aroma

Smelling collectively

You can see the steam rising above the cup in this coffee at Carbon Kopi. But you will have to imagine the aroma.

It is hard to choose the best thing about coffee, so many aspects combine to make a good cup. But one of the key things about drinking coffee, particularly if you have had a difficult meeting or have just come in from the cold, is the aroma that wafts up as you grind the beans, add water to bloom the coffee and then brew. In happier times, we may be walking down the street preoccupied about something that is going on and then suddenly get hit by a fantastic aroma that signals our proximity to a good cafe. We perhaps ‘follow our noses’ to the source of the smell and then breathe in the scents as we enter the cafe. Which brings us, in a round about way, to moths and a recent paper that appeared in Physical Review E.

It is not that moths have been shown to have a particular liking for the smell of coffee. That may be an area of future research for somebody. But they do need a very good sense of smell because they need to be able to ‘follow their noses’ in order to find the source of a smell that they are interested in (typically a pheromone released by a female moth). This female moth may be located 100s of metres away from the male and probably does not emit that much odour, so how do the male moths find her?

In a similar manner to our approach to the aromatic coffee shop, the moths first travel against the wind, aware in some sense that the smell is carried downstream. If they lose the scent, they then fly perpendicular to the wind flow in an attempt to sniff the aroma once more. This pattern of zig-zagging flight allows them to approach the source of the smell fairly quickly*.

Eggs of a large cabbage white butterfly. No real links with coffee and few with moths, but the adult pair may well have had to find each other using the sense of smell.

It’s a clever method that is perfect if the wind flows in one direction without any turbulence. But how many times have you watched as leaves have been swept up in the wind flow and danced a swirling vortex pattern before falling back to the ground? Or, as you approach the side of a tall building, you get hit by a gust of wind that seems to come in a number of directions all at once because of the way that it is being affected by the presence of the building wall? We can see a similar thing in babbling streams and in our coffee as the convection currents swirl in vortices. The real world is not so simple as a linear wind flow, in the real world the wind is turbulent.

And yet still the moths find their way to the source of the smell that they are seeking. How do they do it, and could we design a robot (or robots) to emulate the moths in order to find, for example, chemical leaks? It was these questions that were addressed by the recent paper in Physical Review E. In the study they used mathematical calculations to look, not at the behaviour of an individual moth, but at the behaviour of a swarm of moths, a group of moths all searching for a mate.

In the computer model, each individual moth could discern the wind speed and direction and also detect odour molecules. So, left to their own devices, the individuals in the model would follow the zig-zag pattern of individual moths observed in nature (this was a deliberate element of the model). But the model-moths were given another ‘sense’: the ability to see the behaviour of their fellow model-moths. Which direction were the others going in? How fast were they moving?

The model-moths were then provided with one final behaviour indicator, a parameter, β, which was called a ‘trust’ parameter. If β = 0, the model-moths did not trust what the others were doing at all and relied purely on their own senses to reach the prize. Conversely, if β = 1, the model-moths completely lacked confidence in their own ability to discern where the smell was coming from and followed the behaviour of their peers.

We find our way to a cafe via visual cues or perhaps the sounds of espresso being made. But can we also follow the aroma?

Running the model several times for different wind conditions including a turbulent flow, the authors of the study found that the moths reached the destination smell best if they balanced the information from their own senses with the behaviour of their peers. In fact, the best results were for a trust factor, β ~ 0.8-0.85 meaning that they trusted their peers 80-85% of the time and relied on their own decisions 10-15% of the time. If they did that, they reached the smell source in only just slightly longer than it would take a moth to fly directly to the source of the smell in a straight line. An astonishingly quick result. As the authors phrased it, the study indicated that you (or the moths) should “follow the advice of your neighbours but once every five to seven times ignore them and act based on your own sensations”.

Now it would be tempting to suggest that this study has no relevance for us individuals finding a coffee shop and minimal relevance to coffee. But that I think would be premature. For a start, a similar result was found when the question was not about moths but about the best way for a crowd of people to leave a smoke filled room. If everyone behaved individualistically, or conversely, if everyone behaved in a purely herd like manner, the crowd took longer to escape the room than if people balanced their individualistic needs with a collective behaviour. It is a push to suggest that the same thing may be relevant for us finding cafes, but who knows what may happen post-lockdown(s) as we collectively attempt to find a well made flat white to enjoy outside our homes. Maybe we too need to trust our own senses some of the time but be open to taking the advice of those around us too.

*You can read more details in the paper Durve et al., Phys Rev E, 102, 012402 (2020)

A sense of history at Lundenwic, Aldwych

Lundenwic Aldwych coffee

The bar at Lundenwic

Of all the senses, our sense of smell is probably the one that is most likely to evoke memories that can take us right back to our childhood. One whiff of something as we walk past a café can, almost magically, transport us back many years and to a quite different time and place. This aspect of our sense of smell was brought home to me a few weeks ago on a visit to Lundenwic in Aldwych.

Lundenwic was the Anglo-Saxon name for the settlement that was located between what is now Covent Garden and Aldwych. As time progressed and the population of Lundenwic decreased, the site became known as the old-settlement (Aldwic), from which we get the name Aldwych*. Lundenwic is also the name of a (relatively) new cafe that has opened up near the corner of Aldwych with Drury Lane (incidentally, originally called the Via de Aldwych*). The upstairs seating area is quite small but with Caffeine magazines on hand, and plants dotted around, as well as the bar, there is plenty to watch and to notice while savouring your coffee. The espresso based coffee is sourced from Workshop while the filter option (V60 based) features different guest beans. On the day of our visit there were two filter options available. Opting for the Kenya Kagoumoini AA, I waited for my coffee to be prepared while my cafe-physics review companion had a late lunch of a cheese and ham toasty which quickly filled this small café with the aroma of cooking cheese. The tasting notes for the coffee stated that I should expect “rhubarb and raspberry lemonade”, and while the taste was certainly of lemonade, the aroma seemed to me quite different, almost spicy.

Lundenwic coffee

Kenyan coffee, freshly brewed appealed to all five senses, but each in different ways.

The cooking cheese and the memories evoked by the smells, along with this difference between the smell and the taste of the coffee, suggested that smell ought to be the subject of this cafe-physics review. Indeed, smell turns out to be a very interesting sense. The nerve cells relating to smell are the only type of nerve cell that can regenerate†. It is this ability of these nerve cells to regenerate that recently helped a previously paralysed man to walk again. Nerve cells from his nose were transplanted into his spinal cord where they helped in the regeneration of his spinal cord (for reasons that are not yet fully understood).

But what about those smells in the coffee? That special aroma, that you breathe in and appreciate immediately after you have brewed your cup is due to a fantastic mix of over 1000 volatile aroma chemicals. If you let your coffee stand, those chemicals evaporate off, which means that the just-brewed aroma starts to change. One of the most important chemicals for this coffee aroma is called 2-furfurylthiol. It has been shown that the concentration of 2-furfurylthiol in the coffee decreases by a factor of 4 over the course of an hour‡.  Even after as little as twenty minutes or so, the concentration of these complex aroma molecules starts to decrease significantly and so if you, (horror of horrors), were to let your coffee cool overnight and then zap it in the microwave in the morning, you would no longer regain that freshly-brewed smell that may have attracted you to the coffee in the first place.

durian skins and seeds

What was left after a session eating durian on a durian farm in Penang, Malaysia

This may also be the reason that the coffee at Lundenwic tasted differently to how it smelled. By inhaling the aroma and then tasting the coffee without exhaling (and so pushing the aroma back through the nose), our nerves are sensitive to different sensations. Although we may experience this while tasting many foods, occasionally it is crucial. A few years ago, Hasbean coffee were selling a very unusual coffee. The coffee, from Indonesia was called “Sidikalang”. Looking back at Hasbean’s “Inmymug” video, it is clear that it was very difficult for Hasbean’s Stephen Leighton to come up with tasting notes for the coffee which, in the end was compared with “durian”. The aroma of durian has been described as “turpentine and onions garnished with a gym sock” and yet in South East Asia it is known as the King of Fruits and is highly sought after for its taste. The aroma chemicals found in durian have recently been analysed (by the same group as studied the aroma of coffee). Nonetheless, the inclusion of “durian” in the tasting notes was extremely accurate (and did result in an amusing, if unconventional, attempt at opening one of the fruits in the video). It was accurate not only in terms of the experience of the taste/smell combination of that coffee. The actual taste and smell of the coffee was very similar to that of durian. A very unusual and interesting coffee that I have never yet had the opportunity to experience again.

However, to return to Lundenwic, how do the (lovely and inviting) smells that emanate from that café compare with the smells of the area that had been Aldwych before 1905 (when Aldwych was built, demolishing the slums that had existed there)? Some museums, such as the Canterbury Tales (in Canterbury), use the aromas (odours?) of medieval life to give visitors some idea as to what life was like in years gone by. Recalling a childhood visit to that museum, I would suggest that the smell of freshly brewed coffee and melting cheese is an almost unquantifiable improvement.

Truly we could say that at Lundenwic, it is time to wake up and smell the coffee.

Lundenwic is at 45 Aldwych, WC2B 4DR

*The London Encyclopaedia, 3rd Ed, MacMillan publishers, 2008.

†”On Food and Cooking: The science and lore of the kitchen”, Harold McGee, George Allen & Unwin publishers, 1988.

‡The coffee had been held at 80C in a thermos flask for the duration of the experiment. It may be expected that as your coffee cooled down, the volatile aroma molecules would evaporate more slowly than the time indicated in this study.