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?
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.
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.
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!