Espresso
Roasting
Roasting
is a chemical process by which aromatics, acids, and other
flavor components are either created, balanced, or altered
in a way that should augment the flavor, acidity, aftertaste
and body of the coffee as desired by the roaster.
The first stage is endothermic where the
green beans are slowly dried to become a yellow color and
where the beans begin to smell like toast or popcorn.
The second step, often called the first
crack, occurs at approximately 205 °C (400 °F) in which
the bean doubles in size, becomes a light brown color, and
experiences a weight loss of approximately 5%. The
corresponding Agtron number for this color is between 95-90
(Davids, 68-69).
In the next step the temperature rises from
205 °C to approximately 220 °C, the color changes from light
brown to medium brown (Agtron # 60-50), and a weight loss
of approximately 13% occurs (Davids,
68-69). The resulting chemical process is called
pyrolysis and is characterized by a change in the chemical
composition of the bean as well as a release of CO2.
The second step is followed by a short endothermic
period which is followed by another exothermic step called
the second crack. This second pyrolysis occurs between
225-230°C, and the roast color is defined as medium-dark
brown (Agtron #50-45) (Davids,
68-69). The second pop is much quicker sounding
and the beans take on an oily sheen.
Espresso
potential is maximized in roasting when you maximize the
sweetness and aroma of the coffee while minimizing the bitterness
and acidity. Most people focus on the latter and therefore
roast extremely dark, yet without sweetness and aroma the
espresso will never be palatable. This explains the
unpopularity of straight espresso and the popularity of
espresso based drinks where either milk or other flavors
are used to replace the sweetness that was lost by roasting
darkly.
From 170-200°C the sugars in
coffee begin to caramelize. From tasting pure sugar
versus its caramelized component it is evident that uncaramelized
sugar is much sweeter. The dark color of coffee is
directly related to the caramelization of the sucrose in
coffee. Therefore, to maximize sweetness you want
to minimize the carmelization of sucrose, yet you do not
want to roast too lightly or bitter tasting compounds will
not thermally degrade. Stop the roast somewhere between
the end of the first crack and less than halfway through
the second crack. Do not roast far into the
second crack. We recommend a roasting chamber temperature
somewhere between 205-215°C. Realizing the danger
of the following suggestion we might recommend a color similar
to the one below. Note: All monitors, computers, and
internet browsers will display the color slightly different.
This is only a recommendation to point out that this color
is preferable to the almost black color you will frequently
observe for espresso. To get a better idea of roasting
colors order the Agtron roasting classification kit from
the SCAA.
Related
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Espresso
Blending
Grinding
and Dosing
Tamping
Extracting
Latte
Art
Roasting
and Chemistry by Carl Staub
