Rain drops at Notes, Covent Garden

Notes Covent Garden, rain, puddles

No one wanted to sit outside when we visited Notes at Covent Garden

It was a cold and wet afternoon in early January when I finally had the opportunity to try Notes (Covent Garden branch). Inside, there were plenty of places to sit while warming up and drying off enjoying a coffee. Although it seems small from the outside, inside, the branch feels quite open, with the bar immediately in front of you as you come through the door. One of the attractions of Notes to me, was the fact that I knew that they served different single estate brewed coffees. I think I tried a “La Benedicion” coffee, or at least that is what I seem to have scribbled in my notepad. We took a stool-seat at the window to look out at the rain as my coffee arrived in a 0.25L glass jar. It is always nice to try different single estate coffees and generally, if I know that a café serves single estate coffees I will seek them out to try them for the Daily Grind.

The reflection of the Notes sign board in a cup of tea

The reflections in a cup of tea

Watching the rain form puddles outside, my thoughts were turned to the reflections bouncing off the water in the puddle. It struck me that the appearance of puddles depends on the water molecules behaving both as individual molecules and as molecules within a group. The rain creates ripples in the puddle which can only occur because each molecule is (weakly) attracted to the other water molecules in the puddle, forming a surface tension effect. A ripple is a necessarily collective ‘action’. On the other hand, the reflection of the lights from the street is the response of each individual water molecule to the incoming light. The reflected image is made from the response of many individual molecules. Reflection is more of an individual molecule thing.

Warning sign, train, turbulence

Such turbulence should be familiar to anyone who has stirred a cup of coffee.

I continued thinking about this when I got home where it occurred to me that there was another connection between rain and coffee. It is often said that “rain helps clear the air”, or something similar. Yet this is not quite true. If you have a coffee in front of you at this instant, take a moment to drag a spoon through it. Note the vortices that form behind the spoon. Such vortices form around any object moving through a fluid. In the case of the coffee it is the spoon through the water. For the rain, as the rain drop falls through the air it creates tiny vortices of air behind it. Just as with the coffee spoon, the size of these vortices depend on the speed and size of the falling drop. These vortices pull and trap the atmospheric dust bringing it down to earth more quickly than rain alone could do. The air is cleaned more by this ‘vacuum cleaner’ action than by the ‘wet mop’ of the rain itself.

I’m sure that there are many other coffee-rain connections that you can make if you sit in a café as I did on a rainy day. Let me know your thoughts on this or indeed, on anything that you notice and think interesting while sitting in a café. There is so much to notice if we just put down the phone or close the laptop while enjoying our brew.

Edited to add: Sadly, this article was posted just as Notes Covent Garden was closing down. Notes still has branches at Trafalgar Square and in Moorgate and is opening new branches in Kings Cross and Canary Wharf in February I believe. Hopefully they will all serve single estate brewed coffee and have good window seats from which to observe the rain when it falls.

Getting my teeth into some latte art

LatteArt_CoffeeworksEach year, in the UK, there are approximately 160 000 hip or knee replacements. Additionally, many of us will have a dental implant during our lifetime. How is this linked with coffee? The answer lies in the differences between a latte and a cappuccino.

To support the artwork that can be seen on many a latte, the milk foam used for the drink is a fine “micro-foam”. It is quite a soft structure. On the other hand a cappuccino is more rigid, being made out of a larger foam structure. The different way that a barista froths the milk for a cappuccino compared to a latte means that the peak structures that can be formed in the cappuccino, are far more difficult to create in a latte, the cappuccino has more of a “meringue like” froth.

Joint replacements and dental implants were traditionally made from solid metal. This meant that the majority of the load that was put on the joint (by walking or chewing for example) was carried by the implant. It is thought that this was one of the reasons that joint replacements and dental implants eventually failed; the bond between the bone and the implant became progressively weaker in a process called “aseptic loosening”. In recent years there have been many improvements to joint replacements/implants so as to avoid these problems. One such improvement is to manufacture the implant out of a metallic foam instead of solid metal.

Cappuccino showing peaks in the foamJust like a latte or a cappuccino, the way that the metal foam responds to stress (and its rigidity) is dependent on many factors including the size of the bubbles in the foam and exactly what the foam is made from. (Imagine comparing a cappuccino with a soya milk cappuccino). By manipulating the structure of the metal foam, an implant can be made that behaves almost exactly as bone does when stress is placed on it. Together with the inherently stronger bone-implant bond created by the bone growing into the ‘bubbles’ of the implant foam, this is thought to reduce the risk of implant failure owing to ‘aseptic loosening’.

I am indebted to Michaela and Juan of Poppy’s Place for patiently showing me the art (and science) of making coffee. With good coffee (from Climpson & Sons) and knowledgeable barista-teachers, it is a place that I am sure that I will return to very soon. Michaela and Juan assured me that if I would like to see a properly rigid milk foam I should order a “babyccino”. There are however limits to the amount that I am prepared to ‘suffer for my science’ and the babyccino is it. If you would like to properly investigate the effect of bubble structure on the ability of an implant (dental or otherwise) to take stress, I suggest you compare a latte with a babyccino. If, like me, you like your coffee, a cappuccino will definitely suffice.

 

Red Door, Greenwich

Red Door Greenwich, Red Door

Interior of Red Door cafe, Greenwich

Red Door in Greenwich is a great escape from the bustle of the busy streets surrounding it. Although it was crowded when we visited, it was still possible to find a table and have a conversation without too much background noise. I had heard good things about Red Door and wasn’t disappointed. Good coffee (from Monmouth), nice cake and warm surroundings. Definitely a place to go to when in Greenwich. The music that was playing was coming from a record player in the corner. A proper turn-table playing vinyl records. Suddenly, there were so many possibilities for stories for a Daily Grind article. There was the fact that records are analogue based (as opposed to the digital CDs), or perhaps I could write about the physics of a valve amplifier and how it relates to the evaporation of water from coffee (some of the physics is very similar). However what I started to get obsessed with is: what would happen if you put a coffee on a record player?

Now, I am an experimentalist and I do have a record player at home but before I could say “what would happen if…” my plans for experimentation with the record player were blasted out of the water. So I had to make a model record player out of a rotating spice rack. This probably worked better as I could control the speed of rotation, though it did make taking photographs tricky.

record player, turntable

The record player at Red Door

So, what would happen if we put a coffee at the centre of a turntable? The movement of fluids in cups and on record players is extraordinarily complex and is indeed very far from my ‘area of expertise’. However, we can start to understand what might be happening in the cup by making some approximations. Our first approximation is that the coffee in the mug rotates as a ‘rigid body’, meaning that it rotates as a whole. As the coffee cup rotates about its central axis on the “record player” the coffee inside the cup will (eventually) also rotate at the same angular velocity (speed of rotation). The fact that there is a rotation means that there is a force acting on the particles in the coffee liquid. This force produces an acceleration that increases with increasing distance from the axis of rotation. Each coffee particle is of course also subject to the vertical action of gravity. The combined acceleration means that each particle is simultaneously being pulled downwards and inwards. As the acceleration due to rotation increases with increasing radius, the horizontal acceleration becomes increasingly dominant away from the centre of the cup. This leads to the familiar curved surface (a dip at the centre of the mug) that we see with rotating fluids.

vortices, turbulence, coffee cup physics, coffee cup science

This polystyrene cup was rotated about its axis before being stopped. The water inside continues to rotate causing turbulent layers at the edges. These have been visualised with a small amount of blue ink.

Yet we know that this cannot be the full story. If we suddenly stop rotating the mug, the coffee in the mug continues to rotate for a while but does not do so indefinitely; it slows down. We can understand this by refining our approximation that the coffee inside the mug rotates as a rigid body. In fact, the coffee is a viscous liquid and the viscosity means that the layer of coffee immediately adjacent to the mug walls will move at the same speed as those walls: Stationary wall, stationary coffee. The coffee towards the centre of the cup meanwhile continues to rotate for a while. Imagine suddenly stopping the record player so that the mug is now still but the coffee inside continues to spin around the central axis. Stress is being produced between the stationary ‘layers’ of coffee next to the mug wall and neighbouring ‘layers’ of rotating coffee. This stress leads to turbulence. We can make this turbulence visible if, instead of coffee we use a mug of water. Rotate the mug of water as before and then suddenly stop the mug rotating. As with the coffee, the water continues to rotate. Now drop a tiny amount of water soluble ink or food colouring into the very edge of the water (I used a cocktail stick dipped in ink and held against the mug wall so that a small amount dripped into the water). As the water continues to spin, the ink is caught up in the turbulence and the vortices it produces can be seen. These concepts of boundary layers and turbulence are important for many applications including weather systems and car design. We need to understand how liquids (or gases) flow past each other in order to predict the weather and we need to know how they flow past solid objects in order to make cars more aerodynamic. In the coffee however I think that this turbulence is one of those things that is worth just creating and appreciating. A great demonstration of beauty, art and science in a mug of coffee.

Please do share your pictures of these coffee cup vortices if you manage to create them, particularly if you are able to see the effect with cream in coffee. You could either write about your results in the comments section below or email me photographs of your coffee and I will include them on this page. As always, enjoy your coffee.

My thanks to Kate & Edward of Red Door for sending me the photos of Red Door.

Extra photos of vortices in a rotating coffee:

Rotating coffee

An attempt at visualising the vortices using cream in coffee. Not so successful though you can see at least 2 well defined vortices in the top left of the image. Introducing the second liquid right at the edge of the mug seems critical, not so easy with cream as it is with ink!

Joe’s espresso cafe bar, Victoria

radiant heat, heat loss, heat conduction, infra red, Joe's espresso cafe bar

The slightly ajar door at Joe’s espresso cafe

A few weeks ago I happened to be near Joe’s espresso café bar on the corner of Medway St. and Horseferry Road, with around twenty minutes to spare. Joe’s is an old-style independent café, very focused on their lunch menu and take away coffees. Nonetheless, there is a decent sized seating area in a room adjacent to the ‘bar’ where you can sit with your coffee and watch the world go by on Horseferry Road. It is always nice to come across a friendly café that allows you to sit quietly and people-watch. As I sat and watched the taxis pass by, I became aware of the fact that it had got quite cold. The people who had just left the cafe had left the door to the room slightly open; the cold was ‘getting in‘. Now I know, heat goes out, cold does not come in but sitting there in that café that is not how it felt. Then it struck me, rather than cause me to grumble, the slightly open door should remind me  of the experiments of Carl Wilhelm Scheele (1742-1786).

Scheele was a brilliant chemist but one who performed experiments that would make our university health and safety departments jump up and down spitting blood. Recognised for discovering oxygen in the air (Priestley discovered it a few years later but published first), manganese and chlorine, Scheele also investigated arsenic and cyanide based compounds. It is thought that some of these experiments (he described the taste of cyanide) contributed to his early death in May 1786 at the age of 43. Fortunately, none of this has a connection to Joe’s espresso café. What links Scheele with Joe’s, is Scheele’s discovery of ‘radiant heat’ as he was sitting in front of his stove one day.

Open fire, Carl Wilhelm Scheele, Radiant heat, infra red, convection

Sitting in front of a fire we can observe several different ways that heat moves.

Scheele’s house was presumably very cold in winter. He describes how he could sit in front of his stove with the door slightly ajar and feel its heat directly and yet, as he exhaled, the water vapour in his breath condensed into a cloud in the air. The heat from the stove was evidently heating Scheele, but not the air between Scheele and the stove. He additionally noted that this heat travelled in straight lines, horizontally towards him, as if it were light and without producing the refraction of visible light associated with air movement above a hot stove. Nor was a candle flame, placed between Scheele and the stove, affected by the passage of the heat. Clearly this ‘horizontal’ heat was different from the convective heat above the stove. Scheele called this ‘horizontal form’ of heat, ‘radiant heat’.

A few years later, the astronomer and discoverer of Uranus, William Herschel (1738-1822) was investigating glass-filter materials so that he could better observe the Sun. Using a prism to separate white light into its familiar rainbow spectrum, Herschel measured the temperature of the various parts of the spectrum. Surprisingly, the temperature recorded by the thermometer increased as the thermometer was moved from the violet end to the red end of the spectrum and then kept on rising into the invisible region next to the red. We now recognise Herschel’s observation of infra-red light as responsible for the radiant heat seen by Scheele, though a few more experiments were required at the time before this was confirmed.

sunlight induced chemical reactions, milk

Often milk is now supplied in semi-opaque bottles. Why do you think this is?

Further work by William Hyde Wollaston (1766-1828) and, independently Ritter (1776-1810) & Beckmann not only confirmed Herschel’s infra-red/radiant heat observations but also showed that, at the other end of the spectrum was another invisible ‘light’ that produced chemical reactions. Indeed, milk is often sold in semi-opaque plastic containers because of the fact that the taste and nutritional content of the milk are affected by such sunlight induced chemical reactions.

So, it seems to me that, in addition to an interesting story with which to idle away 20 minutes in a café, this set of thoughts offers a variety of experiments that we could try at home. If we are out, we could try to discern the different ways that heat is transferred from one body to another (as Scheele). If we had a prism, we could perhaps repeat Herschel’s experiment very easily with a cheap (but sensitive) thermocouple and, if we were really ambitious hook it up to a Raspberry Pi so that we could map the temperature as a function of wavelength. Finally, we could investigate how light affects chemical reactions by seeing how milk degrades when stored in the dark, direct sunlight or under different wavelengths. If you do any of these experiments please let me know what you discover in the comments section below. In the meanwhile, take time to enjoy your coffee, perhaps noticing how the hot mug is warming your hands.

Books that you may like to read and that were helpful for this piece:

“From Watt to Clausius”, DSL Cardwell, Heinemann Education Books Ltd, 1971

“On Food and Cooking: The science and lore of the kitchen” H McGee, Unwin Hyman Ltd 1986

Apologies to university H&S departments, you guys do a great job (mostly!) in trying to help to prevent us dying from our own experiments too prematurely.

 

Happy New Year

It is a timely coincidence that today’s Daily Grind falls on New Year’s eve; a perfect opportunity to reflect on how the year has been and to think about the future.

The first thing to reflect upon is the fact that, since starting Bean Thinking in autumn 2014, I have had a great excuse to get out and try many interesting independent coffee shops and tea houses. Obviously there are many more to try and so something to continue with in 2015, but what matters to me is that they are independent. It is an interesting question, at what point does a cafe with multiple branches turn into a chain? Perhaps this is something to worry about another time, for the moment there are still a great many to try (both old and new) that I don’t need to worry about it for a while.

Do you see a wood or the trees?

Do you see a wood or the trees?

Then there has been the opportunity to encounter some very interesting people who, it is fair to say, are quite obsessed with coffee. Some of them are interested in different ways to make the coffee taste good. Some are interested in areas where coffee meets art (3-D latte art anyone?). There are those who are interested in the science of coffee and then those who try to ensure that everyone, from grower to consumer inclusive, gets a good deal for the coffee. In short, science, art, philosophy and coffee. In many ways it reinforces my opinion that a good education is far more than a mere qualification in a narrow specialisation. In the book “History and Philosophy of Science”, LWH Hull suggests an analogy to help us to understand our contemporary tendency to specialisation. Hull suggests that those who specialise are like people exploring different trees. We cannot understand another’s tree by stopping our work (of climbing our tree), and instead climbing “a few feet” up the tree of another. Hull instead suggests that by understanding the history and the roots of our own field of specialisation we can understand that others have similar roots, similar motivations and are seeking similar ends by different means. Understanding our own tree and appreciating how it has come to be, allows us to appreciate the trees of others and thereby allows us to see the wood instead of the trees.

So Happy New Year to all, feel free to leave any comments about art, social justice, science, in fact, anything that you think coffee related and vaguely relevant to this start of the year. I look forward to ‘meeting’ more of you and hearing what you have to say in 2015.

A gift fit for a King?

Adoration of the Magi, Andrea Mantegna, 1431-1506. Digital image courtesy of the Getty's Open Content Program.

Adoration of the Magi, Andrea Mantegna, 1431-1506. Digital image courtesy of the Getty’s Open Content Program.

There is currently a very thought provoking painting on display at the British Museum (although it will soon be gone, the Ming: 50 years exhibition, of which it is a part, ends on 5th January). The painting depicts the moment that the three kings, (or three wise men) present their gifts of gold, frankincense and myrrh to the Christ child. The three kings are on the right of the picture. Notice Melchior however, who is presenting gold to Jesus at the bottom of the painting. He presents his gold gift in a porcelain cup. The painting suggests just how valuable porcelain was to the Europeans of the 15th-16th century.

For many years, the Chinese had the monopoly on porcelain production and they ensured that the recipe was kept secret. Nonetheless, by the 17th century porcelain was being traded with Europe and by the 18th century the Europeans had started to mass produce it. Bramah has argued (in the excellent book “Coffee Makers”) that the explosion in popularity of tea and coffee drinking in Europe during the 17th-18th century was due to the introduction of porcelain into general use and its mass production. So it is worth taking a closer look at one of the key figures in the production of ceramics: Josiah Wedgwood.

Wedgwood painting

Portrait of Josiah Wedgwood, FRS. © Trustees of the British Museum

As a ceramics maker, Wedgwood (1730-1795) was interested in ensuring his pottery came out of the furnace well each time and to do that, he realised that he had to know the temperature of the oven. Other pottery producers of the time judged the furnace temperature by the colour (red hot, white hot etc), Wedgwood asked if there was a better way. Eventually he designed a “pyrometer” (“fire” meter) made from bricks of Cornish clay. Wedgwood used the fact that the clay shrank when fired. The amount that the clay shrank indicated the temperature of the oven. Wedgwood could then quantify what was “red” hot etc. Of course, there were problems. Wedgwood’s thermometer worked at temperatures of around 1000ºC, where ordinary alcohol or mercury based thermometers could not be used. How can the temperature scale (that became known as degrees Wedgwood) be correlated with the temperature scales that we are familiar with (such as degrees Centigrade)? Another, perhaps more significant problem was that the technique was not transferable to other practitioners, different clays shrank by different amounts. The Wedgwood scale required a specific Cornish clay. It was left to Louis-Bernard Guyton de Morveau to improve the pyrometer, basing his high temperature thermometer on the expansion of platinum. Today, we use devices based on electrical properties of metals to measure such high temperatures.

A porcelain bird feeder (1426-1435) from the Ming dynasty. © Trustees of the British Museum

A porcelain bird feeder (1426-1435) from the Ming dynasty. © Trustees of the British Museum

If you are in London, it is worth popping along to the Ming 50 years exhibition before it closes on 5th January 2015. Along with this painting, there are many examples there of excellent Chinese porcelain. One of the things that struck me as I went around the exhibition was just how annoyed visiting European diplomats must have been if they ever visited the Imperial palaces. Not only did the Chinese use this rare and valuable porcelain for cups, they also made exquisitely designed, porcelain, floor tiles and bird feeders. While in Europe we were struggling to make any porcelain, the Chinese were not only walking on bits of this valuable material, they were allowing their birds to feed from it too! An interesting history for next time you take a sip from your favourite mug.

Please leave any comments using the form below. I am very grateful to the image reproduction polices of the British Museum and the Getty Museum for the images shown in this article. Information was taken from:

“Coffee Makers”, Bramah&Bramah, Quillar Press Limited, 2002

“Inventing Temperature”, Hasok Chang, Oxford University Press, 2007

 

Time to enjoy a Beethoven coffee

Portrait bust of Beethoven, Anna EG Hoffman, in the British Museum collection © Trustees of the British Museum

Portrait bust of Beethoven, Anna EG Hoffman, in the British Museum collection © Trustees of the British Museum

It is said that Beethoven prepared his coffee by counting, precisely, 60 beans per cup. Biographies of Beethoven certainly suggest that he had a significant coffee habit. Banned by his doctor from drinking coffee towards the end of his life, there are many references to him frequenting coffee houses in earlier years. Sadly, I have not found the source for the 60 beans story and so would not like to comment on its veracity. Nonetheless, it is a good story and it does link with coffee so, as today (17th December) is the 244th anniversary of his baptism (it is assumed that he was born the day before on 16th December 1770), it is “Beethoven day” on the Daily Grind.

To me, what lends some credibility to the 60 beans story is the fact that, as coffee lovers, we can be very particular about the way we prepare our brew. Some people, for example, weigh the amount of the coffee and the quantity of water and brew their coffee according to instructions from one of the various online brewing tutorials (see here for a good one from Hasbean). Personally, in the morning, I am far too bleary eyed to consider getting the kitchen scales out, nor would I count a certain number of beans. I do however count the number of seconds that I take to grind my coffee with my trusty burr grinder (always set to the same level of grind of course). Can counting the number of seconds for a quantity of grind possibly be a good way of measuring a specific quantity of coffee?

Did Galileo drop balls from the top of the tower?

Did Galileo drop balls from the top of the tower?

Galileo Galilei (1564-1642) died before coffee was properly introduced to Europe. He is relevant to this story though owing to his work on clocks and timing devices. One way that Galileo measured time was to collect water in a jug over the measurement period. It seems that this is almost the reverse of my morning coffee ritual. To check that he was measuring time correctly however, he needed a second, independent method. Of course, Galileo couldn’t use a watch or pendulum because watches hadn’t been developed at the time and Galileo himself was doing the work needed to understand pendulums and make them useful for clocks. So what else could he use to measure time? There is a clue to another method that Galileo used in his experiments on falling balls. Although there are questions as to whether Galileo really did drop balls from the top of the Tower of Pisa, we do know that he did experiments which involved rolling bronze balls down a groove. Along the groove were marks where strings made from gut had been pulled across the groove such that they made a sound as the ball passed, perhaps like the sound of a harp being plucked. By adjusting the position of these strings, the interval between the sounds from different gut strings could be made to match a known rhythm. The time it took for a ball to fall down the groove was being measured by matching its descent to a known tune. This suggests that Galileo sang while he was making his key measurements and that it was this that allowed him to start to understand how bodies fell under gravity. Singing was Galileo’s (surprisingly accurate) method of measuring time.

Which brings me full circle back to Beethoven. Beethoven certainly knew the “mechanician” Mälzel who invented the metronome as we now know it. There are also indications that Beethoven was aware of early versions of Mälzel’s invention. In 1813, the Wiener Vaterländische Blätter wrote “…Herr Beethoven looks upon this invention as a welcome means with which to secure the performance of his brilliant compositions in all places in the tempos conceived by him, which to his regret have so often been misunderstood“.  It seems that in the two hundred years between Galileo and Beethoven, there had been so many improvements to clocks and timing devices that singing, which had started off as a way to measure time, was now itself being regulated by the clocks that singing may have helped to develop.

How many beans go to make your morning coffee?

How many beans go to make your morning coffee?

So how is a Beethoven coffee, assuming that there is any veracity to the legend? Sixty beans works out as 8-10g which, depending on the amount of water in the cup could be weaker (or stronger) than modern brews. In my cup, it was slightly weaker than I am used to. I enjoyed my “Beethoven coffee” while listening to his String Quartet Op 74, “Harp”. As I sipped the coffee while listening to the first movement, I could almost hear the gut strings of Galileo’s experiment being plucked as the balls rolled by. The coffee itself (Costa Rica, Finca Arbar El Manatial, Yellow Honey, Caturra/Catual) was very smooth and rich, as you would expect from a coffee from Has Bean. Described in the tasting notes as “….An amazing caramel and milk chocolate sweetness partnered with delicate peach and apricot acidity…” It was the perfect coffee to enjoy with the Harp quartet piece. Sometimes it is important to take time to go slow and enjoy the coffee.

So why not raise a mug today to Beethoven and savour a Beethoven coffee? Please leave any comments using the form below, especially if you know a reliable reference to Beethoven’s coffee habit or have suggestions as to how to improve my morning brew.

Further reading:

Quotes taken from “Thayer’s life of Beethoven”, Revised and Edited by Elliot Forbes, Princeton University Press, 1967

Information on Galileo and time: “Styles of Knowing, A new history of science from ancient times to the present”, Chunglin Kwa, University of Pittsburgh Press, 2011

Copper latte

Brew&Bread, latte art Sun, KL latte art

Taken at Brew & Bread, One City Mall, Subang, KL, Malaysia

Pop into any cafe and order a latte and chances are you’re going to see some great latte art. With the number of good baristas around competing to produce the best and most consistent latte art, it is easy to see some good art while waking up of a morning. Brew & Bread is a cafe with a couple of outlets in Kuala Lumpur in Malaysia. One of their customers sent me these images of their latte art (via Bean thinking on Facebook), which I think are among the finest examples I have seen of latte art being served, as a matter of course, at cafés. Apparently the people at Brew & Bread take their latte art very seriously, so if you find yourself in One City Mall, Subang or Kota Kemuning in Kuala Lumpur, do take the opportunity to pop in.

Not being a barista I can only guess at the skill that it takes to produce such great images as those at Brew & Bread. As a scientist though I can see some connections between latte art and copper mining. Or rather, the link between good latte art and bad copper mining (and vice versa). How? It’s all about the bubbles.

The small bubbles in the foam on the left trap coffee between them. The larger bubbles in the foam on the right allow coffee particles (and water) to leak and don't trap them so well.

The small bubbles in the foam on the left trap coffee between them. The larger bubbles in the foam on the right allow coffee particles to drain by gravity and don’t trap them so well.

Now, I am on dangerous ground here because I have no experience in making latte art, nor really in steaming milk, so I hope that any baristas out there will leap in and leave comments if I have something awry in my description of how latte art is sustained. However, from various videos and how-to’s available online it seems that a key component for good latte art is making the milk into a micro-foam; a ‘velvety’ structure of tiny bubbles. From a physics perspective this makes sense. As the milk is first introduced into the espresso it picks up the crema on the espresso and captures the coffee-liquid mixture between the surfaces of the bubbles of the froth. A large number of very small bubbles will trap the coffee liquid and particles around the bubbles very well (see diagram). If the milk has too many large bubbles, not only will the mouth-feel get affected, the coffee itself is not held and trapped so well within the bubbles. When the art is about to be created, the barista slows the rate of pouring such that the coffee does not get pulled up with the milk and instead the milk foam is allowed to float on top of the espresso where it remains white. It is this contrast between the trapped coffee in the fast-poured milk and the pure milk of the more slowly poured milk that leads to the contrasts of what is known as latte art.

beer foam, bubble size

The bubbles get larger as they move higher up in a foam column. Shown here in a narrow glass of Corsendonk Agnus (beer)

Now consider copper mining. It is an unfortunate fact that we as a society are very reliant on mined products including copper. Copper is the backbone of our electricity network meaning that if you are reading this at all, you are relying on copper that has been mined somewhere in the world. Mining is a fact of our modern way of life. The question is how to reduce its environmental impact to a minimum. One way to minimise the environmental aspect of mining would be to ensure that it is as efficient as possible. Copper is often found in two forms, a relatively easy to extract oxide and the sulphides of copper which are harder to extract. The ‘froth flotation’ technique has been developed to maximise the extraction of these sulphides by using a foaming vat in a process that is the exact opposite of latte art. The copper sulphide rocks are ground until they are very small (around 0.05mm diameter) whereupon they are reacted with chemicals that make them hydrophobic (resistant to bonding with water). Other particles and rocks, that are mined together with the copper sulphides, do not react with the chemicals and so are less hydrophobic. The resulting ‘grind’ is mixed into a slurry and then introduced into a chamber which is aerated to form bubbles. As they are hydrophobic, the copper sulphide particles attach themselves to the newly formed bubbles to reduce their contact with water. The bubbles are then carried up through the chamber, taking the copper with them. The small bubbles at the bottom of the vat trap a lot of water and waste material between them. As the bubbles move upwards through the vat, they get larger (by combining with each other) and, whereas the copper sulphides, which are chemically attached to the bubbles remain with the larger bubbles, the liquid and waste material drains out towards the bottom of the tank. The copper products can then just be skimmed off the top of the vat. Unlike latte art, larger bubbles are useful in froth flotation in order that particles do not get trapped between the bubbles. What is good for the copper mining is bad for the latte art and vice versa. The more we know about the bubbles in foams (in both latte art and froth flotation) the more efficient and the more aesthetically beautiful our world can be.

Another from Brew & Bread

Art for Christmas, another piece of great latte art from Brew & Bread

I would be very interested to know your thoughts on why a microfoam is needed for good latte art or indeed, any aspect of latte making. Please do feel free to share any good photos of latte art (or cafe recommendations) either here in the comments section or on Bean thinking’s Facebook page. There will be another latte art article in the New Year so new photographs (or cafe recommendations) would be greatly appreciated.

With special thanks to Oh Ying Ying for the photographs from Brew&Bread.

Calming the waves at Brutti & Boni

Brutti And BoniBrutti & Boni is a fairly new Italian cafe in South Kensington. Located at the less busy end of Gloucester Road, it was quiet when we popped in to try it a couple of weeks ago. The bright interior has light coming from a roof window at the back of the shop, though it seems that many people opt to sit outside with their espresso in the morning, watching the traffic go past. They serve Caffe Molinari coffee together with a good selection of Italian food items. All in all, a good place to go if you are in the area visiting the Science, Natural History or Victoria and Albert museums and fancy a break and a relaxed coffee nearby.

Inside, the shelves are stacked with various Italian condiments, pasta and olive oil. It was this that prompted me to visit Clapham Common to retrace the steps of Benjamin Franklin. Franklin of course was one of the founding fathers of the USA. He was also a keen scientist, diplomat, printer, in fact the man in some ways defines the word “polymath”. His interests and importance span so many areas that it is difficult to write a two-sentence description of him. Fortunately, for the purposes of today’s Daily Grind, I do not need to. Today, all that is important is that Franklin did some experiments on Clapham Common with oil.

Shelves of olive oil at Brutti & Boni

Shelves of olive oil at Brutti & Boni

Franklin had been investigating the “old wives tales” that a small amount of oil placed onto water ‘calmed the waves’. In fact, the old wives tales can be traced back to Pliny (the Elder) in his Natural History written in around 77AD. Pliny had written of pearl divers and how they sprinkled oil on their faces so that the water above them became calm, allowing them to see the oysters that they were looking for on the sea bed. Franklin himself describes, in his letter to the Philosophical Transactions (1774), an event that he experienced in 1757 while sailing to the UK. Noticing that the wakes behind two of the boats in the fleet were calm, he describes how he asked his ship’s captain about this curiosity. Replying slightly dismissively, as if to someone who is quite ignorant of the workings of the world, the ship’s captain replied that “The cooks… have I suppose been just emptying their greasy water through the scuppers, which has greased the sides of these ships a little”. Obviously it was common knowledge that oil calmed the waves.

So, one day in the 1760s, Franklin took a walk to Clapham Common and to Mount Pond. Emptying about a tea-spoonful of oil (oleic acid) into the pond he watched as the oil produced an “instant calm [on the pond] over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee [opposite] side, making all that quarter of the pond, perhaps half an acre as smooth as a looking glass.” Oleic acid is the principal component of olive oil. Franklin had effectively calmed the waves on the pond with a mere tea-spoonful of olive oil.

A view over Mount Pond, Clapham Common

A single tea spoon of oil would calm the ripples on Mount Pond, Clapham Common

We can calculate how thin the layer of oil had become by dividing the volume of oil in a teaspoon (5cm³) by the area of half an acre (2023 m²) to get an oil layer that was 2.5 nm thick. To put this in perspective, a coffee bean of width 7 mm would fit nearly 3 million of such oil layers in itself width-wise. Later, more precise, measurements of the thickness of such an oil layer, by Lord Rayleigh and Agnes Pockels, gave 1.6 nm and 1.3 nm respectively. This is approximately the length of a single oil molecule. It seems that the waves on water can be stilled by a single molecular layer of oil. How does this work? Why not let me know what you think in the comment section below.

The attractive power of coffee

Just imagine, you are trying to fill 3 espresso cups at once but all you have is a portafilter with two spouts and a balloon? Ok, that sounds unlikely. The experiment that I’m going to describe however will allow you to bend a stream of coffee with a balloon. Moreover, in order to work it relies on sub-atomic particles. What a party trick; investigating sub-atomic physics while filling two cups with one stream of coffee. It could be mind bending, instead it is coffee bending.

What happens?

When you rub an inflated balloon on your (dry) hair, electrons are transferred from your hair to the rubber balloon. Electrons are, of course, sub-atomic particles and, together with protons and neutrons, they build up atoms. As these electrons carry an electric charge, the balloon becomes the source of a static electric field.

Thanks to Artemisdraws for the schematic

The electric field from the balloon aligns the water molecules such that the coffee gets attracted towards the balloon.

Water molecules are composed of two hydrogen atoms and an oxygen each. They are electrically neutral. However water is also a strongly polar molecule, meaning that when it is subjected to an electric field, the molecules will tend to align such that they are more positively charged closer to the negatively charged balloon and more negatively charged further away from the balloon. This charge distribution means that the stream of water gets attracted towards the balloon. The amount that the coffee stream bends is dependent on the strength of the electric field from the balloon and the mass of the stream which is still being pulled down by gravity.

The video suggests using cold brew coffee when you test this at home. There are two reasons for this. Firstly, if the balloon gets too close to the coffee stream, it can get splashed. There is a chance that this may burst the balloon. Secondly, and more fundamentally, the water molecules are more agitated at higher energies (temperatures). This means that thermal agitation weakens the average dipole moment of the water thereby weakening the attraction between the coffee stream and the balloon. In this effect as in its taste, cold brew is a stronger drink than your ordinary hot filter.

Let me know if you try this and how you get on. It would be particularly interesting to see any attempts made on bending coffee from an espresso machine. My thanks, as always, to artemisdraws for the helpful schematic shown here.

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