Roasted coffee is literally one of the most chemically complex beverages we can put in our mouths and the generation of theses incredible flavourful aromatic compounds is largely down to the chemical changes that occur within the seed of the coffee cherry when it is inside a coffee roaster.
If you’ve ever tried to brew up a cup of the good stuff using raw green beans, the importance of the roasting process becomes apparent very quickly. Firstly, they are structurally much harder and subsequently less soluble than their roasted counterparts making it more difficult to grind and extract flavour from the beans. They also offer very little in the way of that typical coffee flavour that we all know and love, instead tasting vegetal and pea-like.
We roast coffee to ultimately enhance it, allowing this agricultural product to reach it’s full potential with respect to aroma and flavour. Specialty coffee is some of the most tasty and exciting coffee in the world and we strive to showcase the beautiful, inherent characteristics each bean holds thanks to the emphasis on quality within the supply chain. When roasting we are looking to develop positive taste attributes such as aroma, sweetness, acidity and body. Whilst at the same time attempting to avoid negative flavour traits such as bitterness, baked and roasty notes.
Green coffee contains around 250 aromatic molecular compounds, and it’s these compounds that are responsible for a coffees flavour and aroma. Through roasting that figure increases to 800+, giving us a whole host of desirable characteristics. The tasty flavour notes you see on the outside of your coffee bags be it fruity notes, hints of nuts or chocolate, even floral aromas akin to what you’d find in standout African coffees.
Roasting makes this increase happen. Once heat is applied to these seeds a whole host of chemical changes within each bean begins to occur. Hundreds of chemical substances undergo change. Some of them diminish, some disappear, some transform and some combine with each other to form new substances.
On a physical level we see a change in colour, an increase in size, decrease in density and increase in solubility. Both chemical and physical changes that ultimately help to determine that delicious coffees flavour.
The goal of roasting is to take something that’s unpalatable and not only make it palatable but to make this flavour accessible. The roasting process opens up the bean and makes it possible to appreciate the development of beautiful flavours. It make the beans brittle enough to grind easily and porous enough to allow water to access and extract their soluble flavour. In short, roasting not only makes drinking this mighty fine beverage possible, it also creates one hell of a tasty experience for the tongue.
Now the roast process begins with what we call the CHARGE, which refers to the temperature at which the green coffee beans are dropped into a coffee roaster. This is determined depending on batch size and is important as it helps establish the momentum needed for the duration of the roast. Once the temperature within the drum meets the desired charge temperature for that batch, the green beans are dropped into the roaster which signals the beginning of the roast process.
Next up is the turning point, which helps create the s-curve that we see in a typical roast profile. In most roasters temperature data is gathered from a probe within the drum. This measures the temperature of air when empty and beans surface when roasting. When green beans enter the drum from the hopper they are at room temperature and this results in a noticeable drop in temperature within the roast chamber, not surprising seeing as the drum can be in excess of 200 degrees before the charge. Once the temperature of the air and beans equalises it reaches the ‘turning point’ and the curve begins to rise creating the signature ‘tick’ of the roast profile.
Generally, it is common practice to refer to the rest of the roast process in ‘phases’ as a way of explaining and simplifying the changes occurring within the bean throughout the duration of a roast.
The ‘DRYING PHASE’ (as its commonly known) – begins at the charge of the roast and ends as the beans turn from pale green to yellow and adopt a hay-like aroma, which signals the beginning of the Maillard reaction. True to its name, within the drying phase, moisture is driven out of the bean so that certain chemical reactions can take place. And it’s these chemical reactions such as the Maillard reaction and caramelisation that shape the taste of our coffee. It’s important to note that moisture loss does occurs throughout the entirety of a roast, however in the opening 4-6 mins it’s lost to the levels where these reactions can eventually take place. Inadequate drying time can inhibit these reactions and ultimately negatively impact the development of desirable flavours later on in the roast.
The ‘MIDDLE or MAILLARD PHASE’ – This occurs between 145-160 degrees centigrade depending on the roast, signals the beginning of aroma development and sees the bean’s colour turn from yellow, to tan to a light brown. The first noticeable aroma is that of straw, before quickly evolving into that beautiful smell of toasted bread, which is indicative of the Maillard reaction. A chemical reaction between amino acids and reducing sugars that results in a complex sugar browning process. This chemical reaction alone is likely responsible for well over 600 organic flavour compounds within our freshly roasted coffee. In addition, this reaction is also responsible for the production of compounds known as melanoidins. These browning products contribute flavour, body, colour and complexity to the coffee. Most noticeably the body, which is the mouth feel or texture of the brewed liquid on your tongue. From 171 degrees caramelisation deepens the bean’s brown colour and creates fruity, caramel and nutty aromas. The last of the bean’s remaining free sugar is broken down and the coffees bittersweet quality is enhanced. It is also during this stage that the expansion of the beans cause them to shed their chaff.
The final phase kicks in as we approach the end of a roast and this is referred to as ‘DEVELOPMENT TIME‘. This is set to begin at the beginning of first crack, (which is the audible popping sound that can be heard when the beans spontaneously expand due to the release of water vapour and CO2 that has gradually been building within the bean throughout the roast) and ends when the coffee is dropped out of the roasting chamber and into the cooling tray. It’s an incredibly complex phase in which we see sugar caramelisation, organic acid degredation and pyrolysis occurring alongside the on-going Maillard reaction. It is in this phase that we end the roast, halting the bean’s development at the point where we have reached the desired flavour characteristics we wish to highlight. Essentially we are looking to avoid any grassy or vegetal notes that suggest underdevelopment (too little time spent in the development phase) or the baked, roasty notes indicative of overdevelopment (too much time spent in this phase) We’re looking for that sweet spot in between the two where we can showcase the tasty, inherent characteristics of the coffee in question. Once the coffee exits the roaster and enters the tray it is rapidly cooled by a fan, which prevents the beans residual heat from cooking it any further.
If you roast passed first crack, the temperature of the bean continues to rise and CO2 begins to build within the bean again. This pressure forces oils to the surface and a ‘second crack’ occurs, releasing excess CO2 for a second time. We choose not to roast into second crack here at North Star because we feel this destroys much of a coffee’s unique character. Desirable flavour compounds begin to degrade and are eventually lost. Simultaneously, reactions generating unpleasant bitter compounds begin to increase yielding pungent roasty notes that overwhelm whatever subtle flavours survive dark roasting.