Sights, Scents, Subtle Sounds, and the Different Stages of Coffee Roasting
Coffee roasting is essentially a cooking process in which aromatics, acids, and other flavor components augment a coffee’s flavor, acidity, aftertaste, and body as desired by the roaster. It is a balancing act of time, temperature, airflow, and execution to bring out the full flavor potential of each coffee and it is one of the most influential factors that affect the overall flavor. Roasting unlocks over 800 aroma compounds and transforms the lifeless green coffee beans into a lively, brown, aromatic coffee that invigorates the senses.
Before being roasted, washed green (or raw) beans are green and smell like freshly cut grass. Natural and honey processed coffees are a pale yellow and may contain some fruit notes. A range of different coffee-roasting techniques need to be applied to develop and extract the depth of flavors and a distinct aroma. It requires high temperatures (350F to 500F), variation of airflow, constant visual observation, and finally, audible cracks (almost like a popping sound) which are heard as the coffee roasts. The roaster uses all senses to pick up on the sights, scents, and subtle sounds that help determine the bean’s final roast profile. This blog will summarize the three main stages in the roasting process: the drying stage, the browning stage, and the crack or development stage.
The Drying Stage
The first part of the roast is the drying stage. It is anticlimactic, with little change happening to the appearance and the aroma of the green beans. The green beans start with around 8-12% moisture and smell nothing like the finished product. For the actual roasting to start, the humidity inside the bean must evaporate. During the drying stage, the moisture begins to fade. This is the beginning of endothermic reactions in the coffee.
As the green bean absorbs heat, chlorophyll gets driven out, and the starches convert into sugars. Sucrose, a disaccharide composed of glucose and fructose, is the most abundant and vital sugar in coffee. It is the primary carbohydrate in green coffee, making up nearly 10% of its composition, and it contributes to the sweetness and acidity in the cup. Once the moisture is all dried out, the actual roasting starts. The color will change from green to yellow, orange, tan, and finally to brown. Fresh aromas fill the air, awakening all the senses as the coffee enters the browning stage.
The Browning Stage (the fun stage)
The drying stage can be so dull. No colors, no aromas, but enter the browning stage and it all changes. The colors transcend from yellow to brown. Aromas smell familiar like semisweet baked bread. It’s finally starting to smell like something good. Although the coffee is no longer in the drying stage, the coffee will continue to dry throughout the whole roasting process.
The browning stage is when the actual roasting is happening. It also starts the Maillard reaction, an enzymatic browning reaction that will help develop flavors and aromas. It is a chemical reaction between proteins, amino acids, and sugars, making hundreds of different aromas and color compounds known as melanoids. They named the term after French chemist Louis Camille Maillard, who in 1910 first described the process while attempting to reproduce biological protein synthesis. Sucrose caramelizes during the browning stage, turning it into acetic acid, breaking down its sugar molecules, and producing hundreds of new compounds and bitter flavors. The browning stage creates roasted, toasted, baked aromas. The beans are brown and are bittersweet. At the end of this stage, the coffee makes a subtle sound, a slight popping noise, which is the beginning of the first crack and the beginning of the development stage.
The Development Stage or the Crack
Even though the coffee develops throughout the roast, the aroma compounds form in the development stage. The reaction becomes exothermic, as CO2 gets released as a by-product of the chemical reactions inside the coffee bean. All the heat and energy absorbed during the drying and browning stages make the coffee explode or crack. A rapid increase in cell pressure within the coffee bean produces the first crack. The beans will increase by double the size and decrease their density by about twenty percent. Complex sugars fracture into simple sugars, signaling the beginning of pyrolysis, a chemical decomposition in coffee which adds to the caramelization of sugars and other coffee nuances. Any remaining silver skin or chaff left on the bean gets removed. Often roasters will slow down the roast during this stage to fully develop all the flavors. Rushing the roast at this stage will create unwanted smoky flavors and sharp acidity. Adding air helps move the smoke away, preventing the smoky and bitter tastes. After the first crack has subsided, the bean should be a richer brown with a few surface wrinkles.
Second crack is the further pyrolytic conversion of compounds in coffee. This happens around 440-450°F. Nothing good ever happens at or after the second crack. The second crack is a physical fracturing of the cellular matrix of the coffee bean, and as a result, the oils in the coffee bean migrate out to the surface. The coffee is now black and oily and smells like a skunk cooked in a campfire. Your typical mass-market dark roast is roasted past second crack. At this point, there is no need for well-grown, well-processed green beans because all the flavor will be roasted out. This is like buying a beautiful dry-aged steak and cooking it past well-done. Please don’t.
The development stage is the relationship between time and temperature and the balancing of organic acids. Organic acids include the prominent chlorogenic acids: citric, malic, phosphoric, quinic, caffeic, acetic, and formic. Organic acids add acidity, sourness, bitterness, and astringency and account for around 10% of the green coffee bean. The roasting length dramatically transforms these acids so that it can produce a spectrum of flavors from the same bean. If a coffee gets roasted too light, there will be too many remaining chlorogenic acids so that the coffee will taste vegetal and metallic. Without the proper balance, several things can go wrong during the roast. For example, if the temperatures are too low and the time is too short it will cause the coffee to have an underdeveloped flavor characterized by roasted beans that have not fully expanded in volume, an exceedingly light roast color, and grassy, grainy, and raw nut-like flavors. On the other hand, if the temperature is too high with a short roasting time it will cause a disparity in roast levels between the outer bean layer and the inner bean which can cause the outer layer of the bean to get burnt while leaving a raw and underdeveloped inner layer. Roasters will refer to these coffees as being “tipped.” The taste results are a combination of charred, grassy, and grainy. Roasting coffee at too high a temperature for too long will result in burnt coffee. It’s worse than a dirty cigar in a clean ashtray. The flavor has crossed the line from a coffee flavor with carbonyl overtones to a pure charcoal taste. If a coffee gets roasted at too low of a temperature and the roast is too long, it will result in the classic “baked” coffee. Looking at this coffee, it may look just fine, with good bean size, surface texture, and even roast color throughout the bean. However, the flavor and fragrance are undeveloped, resulting in a bland, lifeless, dull brown liquid.
When the lengths of these phases are combined, they form a roast profile or the set of temperature measurements at different times during the roast. Every coffee is different and comes with its challenges. It is what makes coffee roasting so exciting. In covering the three major roast phases, I hope to have provided some insight into the techniques to consider when roasting. Thanks for reading, we’ll see you next time where we’ll explore the degree of roast. Until then, happy sipping.