What happens during the roast?
How does a dense, flavourless seed transform into a delicious, brittle, aromatic coffee bean that we can dissolve in water and enjoy? Heat! Adding heat to a coffee seed creates an incredible amount of change. But what happens and when? Last week we dove into our 3-part series taking you behind the scenes to give you a breakdown on what it means to roast coffee, how we approach it, what is actually happening to the coffee during a roast, and what are some of the variables we manipulate and control to make all that magic happen. This week, we’re learning all about the different stages of the roast, how the roast progresses and the sensitive chemical reactions and colour changes we carefully monitor to bring out the flavours we love in a coffee and avoid the ones we don’t.
So without further adieu, let’s roast...
Stage 1 - Drying
Green coffee is typically between 10-12% moisture, and in order to begin roasting and moving through the necessary chemical reactions the seed needs to shed the majority of that moisture which takes a significant amount of energy. Once the seed has released enough moisture, it will begin to transition from a green to yellow colour and enter the next stage of the roast.
The ‘drying phase’ of the roast is critical in setting the roast up for success, particularly when executing profiles of different total roast times and meeting other goals like the time the coffee will spend in different stages, the timing of first crack, development time, and end temperature throughout the remainder of the roast. The ‘drying phase’ can also have a large impact on the pattern or trajectory of how the roaster is transferring heat to the coffee.
Drying & Flavour - Quicker drying times will result in a more floral, bright cup, where longer drying times can yield a more sweet, balanced, and sometimes less nuanced cup.
Stage 2 - Maillard Reactions
Once enough moisture has been released from the bean the “maillard” stage begins. This starts when the beans begin to turn yellow and the aromatics transform from a green and vegetal quality to become more like hay or dried grass. These complex chemical reactions occur between the amino acids and reducing sugars in the coffee and have a major influence on the final cup character and the brown colour of a roasted coffee. The coveted caramelization of the sucrose in the coffee also begins during this stage producing the caramel tones, bitter compounds and some of the organic acids we all love in roasted coffee. This stage also is thought to contribute to the texture and body of the final cup due to the creation of melanoidins. The Maillard stage comes to a close with the onset of “first crack” but the Maillard reaction continues on for the duration of the roasting process.
Maillard Reactions & Flavour - Shorter times in the Maillard can result in more clarity, the perception of a lighter texture, and less complex sugar browning tones while longer times spent in this stage can allow for more chemical reactions to occur promoting more complex sugar browning tones as well as a heavier body and texture.
Stage 3 - First Crack & Post First Crack Development Time
The momentous first crack marks the beginning of development time. First crack occurs as water vapour builds up in the cellular structure of the coffee and pressure increases dramatically. Eventually, the pressure can no longer be contained and the coffee rapidly expands and releases steam into the environment. It’s called First Crack because an audible cracking sound can be heard similar to the popping of popcorn. This rapid expansion of the coffee allows for heat to penetrate the center of the beans more easily.
During development time, many important and complex processes and reactions are occurring simultaneously. As the temperature of the coffee increases and the time after First Crack progresses, the Maillard reaction and sugar caramelization continue along with the degradation of existing acids, the formation of new acids, and the progression of the colour and roast degree of the coffee. The relationship between the temperature of the coffee and its environment to the amount of time spent roasting after First Crack will dramatically impact a coffee’s final colour and roast degree as well as the flavour in the cup by affecting the acidity, sweetness, balance, body, and bitterness of the coffee.
Development Time and Flavour - A short development time will yield a cup with more acidity and vibrancy and often less sweetness, bitterness from roasting, and body. A longer development time will degrade the acids inherent in the coffee first and develop more bitterness, sweetness, and body imparted by the roasting process. In extreme cases, a very short development time leads to intense acidity and bitterness from chlorogenic acid that hasn’t been broken down enough as well as vegetal, metallic, and hay-like flavours.
Stage 4 - Cooling
Once the coffee has gone through its desired profile and reached its end temperature it’s extremely important to cool the coffee to room temperature as quickly as possible. If the coffee stays warm too long, it can dull the flavours or also continue to cook further and create over roasted characteristics. Our roaster drops the coffee into a large cooling tray where it blasts air and stirs the coffee continuously too cool quickly.