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A builder of unique and responsible homes in the Athens, GA area.
VIDEO GAMES
THE THERMODYNAMICS OF COFFEE ROASTING
I'd like to clarify a modern misconception.
It affects anyone who takes the caffeine prop
erties of coffee seriously enough to choose
a dark roast over a medium roast when they
want a drink with less caffeine. The common
understanding is that dark roasts contain less
caffeine than medium ones, since they are
roasted for a longfcr period of time (and the
caffeine burns off with the roast). The state
ment is both true and misleading.
On one hand, yes, dark roasts do contain
less caffeine. But on the other hand, the
difference is minor. There are about two mil
ligrams more caffeine in a cup of medium-
roast drip coffee than there are in a cup of
dark-roast drip coffee (about 115 mg versus
113 mg). The difference should hopefully not
affect your decision on
which brew to drink on
a given day.
I came to this con
clusion after exploring
the coffee roasting lit
erature for a few months
to find an explanation
for the case either way.
Feeling quite Nancy
Drew about the whole
project, I wanted to
know what had already
been said on the mat
ter. The final scene of a
mystery can be disap
pointing if, when you're
ready to reveal the
verdict, you find someone has already said and
done the same thing. I was surprised, though,
to find a lack of scientific papers on the topic
of Arabica bean physics. Certainly caffeine is
a well-monitored commodity, and there seem
to be many bloggers with online opinions, but
research about the residual quantities of caf
feine due to the roasting process was hard to
find in peer-reviewed journals.
After a moderately exhaustive (and •
exhausting) literature search, I turned to a
"back of the envelope" check to provide some
insight. Physicists—much like detectives—go
through a lot of stationery. It's a great way
to find a rough solu
tion, and the insight it
provides can save many
more pages of work
later. Here's how the
logic goes:
We want to find the
difference in the caf
feine content of two batches of beans that
underwent different roasting times. We assume
the beans are of the Arabica varietal, and
if you're drinking coffee roasted in Athens,
6A, that's a safe bet. Arabica beans typically
weigh 300 mg each. For the sake of the argu
ment, we assume that the beans are identical
spheroids, so that they toast at the same rate.
To roast them, the raw coffee beans are fed
into the top of a drum roaster that has been
pre-heated to 500 degrees Kelvin (we'll treat
it as an isothermal reservoir, neglecting the
effect of any convection currents in cooling.
That saves several envelopes' worth of calcula
tion). The drum rotates the beans continu
ously, encouraging the release of steam from
the beans, much like the way we toss stir-fry
ingredients around to release steam and keep
the ingredients crisp as they cook.
The beans equilibrate with the high-tem-
perature drum, causing several key reactions
to take place. We get into some nice chemistry
at this point. Firstly, the starch converts into
sugar via pyrolysis and the Maillard reaction
(hello, caramel!). Secondly, the protein breaks
down, contributing to the aroma of the final
product. Aroma in roasted coffee traces back
to the levels of free amino acid’s and reduc
ing sugars. Some of these free amino acids
react with carbohydrates to form phenolic
compounds, the aromatics you enjoy when
you smell coffee. Thirdly, caffeol, an oil, is
released. The caffeol is released more with the
duration of the roast, which is why a French
roast bean will appear oilier than a House
roasted one.
The beans are held in these conditions
for 15-20 minutes, throughout which time
steam is driven off. Simultaneously, the high
temperature and pressure inside the bean
crack its walls, expanding the bean's volume
by as much as 20 percent. Since the bean has
become both lighter and larger, the caffeine
content per weight and per volume changes.
The caffeine itself changes very little because
we never exceed the boiling point of caffeine
at the higher temperature and pressure*. The
only caffeine loss comes from sublimation
(evaporation directly from solid state), and
that's minimal.
Since the dark roast beans are in the drum
for a few minutes longer than medium roast
beans, it makes sense that the sublimation
will take place more in
the dark batch. Thus,
dark beans will be
stimulant-challenged
in comparison with
the lighter roasts. The
difference, however, is
about two milligrams,
which you can liken to brewing your coffee
with two beans fewer. It's not a big enough
difference to sway your decision on which
brew to have at 4 p.m., but now you know
the bean of truth that led to this common
misconception.
*The boiling point of caffeine at the bean's
higher internal pressure and temperature
(which is not more than 430 Fahrenheit)
exceeds 532 Kelvin (482 F). See Bothe and
Cammenga, "Phase transitions and thermo
dynamic properties of anhydrous caffeine."
J. Thermal Anal. 16, 2. (1979). I've also seen
the boiling point quoted as being 600 F, but I
couldn't find a reference to support that.
Sarah Dunning
Sarah Dunning is a graduate student in physics at
UGA. The Physicist is an occasional series of columns
in which she explores the physics of everyday life.
On one hand, yes, dark roasts
do contain less caffeine.
But on the other hand, the
difference is minor.
10 FLAGPOLE.COM • AUGUST 19,2009
JACOB HUNT