the art of roasting coffee

The art of roasting coffee

We believe roasting is a unique skill that is easy to pick up, but takes years to master. Thankfully we've been roasting for years. All the coffees you buy from our website are hand-roasted to order. Our talented roasters don't rely on clever machinery to run this key process. Instead they roast using all of their senses. Read on to find out a little bit more about what goes into your bag of Union Hand-Roasted Coffee.

How roasting can affect flavour and tastes

The skill of coffee roasting is so much more than achieving a specific colour shade or “Agtron” number (a defined roasting colour scale). The way in which air circulation and heat is applied will significantly influence the sensory perception of flavour and taste in coffee – this is what controls our parameters for roasting. Once this roast profile is defined, it is the skill of our coffee roasters who make sure we are getting it right for every single batch of coffee we roast.


No one roast fits all

At Union we work with many different Single Origin coffees; coffees grown at varying degrees of altitudes, with different bean densities and of a multitude of varieties, which, in turn are crafted through different techniques of post-harvest processing. Add to this the local weather and temperature of the Roastery which influences atmospheric pressure and our Roasting team are presented with a series of questions when approaching our daily production roasts.

Whether we’re roasting for our online home user customers in our 11kg San Franciscan Roaster or larger batches to produce Revelation espresso on our 50 year old Probat, every roast is approached with the same care and attention. For our data logging we use Cropster software, but there is no automation or computer controls. All our coffees are roasted by-hand, the artisanal way and with our roaster using his skill to create flavour and consistency from the beans.

Essentially each roast can be structured into at least three phases, each phase of the roast affects flavour and taste in different ways...


Stage one

Drying. Here we are aiming to build heat energy within the core of the bean. This will be released at a later stage of the roast, as part of the "first crack". To achieve this energy input we use a lot of power initially in our burners and aim to turn the roast around the turning point, and start gaining heat and temperature quickly. By getting momentum and storing heat within the beans early on, we can reduce the need for heat input during stage two, without fear of the roast stalling.


Stage two

Once the first colour change has been reached, we reduce the heat input and allow the carbohydrates in the coffee beans to start developing into sugars. The Maillard reaction (the same process when baking bread or roasting nuts or cocoa) occurs at 1550C and is a chemical reaction within the beans. It results in the browning of bean. Here we want a gradual increase of temperature within the drum. From our experience we consider that this allows the inherent flavours of the bean to develop. The greater the physical density of bean structure, the more time and heat is required through this stage. The less dense the bean, then less heat input is required to achieve the same rate of change of temperature. The flavours we want to encourage are typically sugar browning; caramels, toffee, brown sugar and chocolate nuances and it’s at this stage where these flavours start to occur. We work hard to avoid a "roller-coast roast", where temperatures dip, or even stall. If this happens the progression of carbohydrates into the sugars is arrested and different chemical reactions occur which will produce non-desirable flavours which often result in a salty, flat and flavourless cup. Again the skill of our roaster is our strength here. Not only are they constantly watching the beans, but also using their senses to detect the point when sweet aromas start to change to bread-like.. The point that we move from hay-like into sweet roasted peanuts we know it’s time to switch to the next stage.


Stage three

This is where we reduce the temperature input to such a level that we are maintaining the forward momentum of the roast, with small increments in temperature against time. This is carefully balanced against preventing the roast from stalling. If Stages one and two are correct, there should be enough latent heat within the beans to start their own little chain reaction of explosions to maintain upward momentum.

During our roasting apprenticeship, we show how a miscalculation at the early stage of roasting continues to impact at the final stage. If continued heat is applied, it is possible to produce coffee roasted to the correct degree of colour, but with carbonisation of the cell wall of the bean. This will produce a roasty, flat and burnt note when we taste the coffee, even though the colour may appear correct.

For perfectly roasted coffee, the physical appearance of the beans change during the roast, expanding in size and smoothing the wrinkles on the surface. This is the point where the beans explode like popcorn and “Crack” open. This noise is caused by the increase in pressure created from water vapour rupturing the cellular structure of the coffee bean and enables naturally occurring oils to reach critical mass and migrate to the outer surface. It’s these oils, not the bean structure, which contribute to the flavour to the drink. This stage in the roast leads to the creation of the fruited notes. These high nuanced notes within the coffee are fragile; introducing too much heat initially can destroy their subtle, sweet tones. The skill is providing just the correct heat input balanced against time so these flavours are highlighted and balanced by the sugar browning which develops through Stage two.


Roasting for purpose

If we’re roasting a filter coffee we might want to end the roast at the end of stage two, and dump the beans into the cooling tray to cool quickly down to ambient temperature. However if we’re roasting for espresso, we aim to keep the roast curve extending for a moment longer for further development.

It is important to cup each roast and adjust the profile where necessary. Through this process we discovered our Gajah Mountain is best developed a little longer as it has some wonderful dried cherry and spice notes which only emerge when roasted with a longer stage one and two. Only then do we get the lovely stewed fruit notes and depth of flavour which we’re aiming to bring out in the espresso shot. It’s these later stages of the roast, where the body and sensation of mouthfeel are created. And here we perceive the sense of sweetness and balance that we can modify. Again, our Roasters are using all their senses, not just looking for the right colour, but are aware of the aromas produced from the beans and time taken during the roast. We continually check the beans via the small trier, pulling a small sample of beans from the drum. All these factors are being judged right up until the point that our Roaster lifts the door lever and drops the beans into the cooling tray.

Throughout the roast progression, our Roasters are continuously anticipating what the coffee is going to be doing, as well as what the beans are actually doing at that moment. Our Roasters are always looking ahead, using the rate of temperature change as a guide to predict what will happen at later stages in the roast. We like to compare this to a conductor of an orchestra who is three strokes ahead of the musicians; our Roasters are acting ahead of what the beans are actually doing, capable of dialling down the temperature if necessary, or adding slight increases of the gas if they predict the roast needs greater impetus.

The different stages of the roast, expressed as temperature against time create the “roast profile’ and we can record this data using Cropster software. This system plots the shape of the roast profile and helps us manage the roasting process by creating a picture of the reactions in the roasting drum. It is very helpful because we can see not just temperature and time, but the rate of change and allows us to refer back to particularly noteworthy roasts and identify how these were created according to the roast profile

It is important to develop the sensory skills which relate to how different roast profiles influence the cup. Our coffees (blends for espresso and estate selections for filter) are produced against different roast profiles. Our Roaster knows from experience, which roast profile will be suitable high- dense-bean, Pico Alto Tarrazu Honey from Costa Rica, for example requires a lot of heat for development and can give the impression of being a darker roast than it actually is; due to sugar caramelization on the bean surface. But for a lower profile for a lower density profile such as Ambiental Fortaleza from Brazil where the roast would be more cooler/ more gentle with heat application during stage one and two.


Post-harvest process and influence on roasting

Freshly picked coffee cherries can be processed to green coffee using different techniques. And each technique may have it's own variants. For example, the artisan producers we source from in Costa Rica may process the same harvest of beans in different forms. One lot might be prepared as “natural” as an intact cherry drying under the sun on raised African beds, whilst another section of the same harvest might have cherry skin removed but with most of the mucilage pulp (layer of sugar around the bean) remaining in place. An alternative variant might be to fully wash the coffee, strip all of the pulp and sugar layer away through the mechanical wet mill (beneficio) followed by drying on raised beds, thus giving a very clean and pure flavour of the coffee bean. We design our specific roast profiles tailored to the post-harvest processes and thereby highlight these different flavours. We seek to bring out the wild strawberry fruits of a natural coffee, jammy redcurrant, sugar cane and brown sugars of the honey process, or the vanilla, chocolate and caramel and fudge of the fully washed process coffees.

A roast profile taking a shorter time, with high temperature input and full air circulation open until first crack, might bring out more fruit flavours and acidity. Whilst greater heat input in stage two can develop more body and sweetness. Longer roast times with more consistent heat application throughout stage one and two will mute the fruit notes of a highly fruited coffee and create darker, bolder flavours like dark chocolate, treacle, chocolate brownie flavours and remain very sweet with little or no fruit but a long bold aftertaste. This roast is very suited to milk-based drinks. The coffee's origin is also important as it can guide how it might perform in the roaster drum. Traditionally, Roasters have approached coffee solely by region or origin basis. However, as the specialty coffee industry has progressed and evolved, regions have started to become less significant and Roasters are now looking at aspects such as varietal, process, altitude more frequently and crafting the roast on these terms rather than specifically on a Country of origin basis.

Variety is another value to consider. To name just a few; Bourbon, Typica, Caturra and Catuai for example can take quite a lot of heat. Whilst Geisha, Eugenioides and Mokka are more delicate and can lose their fruit flavour characteristics easily if exposed to high levels of heat.

The local weather conditions in the roastery must be given consideration when roasting coffee. On a cold day we know that a coffee might roast faster. Hit the first colour change and first crack earlier. Or on a hot moist sunny day the roast might be slower as it takes longer for the moisture to be driven from the beans, which in turn affects the later stages of the profile and delays the different points of the profile.

The flavour and taste of all coffee beans are defined by the agronomy conditions at the farm, processing and the skill of the pickers, farmers, Co-operatives and mill facility. But as the Roaster, we have a pivotal role to play in the journey of the coffee bean from the farm to the consumer. We impart our personality into the coffee through the way we roast the beans. This is where our skill can transform this coffee so the beans sing pure notes of joy. It is the skill of our roasters, gained through years of experience which enable the flavours and taste sensations to be created. The ability to continually reproduce an accurate roast profile, batch after batch, is key to the consistency of our coffee. Having a solid understanding of the different process techniques, origins, bean densities and varietals is established by roasting our single origins coffees at our Micro Roastery. We work hard to maintain the artisanal craft of roasting coffee.

If all that artistry has whet your appetite you might like to browse our coffees to try for yourself.


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