heat

Is sixty the old forty?

Lundenwic coffee

What is the ideal temperature at which to serve coffee?

What is the optimum temperature at which to enjoy a cup of coffee?

A brief check online for the “ideal” serving temperature for coffee suggested a temperature of around 49-60ºC (120-140ºF, 313-333K) for flavour or 70-80ºC (158-176ºF, 343.1-353.1K) for a hot drink. In my own experiments (purely to write this article you understand), I found that I most enjoyed a lovely coffee from The Roasting House (prepared by V60) at around 52ºC. My old chemistry teacher must have been one who enjoyed the flavour of his coffee too. His advice for A-level practicals was that if we wanted to know what 60ºC ‘felt’ like, we should consider that it feels the same on the back of our hand as the underside of our cup of coffee. So, for argument’s sake, let’s say that we serve our coffee at the upper end of the flavour appreciation scale: 60ºC.

But, have you ever stopped to consider what 60ºC means or even, how we arrived at this particular temperature scale? Why do we measure temperature in the way that we do? While there are interesting stories behind the Fahrenheit scale, today’s post concerns the Celsius, or Centigrade, scale. Indeed, we use “degree Celsius” and “degree Centigrade” almost interchangeably to mean that temperature scale that has 0ºC as the melting point, and 100ºC as the boiling point, of water. It is one of those things that has become so habitual that setting 0ºC at the freezing end and 100ºC at the boiling end seems obvious, intuitive, natural.

thermometer in a nun mug

Careful how you drink your coffee if you repeat this experiment!

And yet the temperature scale that Anders Celsius (1701-1744) invented back in 1741 did not, initially, work this way at all¹. Celsius’s scale did indeed count from 0ºC to 100ºC and was defined using the same fixed points we use now. But rather than counting up from the melting point, Celsius’s scale counted up from 0ºC at the boiling point to 100ºC at the freezing point. Rather than degrees of warmth, Celsius’s scale counted degrees of cold. So, in the original Celsius scale, the serving temperature of coffee should be 40ºC: Sixty is indeed the old forty*.

Which immediately begs a question. Why is it that we count temperature up (the numbers get higher as it gets hotter)? A first answer could be that we view that temperature is a form of measurement of ‘heat’ and that heat is an energy. Consequently, something cold has less energy than something hot, “cold” is the absence of “heat” and therefore what we should measure is “heat”. This means that our thermometers need to indicate higher numbers as the temperature gets hotter, and so we are now counting the correct way. While this is good as far as it goes and certainly is our current understanding of ‘heat’, ‘cold’ and temperature, how is it that we have come to think of heat as energy and cold as the absence of heat? It was certainly not clear to scientists in the Renaissance period. Francis Bacon (1561-1626) considered that cold was a form of “contractive motion” while Pierre Gassendi (1592-1655) thought that although ‘caloric’ atoms were needed to explain heat, ‘frigoric’ atoms were also needed to explain cold.

effect of motivation on experience of pleasure while drinking coffee

How heat, rather than visible light, is reflected provides clues as to why we measure temperature ‘up’.

One experiment that helped to show that heat was an energy (and so lent support to the idea of measuring temperature ‘up’) was that of the reflection of heat by mirrors. In the experiment, two concave mirrors are placed facing each other, some distance apart. Each mirror has a focal length of, say, 15 cm. A hot object is placed at the focal length of the first mirror. At the focal point of the second mirror, is placed a thermometer. As soon as both objects are in place, the temperature indicated by the thermometer increases. If the mirror were covered or the thermometer moved away from the focal point, the temperature indicated decreases again to that of the room. It is an experiment which can easily be demonstrated in a lecture hall and which fitted with a view point that cold is the absence of heat.

However, around the same time as this initial demonstration, Marc-Auguste Pictet did another experiment, the (apparent) reflection of cold². The experiment was as before but in Pictet’s second experiment, a flask containing ice replaced the hot object. On repeating the experiment the temperature indicated by the thermometer decreased. Covering the mirror or moving the thermometer from the focal point of the mirror resulted in the indicated temperature increasing again. Just as ‘heat’ was reflected in the mirrors, so too (seemingly) was ‘cold’.

So, the question is, how do you know what you believe you know about heat? Are there experiments that you can design that could help to disprove a theory of ‘frigoric’? And how do you explain the experiments of Pictet? Reader, it’s over to you.

 

*Within ten years of Celsius’s death (of tuberculosis in 1744), his colleagues Martin Strömer and Daniel Ekström had inverted Celsius’s original temperature scale to the form we know today. A similar scale designed by Jean Pierre Christin was also in use by 1743³.

¹”Evolution of the Thermometer 1592-1743″, Henry Carrington Bolton, The Chemical Publishing Company, 1900

²”Inventing Temperature”, Hasok Chang, Oxford University Press, 2008

³”The science of measurement, a historical survey”, Herbert Arthur Klein, Dover Publications Inc. 1988

 

Counting the caloric at Jaz & Jul’s Chocolate House, Chapel Market

Jaz Jules chocolate house

Jaz and Jul’s, The Chocolate House on Chapel Market

The London coffee houses of the seventeenth and eighteenth centuries have entered history as Penny Universities, places of debate and centres of news. Together with the (scientifically based) Grecian, there was Jonathan’s in Exchange Alley (origin of the stock exchange) and Lloyd’s on Tower Street (associated with insurance). But along side these coffee houses there were the chocolate houses, Whites and Ozinda’s on St James’ St and the Cocoa Tree in Pall Mall. White’s in particular developed such a reputation that it features in Hogarth’s The Rake’s Progress (which can be seen at Sir John Soane’s museum).

So it is an interesting bit of history repeating to find Jaz & Jul’s, a chocolate house on Chapel Market. The interior here is very far from Hogarth’s rendering of White’s. Here, light fittings hang from the ceiling like drops of chocolate about to melt into the café while photographs of cocoa plants and farms adorn the walls. Moreover the emphasis on social responsibility, including in sourcing, mean that this establishment is worlds away from the debauched shenanigans at White’s. Their coffee is roasted and supplied by Monmouth while the cakes are hand made and, needless to say, very chocolatey. The light and fluffy chocolate-Pimms cake arrived with my coffee presented on a plate and matching cup that reminded me of a mint-chocolate-chip ice cream.

Interior of Jaz and Jules Chapel Market

The chocolate counter at Jaz and Jul’s

The side of the counter was tiled to resemble a bar of chocolate, which immediately reminded me of the physics and chemistry of chocolate crystallisation. However, the physics connection of this cafe-physics review is a bit more lateral than that. Soon after I had enjoyed my incredibly chocolatey cake at Jaz & Jul’s, a study was released which showed that Britons were significantly under-reporting their daily calorie intake. Could it be that the obesity epidemic is a result of us eating too much rather than merely exercising too little? Apparently, rather than consume the (recommended) levels of 2500 kcal for men and 2000 kcal for women, many people were eating up to 3000 calories per day. Everything in moderation of course and there was plenty of room in my own calorie count for that great piece of cake (honestly). But the word ‘calorie’ turns out to have a connection with chocolate in a more unexpected way.

Calorie comes from the Latin, calor, meaning heat which in turn hints at how we used to think about heat itself. While we now think of heat as energy, which is why it doesn’t even strike us to equate the ‘energy’ in the chocolate cake with the number of kilo-calories in it, this is not how heat was always viewed. In fact, in the eighteenth century, about the time of the old chocolate houses, heat was thought of as a type of fluid, caloric. Caloric was thought to be able to flow in and out of all substances. When something got hot it was because the caloric flowed into it, when something got cold, it was because the caloric had leaked out. Caloric theory was in many ways very successful in understanding heat and heat processes. For example, the theory easily explained thermal expansion, if a fluid had to flow into something in order for that thing to warm up, then surely, the fluid has to occupy some space, the object must expand to hold it!

Mint choc chip cutlery

Coffee with the Chocolate-Pimms cake.

One area that was tricky for caloric theory though was the fact that friction could cause something to heat up. Such heat generation is crucial for our extraction of chocolate. Once harvested from the plant and cleaned, the cocoa bean is first roasted then shelled to leave the cocoa ‘nibs’. These nibs are then ground more finely. As they are being ground, the friction caused by grinding is enough to cause sufficient heat to melt the cocoa butter in the nibs which is then extracted and retained for later use*. How could you explain this heating if you thought of heat as a fluid? The traditional explanation was that as the two objects rubbed against each other (in this case, nib and stone grinder), the caloric fluid would be squeezed out, it would appear as if heat had been generated.

Benjamin Thompson, Count Rumford (1753-1814), disagreed with this explanation of heat. In the course of a colourful career he had been involved in manufacturing cannons in Bavaria. Rumford had noticed that a lot of heat was generated each time a cannon shaft was bored out. The heat produced continued as long as the grinding continued. If the heat were due to the cannon leaking caloric, surely there would be a point at which the cannon stopped getting any hotter. Yet this did not happen. Rumford suggested (correctly) that instead what was happening was that the energy generated by the boring was being transferred into the metal of the cannon, causing microscopic motion.

Although the heat as motion/energy idea eventually caught on, caloric in some ways still survives in the name that we give to our food energy intake. And so we can return to the cake, could it be that spending time thinking about the caloric in the cake can justify the calories consumed eating it? Sadly the jury is out on whether thinking counts as calorie counting exercise. It seems that the brain’s energy consumption is already so great (at 20% of our resting metabolic rate), that intense thinking does not add too much to the energy consumed by the brain. So we’ll need another excuse and I don’t think we have to look far. The coffee and chocolate at Jaz & Jul’s is delicious enough to justify a significant chunk of your daily calorie count, just based on considerations of taste. Everything in moderation!

 

Jaz and Jul’s is at 1 Chapel Market, N1 9EZ

*”Chocolate: A Global History”, by Sarah Moss and Alexander Badenoch, published by Reaktion Books, 2009

 

Seeing things at a kopitiam (coffee shop)

Rocky, Bangsar, KL, Malaysia, koptiam

A kopitiam in Bangsar, Kuala Lumpur, Malaysia

One of the great things about travelling is exploring the different cafe and coffee cultures in different countries. Is it the coffee that is important? Or food, alcohol or maybe just the opportunity for socialising? In Singapore and Malaysia, the “kopitiam” (or coffee shop) is a familiar part of each neighbourhood. Each kopitiam serves local coffee (kopi) and a variety of foods which are usually prepared while you wait, from stalls around the edge of the kopitiam. The kopitiam provides a space for socialisation and meeting people over a bowl of steaming noodles. Inside electric fans are blowing continuously in an effort to lessen the heat. Frankly, the local coffee is not to my taste but there are plenty of other things to eat and drink in each kopitiam. A breakfast of “kueh” and black tea for example is a welcome change from toast at home! In many areas of Singapore, and to a lesser extent Malaysia, local kopitiams are closing to make way for the new style cafés which serve a range of freshly roasted, pour over or espresso based coffee. Not being Malaysian or Singaporean I do not want to comment too much on that, I guess it is similar to the decline of the “caffs” in the UK. Mourned by many in the community but welcomed by others for the improved quality of the coffee.

straw, water, glass

An everyday example of refraction. The water refracts the light to make the straw appear ‘broken’.

However, with so much going on in a kopitiam, the temptation to look at a kopitiam-physics review was too great, especially when I started to “see things” at the edge of the shop. Am I going mad? No, it was not that my imagination was playing with my mind; I saw the ingredients for a mirage. You see, at the edge of the kopitiam the hawkers will cook noodles, or rice dishes etc. and this creates heat. Above some stalls there will be clouds of steam rising as the noodles boil in a pan. The clouds appear white because of the scattering of light by reasonably sized water droplets (more info here and here). Above other stalls, there is no steam but the heat created by the cooking makes the air immediately above the stove warmer (and therefore less dense). This less dense air refracts light less than air at room temperature. It is refraction that causes that straw in your iced coffee to look as if it is broken as you look at it (see picture). In the kopitiam, it means that as you look through this region of warm air you see a wobbly or wavy type pattern as the light from outside is refracted by different amounts depending on the temperature of the air that it goes through. It is this that is the primary ingredient for seeing a mirage.

The fact that air at different temperatures refracts light by different amounts is the reason for mirages in the desert. Frequently, warm air is trapped at ground level by a layer of cold air above it. The light is bent as it travels through these layers (see diagram here) and so it may appear as if they sky is on the ground (which the brain will interpret as a pool of water on the ground). Conversely, if there is a layer of cold air trapped beneath a layer of warm air, the light is bent downwards and so objects that are usually below the horizon due to the curvature of the earth can be seen (illustrated by the diagram here).

Edmond Halley, Canary Wharf, Isle of Dogs, view from Greenwich

The view towards the Isle of Dogs (and Canary Wharf) from Greenwich. Things have changed a little since Halley’s time.

Back in 1694 Edmond Halley (who drank coffee with Isaac Newton at the Grecian) was investigating the evaporation of water as a function of temperature. He wanted to see if evaporation alone could explain the rainfall and the quantity of water in the river system. As he did so he noticed that, in still air, there was a layer of water vapour that formed above the bowl of evaporating water. He noticed this because it refracted the light in an unusual manner. At the time, there was reported to be an unusual phenomenon that occurred at high tide near Greenwich. It seems that cows used to graze on the Isle of Dogs in London. Ordinarily the cows could not be seen from Greenwich because they were too far away, but occasionally, at high tide, the cows would be visible. Putting together what he knew about the evaporating water Halley wrote “This fleece of vapour in still weather… may give a tolerable Account of what I have heard of seeing the Cattle at High-water-time in the Isle of Dogs from Greenwich, when none are to be seen at low-water (which some have endeavoured to explain by supposing the Isle of Dogs to have been lifted by the Tide coming under it.) But the evaporous effluvia of water, having a greater degree of refraction than the Common Air, may suffice to bring these Beams down to the Eye, which when the Water is retired, and the vapours subsided with it, pass above, and consequently the Objects seen at the one time, may be conceived to disappear at the other”*. I think that although he had the mechanism correct (in terms of refraction), the cause of this odd refraction was temperature inversion and a layer of cold air immediately above the Thames rather than water vapour but what do you think? Let me know in the comments section below.

 

* Punctuation and capitalisation kept as in original. Taken from Edmond Halley, “An Account of the Evaporation of Water, as It Was Experimented in Gresham Colledge [sic] in the Year 1693. With Some Observations Thereon” Phil. Trans. 18, 183-190, 1694″