Roasting is one of the areas where I lack the most personal experience. I have assisted in roasting and I have roasted at home in the toaster-oven-like Behmor coffee roasters, but my hands-on experience in this realm is lesser than other fields. Regardless, I will share what I know about the subject.

When steeped in hot water, green coffee beans offer little in taste or aroma. Roasting coffee creates a series of complex and drastic changes in this hard, grassy seed. Roasting breaks down thousands of compounds within the coffee, and it’s the job of the specialty coffee roaster to bring out the beautiful, intrinsic characteristics of the coffee of subject. Some of the effects roasting has on coffee beans:

  • Change in color from green, to yellow, to tan, to brown, to black

  • Nearly double in size

  • Become half as dense

  • Gain, then lose sweetness.

  • Become much more acidic (perceived)

  • Develop upwards of 900 aroma compounds

  • Pop loudly as they release pressurized gases and water vapors (pops will be simultaneous, like popcorn. This is called ‘first crack’, if pushed further into development, you will hear this happen again, known as ‘second crack’. Second crack is rarely seen in specialty coffee, as it tends to signify an over-roasted bean)

The goal of roasting is to optimize the flavors of coffee’s soluble chemistry. Dissolved solids make up for brewed coffee’s taste, while dissolved solids, oils, and suspended particles, primarily fragments of the bean cellulose, create coffee’s body.

Color Changes During Roasting

The first stage of roasting is commonly known as the “drying phase,” although beans lose moisture at similar rates throughout most of the roasting process. During the first few minutes of roasting, degradation of chlorophyll causes beans to change color from green to yellow. As roasting progresses, the beans change from yellow to tan to light brown, primarily due to Maillard reactions. Late in roast, as beans approach first crack, the brown color deepens due to caramelization. In a dark roast, carbonization may turn beans black. Roasts generally last 11-15 minutes.

As bean development continues, the outer bean becomes more brittle and the inner bean releases water vapor and gases during the cracking phases. Coffee loses 12% - 24% of its weight during roasting, depending on the initial moisture content.

Cooling

Once desired roast profile has been reached, beans are immediately emptied from the roaster into a cooling drum, where they are fanned and rotated to encourage a consistent and steady temperature drop. Though slow, development will continue until the beans are cooled appropriately.