The Roasting Process
The roasting of coffee beans is a cooking process that produces chemical changes
within the bean. The coffee bean is composed of carbohydrates, proteins, sugars,
acids and countless other organic compounds. There are over one thousand identified
flavoring chemicals in each coffee bean. The coffee roasting process will transform,
breakdown, evaporate and oxidize many of these chemicals into other chemical compounds
and gas such as carbon dioxide . The progression of the roasting process from beginning
to end will follow some roast profile and at the end the coffee will be roasted
to some degree of roast. The result of this process will determine to a large extent
the flavor of the brewed coffee.
The roasting process begins by loading the green beans into a pre-heated drum roaster
or fluid bed roaster. As the beans go through the roasting process the beans go
through both endothermic and exothermic phases where the bean either absorbs heat
from the oven and drum or produces heat as a result of chemical reactions within
the bean. During the initial phase of roasting free water is driven off from
the bean as the temperature rises from room temperature through 212 degrees F. During
this initial part of the roast cycle there is no visual change in the appearance
of the bean and no chemical reactions occur and is strictly an endothermic process.
As the bean increases in temperature the color of the bean changes from its natural
pale green or bluish green color to a pale yellow color. At this point in
the roast the bean is passing through the 270 degrees F temperature range. The smell
of baked bread is perhaps the first recognized aroma of the roasting process but
the coffee is not palatable yet.
Some chemical changes occur to cause the color change and the bean is still absorbing
heat. Many of the changes that occur during roasting are not fully understood
but what is known is that the sugars caramelize and some are broken down into a
number of acids and carbon dioxide gas. The carbohydrates are also broken
down into acids. Also some of the acids that were present in the green bean
are broken down into other organic compounds or acids and some boil off and escape
as a gas. As the bean is heated toward the first crack the complexity of the chemical
changes increase. The roast profile and degree of roast will determine what types
of acids remain and the amount that remain and will determine the flavor and
aroma of the roasted coffee.
As the roast progresses and the bean temperature moves through the 390 to 410 degrees
F temperature range the first crack occurs and a popping or cracking sound is heard.
The sound is caused by the cell walls rupturing from the liberation of carbon dioxide gas and the bean increases in size. The exothermic reaction begins here liberating
heat within the bean as most of the sucrose is altered. This is a critical point
in the roasting process and most of the bean flavor development occurs here. The
sugar browning cases the color of the bean to change from the cinnamon or very light
brown color and will continue towards a darker and darker brown if the roasting
continues. The next few minutes will determine the degree of roast of the coffee
and the final cup quality of the brew.
Acids that are present In the green bean and acids that are created as a result
of the roasting process are now in constant change. Early in the first crack
the acidity of the final brew will peak but will begin to diminish as the roast
moves toward the darker end of the spectrum. These processes are complex and
only a fraction of a minute can change the perceived acidity and sweetness of the
final brew. Once past the first crack the perceived acidity begins to drop off until
the end of the roast. If a light roast is desired then the coffee is finished at
this point. The roast level and the optimum cup quality for any single coffee
will be somewhere in this narrow time span of the roast between the first crack
and second crack.
The sugar content of the coffee will be at its highest before the first crack but
the flavor development of the roasted coffee has not progressed to a point yet where
the brewed coffee is palatable. As the pyrolysys of the sucrose begins around 390
degrees and the first crack occurs the optimum degree of roast for almost all coffees
is soon to come. Since the desired flavor profile of a given coffee will determine
the degree of roast, a coffee with a high acidity and sweetness will usually be
roasted to a light to medium light roast. A coffee with less acidity and less
sweetness will be roasted to a medium roast towards a medium dark roast. The
roast levels past the medium dark degree of roast will begin to loose any detectable
sweetness and the burnt flavor begins to dominate.
If the roasting is allowed to progress towards the darker end of the spectrum the
beginning of the second crack begins. At this point of the roast the acidity and
sweetness have passed their peaks and the burnt taste of dark roasted coffee begin
to prevail. Coffee roasted past this point will begin to show spotting or
the emergence of oil onto the surface of the bean. At first is will be visible
as a drop or two at the end of the bean and as the roasting progresses the entire
bean will begin to show a sheen and then a visible wetting of the surface.
For the most part coffee roasted this dark no longer posses the flavor and aroma
of fine coffee and the charred taste is most prevalent. The bean has been roasted
to the extreme and is near the burning point.
When the coffee is finished it goes through a cooling process where the roasting
process is quickly stopped by dumping it into a cooling bin and either cooled with
air or a combination of air and water quenching. The cooling process will
also have an influence on the taste of the coffee and if the coffee is cooled too
slowly the flavor will be affected.
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