Below is a sketch that explains how the taste of an espresso roast changes over time
The horizontal line represents the level of burnt material in the roast. We are not suggesting an espresso roast should taste burnt, only that even in a light espresso roast there will be a small percentage of the material in the bean that has gone past the point of caramelisation. If your roast were to have no burnt material whatsoever, we would argue that it isn’t an espresso roast. We can argue all day about how high the line should be drawn, but this sketch is only indicative to convey what is happening
Having a very low level of burnt material in the roast combines with the acidity (high notes) and the caramelised sugars under pressure to create the magical taste that separates great espresso from just coffee. To give you an idea of how carefully we manage this, a reduction in the end point temperature of the roast of just 2F will create a difference in the cup that you can easily detect – i.e. the difference in taste from a 2F change in end temperature is not merely an academic or theoretical difference
Then we have the classic decay curve. This represents how the volatile components in the coffee rapidly die off as they oxidise. As a result we can taste the changes in the acidity of the coffee as it ages. The decay curve represents the ‘average’ level of acidity in the coffee at any point in time. We have also included a rough candlestick chart to convey what is happening ‘behind the average’. As you can see, immediately post roast the taste is pretty unpleasant to most people’s palate – the acidity is very fragmented, with what you might call extreme complexity, but too much so – there is no harmony – it all clashes on your tongue and in your nose. You also have a lot of carbon dioxide still trapped in the coffee as it has not had an opportunity to de-gas
As time progresses, the extreme fragmented nature of all different tastes in the roast converge. At some point the convergence is sufficient that the sum of all the individual taste elements in the espresso is pleasing to our palate, and this is typically very pronounced – a coffee that was unpleasant one day will suddenly become enjoyable the next, like someone has flicked a switch
Of course you can criticise this chart for being overly simplistic because there are thousands of components that make up the average that is the ‘taste’ of your espresso, but none the less, we think it is adequate for demonstrating the changes that are occurring
The coffee will continue to improve for a few more days as it travels through the ‘sweet spot’, then like the tide it will start to wane, although still within the zone that we might term the sweet spot where we find the coffee pleasant
As time progresses the coffee will travel beyond the sweet spot and start to taste flat and tired and lacking in complexity. Beyond this the volatile components will continue to decay until they drop below the level of the underlying burnt components in the roast
When this occurs the coffee tastes burnt, dead, flat, and ashy. In our experience it will eventually happen to even the most vibrant of espresso. The brighter and more acidic the coffee is immediately post roast, the more time it takes for the coffee to travel along the decay curve. The shape of the curve is about the same for all coffees, it just the progression along the curve occurs more slowly
Since we ran our very first roast in April 2004 we have progressively lengthened what we think is the optimum ‘average’ rest period for a coffee. When we started it was probably around the 5-7 day mark. For the last couple of years it has probably hovered around the 10 day mark. This year we have found ourselves preferring coffee at the 14 day mark more and more often. The concept of averaging is vital to espresso roasting we think. Not much weight can be placed on your views on any given day, you have to taste the coffee again and again, and form an opinion based on your ‘average’ experience. We started favouring the 14 day rest period for a number of roasts earlier this year, but only after several months of repeated testing are we more confident that 14 days is a more suitable rest period for many espresso roasts
You can accelerate the rest period by opening the bag early and the constant contact with moving air will accelerate the ageing process, but for some reason which i can’t explain a coffee that has been allowed to rest slowly, i.e. with the bag sealed, tastes better than a coffee that has been allowed to rest for say 3 days by spreading it out flat on a tray to maximise the exposed surface area
