Coffee roasting has been around for hundreds of years, yet very minimal documentation exists about the techniques and science. Many roasters learn the craft through the guidance of master roasters, but “trial and error” is the most common instructor of roasting. Due to the scientific nature of roasting and the versatility of coffee beans, experimentation with time, temperature, and taste palate are consistently utilized.
The variability of coffee and finding the perfect roast for each bean type is what makes coffee roasting not just a science, but an art form. While experimenting, I highly advise taking diligent notes. The best part of home roasting is creating that perfect cup of coffee. In contrast, the worst part is never being able to do it again. Measure and record and roast to your palate. After 12 years of honing my craft, I am still learning something new about coffee.
Although the process of roasting is largely dynamic, with recent technological advances there can be a systematic approach that can apply to all beans and roasting machines. These technologies help make roasting more consistent and predictable. I have used data-logging software in the past and still do on occasion, but nothing beats using my three senses (sound, smell, sight) to discover the perfect roast. There may be a time, probably sooner than later, when I will need to use my software more frequently, but for now, I will continue the romance of manual adjustments and trusting my senses over the computers.
When I roast, I rely on my three senses (sound, smell and sight). When it comes to sound, I am listening for the first and second cracks. These cracking noises act as timers during the roasting process. Much like roasting popcorn, coffee beans will release an audible popping, or cracking, noise when it expands. The release of water vapor and carbon dioxide pressure will create this initial popping sound known as the First Crack.
Once the first crack completes, there is a quiet period where carbon dioxide pressure begins to renew in the core of the bean. That added pressure to the bean core forces the oils to the surface due to the weakened structure form the first crack. As soon as the oils begin to appear, the sounds of the Second Crack will commence. The second crack tends to be a little softer, faster, and is more like a snapping sound than a pop.
While many coffee chains roast into the second crack, I personally try to stay between the first and early second crack. Roasting past that stage can destroy the unique characteristics of the coffee beans. The increased level of caramelization caused by roasting into the second crack can create a pungent, "roasty" essence that covers up the beans’ natural flavor profile. Even though that is my personal preference, I understand that there are many of those who enjoy the heavy, smoky flavors of darkly roasted coffee. Ultimately, the roast profile of coffee is a personal choice and does not have a right or wrong answer for enjoyment.
Your sense of smell is another important factor when it comes to distinguishing roast profiles. The volatile aromatic compounds found in coffees’ soluble chemistry is responsible for the aroma of coffee. Although green coffee contains aromatic compounds, they are not distinguishable until the moisture secretion and caramelization of the beans occur. The aroma of coffee will shift as the coffee is roasted and these changes will become more noticeable with practice.
Halfway through the process when the beans begin to yellow, a grassy smell will release and as it tans, the beans will smell like toasted bread. As you transition into the first crack, the beans will give off a sweet, fragrant aroma and once you hit the second crack, the beans will smell like burnt sugar or roasted nuts. The aromatic properties of coffee peaks at a light or medium roast. Darker roasts will lose aromatics more quickly, becoming smokier and more poignant.
The best way to determine roast levels aromatically is to grind the coffee afterward. Although there are some coffees that I can know it's perfect when I start to small marshmellow, sometimes brown sugar, and on a few Grandma's pecan pie. The lighter roasts will smell sweet, malty, and sometimes floral. The medium and slightly dark beans will have a bittersweet aroma like chocolate with fruit characteristics.
In addition to audible cues and aromas, looking at the beans and its progression of color is another informative tool used in roasting. While roasting, it is important to pay attention to the color, bean size, and surface texture. These factors will all correspond to the degree of roast.
At the early stage of roasting, the green beans will begin to pale and turn yellow. During this phase, the coffee beans will lose moisture and the degeneration of chlorophyll will cause beans to change color from their natural green state to the brown colored bean we all know and love. When the yellow shade transitions to the brown stage, the beans will expand and experience the Maillard Reaction. This reaction is similar to caramelization and is the same reaction that occurs in the browning of meats. Shortly about the browning occurs, the popping sounds of the first crack begins. The coffee will still be uneven in color and visible cracks will be shown. The completion of the first crack is when we start categorizing coffee into different roast profiles.
City Roast (Light Roast): Beans are considered City Roast when they are stopped just after the completion of the first crack. City roast coffees are light-bodied and very acidic. With a City Roast level, you tend to get the flavors of the origin of the coffee better (so a Sumatran is quite earthy, Latin American more creamy and African bolder). It also is the toughest level to get right, because if you miss it, the coffee can be bitter, sour and underdeveloped.
Sight: Splotchy/light brown and not oily at all.
Sound: City roast comes right after the first crack.
Taste: Bright, sweet and juicy.
Smell: Maltiness is very distinct.
Touch: Bumpy, uneven surface.
City Plus Roast (Medium Roast): After the coffee has cleared the first crack, it has reached the City Plus roast. The beans have developed a more even surface and the edges are softer. City Plus is a very versatile roast level, and a ton of coffees have great flavor at this level. If done right, you haven't yet lost some of the uniqueness of the cup and get the local flavors that we try to develop in the beans.
Sight: Slight splotches with white stripe in crack (on washed coffees).
Sound: 10-60 seconds after the first crack.
Taste: Bright, sweet and juicy, less sour than City.
Smell: Sweet, floral depending on bean.
Touch: Still uneven, no shininess at all.
Full City Roast (Medium-Dark Roast): At this stage, the beans are on the verge of second crack. The color of the coffee beans starts to become more even, but there is still none of the oil that you see from darker roasts.
The flavors are more balanced and depending on the coffee, this is where it starts to transition from nutty to fruity notes. Full City is an excellent roast level for a lot of coffees. Some African and Indonesian beans really shine at this level. There is a temptation to move to a darker roast from here and get more of the chocolate out of the beans, but I love the blend of light fruited notes with lighter chocolate notes. That's the main reason I stay here with several coffees that other roasters may roast a little darker.
Sight: More uniform surface and no visible oil
Sound: Fully City roast comes right before the second crack.
Taste: Mellow caramels, fruity, chocolatey.
Smell: Bright, sweet and chocolatey.
Touch: Smooth, soft edges and no oily sheen.
Full City Plus (Dark Roast): Coffee has just entered the second crack and the roast is stopped after a few snaps. The surface of the bean is reaching a darker shade of brown (not black) and a slight sheen of oil may appear. This is as dark as I like to roast the beans because you still get the heavy body and boldness of a dark roast without losing the unique flavor profiles.
Sight: Fuller bean, more small cracks, slight oil sheen.