tension of the condensing droplets, causing them to flatten out and readily coalesce into a film. This is the basis of the anti-misting fluids which are used for treating spectacle lenses and car windscreens.
Tony Finn
Hull, East Yorkshire, UK
When you draw an image in the condensation mist, you leave traces of finger grease (or, if you have just washed, grease plus shampoo or soap). The film is transparent, so you don’t see it when the condensation clears. The next time water vapour condenses on the cold mirror, there is a difference in droplet size between condensation on clean glass and on contaminated glass.
In some cases, it is the contaminated glass that encourages droplet formation, and then you see the image as positive rather than negative. But usually water-loving surfactants such as soap reduce the formation of droplets and generate a smoother, clear film of water, contrasting with the grey mist on the surrounding glass.
Hugh Wolfson
Altrincham, Cheshire, UK
Whisking disaster
For years, whenever my family have come to visit, I have made meringues, which involves whisking egg whites until they are thick. I have always used free-range eggs, but recently I bought organic free-range ones, and no matter how much I whisked these whites they would not thicken. Why should organic eggs behave in this way? Is something missing from the birds’ organic diet that prevents the whites of their eggs thickening?
Vera Gaylor
Billericay, Essex, UK
Your correspondent may have jumped to an unwarranted conclusion from a single instance. I regularly whisk organic free-range egg whites without any problem, and bearing in mind that in culinary history all eggs could once have been so described, it seems unlikely the egg is the problem.
J. Oldaker
Nuneaton, Warwickshire, UK
A good, stiff meringue froth demands a complex interconnection of suitably distorted protein molecules. Anything that interferes with the interlinking of the molecules leaves the whites an unappetising slush. Oil is the usual culprit. Use clean, dry utensils, free of detergent. Before the whites have formed a stiff froth, the merest drop of cooking oil, cream or oily yolk getting into the whites can ruin the meringue.
Jon Richfield
Somerset West, South Africa
The eggs from our pet chickens, while not certified organic, are as close as we can get, and make wonderful meringues. The problem, I suspect, is freshness. Whites from eggs less than 5 or 6 days old will not whip up. Supermarket eggs can be up to 2 weeks old, but the organic ones were probably newly laid.
This raises the question of what changes eggs undergo as they age to allow them to make successful meringues.
Phil Baker
Uxbridge, Middlesex, UK
The eggs may have been too young. I assume that the protein molecules develop cross-linkages as the egg ages, enabling the albumen to contain the air bubbles when whipped.
Lorna English
By email, no address supplied
Concerned consumer
I always use blue toilet paper because it matches my bathroom decor. However, a friend told me that I should only use white, because coloured paper is more damaging to the environment. My local supermarket sells a huge variety of colours with any number of patterned varieties too. Is it true that some varieties are more environmentally damaging? And if so, why? Is kitchen roll even worse than toilet paper?
John Shaw
Driffield, East Yorkshire, UK
If your friend means that the dyes are ecologically harmful, forget it. Chemically active groups on the dye molecules cling to the cellulose, which is why the colours don’t run and leave you fundamentally decorative after you apply them. The dyes are like a mousetrap that has caught a mouse: the mouse, in demonstrating its bite, has become harmless. Much as the trap is hard to reset, the dyes are hard to release from the paper.
Dyes are expensive, and toilet paper requires only traces, so even the most environmentally unaware manufacturer will prefer safe dyes that are simple to handle, and can be applied stingily, typically in parts per million. When the paper reaches the sewage works, the immobilised molecules soon succumb to bacteria, so they do not accumulate in the environment.
If you doubt this, buy a job lot of toilet paper, fold wads of say 10 squares, each of a single colour, bury them separately in moist garden soil, and in a month or two exhume them and observe the result. In good soil you will do well even to detect your test pieces after the earthworms have done their work.
Much the same applies to kitchen paper, except that its strength while it is wet may mean it breaks down more slowly. Its persistence probably does more to provide bacteria with a durable home than harms the environment in any way.
Anyway, what about the bleaches necessary for producing white toilet paper? If you really want to be politically correct, go for garbage grey.
Jon Richfield
Somerset West, South Africa
Wood is brown. Unbleached paper is brown. White paper improves the contrast between text and background to aid reading, so most people prefer it. To make paper white, it is usually bleached with chlorine, which can form carcinogenic dioxins. The paper industry has substantially cut down the quantity of dioxin by-product it produces, and there are initiatives to eliminate it totally by using only hydrogen peroxide and ozone bleaches, which are somewhat more expensive.
Incidentally, what we consider brilliant white is actually slightly blue. Many papers therefore contain fluorescent whitening agents (FWAs), that re-emit UV light as blue light, plus some blue dye. You may have seen clothes and paper containing FWAs glowing under ‘black lights’.
Brady Hauth
Salt Lake City, Utah, US
Pickled poser
I love pickles and chutneys. But I’d like to know what nutrition is conserved and what is lost when vegetables are prepared and preserved in this manner.
Aidan Hancock
London, UK
Pickles and chutneys were originally a means of preserving fruit and vegetables using a combination of heat processing to kill bacteria, fungi and yeasts, with added sugar, acid (in the form of vinegar) and salt acting as preservatives.
In some cases the raw fruit or vegetable is fermented for weeks or months in a brine containing lactobacillus bacteria, which produce the natural preservative lactic acid. Mango and cabbage can be preserved in this way. This also develops the texture and flavour of the fruit or vegetable.
Vitamins, minerals and nutrients are lost when the cooked or fermented plants are washed during processing. Unstable vitamins start to break down soon after harvesting, a process which is accelerated at high cooking temperatures and at the acidic pH typical for these types of product.
Some pickles and chutneys are prepared in oil rather than a sweet sauce, and there is generally less vitamin breakdown in the oily type because cooking is less severe. If we take mango as an example, the raw fruit contains about 40 milligrams of vitamin C per 100 grams of fruit, while oily mango chutney only contains about 1 milligram and in sweet chutney vitamin C is barely detectable.
Mark Wareing
Sedbergh, Cumbria, UK
Many pickles are lightly cooked, or even just blanched or fermented. But chutneys are nutritionally different, as they are cooked almost as aggressively as jam. Both are products of pre-refrigerator preservation technologies, but their preparation and storage cause nutrient loss in four main ways: leaching, heating, oxidation and degradation. The main losses are of soluble or unstable nutrients such as some vitamins, antioxidants and minerals.
Pickling fluid itself causes hardly any degradation. How much leaching occurs depends on how the pickles are cooked and stored in liquid. For instance, large chunks leach less than grated material, which has a larger surface area. Using pickle fluid in soups or stews is a tasty way to reduce this loss. Bulk nutrients such as starches and proteins are not much affected, and in fact processing may improve their digestibility.
In modern pickling mild preservatives prevent decay. Manufacturers also rely on opened containers being kept cold to slow degradation and decay. Darkness also protects light-sensitive vitamins such as A and C. To prevent oxidation, jars of preserves should be closed tightly and used soon after opening.
Pickles have an honourable nutritional history. As well as simply combating starvation, the likes of sauerkraut have prevented many a case of scurvy during northern winters.
Antony David
Cork, Ireland
Dunking dumplings
Whenever I am preparing Italian potato dumplings, or gnocchi, I notice that they behave strangely. When I put the frozen gnocchi into lightly salted boiling water, they immediately sink to the bottom. But the main ingredient of frozen gnocchi is frozen water, whose density is about 0.92 kilograms per litre, and the density of boiling water is 0.97 kilograms per litre, so shouldn’t the gnocchi stay afloat until the ice melts and then sink to the bottom? Instead, they rise to the surface after 2 minutes and all float when cooked, when they should be heavier than water. What is going on?
Radko Istenic
Ljubljana, Slovenia
When the frozen gnocchi are placed in hot water the combined density of all the ingredients is greater than the density of boiling water, and therefore the gnocchi sink. As the gnocchi warm up it’s a bit like inflating a rubber dinghy at the bottom of a swimming pool. The air trapped in the dough expands and the combined density of all the ingredients becomes less than the density of boiling water, causing the gnocchi to rise to the top.
Martin Garrod
Portsmouth, UK
I’ve been making gnocchi for a long time and, as I had some frozen ones, I decided to do some rudimentary measurements in my kitchen.
First, my frozen gnocchi had a density of 1.1 grams/ millilitre and they duly sank in plain boiling water. When they came to the surface and were well cooked, I scooped them up, drained them on a towel and took the same measurements again. This stage was very fiddly so the results must be taken with a pinch of salt. There was a 14 per cent increase in volume, an 8 per cent increase in weight and their density was reduced by 5.5 per cent. Curiously, when I placed them in cold tap water to measure the volume, my cooked gnocchi sank.
Gnocchi sink because they are denser than water. However, the dough of well-made gnocchi has many small bubbles of air which stay there and expand when placed in boiling water, so they come to the surface. My cooked gnocchi sank in cold water because the air trapped inside contracted slightly.
Maria Fremlin
Colchester, UK
Spice attack
Why does turmeric stain everything an indelible yellow, including surfaces that appear impermeable to other substances? Other powdered spices such as cinnamon, paprika and chilli do not leave the same legacy. And what is the best way to remove turmeric stains?
Hefin Loxton
Huddersfield, West Yorkshire, UK
Turmeric, the powdered rhizome of Curcuma longa, and paprika, which is obtained from the fruits of sweet peppers, Capsicum annuum, are examples of spices used in cooking as much for their colour as for flavour.
The yellow colour of turmeric is caused by curcumin, which makes up around 5 per cent of the dry powder. The red pigments in paprika are a mixture of carotenoids, principally capsanthin and capsorubin, and in dried paprika they amount to a maximum of 0.5 per cent of the weight.
The red carotenoids, which consist of long, chain-like molecules, are soluble in organic solvents such as petroleum spirit. Curcumin consists of smaller molecules with terminating phenyl groups. It is insoluble in water but dissolves in solvents like methanol. Therefore you might expect that both paprika and turmeric would stain paintwork and plastics, because they dissolve in organic solvents. You would also expect them to migrate to the oily part of food during cooking.
To compare their colouring properties, place a good pinch of turmeric into two small glass spice jars and do the same with paprika. Add a dessertspoon of methylated spirit to one set and the same amount of white spirit to the other (you can repeat the experiment with cinnamon and chilli powder). Upon shaking the mixtures you will see a vivid yellow colour appear instantly in the meths from the turmeric powder and the white spirit turn red from the paprika. When you place a drop of extract from each of the four jars on to a clean white plate, you will see that the turmeric-meths extract has much the strongest colour followed by the white spirit and paprika. The same experiment can be done with acetone (nail varnish remover).
This demonstrates the principal reason why turmeric stains more than other spices – it simply has more extractable colouring material in it. Other reasons will reflect the different physical properties of curcumin and the red carotenoids, as demonstrated in our solubility experiment, and differences in the way the dyes react chemically with solid materials. Drops of your extract can be placed on various surfaces to test their staining ability but do ask first.
Curcumin is stable when heated but is not stable when exposed to light. So to remove a turmeric stain, first clean with methylated spirit and then place the object in sunlight.
Michael Elphick
Nottingham, UK
Rubber horror
Why do rubber bands spontaneously melt? Often I find an ageing one on my desk that has turned into a sticky mess. After a few more months, the sticky mass solidifies and becomes brittle. Why?
Stuart Arnold
Munich, Germany
Natural rubber is made of polyisoprene chains that slip past each other when the material is stretched. When raw, the substance is too sticky and soft to be of much use, so it is toughened with the addition of chemicals such as sulphur that create cross-links between the chains, making the rubber stiffer and less sticky. This process is called vulcanisation.
With time, ultraviolet light and oxygen in the air react with the rubber, creating reactive radicals that snip the polyisoprene chains into shorter segments. This returns the rubber to something like its original state – soft and sticky. Meanwhile, these radicals can also form new, short cross-links between chains. This hardens the rubber and eventually it turns brittle. Any vulcanisation agents left in the rubber contribute to the process.
Whether a rubber band goes sticky or hard depends on the relative rates of these processes, and these rates in turn depend on the rubber’s quality such as what additives, fillers and dyes it contains – and how it is stored. Heat and light speed up the reactions (for example, a 10 °C rise in temperature will roughly double reaction rates), and the presence of strong oxidisers such as ozone creates even more radicals. The eventual fate of your rubber band depends on the temperature in the room, and whether you have a desk by a window or near a machine such as a photocopier that creates ozone.
How much light and heat is required for these changes? The polymer chemistry of rubber is fairly messy and so this is difficult to answer precisely. Obviously, the chemical reactions run slowly if the rubber is in a fridge, more quickly if left on a sunny desktop. A rule of thumb is that reaction rates roughly double for a 10 °C rise in temperature, but this is complicated when you take oxygen and light into consideration. The quality of the rubber is also important, such as whether it contains additives, fillers or dyes that absorb light energy or help transfer radicals. The final factors that influence the change are ozone concentration, UV light intensity and whether the band is stretched or not – stretching brings chains closer together, allowing radicals to jump from one chain to another more easily, and to create new bonds between chains — Ed.
Citric secret
Why does lemon juice stop cut apples and pears from browning?
Brian Dobson
Alton, Hampshire, UK
To answer this question first we need to understand why some plant tissues go brown when cut. Plant cells have various compartments, including vacuoles and plastids, which are separated from each other by membranes. The vacuoles contain phenolic compounds which are sometimes coloured but usually colourless, while other compartments of the cell house enzymes called phenol oxidases.
In a healthy plant cell, membranes separate the phenolics and the oxidases. However, when the cell is damaged – by cutting into an apple, for example – phenolics can leak from the vacuoles through the punctured membrane and come into contact with the oxidases. In the presence of oxygen from the surrounding air these enzymes oxidise the phenolics to give products which may help protect the plant, favouring wound healing, but also turning the plant material brown.
The browning reaction can be blocked by one of two agents, both of which are present in lemon juice. The first is vitamin C, a biological antioxidant that is oxidised to colourless products instead of the apple’s phenolics. The second agents are organic acids, especially citric acid, which make the pH lower than the oxidases’ optimum level and thus slow the browning.
Lemon juice has more than 50 times the vitamin C content of apples and pears. And lemon juice, with a pH of less than 2, is much more acidic than apple juice as a quick taste will tell you. So lemon juice will immediately prevent browning.
You could also prevent cut apples browning, even without lemon juice, by putting them in an atmosphere of nitrogen or carbon dioxide, thus excluding the oxygen required by the oxidases.
An excellent vegetable for observing browning is celeriac. It is possible to cut a large, relatively uniform slice of this root tissue and then lay several small filter paper discs on the cut surface, each soaked in a different solution such as lemon juice, apple juice, vitamin C, other antioxidants, citric acid, other acids and suchlike. Adisc soaked with an agent that blocks the action of oxidases will leave a white circle on an otherwise brown surface.
Stephen C. Fry
Institute of Cell and Molecular Biology
University of Edinburgh, UK
Polyphenol oxidase (PPO) was discovered in mushrooms in 1856 by Christian Schoenbein. It is widespread in nature and found in humans, most animals and many plants. In plants its function is to protect against insects and micro-organisms when the skin of the fruit is damaged. The dark brown surface formed by the skin is not attractive to insects or other animals, and the compounds formed during the browning process have an antibacterial effect.
In some foodstuffs made from plants this browning effect is desirable. For example, in tea, coffee or chocolate it produces their characteristic flavour. However, in other plants or fruits such as avocado, apples and pears, browning is an economic problem for farmers, because brown fruit is not acceptable to consumers and it doesn’t taste good.
Angeles Hernández Y Hernández
Laboratory of Crystallography
Andalusian Institute of Earth Sciences, Granada, Spain
The black stuff?
When I buy a pint of Guinness there is no doubt the liquid is black. Yet the bubbles that settle on top, which are made of the same stuff, are white. The same is true of many types of beer. Why?
Stewart Brown
Bristol, UK
In the interests of science I poured myself a Guinness and waited until the rising bubbles had formed a creamy head. I put a little of this in a dish and examined it through a low-powered microscope. Unlike bath foam, which has many semi-coalesced bubbles, Guinness foam is made mainly of uniformly sized, spherical bubbles of about 0.1 to 0.2 millimetres in diameter, suspended in the good fluid itself.
Near the edge of the drop of foam it was possible to find isolated examples of bubbles, and by viewing objects held behind these it was clear that they were acting as tiny divergent lenses. Just as a clear spherical marble, which has a higher refractive index than the surrounding air, can act as a strong magnifying glass, so spherical bubbles in beer diverge light because the air they contain has a lower refractive index than the surrounding fluid.
As a result, light entering the surface of the foam is rapidly scattered in different directions by multiple encounters with the bubbles. Reflections from the bubbles’ surfaces also contribute to this scattering. Some of the light finds its way back to the surface and because all wavelengths are affected in the same way we see the foam as white. Light scattering from foam is akin to the scattering from water droplets that causes clouds to be white. This is called Mie scattering.
I sat back and drained the glass. On closer inspection, the head of Guinness is actually creamy coloured, and a drop or two that remained in the bottom of the glass had a light brown colour. Although bulk Guinness appears black, it is not opaque. In the foam there is not so much liquid – most of the space is taken up by air. But because light is scattered from bubble to bubble the intervening brew does absorb some of it, providing a touch of colour.
Needless to say, to ensure reproducibility the experiment was repeated several times.
Martin Whittle
Sheffield, UK
Light bite
Aero, a famous brand of chocolate bar, contains bubbles in a chocolate matrix. The bubbles are evenly sized and distributed throughout the whole bar. How do the manufacturers produce this effect? Why don’t the bubbles rise to the surface as the chocolate solidifies?
Natasha Thomas
Watford, Hertfordshire, UK
The way in which the unique Aero bubbles are added is a top-secret process closely guarded by Nestlé Rowntree. We can tell you, however, that there are approximately 2200 bubbles in one Aero chunky bar!
Marie Fagan
Press and PR officer, Nestlé UK
Secret details may be absent but the broad answer is in Rowntree’s British patent GB 459583 from 1935.
The chocolate is heated until it is in a fluid or semi-fluid state, then it is aerated, for example using a whisk, to produce many tiny air bubbles distributed throughout the chocolate. This is poured into moulds and the air pressure greatly reduced as the chocolate is cooled. The reduced air pressure causes the tiny bubbles to grow and gives the finished chocolate its frozen bubbles appearance. The solid chocolate coating on the surrounding surfaces of the bar is placed into the mould before the aerated fluid chocolate is poured in.
The patent gives no clues on how the bubbles are prevented from rising to the surface during manufacture, but this may be due to the high viscosity of the semi-fluid chocolate and the rapid rate of cooling.
Patents provide a great source of technical information. It has been suggested that 80 per cent of technical disclosures appear in patents and nowhere else. You can view and print GB 459583 using the service on the Patent Office website, www.patent.gov.uk. The service provides an interface to British and European patent offices for you to search their databases.
Melvyn Rees
Marketing and Information Division
The Patent Office, London, UK
It’s not the chocolate answer that your reader was looking for, but I was once told that a soap manufacturer used the same process to make floating soap. The experiments were a technical success, inasmuch as the soap floated, but the product was not commercially viable because it dissolved too quickly.
Mike Dignen
Norwich, Norfolk, UK
David Bailey of Brookes Batchellor patent attorneys in Tunbridge Wells, Kent, UK, also picked up on another patent, GB 459582, and so did Armen Khachikian of the British Library’s patents information section. This was filed by Rowntree on the same day as the one mentioned above and contains the ‘Aero’ concept. The chocolate makers clearly knew what they were about. Khachikian points out that eight days before lodging the patent, the Aero name was trademarked. Although British patents expire 20 years after they are filed, the trademark on the name Aero is still in force.
Thanks also to Tom Jackson of Wigton, Cumbria, UK, for ferreting out US patent 4272558 and British patent GB 480951 from 1938 on ‘Improvements in confections for eating or for making into beverages’ filed by Sydney Phillips and Arthur Whittaker. This patent contains information on making bubbles in molten chocolate with pressurised gas and then discharging the gas through a nozzle. It states: ‘The releasing of the chocolate into a region at atmospheric pressure causes the gas to escape from or expand within the chocolate, giving the chocolate a porous, cellular, honeycomb-like open structure.’ — Ed.
Cream on
One of the ways of drinking the liqueur Tia Maria is to sip it through a thin layer of cream. If the cream is poured on to the surface of the drink, to a depth of about 2 millimetres, and left to stand for about two minutes, the surface begins to break up into a number of toroidal cells. These cells develop a rapid circulation pattern which continues even if some of the Tia Maria is sipped through the cream. How and why do these cells develop and what is the energy source?
Geoffrey Sherlock
Amersham, Buckinghamshire, UK
We are glad to see that this Last Word question from 1995 inspired a research project by Julyan Cartwright at the Laboratory for Crystallographic Studies in Granada, Spain; Oreste Piro at the Mediterranean Institute of Advanced Studies in Majorca, Spain; and Ana Villacampa at the Lawrence Livermore National Laboratory in California. Their paper ‘Pattern formation in solutal convection: vermiculated rolls and isolated cells’ was published in Physica A, (vol. 314, p. 291). Over the years since 1995, a number of theories were sent to the New Scientist offices and the consensus was a reaction between alcohol and fat in the cream was responsible. Now we know the real answer, and the first author of the Physica A paper has sent us the following account — Ed.
We tried this and were hooked. It is beautiful to watch these patterns form, and how different patterns form in layers of cream of different thickness.
This is all caused by convection. Convection is the bulk movement of fluid, often associated with temperature differences thermal convection. In Tia Maria and cream the convection is driven by a difference in concentration, and is called solutal convection.
The important component is the alcohol in the Tia Maria. After the cream is poured on top of the liqueur, the alcohol begins to diffuse through the cream layer. When it reaches the surface it alters the surface tension: the more alcohol at the surface, the lower the surface tension. Regions of higher surface tension then pull liquid towards them from the regions of low surface tension. As the surface liquid is pulled away, the liquid beneath these regions of low surface tension takes its place.
But this liquid contains yet more alcohol, because it has come from the part of the cream nearer to the Tia Maria below. It has an even lower surface tension and in turn gets dragged away. This positive feedback mechanism creates convection, which continues as long as there is a concentration difference to sustain it.
This type of convection, driven by surface tension, is called B’enard-Marangoni convection and it is particularly relevant to thin layers of fluid. It is important in situations like drying paint, and the same capillary or surface-tension forces also cause other types of patterns in alcoholic drinks, like the tears in glasses of wine.
The other important mechanism that can cause convection is buoyancy. But buoyancy-driven, or Rayleigh-B’enard convection, cannot be causing the patterns in Tia Maria because cream is lighter than Tia Maria, so Tia Maria with cream on top is buoyantly stable.
The patterns that form when a fluid starts to convect by either of these mechanisms have been well studied in situations such as rolls of clouds in the sky, or hexagons formed in a frying pan when a layer of oil is heated. Tia Maria is an oddity because the patterns are not normal rolls or hexagons.
Similar patterns have also been reported in the scientific literature, in particular in papers written in the early decades of the 20th century. The worm-like patterns in thin layers of cream were called vermiculated rolls, and the toroidal cells in thicker layers, isolated cells. Both appear when there is a surface film on top of the convecting substance that hinders movement between the surface and the bulk of the liquid. In this case, the fatty cream is partially blocking the surface, so these patterns appear.
More recent convection research has tended to ignore these types of patterns, and the old experiments have often been considered as inaccurate because the fluids were impure; the different patterns were thought to be the result of impurity. We have tried to redress this. After seeing the patterns in Tia Maria, we carried out solutal convection experiments with simpler pure fluids that still show the same patterns. You can find a thorough account, both with Tia Maria and more conventional lab chemicals, in Physica A as detailed above.
Julyan Cartwright, Oreste Piro, Ana Villacampa
Spain & US
Honey monster
How can an unopened jar of runny, clear honey suddenly begin to turn into a hardened block of sugar with no obvious external stimulus? Jars that have remained clear for years can, over the space of a couple of weeks, change into solid sugar while the jar remains motionless on its shelf. Temperature does not seem to be a factor – the process can occur in winter or summer.
Billy Gilligan
Reading, Berkshire, UK
Bee-keepers argue about this, as honeys from different sources behave differently. Honey is a supersaturated solution of various proportions of sugars (mainly glucose and fructose), and is full of insect scales, pollen grains and organic molecules that encourage or interfere with crystallisation. Glucose crystallises readily, while fructose stubbornly stays in solution. Honeys like aloe honey, which is rich in glucose and nucleating particles, go grainy, while some kinds of eucalyptus honey stay sweet and liquid for years.
Unpredictably delayed crystallisation means a nucleation centre has formed by microbes, local drying, oxidation or other chemical reactions. Crystallisation can also be purely spontaneous, starting whenever enough molecules meet and form a seed crystal. Some sugars do this easily, others very rarely.
By seeding honey with crystals, or violently stirring air into it, you can force crystallisation. Products made this way are sold as ‘creamed’ honey. The syrup between the sludge crystals is runnier and less sweet than the original honey, because its sugar is locked into crystals. Gently warm some creamed honey in a microwave until it dissolves, compare the taste of the syrup with the sludge – you will be astonished.
Jon Richfield
Somerset West, South Africa
I have seen this happen many times. The time before crystallisation starts seems to depend on the source of the nectar the honey is made from. Oilseed rape honey will crystallise within a week or two of the bees making it. Heather honey never seems to crystallise. Fuchsia honey is extremely runny and, unlike any other I have seen, seems prone to fermentation, even when all the extracted honey comes from cells capped by the bees for storage. Even this crystallises after a year or two.
Pat Doncaster
Cork, Ireland
Gurgle time
Does liquid pouring from an inverted bottle flow faster at the beginning and end of its expulsion or when it reaches the ‘glug-glug’ point somewhere in the middle? And, whatever the case, what is the explanation for the different velocities?
Randy Baron
Basle, Switzeland
Water flowing from an inverted bottle has no free surface. So the water coming out has to be replaced by something else because liquids do not expand or contract very much when the pressure changes. In the case of a thin-walled plastic bottle, the volume can be replaced by the walls of the bottle being pushed in by air pressure as the water runs out, so this is what happens first.
Once this has happened (or in the case of a glass bottle, immediately) another replacement mechanism is needed and bubbles of air have to enter through the neck of the bottle. Essentially, the bubbles and the water escaping have to take turns, coming in and out, respectively, giving rise to the glug-glug effect.
Two other important factors affect the rate of flow. Firstly, if there is a significant amount of gas inside the bottle above the liquid, this can expand to replace the volume of water lost. This process goes on until the reduced pressure of the gas is just sufficient to support the height of liquid above the exit. Then glug-glug starts again.
The second factor is swirl. If the bottle is handled so that the liquid swirls, the water moves to the outside of the neck and allows a column of air relatively free passage up the centre. By actually moving the bottle in small circles before releasing the liquid, you can get a very effective tornado in a bottle.
These effects are important in industrial separating devices called hydrocyclones, which are shaped something like an inverted milk bottle. It is possible to tell how effectively these devices are working simply by viewing the pattern of discharge from the bottom, described as rope, cone or spray.
Martin Pitt
Department of Chemical and Process Engineering
University of Sheffield, UK
Note to bar staff in a hurry: A little experimentation in the New Scientist laboratory confirms that whichever angle the bottle is held at, flow is fastest when the bottle is full, because this is when there is greatest pressure on the liquid at the mouth of the bottle.
To pour out the contents of a bottle quickly, it is far more effective to keep it at an angle than simply turn it upside down. This is because the angled pour avoids the gurgling that slows the passage of liquid through the neck of the bottle. For an extra edge, follow Martin Pitt’s advice: spin the bottle and then invert it while continuing to rotate it rapidly about its axis.
We found that a 750-millilitre wine bottle emptied in 9.9 seconds if inverted, but in 8.1 seconds if held at 45 degrees. Swirling the bottle so that a little tornado forms in its neck, allowing air to enter continuously and replace the liquid, brings pouring time down to just 7.7 seconds.
In all cases, the rate of emission slows as the head of water above the neck falls. Dividing the volume of water in the bottle into equal thirds, the first, second and third volumes left the inverted bottle in 2.5, 3.5 and 3.8 seconds; the angled bottle in 2.0, 2.4 and 3.7 seconds, and the swirling bottle in 2.0, 2.3 and 3.3 seconds. The swirling technique, although very smooth once perfected, is not recommended for high-speed pouring of beer or any drink containing gas — Ed.
Changing tastes
Monosodium glutamate is a common flavour enhancer that is used particularly in Chinese and Japanese cooking. Why is it so popular in these cuisines and, more pertinently, how does it enhance the flavour of food?
Michael Stuart
Goole, East Yorkshire, UK
Monosodium glutamate or MSG is presumably most commonly used in oriental cooking for traditional reasons. For thousands of years the Japanese have incorporated a type of seaweed known as kombu in their cooking to make food taste better. It was not until 1908, however, that the actual ingredient in kombu responsible for improvement in flavour was identified as glutamate.
From then until 1956, glutamate was produced commercially in Japan by a very slow and expensive means of extraction. Then large-scale industrial production began and has continued, mainly involving the fermentation of natural substances such as molasses from sugar beet or sugar cane. Today, hundreds of thousands of tonnes of MSG are produced all over the world.
Monosodium glutamate contains 78.2 per cent glutamate, 12.2 per cent sodium and 9.6 per cent water. Glutamate, or free glutamic acid, is an amino acid that can be found naturally in protein-containing foods such as meat, vegetable, poultry and milk. Roquefort and Parmesan cheese contain a lot of it. The glutamate in commercially produced MSG, however, is different from that found in plants and animals. Natural glutamate consists solely of L-glutamic acid, whereas the artificial variety contains L-glutamic acid plus D-glutamic acid, pyroglutamic acid and other chemicals.
It is widely known that Chinese and Japanese food contains MSG, but people don’t seem to be aware that it is also used in foods in other parts of the world. In Italy, for example, it is used in pizzas and lasagne; in the US it is used in chowders and stews, and in Britain it can be found in snack foods such as potato crisps and cereals.
It is thought that MSG intensifies the naturally occurring ‘fifth taste’ in some food – the other, better known, four tastes being sweet, sour, bitter and salt. This fifth taste is known as umami in Japanese, and is often described as a savoury, broth-like or meaty taste.
Umami was first identified as a taste in 1908 by Kikunae Ikeda of the Tokyo Imperial University, at the same time that glutamate was discovered in kombu. It makes good evolutionary sense that we should have the ability to taste glutamate, because it is the most abundant amino acid found in natural foods.
John Prescott, associate professor at the Sensory Research Science Center at the University of Chicago, suggests that umami signals the presence of protein in food, just as sweetness indicates energy-giving carbohydrates, bitterness alerts us to toxins, saltiness to a need for minerals and sourness to spoilage. A team of scientists has even identified a receptor for umami, which is a modified form of a molecule known as mGluR4.
Mark Bollie
Amersham, Buckinghamshire, UK
Curious cuppa
When you add a few drops of lemon juice to a cup of black tea, the colour of the tea lightens considerably and very quickly. Why?
Stuart Robb
Strathaven, Lanarkshire, UK
The simple answer to this question is that adding lemon juice alters the acidity of the tea and the colour change is an indication of this, in the same way that litmus paper changes colour.
A similar effect can be observed by substituting the tea with some cooked red cabbage juice.
Aron
Toronto, Canada
Tea leaves are rich in a group of chemicals known as polyphenols that amazingly account for almost one-third of the weight of the dried leaf. Both the colour of the tea and much of its taste are due to these compounds.
One group of polyphenols, the thearubigins, are the red-brown pigments found in black tea and constitute between 7 per cent and 20 per cent of the weight of dried black tea.
The colour of black tea is also influenced by the concentration of hydrogen ions in the water. Thearubigins in tea are weakly ionising acids and the anions (negatively charged ions) they produce are highly coloured. If the water used to brew tea is alkaline, the colour of the tea will be deeper due to greater ionisation of the thearubigins.
If lemon juice, which is an acid, is added to the tea, the hydrogen ions suppress the ionisation of thearubigins, and that makes the tea lighter.
Interestingly, the theaflavins – the yellow-coloured polyphenols in black tea – are not involved in the change in colour that is associated with a change in acidity.
Johan Uys
Bellville, South Africa
Indestructible wine
I’ve just returned from a holiday in Madeira. I learned that old bottles of Madeira wine – a fortified wine similar to port and sherry – should be stored in an upright position. Bottles stored in this way are still drinkable centuries later. However, most other bottles of wine should be stored lying down to keep the cork moist and intact. Why is Madeira different, surely its cork will dry out too?
Cristina Mariana
Lisbon, Portugal
‘Old bottles of Madeira wine’ don’t have to be stored in an upright position, but unlike other wines, it won’t do them much harm.
Once wines have been bottled, oxygen becomes the enemy. It oxidises the wine, resulting in an unpleasant odour and taste. The purpose of the cork is to keep out all oxygen except the small amount in the neck of the bottle. But because corks dry out and shrink, bottles stored upright will eventually let air in to oxidise the wine. Hence the typical advice to store wine bottles on their sides, keeping the cork moist.
Madeira wine, like sherry and port, is fortified by the addition of brandy before fermentation is complete. This means some residual sugar remains in the wine because the increased alcohol concentration kills off the yeast. Another result of this process is, of course, to make a more alcoholic wine (usually between 16 and 20 per cent by volume instead of between 10 and 13 percent).
This increased alcohol and sugar content tends to protect fortified wine from oxidation, so the danger is lessened.
However, some oxidation will still occur if oxygen is present.
Madeira, however, is a special case. It tastes better when it’s somewhat oxidised, a characteristic that was accidentally discovered and then deliberately exploited in the 18th century by shipping barrels of it on sailing vessels on long journeys through tropical regions. Indeed, the term used for an oxidised dry wine is ‘maderised’, obviously derived from ‘Madeira’. Therefore, the risk of further maderisation to a bottle of Madeira – from a dried-out cork, say – is not as serious as it would be with other wines.
Why, though, might upright storage be recommended? Between 5 and 10 per cent of wine corks rot when kept wet, and bottles sealed with those corks will eventually acquire the mouldy smell of rotten cork. Such a bottle is called ‘corked’ when it is opened and sampled, hence the routine of smelling the cork before pouring the wine. If a bottle of Madeira is stored upright, the cork will never be wet and the bottle will never be corked. So if the risk of oxidation is considered a matter of little concern, and the risk of a mouldy cork is a matter of greater concern, then the bottle should be stored upright. Of course, the very best solution would be to use only superior corks when bottling Madeira.
Will it still be drinkable centuries later? A couple of years ago I had the privilege of opening, decanting and tasting about 50 millilitres of an 1814 Madeira. It was still drinkable – not fine, but drinkable nonetheless. It had been recorked every 25 years or so. Its name was ‘Violet’, the same as my wife, so I kept the bottle. In those days, Madeiras were often labelled with the name of the ship in which they were transported to the US.
Edward Hobbs
Wine consultant
Wellesley College, Massachusetts, US
Vintage Madeira is quite capable of outlasting its cork. The practice therefore is to recork each bottle every few decades. A few shippers even list these recorking dates on their labels, in addition to the vintage date and the grape type. The oxidised state of the wine allows the process to be carried out with a fair degree of confidence, whereas the same process applied to port, sherry or an unfortified wine would risk spoiling the contents.
The process by which Madeira is deliberately allowed to oxidise, known as estufagem, was discovered by accident after barrels of wine that had been sent on the long journey across the tropics to the New World were found to take on a pleasant colour and taste.
For centuries, producers continued to send out their Madeira in barrels to act as ballast for ships and to improve its flavour. Now the barrels are simply kept at tropical temperatures of up to 50 °C for about three months in the island lofts of the wine shippers.
Mark McKegrow
Cheltenham, Gloucestershire, UK
A long drink
What is the maximum length of a vertical straw with which you can drink cola?
Bhargav
Hyderabad, India
If you applied an absolute vacuum above a non-volatile liquid, then the maximum height you could suck it up a vertical pipe would be reached when the hydrostatic head pressure of the column of liquid equals one atmosphere (101,325 pascals). This pressure is given by p × g × h, where p is the fluid density, g is the gravitational acceleration (9.81 metres per second per second) and h is height in metres. For water, which has a density of 1000 kilograms per cubic metre, this gives a maximum height of about 10.3 metres.
However, because water has a vapour pressure of 3536 pascals (at 27 °C), it will begin to boil before you reach a perfect vacuum. So the maximum vacuum pressure that you could apply is 101,325 3536 = 97,789 pascals. This gives a maximum height of 9.97 metres.
In the case of a soft drink, things are more complicated, because the dissolved carbon dioxide will start to ‘boil’ out of solution under vacuum. If you sucked extremely slowly, first of all you would only get CO2 and then, when you had removed the gas, you would get flat soft drink. If you sucked very quickly, then you might get the drink to rise up before the CO2 nucleated and formed bubbles. More likely you would get a froth of liquid and CO2 bubbles, and you might actually be able to suck this up to a much greater height because the effective density of the foamy mixture would be lower than pure liquid water. At intermediate suction rates, the foam bubbles would coalesce and you would be limited to a lower column height.
The exact answer depends on how much dissolved CO2 you want left in your drink and the maximum rate at which you can suck. You would also need more than an ordinary drinking straw, because plastic ones collapse under moderate vacuum pressures.
Simon Iveson
Department of Chemical Engineering
University of Newcastle, New South Wales, Australia
Using a very long, thick-walled, plastic pressure tube, 15-year-old pupils can usually manage a 2-metre suck to get a drink. By sucking, sealing the tube with the tongue, breathing and sucking repeatedly, a 4-metre lift is easily achieved. This is the highest my pupils have managed because the next option is to stand on a step ladder at the top of a stairwell and this not a good idea when you have a class of 30.
I suppose that this height is approaching the limit. The pressure reduction in the mouth is the same as that in the top of the tube, so it becomes difficult to suck against the external pressure, and it also becomes difficult to get your tongue out of the end of the tube.
There is also the problem of internal pressure in the lungs which, if the throat is opened and air is expelled into the tube vacuum, can fall dramatically. It is wise to stop before this point.
Keith Sherratt
Nottingham, UK
Be careful experimenting with the limits of your ability to suck up fluids. Not only could you choke unpleasantly, but strong suction can cause blood blisters in your mouth.
In the Kalahari, as recently as a decade or two ago, the !Kung people sometimes had to suck water out of narrow holes in rocks. In dry seasons, they would join reeds into long straws and the men who were able to suck it up far enough would spit it into a communal container.
Jon Richfield
Somerset West, South Africa
Shock value
Could someone please tell me why and how fabric conditioners reduce the amount of static electricity in clothes?
Johanna
By email, no address supplied
Static electricity is an imbalance of electric charge: a lack or overabundance of electrons on the surface of the material. This typically occurs by ‘tribocharging’ when two materials are brought into contact then separated, electrons are exchanged by the materials, leaving one with a positive charge and the other with a negative charge. Friction between the two materials can enhance this charge-separation process.
Under normal atmospheric conditions, fibres such as cotton and wool have a relatively high moisture content, which makes them slightly conductive. This prevents the charge separation from occurring by allowing static electricity to be conducted away. However, synthetic materials have a high surface electrical resistance particularly when humidity is low – and this prevents the charge dissipating. A layer of fabric conditioner simply reduces the electrical resistance of the surface of fabrics.
Paul Thompson
Twickenham, Middlesex, UK
Static build-up in clothing is caused by fibre-to-fibre, fibre-to-person and even fibre-to-air friction, and depends on the type of fibre from which the garment is made. The amount of static build-up is also highly dependent on the relative humidity – the higher the humidity the lower the charge. Fibres such as rayon, silk, wool, cotton and linen have high moisture ‘regain’ – their fibres absorb a great deal of moisture at a given humidity from a bone dry condition – and are low in static. Fibres such as polyester, acrylic and polypropylene, having low moisture regain, are high in static.
Anti-static finishes or sprays come in two types. The first are made of molecules that contain polar groups, in which charge is unevenly distributed, and these act as conductors to dissipate static charge. The second type are humectants, or water-absorbing materials, that also permit the textile to dissipate static electricity. The increased moisture present on the surface or within the fibre itself increases electrical conductance, helping to drain away the charge.
Textile technologists can design fibres and fabrics to minimise static electricity. In carpets, a small percentage of fibres (up to 3 per cent) can have either a carbon core or a carbon strip to drain away static charge. Carpets and upholstery fabrics may also be made with carbon lampblack mixed into the latex or hot-melt backing material for the same purpose.
If the carpet is made of yarn spun from staple fibres, a small percentage of stainless steel fibres or fibres coated with aluminium or silver vapour may be incorporated into the blend to reduce static electricity. However, less than 5 per cent of this type of fibre can be used because otherwise the fabric takes on a grey tinge.
Bob Wagner
Plymouth Meeting, Pennsylvania, US
Fabric conditioners contain a type of compound called a surface active agent or surfactant. It’s a cationic surfactant, meaning it’s a long molecule (rather like an oil or a fat) with a positive charge at one end. Often the surfactant used is an ammonium compound in which the nitrogen atom is surrounded by four organic groups.
During the washing process the negative charge that forms on the surface of the fabric draws the positive end of the surfactant molecules to itself. These long oily molecules then lubricate the fibres to prevent the friction that causes static cling. It makes ironing easier, and allows the weave to relax and supply that soft, fluffy feel.
Richard Phillips
Fayetteville, Arizona, US
Honey, I’m bendy
Why does a slice of bread spread with honey gradually become concave?
Donal Trollope
Stonehouse, Gloucestershire, UK
My wife has assured me that her bread doesn’t have time to go concave when spread with honey. However, for those folk who chomp their honeyed bread in a more leisurely fashion, there is a simple explanation.
Bread is approximately 40 per cent water while honey is a strong solution containing approximately 80 per cent sugar. This means that moisture is drawn out of the bread and into the honey by osmosis. Removing the water makes the bread shrink, but only on the side exposed to the honey. This causes the bread to become concave.
This is less likely to happen, of course, if you butter your bread before spreading the honey. Butter forms a water-impermeable layer that protects the bread from dehydration by the honey.
Peter Bursztyn
Barrie, Ontario, Canada
Grey matter
The surfaces of the incandescent light bulbs where I work become progressively greyer over time. Why?
Kirsty Rhode
Manchester, UK
The greying of the inner surfaces of incandescent bulbs is the result of gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament so thin it burns out.
Various methods have been developed to reduce greying. Filaments of the first incandescent lamps burnt in a vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of greying. A mixture of nitrogen and argon is used today. In addition, ‘getters’ reactive metals such as tantalum and titanium can be placed near the filament to attract the tungsten so that it is not deposited on the glass. Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally will remove the grey coating from the surface of the glass.
Greying can be almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then redeposit the tungsten on the filament. This keeps the bulb wall clean. To prevent the tungsten halides from condensing on the bulb and breaking the cycle, the temperature of the bulb wall must be at least 500 °C. This is too hot for glass bulbs, which normally operate at about 150 °C, so fused quartz (silicon dioxide) must be used instead.
Compared with ordinary incandescent lamps, quartz-halogen lamps have longer lives and maintain their light output over time. For example, a quartz-halogen lamp with a 2000-hour life will have dimmed by less than 5 per cent by the time it burns out. When an incandescent lamp with a 1000-hour life burns out, it will have dimmed by more than 15 per cent.
Ross Firestone
Winnetka, Illinois, US
This can be explained by the fact that lights work not by emitting light but by sucking dark. ‘Dark sucker’ theory is too complex to be described here in detail, but it proves the existence of dark, that dark is heavier than light, that dark is coloured, and that it travels faster than light.
To answer your question, a bulb becomes darker over time because of all the dark it has sucked in. Similarly, a candle, which is a primitive type of dark sucker, has a white wick when new and this becomes black when used, due to all the dark which has been sucked into it.
Ken Walke
Wigan, Lancashire, UK
Readers should note that the revolutionary ‘dark sucker’ theory has yet to win widespread support from the scientific community — Ed.
Heated hop
I was sitting with a group of friends in the pub last night and, as we contemplated our pints of bitter, we wondered: why does beer go flat when it gets warm? Not only that, but the effect seems to be more pronounced with lager than with bitter.
Jon Shaw
Brighouse, West Yorkshire, UK
The answer lies in the behaviour of gases, and their solubility in water. Most beers are dilute solutions of sugars, gases, organic acids and other complex compounds, and (hopefully) alcohols.
The gas which gives fizzy drinks their distinctive fizz is carbon dioxide. In the case of British-style real ales, the CO2 is generated by the action of yeasts on residual sugars in the drink, whereas in most beers, including British-made lagers, the conditioning gas is added artificially at the brewery or at the point of sale.
The problem arises because the solubility of CO2 is related to the temperature of the solvent in which it is dissolved, in this case the beer. More gas can dissolve in cold beer than in warm beer. This is also why fish such as trout and salmon, which need a lot of oxygen, live in cold mountain streams and rivers, because the amount of oxygen dissolved in these environments is so much higher.
When the beer is served from the pump it will contain a certain concentration of dissolved CO2, but, as the drink heats up under the influence of a sweaty hand in a warm room, its ability to hold its CO2 in solution decreases.
This excess gas is then released into the atmosphere through the bubbles that you see rising in the beer, and the drink consequently goes flat. Other volatile compounds from the malt and the hops vaporise faster and you may notice that the beer also smells different.
The difference your correspondent observes between lager and bitter is mainly caused by two factors. The first is that lagers are generally served much colder than bitters, to disguise the fact that they have less taste (this is because they contain fewer fruity esters and longer-chain alcohols, as a result of the lower fermentation temperatures and different yeast strains that are employed in their manufacture). The bigger temperature difference between the beer and the air means lagers warm up faster than bitters. Their rate of loss of CO2 is consequently much higher, and they go flat faster.
Secondly, lagers are generally more carbonated than bitters, so are fizzier at the point of purchase and therefore have more CO2 to lose in the first place. This extra carbonation, like the lower serving temperature, is usually used to camouflage the lack of flavour that is found in most British-brewed lagers.
The answer to the problem, of course, is to drink your beer faster, or sit in cooler pubs.
Geoff Nicholson
Newcastle upon Tyne, UK