The answer, unfortunately, is not clear. The available data are all over the place.
Some research shows that the concentration of caffeine increases with darker
roasts while other research shows that it decreases. Some research even shows
no changes at all! What are we to make of all this—how can we see completely
opposite patterns with something that seems so cut and dry? If we consider what
we know about roasting and add to it some details of how caffeine behaves in the
universe, we might be able to guess at the answers.
As coffee is roasted longer and darker, it loses mass: gaseous molecules are
created during roasting and they leave the bean. Longer roast times produce
more gases, which mean lower weights. Some molecules in the beans, however,
don’t change at all during roasting. Consequently, as roast levels darken, these static compounds increase in concentration. We can demonstrate this with an
example using mythical compound q.
Let’s say the concentration of q in the unroasted bean was 5 parts q to 100 parts
bean.
In a light roast, some of the bean vaporizes leaving only 85 parts bean but q
stays the same. So, now the concentration is 5 q/85 bean. If the roast darkens a
lot, the bean may only have 75
parts left, making q much more concentrated merely because it could tolerate the
heat!
This behavior would certainly help explain how the concentration of caffeine
increases in darker roasts. Its actual content remains constant while lots of stuff
around it is leaving.
If this were always the case, then we’d always see an increase in caffeine
concentration with darker roasts. But, that’s not what we find.
Caffeine seems to be a fairly stable molecule in coffee. In other words, it doesn’t
seem to combine or interact with other molecules, though there isn’t any
research exploring whether this is true or not.
However, it does have a quirky trait
whereby it tends to not obey the typical transition steps between phase changes.
So, instead of changing from a
solid to a liquid to a gas, it often skips the liquid phase and turns directly into a
gas, a process called sublimation. Sublimation for caffeine can begin at 178°C
(352°F). While it is very difficult to measure the actual internal bean temperature
during roasting, it is simple to measure the temperature of the mass of beans,
which is probably near the temperature inside a bean.
As most roasts easily
exceed bean mass temperatures of 215°C
(419°F) and can go as high as 235°C (455°F), it is perfectly reasonable to suspect
that some caffeine in the bean sublimates and drifts away from the bean.
If this happens, then it explains the caffeine decrease as roasts become darker. In
fact, some research does indeed show that total caffeine content decreases with
darker roasts.
What about the data that demonstrated no change in caffeine concentration in
either direction? Well, it is possible that both of those phenomena occurred
simultaneously at just the right levels as to maintain a constant caffeine concentration. I don’t think it is that straightforward, though.
There are several
reports where beans were processed differently or were of different quality
grades and their caffeine contents were different. This suggests that some kind of
interaction between caffeine and biological and/or chemical processes exists. The
effect of this interaction may be the unpredictability of how caffeine behaves
during the roasting process.
At the end of the day, all this discussion of how the caffeine concentration is
changing is probably moot. In all cases, the changes in concentration are pretty
small, amounting to 0.1 percent or less of a difference from the lightest to the
darkest roast.
Thus, in a practical, real-world sense, on a per-cup basis, the
amount of caffeine in a cup produced from a very light roast compared to that of
a cup produced from a very dark roast is pretty small. It is so small, in fact, that a
person who drinks a cup of coffee a day would probably experience no
physiological difference between the two cups based upon their caffeine content!
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