The Science (and Art) of Roasting Coffee

It is said that coffee roasting is ‘part science, part art, and part magic.’ This indeed is true. In fact, the science involved in the process of roasting coffee beans is nothing short of astonishing.

Witnessing the process for the very first time, this is my personal account of what happens. Now, I do have a confession to make. In no way do I claim to be knowledgeable or experienced in this fascinating field (far from it) but the following description is a humble attempt to record my observations and what I have learned so far. So here goes…

“It takes 400 man hours for every pound of unroasted beans to reach this point. Now is not the time to get it wrong”, said artisan roaster, Ian, as he excitedly fires up the trusty 14lb (in old money) capacity Uno. With a ‘pop’ and a ‘whoosh’, the blue flame from its DNA-like helical gas burner breathes into life. Allowing time for the post-war roasting machine to warm up, he methodically inspects the six kilo batch of hard, tasteless Ethiopian Yirgacheffe (produced in the lush, deep soils of the high-rolling mountains of the Sidamo region, southern Ethiopia) for any ‘offenders’ that do not make the grade and could potentially spoil the entire process.

To the nose, the beans give off a delicate profusion of freshly mown hay or the smell of the earth just when the first raindrops fall on the dry, sun-kissed ground in summer. Only when the roaster is completely satisfied with the overall consistency in quality and size, does the Uno’s vintage belt-driven motor hum into action. The beans are then carefully poured into its horizontal-axis chamber. To a soft percussive rhythm, the lightly green and blue-hued beans become a blur as they brush the insides of the rotating drum.

As each second passes, the unhurried alchemy of extracting the delightful flavour from the coffee gradually reveals itself.

In a seeming state of perpetual free fall, the – as yet – unroasted beans begin to absorb the heat from the Uno’s centrifugal fire. Five-or-so expectant minutes pass. Nothing much of note happens to the untrained eye during this preliminary ‘drying off’ phase. And then, the first wisp of a feint smoky aroma, like the smell of toast, begins to emanate; a sensory prologue to the action that is to follow.

At this point the core temperature inside the coffee bean has broken through the threshold of 150-160 degrees centigrade, and is steadily climbing. At the molecular level, this magical moment is coupled with an explosion of cellular activity as the bean’s very chemical composition starts to dramatically change in a series of volatile runaway reactions. In physical terms, this metamorphosis is the transition from an endothermic (to take in heat energy) state to becoming exothermic (to give out heat energy).

It is shortly after this stage when the first ‘crack’ – not dissimilar to the sound of the popping or splitting of popcorn – of the coffee bean occurs and the effects of the increased energy inside the bean becomes visible. By now, they are starting to release their hidden aromatic oils that have been conspicuously concealed so well since their early days of infancy as an unripened green berry on the bush. In the following minutes, the temperature rises further to approximately 220 degrees. The pressure increases too; creating the optimum conditions for the necessary exogenous chemical reactions to take place.

Cascading inside the Uno’s spinning drum, the beans take on a yellow-orange colour as they start to give up their locked-in moisture. Water vapour and carbon dioxide is released in extraordinary quantities through the ‘fissure’ which  runs through the centre line of the bean. This leads to a rapid expansion in volume (up to twice the bean’s original size), accompanied by a marked loss in weight and density. In the parlance of the chemistry class, it is known as the Maillard Reaction. From a personal perspective, it is as though the beans are bursting back into life after a suspended period of deep hibernation.

The temperature still rising, further complex chemical reactions occur between the amino acids, carbohydrate and sucrose compounds (more than 800 hundred have been identified so far). By now, the beans have turned to a light brown colour signaling the ‘carimalisation’ of the natural oils and sugars that give the coffee bean its distinctive colour and rich flavour.

The skill – or ‘art’ to be more exact – of the speciality roaster is the intuitive use of his or her senses. Without a temperature gauge or timer in sight, the roaster falls into a spell of intense concentration as the final critical moments approach. Like a Michelin-starred chef or master wine blender, he is using all his senses to bring out and enhance the best characteristics that the Yirgacheffe has to offer. Meanwhile, the beans take on a dark chestnut colour as the premises of J. Atkinson & Co. fill with their delicious, almost floral, nutty aroma.

Asked if he is ever afraid of the beans catching fire, Ian enigmatically responds, “every roaster is christened with his first fire at some stage, I decided to get mine in early”.

It only takes a mesmerising 12 minutes from start to finish. With a final flourish of the roaster’s hand to bring the Uno’s rhythmic mechanical overture to a close; the drum comes to a stop, the helix flame is extinguished and the roasted Ethiopian Yirgacheffe beans spill out into the perforated cooling tray below. Lightly coated with their natural coffee-flavoured oils, they glisten in the low mid-afternoon sun, as they are patiently stirred to bring their temperature back down to an ambient level.

Depending on the specific roast desired (low, medium, high; full roast, double roast), the bean can go through a further two or three ‘cracks’ until the roaster decides to terminate the process. Done with expert split-second timing, this is to ensure that the balance of flavour, complexity of taste and acidity levels all add up to the holy grail of the experienced Roastmaster: Perfection.

Coffee roasting is a centuries-old craft. In today’s age of impressive scientific discovery, we still do not fully understand the massive thermodynamic changes that roasting brings about inside the bean. One thing is for sure however, I now understand why its is affectionately referred to as an ‘art’.

And what about the ‘magic’ I hear you ask?

Well, all I can say is that you only have to taste the sublime Ethiopian Yirgacheffe coffee for yourself to discover the answer.