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Food Physics

Have you seen our beer mats and sandwich bags in the pubs and cafes of Norwich? You can let us know what you think by clicking on the mats and bags below - answer the questions and find out what physics has got to do with it.

We would like to thank the Institute of Food Research in Norwich for all their help in putting this campaign together.

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Click on the beer mat to the left to start the quiz and follow the links below to find out more pint sized physics.

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Click on the sandwich bag to the left to start the quiz and follow the links below to discover what physics has got to do with it.

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Click on the headers below to find out about the physics of food and drink. We would like to thank the Institute of Food Research in Norwich for all their help in putting this campaign together.

Dunking Biscuits

Who will win and who will crumble in the Ultimate Biscuit Dunking Challenge? And what's physics got to do with it?

Which biscuit is the tastiest?

Biscuits are made mostly from sugar, flour (which is 60% starch) and fats. When you dunk your biscuit into your drink, the liquid is drawn up through the tiny channels and holes in the biscuit by capillary action. The liquid starts to dissolve the sugar that is holding the biscuit together and makes the starch swell. If you continue to dunk your biscuit without taking a bite, it soon won't be able to hold itself together - leading to a dunking disaster.

As all biscuits have a slightly different structure, they can survive a different number of dunks. Len Fisher from the University of Bristol found that you need to have a thin dry layer of biscuit within your dunked biscuit for it to stay in one piece. He also found that dunking your biscuit sideways gives you the most dunking opportunities as the drink takes longer to saturate the biscuit when soaking it from only one side.

Len Fisher went on to discover that milk was the best beverage in which to dunk a biscuit if you want maximum taste. Dunking your biscuit in milk releases up to 11 times more flavour than eating a dry biscuit. Milk is full of fat droplets suspended in water and fat droplets are very good at absorbing the flavour molecules that excite your taste buds. They also hang around in your mouth so that the flavour and aroma chemicals can sit on your tongue and be released up your nose giving you a better taste sensation.

So which biscuit is the tastiest? It's all down to personal preference, but the best beverage to use is a glass of milk!

Find related websites about biscuit dunking with physics.org

What's the difference between a cake and a biscuit?

A few years back the tax man was trying to add VAT to the Jaffa Cake claiming that it was not, as it said on the packet, a cake which is VAT exempt, but a luxury chocolate covered biscuit for which VAT has to be paid.

The case went to court, so what is the technical difference between a cake and a biscuit?

Biscuits are baked starch and fat held together by sugar, whereas cake has a structure like a sponge. During baking the bubbles in cake mix expand giving the cake a foam-like structure. As the cake continues baking this foam starts to set and form a sponge. If you open the oven door too soon, the bubbles in the delicate foam stage collapse, giving you a sunken cake!

Another difference between cakes and biscuits becomes evident when you leave the lid off the biscuit - or cake - tin. Because of the way they are made, biscuits are very dry (before they are dunked) and so absorb moisture from the air. Cake on the other hand is quite moist, so water is released into the atmosphere if it's left out, making the cake go hard.

To solve the Jaffa Cake cake/biscuit conundrum, McVities made a giant Jaffa Cake to bring to the court's attention the sponge-like structure of Jaffa Cakes and the fact that they go hard when left out. The court ruled that Jaffa Cakes are indeed a cake and should, if you don't want hard cake, be kept in a tin.

Find related websites about humidity with physics.org

How to make reduced fat biscuits

Making lower fat foods is easier said than done. If you simply reduce the fat content of a food you change the structure, texture and flavour release of that food. Fats give us unique sensory stimuli which are very hard to mimic with non-fat ingredients.

So scientists at the Institute of Food Research have been researching how to make reduced fat foods, but without sacrificing the role that fats play. Many reduced fat foods rely on replacing the fats with a starch such as corn, potato or tapioca, or proteins such as egg white or gelatine. All of these ingredients bind with water to produce a thickened gel, recreating that creamy feel we associate with fats.

However, this type of fat substitution doesn't work for all food stuffs, including biscuits and chocolate. Reduced fat biscuits depend on some clever engineering which reduces the overall fat content, whilst retaining the sensation of fat in the mouth. By taking a small amount of fat and wrapping it around a droplet of another liquid, then the overall surface area will be the same as a droplet made entirely of fat. This fools your tongue by mimicking the taste and texture of the fat in a normal biscuit, but the overall amount of fat is reduced.

This technique is called WOW (water-in-oil-in-water) and is currently being developed and due to be used in low fat products over the next few years.

Read more about reduced fat foods from the Institute of Food Research

Physics of chocolate

Physics plays a part in everyone's favourite treat - chocolate. One of the reasons that chocolate is incredibly good to eat is that its melting point is just below body temperature. When you place a piece of chocolate on your tongue it immediately starts to melt, spreading the sweet, satisfyingly fatty taste across your mouth. Chocolate also contains chemicals such as caffeine and theobromine which are thought to contribute to that feel-good lift that keeps chocolate addicts coming back for more.

Chocolate can behave in a rather strange way when heated and cooled. If you've ever cooked with chocolate you may have found that once heated and then cooled it takes on a dull appearance rather than the glossy smooth texture that it started off with. This is because the structure of chocolate can change when heated.

Chocolate crystals

One of the main ingredients of chocolate is cocoa butter which has a crystal structure. When chocolate is heated and then cooled, the cocoa butter sometimes recrystalises with a different structure giving the chocolate a dull, matt and sometimes chalky appearance.

This can cause major problems for chocolate manufacturers as it makes the chocolate look less appetising. So to prevent it, chocolate goes through a process called tempering - it is heated up, then rapidly cooled and then gently warmed again. This forces the cocoa butter to crystallise with the smooth glossy structure that we all find appealing.

In fact, knowing when the wrong crystal structure is forming is tricky, but imaging techniques such as ultrasound can help identify when these changes are going to take place.

Find related websites about crystals with physics.org

The Perfect Cuppa

How do you make the perfect cuppa? And what's physics got to do with it?

Do you add the milk first?

When tea was first introduced to the UK it was often drunk out of fine porcelain cups. These cups were so fine that if you poured very hot water into them they would crack. So the milk was added first to help protect the porcelain from the hot tea.

But there's a reason to add milk first even if you're not using your finest porcelain. Milk contains proteins that change shape, or become denatured, when heated and when the proteins change shape, the taste of the milk also changes. This is why UHT, or Ultra High Temperature, milk tastes a bit strange.

As milk proteins denature, there is a risk that the milk will clot. The change in shape exposes the hydrophobic, or water-hating, parts of a protein and these are attracted to the hydrophobic parts of other proteins and lumps of denatured protein and fat form. The risk of clotting increases as milk gets older, so milk that still smells just about okay can produce unappetising blobs when you add it to your tea. However, if you add chilled milk of the same age to your cup first, there is less risk of the hot tea denaturing the milk and producing the offensive floaters.

Adding milk to your cuppa cools it down whether you add it before or after the tea, but the curious case of Newton's cooling curve means that your brew stays at a drinkable temperature for longer than you might expect.

The initial cooling effect of adding milk is very noticeable and it takes only a few minutes for your tea to go from being boiling hot to drinkable. But because the rate of cooling, or how fast the tea loses heat to its surroundings, isn't steady it then takes a lot longer to go from drinkable to stone cold.

Find related websites on Newton's cooling curve

Do you use freshly boiled water and warm the pot?

Many people swear that the only way to bring out the full active flavours of tea is to use freshly drawn boiling water. Simply being hot isn't enough and warming the pot first stops the boiling water cooling too much as it hits the pot.

But there are times when even boiling water isn't hot enough - up a mountain for instance.

Water boils when its molecules have enough energy to turn from a liquid into a gas. If the air pressure surrounding the liquid is reduced, then the molecules are able to turn into a gas much more easily, lowering the boiling point.

Atmospheric pressure decreases as altitude increases so water boils at a lower temperature up a mountain. At sea level, water boils at 100°C but at the top of Everest your kettle will boil at just 72°C, making for a pretty horrid cup of tea just when you would really need a good one!

Find related websites on the physics of boiling water

Do you only ever drink out of a china cup?

What you drink your tea out of really comes down to personal preference. Many people hate the sensation of drinking out of a polystyrene cup, but it will keep your tea hotter for much longer than your favourite china cup since polystyrene is made up of lots of pockets of air which act as a good insulator.

But your personal preferences may well be good for your health. Drinking out of your favourite cup may be relaxing and soothing which can be as beneficial as the chemicals in the tea.

Scientists at the Institute of Food Research in Norwich have looked at the health benefits of drinking tea and found that green tea in particular contains chemicals, called polyphenols, which are known to help prevent certain diseases. Polyphenols help to mop up free radicals which are associated with ageing, cancer and heart disease.

Find related websites on heat transfer

Whatever you do, don't…

… brew your tea for too long. Brewing tea for longer releases more tannin, a naturally occurring substance found in tea. Too much tannin and your tea tastes bitter, but it has also been found that tannin inhibits the absorption of iron into the blood stream.

… use hard water. The minerals in hard water may contribute to an unpleasant scum on the surface of your tea.

… superheat your water. Superheating is when a liquid is heated beyond the boiling point and this can easily happen if you heat water in a cup in a microwave oven.

Superheating occurs when there are few nucleation points, the rough places on a surface where bubbles can form easily. If no bubbles form and the water is heated without being disturbed, then when you take it out of the microwave and stir it, the water is likely to boil suddenly, spraying steam and hot water everywhere.

Find related websites on superheating

Paper Folding

Who says you can't fold this paper bag in half more than seven times? And what's physics got to do with it?

Wider or thicker?

When you fold your paper in two you are halving the width of the paper, but doubling the thickness. So if you start off with a piece of paper that was 0.05 cm thick but 30 cm wide, by the time you reach your eighth fold, the paper is a massive 6.4 cm thick but less than 1 cm wide!

So the trick is to try and keep the width bigger than the thickness, which is exactly what Britney Gallivan did in 2005. Britney was a high school student in America who, for extra Maths credit, was given the problem of folding a piece of paper more than 7 times.

Britney worked out that it would be better to fold the paper in the same direction, rather than changing directions for each fold. Her calculations also showed that she would need a piece of paper 1.2 km long to be able fold it 12 times. But proving her calculations took some dedication.

Britney finally found a roll of toilet paper which fitted the bill and she took it to the local shopping mall where she laid it out on the floor. After marking the half way point she started the first fold. With paper 1.2 km long, there was quite a lot of walking involved so it took her seven hours to fold the paper 11 times after which she was left with a slab of paper 80 cm wide and 40 cm thick which she folded once more to make it twelve.

Britney broke the world record for paper folding, and she got her extra credit in maths!

See Britney's paper folding equations here

Plastic Fantastic

As you eat your lunch you may well be looking at the paper bag that your sandwich came in and maybe a packet of crisps, chocolate bar or bottle of drink. Or maybe you've bought in some food from home in one of those takeaway boxes?

Whatever you're eating, the chances are that there's some plastic involved. Plastics are widely used in packaging because they're cheap to make and keep the food fresher for longer.

But plastics degrade very slowly, which is bad news for landfill so reducing the amount of plastic used in food packaging and increasing the amount of plastic that can be recycled is a good idea.

As there are so many different types of plastics, one of the challenges of recycling is sorting out the different types - currently it can be extremely labour intensive and therefore expensive. One way to reduce the amount of manual labour involved is to use an optical sorting machine.

This machine shines either infrared or visible light on to the plastic as it moves along a conveyer belt. By analysing the wavelength of light that is reflected, the machine is able to detect and sort the different plastics. Unfortunately, it still has some trouble identifying anything black as black objects don't reflect light - which is a bit of a hitch given that there are so many black plastic bags.

Find out more about recycling here

Packaging gets smart

How would you feel if your food could tell you that it was going off? Or if your shopping could say it was too warm?

Smart packaging detects changes in its contents and tells you about them. Some materials change colour when they are heated or cooled and there's already beer cans that change colour when cooled to a satisfying drinking temperature.

Plastic packaging containing nanoparticles is being developed that will let you know when food has gone off. The packing changes colour when it comes in to contact with the chemicals that are responsible for 'bad food' odours. And zinc oxide nanoparticles can be incorporated into plastic packaging to help block ultra violet light as well as provide anti bacterial protection.

Manufacturers and distributers can track their products using Radio Frequency Identification (RFID) tags. RFID tags transmit radio signals so that the location of any item can be known at any time. RIFD tagged products stored on smart shelves can tell when a product is out of stock, making restock orders more accurate.

But there are concerns about the use of RFID tags. If they're not deactivated they can continue to transmit their whereabouts even after you've bought them and left the store. So your shopping could be telling the retailers and manufacturers where you live and what you've bought.

Find related websites about smart packaging with physics.org

Stronger than a shield of steel

When you think of strong materials, paper isn't normally anywhere near the top of the list. Paper is made from wood, but the pulping process damages the long cellulose fibres that give wood its strength.

However, scientists in Sweden have developed a way of making paper which protects the cellulose fibres and produces paper with a structure that makes it stronger than the wood it came from.

Their method uses an enzyme and a mechanical beater to gently break down the wood pulp into fibres. As they dry out, the undamaged cellulose fibres then bind together in such a way that they are able to slip and slide over each other to dissipate stresses and strains.

This new, so called nanopaper has been shown to have the tensile strength of cast iron and be almost as strong as structural steel. Although why you'd want paper that strong is another question.

Read more about nanopaper here

Top

Bubbles in Stout

What's happening to the bubbles in your pint of stout? And what's physics got to do with it?

Sinking or an illusion

It might be hard to believe your eyes, but the bubbles in your pint of stout really are sinking. But everyone knows that bubbles float to the surface, so what is going on?

As your pint of stout settles, the bubbles in the centre of the glass (where you can't see them) ARE rising to the surface, but at the same time the bubbles touching the wall of the glass experience some drag which prevents them from floating upwards.

As the bubbles in the centre of the glass rise to the top they create a fountain-like circulating current in the liquid. This current flows up through the centre of the glass to the surface, then across the surface towards the sides of the glass before heading back down to the bottom of the glass. The bubbles in the stout are swept through the drink by this current so that what you see are the sinking bubbles close to the sides of the glass.

This can happen in all fizzy liquids, but it is particularly noticeable in a pint of stout, such as Guinness, because of the colour of the liquid and the small bubbles which are easily moved by the current.

Find related websites about the bubbles in your pint with physics.org

What else is happening in your pint glass?

Your typical pint of stout, as well as some bitters, contains nitrogen bubbles which have been released at high pressure through small holes in the tap when the beer is pulled. Nitrogen is less soluble in water (or beer) than carbon dioxide so nitrogen bubbles last longer and create a stable foam which you experience as the beer's characteristic creamy head and 'smooth' taste. Most lagers on the other hand use dissolved carbon dioxide which produces less stable bubbles than those found in stout. The carbon dioxide dissolves in the lager so the bubbles and the head gradually shrink, and then disappear.

But there are ways of making the bubbles and head of a pint of lager last much longer.

Next time you are in the pub and order a pint of lager, have a look at the very bottom of the glass. You may well find a shape etched into the glass and lots of bubbles streaming off it.

These are nucleation points which act as a rough surface where the dissolved carbon dioxide can form into bubbles. Different glasses often have different shapes etched into the base of them, so when you have just a few centimetres of lager left, leave it undisturbed for a while to see if you can get the same shape in bubbles on the surface of your drink!

Even if your glass doesn't have any nucleation points etched in, bubbles will still form around tiny particles of dust, fibres from a dish cloth or imperfections in the glass. To see what would happen without them, try pouring beer or a fizzy drink into a glass coated in olive oil. The result? A very flat, and undoubtedly quite disgusting pint.

Find related websites about the physics of beer with physics.org

A fizzy drink is like the bends

Soft drinks get their fizz from the carbon dioxide that is pumped into them under pressure. At high pressure, the gas dissolves in the liquid, but as soon as you take the lid off a bottle of pop, the pressure drops and the dissolved gas is released as bubbles.

If you want your bottle of pop to stay fizzy, the fridge is the best home for it. Carbon dioxide is more soluble at colder temperatures, so it can't escape until you next fancy a drink.

When you shake a can of pop, some bubbles of carbon dioxide form and some of these bubbles will be clinging to the side of the can. As soon as you open the can, the bubbles expand rapidly because of the lowered pressure. They rise quickly to the surface, producing more bubbles as they go and pushing any liquid out of their way - resulting in a frothy mess of sticky pop.

Scuba divers also have to be careful about dissolved gases forming bubbles, but in their case it can be life threatening rather than just a bit messy. Divers experience extreme pressures due to the weight of the water and the compressed air that they breathe is also under pressure - it wouldn't come out of the tank if it wasn't.

When a diver is in deep water for a long time, some of the high pressure nitrogen in the air that they're breathing will dissolve in the water in their body. This becomes a problem if the diver then surfaces too quickly; as the pressure plummets, the nitrogen comes out of solution and forms painful and potentially life threatening bubbles of gas. To avoid getting the bends divers aim to surface slowly, allowing the dissolved nitrogen to be released slowly as the pressure gradually decreases.

Find related websites about the bends with physics.org

Diet Coke and Mentos…. (and shandy)

If you drop a tube of small chewy sweets called Mentos into a two litre bottle of diet Coke, you get a fountain of diet Coke whooshing out up to ten metres into the air. There's a great film of this here (link to http://www.youtube.com/watch?v=hKoB0MHVBvM )

Mentos have lots of little ridges on them which act as nucleation points where the carbon dioxide dissolved in the diet Coke rapidly forms bubbles. As the Mentos sink to the bottom of the bottle these bubbles rise through the liquid and agitate the diet Coke making more bubbles form.

The fountain effect works best with diet Coke because it has a lower surface tension then that sugary Coke. The lower surface tension allows the bubbles to grow more rapidly and the gum arabic which coats the Mentos acts as a surfactant, lowering the surface tension of the liquid further.

Another sticky, fizzy mess can made if you don't know whether to pour the beer or lemonade in first when making a shandy. Experienced bar staff know that you need to add the beer to the lemonade, but why?

The bubbles in beer are very stable. They have a protein skin which strengthens them and lets them form a stable foam. But lemonade doesn't contain any stabilising proteins so although it's very fizzy, the bubbles and foam are short lived.

If you add the lemonade to the beer, you get lots of lemonade bubbles which become stabilised by the proteins in the beer resulting in a massive amount of foam. But adding the beer to the lemonade means that the bubbles in the lemonade have already mostly popped before the beer is added. In addition, the beer proteins are diluted by all the lemonade, so those lemonade bubbles that are left aren't stabilised and you end up with a head that's just about right.

Find related websites about surface tension with physics.org

Lipstick and Beer

How does wearing lipstick affect your beer drinking? And what's physics got to do with it?

Why does lipstick destroy foam?

It's not just the fats in lipstick that destroy foam - the fats in crisps, nuts and pork scratchings, as well as the natural oils in moustaches all diminish the head of a beer.

The head is created by bubbles of gas, often carbon dioxide or nitrogen, that are released as your pint is being pulled. These bubbles are coated with a strong skin of proteins that originate from the malted barley used during the brewing process and which helps the bubbles to form a stable foam. But when fats or detergents come into contact with the foam, they can literally punch holes in the protein skin, weakening and destabilising the bubbles and destroying the head.

A technique known as Atomic Force Microscopy, which can study the surface of bubbles at very small scales, has shown these holes in the surface of the protein skin.

Scientists at the Institute of Food Research in Norwich have been investigating how beer foam can be made to resist the ravages of oils and fats. They've found that the protein skin can be made stronger by adding compounds called hop acids which help to bind the beer proteins together. Another additive is a seaweed extract which thickens and strengthens the protein skin. And looking to the future, the researchers have recently patented a method of extracting a natural molecule from excess grain that is normally thrown away in the hope of using it to make more stable beer foams.

But preserving your head doesn't have to involve strengthening the bubbles. Another method is to keep the fats away from the foam in the first place. Barley proteins are thought to have small pockets in which fat molecules can sit. So by ensuring that barley proteins aren't lost in the brewing process, brewers can ensure that there are fewer fat molecules available to go on a bubble bursting rampage.

Find related websites about foam with physics.org

Why is foam white?

Your beer is brown and the head is…. white!? They're made out of the same stuff, so why the change in colour?

It's all to do with the way that light travels through the foamy head. The foam is made up of loads of bubbles crammed in close together and each individual bubble is a pocket of gas surrounded by a thin wall of beer. Light travelling through the foam passes through a large succession of bubble walls and each time it passes through a wall it is scattered, reflected and refracted in different directions. This means that the light that exits the foam and enters your eyes is a mix of all different wavelengths and appears white.

This is why many things look white, even if they're not really. Mayonnaise, clouds and polar bears all look white because of the way that light is scattered and then mixed before it reaches your eyes.

Find related websites about light scattering with physics.org

Widgets

Not so long ago, the only way to enjoy the pleasures of a foamy head on your beer was to go to the pub. But what about when you were at a house party or relaxing at home and only had a can of beer?

The answer is the widget. A widget agitates the beer as it is poured to release the carbon dioxide that is dissolved in the liquid so that a head can form.

Widgets are added during the canning process, but before the can is sealed a shot of liquid nitrogen is also added to the can. As the liquid nitrogen evaporates, it increases the pressure in the can forcing some of the beer through a tiny hole in the widget. When the can is opened, the pressure is released and the beer rushes out of the widget at a very high speed. This agitates the rest of the liquid, releasing carbon dioxide and producing lots of small bubbles that form the head.

Some of the early widgets were simply pieces of filter paper mounted to the inside of the can so that bubbles would form on the rough surface. However, problems occurred when cans that were too warm or that had been shaken were opened - too many bubbles would form, making the beer gush uncontrollably out of the can.

Rough surfaces, whether on a paper widget or a glass, have very small holes which encourage bubbles to form. As the bubbles grow, they break away from the surface and rise to the top of the liquid. But the bubble will leave a pocket of gas in the hole which enables another bubble to start growing. This is why you often see a stream of bubbles coming from the same place in a glass of bubbly.

Find related websites about pressure with physics.org

Bubbly milk

If you have a glass of full fat milk, a glass of semi skimmed milk and a glass of skimmed milk, which one is going to be the easiest to blow bubbles in? Why not try it and see?

The glass of skimmed milk will bubble up the easiest and quickest because of its low fat content. Fats destroy the bubbles in the milk just like they do in your pint of beer. The higher the fat content of the milk, the less likely you are to get a bubbly head on it.

If you don't have the different types of milk, use just whole or semi skimmed milk and compare blowing bubbles in a glass of warm (about body temperature) milk with a glass of chilled milk. In the chilled milk the fat is solid and so isn't very effective at destroying the foam. In the warm milk, the fat has melted and can spread out across the surfaces of the bubbles, thus destroying them more effectively.

Find related websites about experiments to try at home with physics.org

The tallest straw

What's the tallest straw you can drink out of? And what's physics got to do with it?

How many metres?

Using a straw to drink is second nature to most of us, but how does the liquid get up the straw? And why would it stop at 10 metres?

When you suck on the end of a straw you're lowering the air pressure inside the straw above the liquid. But that's not enough to make the liquid travel up the straw into your mouth. It needs to be pushed up the straw and this is done by atmospheric pressure pushing on the liquid in the glass.

Although all the molecules that make up the air around us are very small, they do actually weigh something and we feel this as atmospheric pressure. The atmospheric pressure at sea level can only push water up vertically 10 metres. This is because the pressure at the bottom of a 10 metre column of water is equal to that of atmospheric pressure - so even if you had a straw taller than this, atmospheric pressure would not be able to push the water any further up.

Of course it would be a bit tricky to actually suck drink up a straw vertically using your mouth, but you would be able to do this using a vacuum pump. And if you had a drinking straw horizontally on the ground, or a curly whirly straw then you would be able to make it longer than 10 metres as this would be unaffected by atmospheric pressure - but again, you'd probably need a vacuum pump!

Find related websites about atmospheric pressure with physics.org

How do tall trees suck up water?

Trees grow higher than 10 metres, so how do they get water to the top branches? This is to do with the nature of water. Water molecules like to stick together, especially at the surface. This is called surface tension and explains why water droplets form - all the water molecules pull together at the surface to form a sphere.

Next time you have a glass of water in front of you, look carefully and see if you can see the water clinging to the side of the glass - the slight upward curve of the water surface is called a meniscus. This is because water molecules also stick to some materials, including glass.

If you place a glass tube into some water, the water begins to creep up the sides of the tube. And if the tube is thin enough, the force of attraction of the water molecules will exceed the weight of the water in the tube, and drag the water further up the tube - this is capillary action.

Trees are made up of many tiny tubes of cellulose (which water also loves to stick to) called xylem. Water is drawn through these tubes by the process of transpiration. As water evaporates through the tree's leaves, water molecules are pulled up through the tiny capillary like xylem. This process can draw the water well over 10 metres and even up to 100 metres, the height of some American Redwoods.

Cellulose is present in things made out of trees, such as kitchen towels, beer mats, and paper all of which draw up water in the same way a tree does - by tiny capillaries.

Try doing some DIY chromatography using kitchen towels with Marvin and Milo.

Find related websites about atmospheric pressure with physics.org

Getting beer up from the cellar

Traditionally real ale was held in casks that were placed on the back of the bar and had a tap in the bottom where the ale was drawn off. But in 1797, locksmith and engineer Joseph Bramah invented the beer engine, a tall hand pump designed to draw beer up from the cooler cellar directly into a waiting glass. The beer engine relies on atmospheric pressure to force the beer up through the tap.

Serving beer through a hand pump can agitate the beer and aerate it, and there are some nozzles, such as a 'sparkler', that forces the beer through tight holes to help produce a thick creamy head.

Beer served from casks needs special attention. The casks need to be well ventilated to allow air in for continued fermentation and stored in the beer cellar for a certain amount of time after being delivered by the brewery to allow the sediment to settle.

Nowadays many beers are pulled by using keg beer dispensers which does away with the need for this special care and doesn't use a beer engine. Most modern beer is delivered in kegs which are hermetically sealed and pressurised in the brewery. The beer is propelled from the cellar to the beer taps by compressed gas and cooled as it travels through the pipes on the way to the bar.

Find related websites about pumps with physics.org

Pub Tricks

We couldn't have a piece about physics and beer without including some pub tricks (that use physics of course).

Try this: take a straw and using a drawing pin punch a small hole near the top. Now challenge someone to drink through the straw. You'll find that they will struggle to drink a drop!

The pressure inside the straw needs to be lower than atmospheric pressure for the drink to be sucked up, but the air getting into the straw through the hole keeps the pressure the same, making it impossible to create suction. The only way to get a drink is to cover up the hole in the straw.

Another clever trick that involves atmospheric pressure is the amazing antigravity beer mat. Take a pint glass and fill it almost to the top with water. Place a beer mat over the glass and hold it in place while carefully turning the whole lot upside down. The key to this is to try and make sure that the seal around the glass and mat is tight.

Now ask your friends what you think will happen if you take the hand holding the beer mat away. There are two options: either the mat will stay where it is and the water will stay in the glass, or the mat will come away and someone will get their feet wet.

If the mat is carefully sealed over the glass then the water will stay in the glass. This is because the surface tension of the water behind the mat is pulling on the mat and holding it in place. But at the same time there is difference between the atmospheric pressure on the outside of the mat and the pressure inside the glass. This difference in pressure is enough to hold the mat and the water in place - so your feet don't get wet!

Try more experiments at home with Marvin & Milo

Watch our Physics to Go experiments

Find related websites about pub tricks with physics.org