When it comes to roasting great coffee, few do it better than award-winning Union. that's why Founders Jeremy and Steven will be presenting live roasting demonstrations in The Roastery at this year's Lcf. we sat down with them ahead of the event to learn about the art of roasting.

"We believe roasting is a unique skill that's easy to pick up, but takes years to master. Our talented roasters don't rely on clever machinery to run this key process. Instead they roast using all of their senses...

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 talent of our coffee roasters who make sure we're getting it right for every single batch of coffee.
 

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.
 
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 initially use a lot of power in our burners and aim to turn the roast around the turning point, and start gaining heat and temperature quickly.

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. 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 — often resulting 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.

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.

We continually check the beans via the small trier, pulling a small sample of beans from the drum. 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.
 
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 caramelisaation on the bean surface. But, for a lower density profile such as Ambiental Fortaleza from Brazil where the roast would be more cooler/gentle with heat application during stage one and two.
 

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 you'd like to hear more from Jeremy and Steven, make sure you visit The London Coffee Festival this April, where they will be doing live demonstrations in The Roastery. Tickets are available here.