concentration

New findings in arabica…

coffee and Caffeine at Sharps

Things are not always as they seem. Speciality coffee helped scientists to develop a method for detecting fraud in coffee sold as 100% arabica. But how?

…or why adding chloroform to coffee can be a good idea.

Perhaps you remember a story from a few weeks ago that up to one in ten bags of “100% arabica ground coffee” on sale contained “substantial” amounts of robusta coffee?

The story suggested that, perhaps unsurprisingly, fraud is quite common in the coffee industry with cheaper robusta being substituted for the more coveted arabica in a substantial number of packs of pre-ground coffee. But how did the authors of the paper measure this and why did speciality coffee play an important role in the study?

The study used ¹H NMR (nuclear magnetic resonance) spectroscopy to measure the concentration of a particular “finger print” chemical known as 16-O-methylcafestol or 16OMC for short. This aspect of the study was not new. The compound 16OMC was known to be found in robusta (Coffea canephora) while it had not been previously found in arabica (Coffea arabica) beans. It had therefore been considered an excellent marker chemical as to whether a sample of arabica beans had been contaminated with a cheaper robusta.

Previous studies had also used NMR to check for 16OMC but in those studies, they had used a conventional NMR machine and the data collection and analysis had taken a long time. It was also expensive, which meant that it had shortcomings as a technique for quickly investigating fraud within the industry.

The difference in this new report was that firstly, the scientific team investigating the coffees used an NMR machine that fits on a lab table-top: portable, commercially available, and so a possible tool to quickly detect fraud. But secondly, the authors ‘double brewed’ the coffee using chloroform. They first dissolved the ground coffee in chloroform which was filtered using filter paper and then dried and re-dissolved in fresh chloroform to produce a super-concentrated coffee-chloroform brew. This super-concentrated chloroform coffee enabled the authors to obtain a much better signal to noise ratio on the data and so improve the reliability of the detection of any rogue robusta.

neon sign, light emission

NMR uses the light emitted/absorbed from energy levels in atomic nuclei. An analogous effect causes light to be emitted/absorbed by energy levels in electrons. An effect you will have seen on many a high street in these neon signs.

But why could this group use a portable NMR machine whereas previous studies required far more expensive and bulky pieces of kit? NMR works because, just like electrons, atomic nuclei (protons, neutrons) have a property called spin. This spin gives rise to a magnetic moment which means that when you apply an external magnetic field to the sample, some nuclear magnetic moments are parallel, some perpendicular and some antiparallel to the applied field. Consequently, the different moments have different energies which, being on the atomic scale, are quantised meaning that they form discrete levels. This difference in discrete energy levels means that the nuclei will emit/absorb energy (i.e. light) at specific frequencies, which we can calculate. Moreover, the frequency is directly proportional to the applied magnetic field (because the larger the field, the bigger the energy difference between the levels): increase the field applied and you increase the resonance frequency of the nuclei.

But there is one more detail. The nuclei do not exist in isolation, they are affected by the chemical environment that they are in. So a proton in 16OMC will respond slightly differently to an applied magnetic field than a proton in say, water. Rather than be at the resonance frequency we have calculated, the frequency will shift as a consequence of the chemical environment surrounding the proton. As you may expect, this shift is small, but it is significant. It is partly because of this effect that NMR is such a fantastic tool for chemical analysis¹. Typically, the shift is of the order of parts-per-million from the non-shifted resonance frequency. So, in the coffee study discussed here, the interesting “fingerprint” peak is at 3.16ppm. Given that the machine was operating at 60MHz, this means that the scientists were looking at shifts of 189.6 Hz to the non-shifted resonance signal.

It seems sensible that the bigger the shift, the easier it would be to resolve these chemical fingerprints. To get the larger shift requires using a higher operating frequency which is exactly what more traditional NMR spectrometers used. However, given what we know about nuclear energy levels (above), a large energy level split (i.e. high operating frequency) requires a large magnetic field, and large magnetic fields require expensive and bulky pieces of kit. To put this all in perspective, the magnetic field of the Earth at its surface is (variable but around) 0.00005T. A fridge magnet has a field about 0.01T. Commercially available, small rare earth magnets can have fields about 0.3T. A 60 MHz NMR spectrometer looking at ¹H nuclei would need 1.5 T, higher frequency NMR spectroscopy would require still higher fields. The sort of magnetic fields that would be needed for the more traditional NMR technique therefore require large superconducting magnets which are bulky and require expensive cooling. Being able to use a lower frequency for such sensitive measurements is a significant engineering, as well as scientific, achievement.

Look carefully next time you add milk to your coffee. The ‘milk rings’ that can form offer an historical connection to understanding energy transitions in atoms as you can read about here.

So where does the speciality coffee come in? Well, it turns out that by measuring speciality coffee the team uncovered a surprising result: 16OMC was present in arabica beans too.

In order to calibrate the technique, the study had obtained traceable coffees known to be purely arabica or purely robusta. Some of these coffees were sourced from Ethiopia and were grown far away from any possible robusta hybridisation. They were speciality coffee. When the team measured these samples with their concentrated coffee extraction technique, they found that these too contained a small peak at 3.16ppm. Previous studies had missed this because it is such a small quantity. So, as well as determining a technique to quickly establish whether a given coffee on sale is fraudulently being marketed as 100% arabica, this new technique enabled the scientific community to learn something new about arabica. The coffee is more chemically rich than was realised.

If you would like to read more about the study, the authors have summarised it here as well as publishing the paper as open access (so you can download it for free) here. A summary of the results by the company that made the spectrometer can be found here. You can learn more about NMR spectroscopy online, or by obtaining a book from the library such as:

¹ Nuclear Magnetic Resonance Spectroscopy, Robin K Harris, Longman, (1983,1986)

The idea of a coffee at A Wanted Man

We cannot do without a view, and we put up with an illusion, when we cannot get at a truth“.

A wanted man, Chelsea, coffee cup

A wanted man becomes visible under thin coffee.

A Wanted Man on Chelsea’s Kings Road is unusual in many respects. Firstly, never before have I been to an espresso ‘canteen’, but then, neither have I had a coffee in a café that is part coffee-shop part waxing salon. While both wax based hair removal and coffee rely on bees, this is surely not the connection between these two enterprises. Nonetheless, once your coffee-loyalty card is full, you can choose: free brow shape, bikini wax or coffee. The coffee comes from Common Man Coffee Roasters in Singapore so it would be interesting to know how it was transported to Chelsea in order to retain its freshness, surely each batch is not flown in? On our first visit, we had a rich and smooth long black, a lovely aromatic banana bread and a good hot chocolate (with soy milk). There is plenty of seating in the front of the café and some more towards the back near the bar which was all fairly empty on our first visit but far more crowded (with singly-occupied tables) on my second visit (see below).

As I drank my coffee, hidden wording became visible at the bottom of the cup. “A wanted man” appeared beneath the coffee when the coffee was sufficiently thin. By tilting the cup, this “critical” thickness could be estimated, as you can see in the photos. Ah-ha I thought, the physics bit of this cafe-physics-review will be easy! The absorption of light (which we could measure by the visibility of the writing at the bottom of the cup) is directly proportional to the thickness of the absorbing liquid, the coffee. This is the Beer-Lambert law which describes how light is absorbed through substances such as coffee in which there are molecules and bits of sediment that absorb light (which is ultimately why coffee appears brown). Could I experimentally verify this bit of the Beer-Lambert law by somehow quantifying the visibility of the wording as a function of cup-tilt angle?

a tilted coffee cup at a wanted man

Absorption is a function of thickness and concentration

Before I had thought that far, I had finished the coffee, however the second part of the Beer-Lambert law could be tested by having another coffee on a separate occasion. The other part of the Beer-Lambert law states that the absorption (that’s the (in)visibility of the wording on the cup in this case) is also directly proportional to the concentration of the absorbing molecules/sediment. This makes sense, weak coffee is far more transparent than overly extracted coffee. On my second visit, the coffee tasted slightly stronger, a bit different from my memories of the first occasion. Did the “A wanted man” become visible at a different tilt angle? I would guess – or perhaps that should read ‘hypothes-ise’ – that the angle on the second occasion would have to be lower (that the coffee would have to be thinner generally).

However, while sipping my coffee (before getting to the tilt-angle-test) and looking around the second time I noticed that all along the wall where previously there had been plenty of empty tables, each one was now singly occupied by somebody using a laptop, a phone/tablet or in one case, both of these items together. This second time, my mind started wandering into more social issues, while looking at our screens and immersed in social media, are we able to see more or less, than our less absorbed fellow citizens? Does social media clarify the detail or cloud important aspects of our understanding?

Beer-Lambert applied to twitter and Facebook

Does social media do this to you? The light absorption of a coffee is determined by the thickness of the coffee and concentration of absorption sites within it.

After considering these two points, it became clear that in some ways they are connected. Admittedly a loose connection, and not one that is strictly scientific but perhaps it’s worth ‘running with it’ for a bit and seeing if it leads anywhere. Just as with the Beer-Lambert law with coffee, the more ‘interacting sites’ (or absorption sites) we encounter on social media, the harder it is to see through to the bottom. Twitter, Facebook etc. can be enormously helpful for widening our networks and learning about new things. But, as has been frequently pointed out elsewhere, they can also become quite unhelpful when we are in an “echo chamber” or when we think that points can be made in mere soundbites. Is it possible that the more absorbing and reflecting sites that we encounter, the harder it is to see anything to any greater depth? What we need is time-out, for self-reflection and for considering points made by others, on Twitter, Facebook and elsewhere.

Perhaps the best way to end such a post is with a long quote by somebody else. In fact, the same person (and in the same book) as was quoted at the beginning of this article. Perhaps it would be something to consider while we drink our coffees and hover over the ‘retweet’ or ‘share’ button. Are we helping to probe the depths of our cup by the links we share, or are we merely adding to absorption sites in soundbites in our networks?

It requires a great deal of reading, or a wide range of information, to warrant us in putting forth our opinions on any serious subject; and without such learning the most original mind may be able indeed to dazzle, to amuse, to refute, to perplex, but not to come to any useful result or any trustworthy conclusion. There are indeed persons who profess a different view of the matter, and even act upon it. Every now and then you will find a person of vigorous or fertile mind, who relies upon his own resources, despises all former authors, and gives the world, with the utmost fearlessness, his views upon religion, or history, or any other popular subject. And his works may sell for a while; he may get a name in  his day; but this will be all. His readers are sure to find on the long run that his doctrines are mere theories, and not the expression of facts, that they are chaff instead of bread, and then his popularity drops as suddenly as it rose.

John Henry Newman, The idea of a university.

A Wanted Man can be found at 330 Kings Road, London