Aroma and batch brew

Isn’t it great to find a lovely, freshly brewed, hot cup of aromatic coffee in a quirky little café? Which bit do you enjoy most? That special aroma as you inhale the steam above your cup before sipping the coffee to compare the taste with the smell?


Representation of 2-furfurylthiol. Amazing what can be found (briefly) above your coffee cup.

As you may imagine, a fair bit of research has gone into working out which chemicals are responsible for that just brewed aroma (for a review see here). More than 800 volatile chemicals have been identified as key to the aroma of coffee of which the most important for that freshly roasted and brewed coffee smell seems to be 2-furfurylthiol. Although it has a complicated name, it’s got a fairly simple chemical representation (shown right). Responsible for the “roast-y, sulphur-y” smell in freshly brewed coffee the problem for us, and for 2-furfurylthiol, is that it is not very stable. In fact, in experiments in which a freshly brewed coffee was stored in a thermos flask to keep it warm, the concentration of 2-furfurylthiol in the space just above the coffee decreased by more than 50% within 20 minutes of storage. After an hour, the concentration of 2-furfurylthiol had decreased to less than a quarter of its original amount and shortly after that, it was gone completely (study can be found here). (Other volatile aromatics decreased similarly (here)).

So if you were to brew a coffee, put it in a flask to keep it warm and then drink it within 20 minutes, you will have lost more than half of the lovely coffee smell. And if, heaven forbid, you were to take it from its thermos 1hr after brewing, almost all those wonderful aromatics would have decayed away.

Lundenwic coffee

This was not a batch!
Could you taste the difference between freshly made drip brewed coffee and batch brew?

Why is this important? Well, it’s about batch brew. You may have noticed that batch brew is increasingly popular in many cafés. Offered as a way of getting a filter coffee ‘freshly’ prepared for you without the hassle of actually having to have the filter made there and then. Different establishments try to get around the inevitable aromatic loss by changing the batch every 30 minutes or storing it in a ‘low oxygen’ environment, but is this enough? Do we need some blind taste-tests on batch brew?

A problem is that the decay of 2-furfurylthiol is not just due to oxidisation. Sadly for us, its decay seems to be intimately tied to other qualities that we appreciate in the coffee, the melanoidins (that make the coffee brown) and other chemicals formed during the roasting process (the phenols and the quinones). So even in a low oxygen environment, that aromatic 2-furfurylthiol is going to react with the other chemicals that make coffee great to make batch brew less great.

weather, bubbles, coffee, coffee physics, weather prediction, meteorology

It’s all in the 2-furfurylthiol. That fantastic coffee aroma is due to a number of unstable aromatic compounds that rapidly decay after the coffee is brewed.

That’s the theory. Clearly many cafés have taste-tested the batch brew and found that it doesn’t make enough difference to be concerned about. And in practice there are many other factors that may make a batch brew better than a fresh drip coffee you can make at home (though it would be great if someone could point some of these out for me!), what we need is a citizen science type taste test. A blind test of the same bean, prepared as a fresh filter and a cup at the end of the storage life of the batch. They will most likely have different temperatures so this would need to be considered, either by pouring very little of each (so the fresh-filter cools quickly), or waiting for 5 minutes for your cup of fresh-filter to cool to the batch temperature. Do they taste the same? Do they smell the same?

So this is a call for some science experiments “in the field” (and seemingly for everyone to drink more coffee). If you enjoy a cup of “batch” and are a regular at a café, please do drop me a note to share your blind taste-test experiences. If you are a café, any tips you have as to how to store warm coffee for longer than 20 minutes without compromising the aroma would be very interesting to hear (though if you find a café storing batch for longer than approx. 30 minutes, I would seriously consider going somewhere else!). And if you just drink coffee at home, why not get involved too, prepare a filter coffee that you store in a thermos and another a bit later ‘fresh’, get someone to help you so that you taste them ‘blind’ and let me know what you think. The comments section below is always available, otherwise I can be found on Twitter and Facebook and will happily debate there.

Enjoy your coffee!


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.