beans are roasted,
these acids break
down, making darker
roasts less acidic and
thus more bitter.
During roasting, the
within the coffee bean
are broken down.
Recent research has
shown that the bitterness of coffee in
medium roasts is due
to chlorogenic acid
The browning of the beans is due to a fascinating chemical change called the Maillard reaction. In this reaction, carbohydrates
react with amino acids at high temperatures,
producing chemical compounds that are
brown in color. When you toast bread or grill
a hamburger, the browning that occurs is due
to the Maillard reaction. This reaction occurs
faster in an alkaline environment, which is
present in the coffee bean.
Another chemical reaction occurs during
roasting, called pyrolysis. It is a decomposition reaction that occurs at high temperatures
in the absence of oxygen. During pyrolysis,
fats in the coffee beans are converted into
aromatic oils, producing coffee’s distinctive
aroma. Along with pyrolysis, another set of
chemical reactions, called caramelization,
occurs. In these reactions, sugars turn brown
and develop a caramel taste.
During roasting, heating causes the molecules to speed up and move farther apart, so
the coffee bean expands. Heating also drives
out the water, as it boils and is released as
steam. A similar process occurs when a corn
kernel is popped and popcorn is formed. During roasting, an audible crack can be heard at
two different temperatures—once at around
196 °C and again at around 224 °C.
Why is coffee bitter?
If you were to chew a raw green coffee
bean, it would taste quite bitter. This bitterness is mainly due to the presence of alkaloid
compounds, such as trigonelline (C7H7NO2)
(Fig. 1(a)) and caffeine (C8H10N4O2) (Fig.
1(b)). Alkaloids are naturally occurring
organic compounds with an alkaline pH and
one or more nitrogen-containing rings. They
are often produced by plants and can be toxic,
warding off predators.
The bitter taste
of coffee is due
The darker the
roast, the more
bitter the coffee
tends to be.
into lactones—complex organic compounds
with a ringed structure. Further roasting
breaks down these lactones into phenylin-
danes, another type of complex organic com-
pound. Both lactones and phenylindanes have
a very bitter taste.
To make coffee, whole coffee beans are
ground up, greatly increasing their surface
area. When you scoop the grounds into a coffee filter and add water, you get your morning
cup of joe! But what exactly is happening
when coffee is brewing? Most of today’s
household coffee makers work by dripping
very hot water—just shy of boiling—through
the grounds that are resting in the filter. Once
this hot water passes through the grounds, it
becomes coffee and drains into the coffee pot,
which rests on a hot plate.
Making coffee is basically an exercise in
solubility. As the hot water drips through
the grounds, soluble components from the
grounds are dissolved in the water, producing
a transparent solution. (Yes, coffee is transparent, and light will pass through it undistorted sometimes.) About 30% of the coffee
bean is soluble in water, but a typical cup of
coffee only contains 1.20% to 1.45% of total
dissolved solids. The stronger the coffee, the
greater the amount of total dissolved solids
within the coffee. To make stronger coffee,
more grounds need to be used.
Why coffee smells so good
Hundreds of different chemical compounds
have been identified within the coffee bean.
The smell that your nose detects is due to
molecules in the gaseous state entering your
nose and stimulating olfactory receptors. Coffee’s pleasing aroma is due to aromatic oils,
mostly aldehydes and ketones, which are pro-
duced during the roasting process. Aldehydes
and ketones contain the carbonyl functional
group (C=O). If you touch a coffee bean, you
will notice the oily feel of these compounds.
These volatile oils readily turn into a gas
upon heating. The aroma is stronger during
brewing, because heating overcomes inter-
molecular forces of attraction between the
molecules, sending them into the gaseous
state. Coffee should not be made with boiling
water, as boiling drives off too many of these
aromatic molecules, resulting in coffee with-
out much taste. Figure 1. Chemical structures of (a) trigonelline and (b) caffeine
(b) (a) O