For me, the roasting process is one of the most fascinating aspects of the coffee industry. By taking the raw seed from the coffee cherry (which has almost no flavour beyond a mild vegetal taste) and applying heat, it’s transformed into the aromatic and complex bean we brew and drink every day. Although this may sound straightforward, there are lots of variables to consider and the whole process is more scientific than you might think. In my experience, keeping an open mind, collecting data, and being methodical is key to roasting tasty coffee and improving quality over time.
We want all our coffees to be the best expression of variety and the environment where it was grown. This means avoiding any negative flavours associated with the roasting process itself. Under-roasting can produce coffee that tastes savoury, grassy, and sour, whereas over-roasting can result in heavy, bitter, and even smokey flavours. We’re constantly striving for that holy grail, where the coffee is full of flavour, sweet, and most importantly, balanced.
We use a classic drum roaster, which consists of a solid, rotating, cylindrical drum laid horizontally on its axis with an open flame below. In this system, the coffee beans are heated primarily by the hot air inside the drum (convection), but also directly from the drum wall itself (conduction). By controlling the amount of gas supplied to the burner, we can manipulate the temperature inside the drum, and we can end roasting at any time by emptying the batch into the cooling tray.
During roasting, maillard reactions (a chemical reaction between amino acids and sugars) and caramelisation turn the beans brown and create hundreds of new taste and aroma compounds. The two most important factors to consider when roasting coffee are temperature and time. How long a coffee is roasted for and what temperatures it’s roasted at all influence the flavours in the final cup. Two coffees roasted to the exact same end temperature can taste wildly different, if one roast was relatively fast and the other one was quite slow. By controlling the use of heat and timing, it’s our job to balance the acidity, sweetness, and bitterness of the beans we’re roasting. Once we find a combination of heat and timing (or recipe) we’re happy with, this can be replicated and is known as the roast profile.
Every coffee is different, from moisture content and density, to variety and growing conditions, and all of these factors will influence how a coffee will roast. You can’t apply the roast profile of one coffee to another and expect the same results, so we develop a unique recipe for each coffee to get the best out of our beans. The more data we can collect, the better. This enables us to make more informed decisions on how to approach a particular coffee, based on “similar” coffees we’ve roasted in the past. Calendar is just over a year old, and we’re beginning to see the same or statistically similar coffees come back around. The time we spent in our first year collecting data is beginning to pay off, as we can now develop our profiles faster and reduce our waste with fewer test batches.
Espresso and filter coffee use fundamentally different brew methods, so we prefer to tailor our roasts to one or the other. Whether we’re roasting for filter or espresso, the end temperature is often similar, but the total roast time and how we apply gas can be really different. Acidity gives coffee its liveliness, delicacy, complexity, and brightness, but too much can be overbearing (especially in espresso) and too little can make the coffee taste flat and boring. Roasting coffee for longer reduces acidity and increases body, which we think is better suited for espresso. For our filter coffees, we prefer a little more acidity so we keep the roasts short and the beans are sometimes lighter in appearance. In both cases, it’s important that the coffee is roasted evenly. Beans that are cooked more on the outside compared to the inside are considered ‘underdeveloped’, which produces grassy flavours and is one of the most common causes of sour espresso.
Roasting coffee is a complex process and it’s surprising how little information there is on how to do it really well, especially when it comes to retaining those flavours unique to variety and terroir. I’m lucky enough to have been roasting coffee for over five years, and even in that short time, I’ve noticed huge improvements in the quality and consistency of roasted coffee. Above all, it’s great to see more roasters choosing to source and roast their coffee seasonally. Coffee roasting still has a long way to go, but no matter how advanced roasting becomes, the quality and freshness of the green coffee will always be most important.