latte art

Latte Art

Latte art scutoid tulip
The physics of bubbles. What links latte art to the shape of cells as an embryo develops?

An odd one out competition: which of the following is not a type of latte art? Tulip, heart, swan or scutoid? You may well ask, “what on earth is a scutoid?” and so identify this as the odd one out and, to some extent you would be right. Scutoids are not a type of latte art. But I would wager that you can still occasionally see them in your coffee.

Twitter can be a great thing and I was recently alerted there to a New York Times article about Karen Uhlenbeck by @Bob_Mat_Phys. Uhlenbeck is a mathematician at the University of Texas who has just won the Abel Prize in mathematics for her work on the maths of bubbles. The article was fascinating in itself but also mentioned in the article was the fact that there may be, on occasion, a connection between a cup of coffee and the cell structures seen in foetal development. And while I’m very well aware of the extraordinary number of connections that can be made between coffee and the science of the everyday world, I’ll admit, that one surprised me.

Metal jug and transparent glass
More bubbles in your coffee. But what determines their shape? And what shape are they?

By this point you may be unsurprised to hear that the connection is made via the scutoids, but what are they? A new type of shape, they were first described in a Nature Communications article about the development of cells as organisms such as fruit flies grew. Scutoids formed as the embryonic cells grew to form tubes or egg shapes. On one surface of the tube the cell was contacting a different number of cells to that which it contacted on the other surface (so perhaps the cell looked like a pentagon on the top and a hexagon on the bottom). In order for the cell to do this, it formed a further triangular face along one side of the cell and it is this cellular shape that is the scutoid.

Where is the connection with a coffee? Well, the amazing thing is that this shape can be the result of the physics that determines the shape of bubbles, in this case when they are confined between two curved surfaces, such as two cylinders. The shape of a bubble is the result of the minimisation of the surface energy of the bubble. So, in free space, the bubble will be spherical but somehow squash bubbles into a box and you can form a cube shaped bubble in the middle of the box. The shapes that form are the result of the minimum surface energy of the bubble surface. Now, if we return to the curved surfaces and the scutoids. The idea is that if there is a single layer of bubbles between two curved surfaces and that these surfaces are then moved away from each other, the bubbles will first resemble prisms and then, as the surfaces are stretched further, some bubbles will form a prism shape but with a triangular surface at one of the bounding walls: a scutoid.

latte art by Mace, Eiffel Tower and hot air balloon
It is astonishing what you can see in a coffee when you look closely enough.

The paper that showed this (published in Philosophical Transactions but you can read the full version here) combined mathematical modelling of the minimisation of surface energy with experiments involving two cylinders and some soap suds. They then photographed the resultant bubble structures. The results suggest that the minimisation of energy (ie. the physics of the bubble shape) could be a first approximation for explaining the cell structures that form in foetal development. But can you see them in your coffee?

You would need a coffee mug or French press and a smaller cylinder that fits neatly inside it. You would then need to form a foam somehow. Soap suds are obvious, some form of milk texturing would be more interesting. You can then look closely and see, can you in fact see scutoids in your latte art?

Something in the air at Mace by Coffee Chemistry Signature, KL

3D hot chocolate art on an iced chocolate, Mace, Mace KL, dogs in a chocolate

Drinking an iced chocolate with friends.

Perhaps Mace by Coffee Chemistry Signature in Kuala Lumpur should really have a “cafe-art” review rather than a “cafe-physics” review. Indeed, it was because of its latte art that Mace, which operates from a light and airy building in Damansara Uptown, Kuala Lumpur, had been recommended to me. With a comfortable interior and friendly staff, Mace is an interesting place in which to spend some time. But it is certainly the artistic endeavours that are the striking thing about Mace. Nor is it just ‘latte art’. The cakes at Mace arrive at the table decorated into an artwork. It is interesting that every visit to Mace will provide a different creation to enjoy, providing a place that you could return to again and again.

Nonetheless, this is a cafe-physics review website and there is also plenty of science to be found in latte art. For example, one of our drinks arrived with a 3D latte art sculpture floating on its surface. This piece requires manipulation of the rigidity of the milk foam, a topic that has been covered previously on the Daily Grind. However this time, it may be worth looking a little deeper into our frothy coffee: What makes a bubble?

The answer may seem obvious, inside the bubble is “air” with the bubble surfaces being formed from the water and proteins in the milk∗. But it is the question of what air is, and the implications of that, that is today’s Daily Grind.

Tweetie pie with a cake at Mace, KL

Cakes can be shared with cartoon characters at Mace

It appears that it was Empedocles (492 – 432 BC) who first recognised that air was a substance†. A thing that existed all around us. But it took until the seventeenth century and the invention of the air pump by Otto von Guericke (1602 – 1686) before people recognised that air was heavy. Guericke was responsible for the spheres of Magdeburg demonstration about the strength of a vacuum. He had fashioned two hemispheres of copper. Each hemisphere fitted very closely to the other. He then used his air pump to pump the air out of the spheres (ie. make a vacuum) and tried to pull the two hemispheres apart. Accounts vary but it is said that teams of 8-15 horses tethered to each hemisphere were unable to pull the spheres apart because of the vacuum created within the spheres†.

It was von Guericke’s air pump, together with the work of Boyle on gases and Torricelli’s invention of the barometer that prompted Francesco Lana-Terzi, SJ (1631-1687) to design an ‘air ship’. The idea was simple: If air had a weight and it is possible to make something lighter than air (by making a space inside a copper sphere a vacuum), then it should be possible to make something lighter than air such that it would float, just as objects that are less dense than water float. What differentiates Lana-Terzi’s design from previous fantasies about flight (such as Daedalus and Icarus) was that Lana-Terzi based his ideas on solid principles of mathematics and physics. He calculated how heavy the air was and balanced that with the amount of air that he would have to pump out of four hollow spheres of copper in order that they could lift a gondola full of people.

latte art by Mace, Eiffel Tower and hot air balloon

Art on a cafe latte at Mace

Although there were practical problems with Lana-Terzi’s idea of an air-ship based on four hollow copper spheres, his ideas were correct and eventually led to the development of the hot air balloon. And it is with the hot air balloon that we return to coffee, to Mace and find a connection with a London cafe. The artwork on my cafe latte was not, ‘latte art’ in the sense to which we have recently become accustomed. It was however very much art on a latte, with a scene featuring the Eiffel Tower depicted in chocolate. Just to the right of the Eiffel Tower and suspended in the milky sky was a hot air balloon, floating away exactly as Lana-Terzi had envisaged. Back in 1783, on the corner of Euston Road with Tottenham Court Road, there used to be a pub/coffee house called the Adam and Eve. It was renowned for its cakes and cream and its large tea garden. As far as I can work out, the tea gardens extended to around what is now Brock St and the site of a Beany Green. It was here, in 1783 that the balloonist Vincenzo Lunardi (1759-1806) “fell with his burst balloon, and was but slightly injured”‡. Fortunately for Lunardi, and for ballooning in general, it was only a slight setback. Lunardi went on to make a number of balloon flights, including the UK’s first successful hot air balloon flight.

So next time you are in Kuala Lumpur, why not spend a while at Mace imagining floating in on Lana-Terzi’s air ship gondola while you enjoy a gorgeously frothy iced chocolate. Who knows, one day Lana-Terzi’s air ship gondola may even feature on their latte art, I’d love to see that picture!

Mace by Coffee Chemistry Signature is at Damansara Uptown, Kuala Lumpur.

∗ On Food and Cooking, The science and lore of the kitchen, H. McGee, Unwin paperbacks, 1984

† History and philosophy of science, LWH Hull, Longmans, Green and Co, 1959

‡ Quote from London Coffee Houses, Bryant Lillywhite, 1963


The hot chocolate effect

hot chocolate effect, Raphas

A ready prepared hot chocolate

This is an effect that reveals how sound travels in liquids. It enables us to understand the milk steaming process involved in making lattes and yet, it can be studied in your kitchen. It has an alternative name, “The instant coffee effect”, but we won’t mention that on this website any further. To study it you will need,

1) a mug (cylindrical is preferable),
2) some hot chocolate powder (no, instant coffee really will not do even if it does work)
3) a teaspoon
4) a wooden chopstick (optional, you can use your knuckle)

Make the hot chocolate as you usually would and stir. Then, remove the spoon and repeatedly tap on the bottom of the mug with the wooden chopstick (you could instead use your knuckle). Over the course of about a minute, you will hear the note made by the chopstick rise (not having a musical ear, I will have to trust that this can be by as much as three octaves).

resonator, mouth organ

The length of the pipes in this mouth organ determine the note heard. Photo © The Trustees of the British Museum

What is happening? Well, just like an organ pipe, the hot chocolate mug acts as a resonator. As the bottom surface of the hot chocolate is fixed in the mug and the top surface is open to the air, the lowest frequency of sound wave that the hot chocolate resonator sustains is a quarter wavelength. The note that you hear depends not just on the wavelength, but also on the speed of sound in the hot chocolate, and it is this last bit that is changing. When you put in the water and stir, you introduce air bubbles into the drink. With time (and with tapping the bottom surface), the air bubbles leave the hot chocolate. The speed of sound in a hot chocolate/air bubble mixture is lower than the speed of sound in hot chocolate without air bubbles. Consequently, the frequency of the note you hear is higher in the hot chocolate without bubbles than in the former case.

Let’s use this to make a prediction about what happens when a barista steams milk ready for a latte. At first, the steam wand introduces air and bubbles into the mixture but it is not yet warming the milk considerably. From above, we expect that the speed of sound will decrease as the bubbles are introduced. This will have the effect of making the ‘note’ that you hear on steaming the milk, lower. At the same time the resonator size is increasing (as the new bubbles push the liquid up the sides of the pitcher). This too will act to decrease the note that is heard as you steam (though the froth will also act to damp the vibration, we’ll neglect this effect for the first approximation). At a certain point, the steam wand will start to heat the milk. The speed of sound increases with the temperature of the milk and so the note will get higher as the milk gets warmer.

So this is my prediction, musically inclined baristas can tell me if there is any truth in this:

1) On initially putting the steam wand into the cold milk, the tone of the note heard as the milk is steamed, will decrease.
2) This decrease will continue for some time until the milk starts to get warm when the note increases again.
3) Towards the end of the process, the note heard on steaming the milk will continue to increase until you stop frothing.
4) It should be possible, by listening to the milk being steamed, to know when the milk is ready for your latte just by listening to it (if you are experienced and always use similar amounts of milk per latte drink).

So, let me know if this is right and, if it is wrong, why not let me know what you think is happening instead. I’d be interested to know your insights into the hot chocolate effect in a milk pitcher.

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.