The process of coffee roasting is basically the chemical decomposition of green coffee beans by adding heat over a period of time. Throughout the roasting process certain aspects of the coffee are enhanced, lost or created. As the roast’s timeline extends noticeable differences in the coffee’s aroma, flavor, sweetness and color become apparent. Although the age of the coffee, the processing method, the grind, and brewing method all affect the taste of coffee, the roast degree creates the baseline flavour that can be anticipated.
The changes to green coffee beans brought on by time and heat occur in three main categories: Maillard reactions, Strecker Degradations and Caramelization. The Maillard is a browning reaction similar to what makes toast taste different than stale bread. Small changes in the temperature and length of time spent in the Maillard reaction can have a big impact on the final profile of a coffee. Strecker Degradations is when Amino acids react with carbonyl-grouped molecules to create compounds such as aldehydes and ketones. Who knows what that really means, but it is the process which enhances flavor and aroma. Caramelization is the breakdown of sugar molecules under high heat, creating an array of sweet, bitter and nutty flavor molecules.
Many of these reactions are sensitive to variations in temperature and length of exposure to heat. So a small change in roasting technique can have a profound impact on profile. Generally speaking, beans roasted over a shorter period of time will have more acidic, fruity, floral notes. Comparatively, beans roasted over a longer period of time will have more body, earthy and chocolatey flavours.
The overall goal of roasting specialty coffee is to highlight the bean’s origin characteristics such as acidity, floral notes, chocolate, molasses, and earth, without overwhelming these aspects with the greater roast flavour. The longer the roast carries on, the unique characteristics of the bean will burn off and the taste of the roast will become more apparent until the beans taste entirely of smoke, char and tar.
Light roasts are light brown in color, with a light body and no oil on the surface of the beans. Characteristics from bean origin are best expressed at this stage (the flavours that reflect terroir, varietal, and processing method). Light roasts have a toasted grain taste and pronounced acidity with floral and fruity notes.
Light roasted beans generally reach an internal temperature of 180°C – 205°C (356°F – 401°F). When the beans reach a certain temperature, moisture and gas get released along the seam of the bean and this results in a popping or cracking sound known as first crack. Once first crack is reached the roast is finished. Some common roast names within the Light Roast category are Light City and Half City.
Medium roasted coffees are medium brown color with more body than light roasts. At this roast degree body develops and acidity is muted, creating balance and more pronounced aromas of caramel, roasted nuts, cocoa, and perhaps chocolate.
Medium roasts reach internal temperatures between 210°C (410°F) and 220°C (428°F) between the end of the first crack and just before the beginning of the second crack. Common roast names within the Medium Roast level include Regular Roast, American Roast, City Roast and Breakfast Roast.
Dark roasted coffees are dark brown in color, or sometimes almost black. They have a sheen of oil on the surface, which is usually evident in the cup when brewed. By this point of the roast, subtle flavours from origin are overpowered by the flavors of the roasting process.
Dark roast coffee has an internal temperature of 240°C (464°F) at the end of the second crack and beyond. Around this temperature the roaster needs to be very attentive as the risk of the beans combustion grows immensely. Dark roasts go by many names, some of the more popular designations for a dark roast include French Roast, Italian Roast, Espresso Roast. Many dark roasts are used for espresso blends.