|Green coffee beans taste woody, grassy and uninteresting. Although all the natural chemicals are already present in the green coffee, it is the roasting process that caramelizes the sugars, drives out some of the moisture, and converts the flavor chemicals. The flavor chemicals become trapped in the intercellular spaces in the form of an oily substance. They are released when ground and brewed.
The same coffee roasted to a final stage or brought to the same stage in a different time to temperature profile can taste vastly different. For the purpose of describing the roast style, one can go through the roasting spectrum, starting from green beans, and ending in the final stage of charcoal. Stages of roasting are often referred to as roast styles.
Roast style is generally associated with the color of roasted beans. Since this often becomes subjective, and the range of browns overlaps, much work is done to quantify the color shades. The Specialty Coffee Association of America has come up with 8 different shades, numbered from 95 (lightest) to 25 (darkest), in increments of 10. The ground coffee is compared with these 8 color tiles to give it a number. This is referred to as the Agtron number.
When green coffee undergoes roasting, it first gives off trapped moisture. After that the coffee changes color from greenish yellow to cinnamon brown (at about 380F), which corresponds to Agtron #95, this stage is often referred to as a cinnamon roast. After this, one hears the "first crack" when the steam in the beans creates enough pressure to break the cell walls. This happens between 380 and 400F. Around 400F it is moderately light brown (Agtron # 85), and is called light roast. Between 415 and 435F it becomes medium brown (Agtron #55) and is known as medium roast. At this point the sugars caramelize and the chemical reaction of pyrolisis gives off carbon dioxide. The gases create pressure that break the cell structure with a distinct second crack, which is louder than the first crack. The pyrolitic reaction produces its own heat and as the temperature rises to about 440F, the beans become medium dark brown (Agtron #45), often called Viennese, or full city roast. As the pyrolisis continues and the temperature is brought up to about 445F, the color becomes dark brown (Agtron #35) and the surface become shiny with flavoring oils oozing out to the surface. This is often called French, Italian, or Espresso roast. As the temperature is raised to about 470F the beans are very dark brown, very shiny and called Italian or Dark French roast. As the temperature is raised to 480F the beans are very dark and shiny with drops of oil on the surface (Agtron #25), also called dark French. After this the coffee is burnt or is now charcoal and of no value.
Most inherent taste characteristics, such as acidity, body, aroma, complexity etc., enhance as roasting proceeds until medium to full city roast (Agtron #50). After that they start to diminish again, except sweetness and pungency which are most pronounced in darker espresso or French roast (Agtron #40)