One of life’s simple pleasures is really quite complicatedFor sheer sensory enjoyment, few everyday experiences can Coffee cultivation entails myriad variables that must be moni-
compete with a good cup of coffee. The alluring aroma of
tored and regulated. Once a coffee bean is grown, nothing can
steaming hot coffee just brewed from freshly roasted
be added or removed: the quality must already be present. For
beans can drag sleepers from bed and pedestrians into cafés.
a single portion of espresso, 50 to 55 roasted coffee beans are
And many millions worldwide would find getting through the
required; a single imperfect bean will taint the whole sufficiently
day difficult without the jolt of mental clarity imparted by the
to be noticeable. This is because human olfaction and taste
caffeine in coffee. But underlying this seemingly commonplace
senses originated as defense mechanisms that protected our an-
beverage is a profound chemical complexity. Without a deep
cestors from rotten—hence, unhealthy—foods. Only through
understanding of how the vagaries of bean production, roast-
modern technology can one economically and consistently
ing and preparation minutely affect the hundreds of com-
pounds that define coffee’s flavor, aroma and body, a qualitycup would be an infrequent and random occurrence. Growing Coffee
Connoisseurs agree that the quintessential expression of
R A W C O F F E E B E A N S are the seeds of plants belonging to the
coffee is espresso: that diminutive heavy china cup half-filled
Rubiaceae family, which comprises at least 66 species of the
with a dark, opaque brew topped by a velvety thick, reddish-
genus Coffea. The two species that are commercially exploit-
brown froth called crema. Composed of tiny gas bubbles en-
ed are Coffea arabica, which accounts for two thirds of world
cased in thin films, the surprisingly persistent crema locks in the
production, and C. canephora, often called robusta coffee, with
coffee’s distinctive flavors and aromas and much of its heat as
one third of global output. Robusta coffee plants and all wild
well. Espresso—the word refers to a serving made on request
coffee species have 22 chromosomes, whereas arabica has 44.
expressly for the occasion—is brewed by rapidly percolating a
Therefore, arabica and other coffee species cannot be crossed
small quantity of pressurized, heated water through a com-
pressed cake of finely ground roasted coffee. The resulting con-
Robusta is a high-yielding and disease-resistant tree stand-
centrated liquor contains not only soluble solids but also a di-
ing up to 12 meters tall that grows best in warm, humid climes.
verse array of aromatic substances in a dispersed emulsion of
It produces a cup featuring substantial body, a relatively harsh,
tiny oil droplets, which together give espresso its uniquely rich
earthy aroma, and an elevated caffeine content that ranges from
2.4 to 2.8 percent by weight. Although robusta is sold by many
Aficionados consider perfectly brewed espresso to be the ul-
purveyors, it does not give rise to the highest-quality coffee.
timate in coffee because its special preparation amplifies and
Arabica, which originated in the Ethiopian highlands, is a
exhibits the inherent characteristics of the beans. Espresso is
medium- to low-yielding, rather delicate tree from five to six
useful for our purposes as it is in effect a distillation of all the
meters tall that requires a temperate climate and considerable
numerous techniques by which coffee can be made, including
growing care. Commercially grown coffee bushes are pruned
the Turkish method and various infusion and filter drip pro-
to a height of 1.5 to 2.0 meters. Coffee made from arabica
cesses [see box on page 91 for descriptions of alternative cof-
beans has an intense, intricate aroma that can be reminiscent of
fee-preparation methods]. To know espresso is to know cof-
flowers, fruit, honey, chocolate, caramel or toasted bread. Its
caffeine content never exceeds 1.5 percent by weight. Because
High-quality coffee arises from maintaining close control
of its superior quality and taste, arabica sells for a higher price
over a multitude of factors in the field, in the plant and in the cup. 86 SCIENTIFIC AMERICAN
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.
plants to blossom, and some 210 days af-terward red or yellow fruit called cherriesappear. Each cherry contains two oblongseeds—the coffee beans. Because bothflower and fruit can be present simulta-neously on the same branch, the picker’sforefinger and thumb are the best tools togather just the ripe cherries. Stripping en-tire branches by hand or using automat-ed harvesting machines does not discrim-inate between the ripe and the unripecherries.
coffee beans depends on the genetics ofthe plant, the soil in which it grows andthe microclimate, which encompasses fac-tors such as altitude, the amount of rain-fall and sunlight, and daily temperaturefluctuations. Along with the roasting pro-cesses that are applied, these agriculturaland geographical considerations are re-sponsible for the taste differences amongthe many varieties of coffee beans thatsuppliers combine to produce the variousdistinctive blends one can purchase.
Processing Coffee coffee cherries must be processed immediately after harvest to prevent spoilage. Producers employ two process- ing methods: sun-drying and washing. Effective sun-drying is accomplished by spreading the cherries out on a patio and stirring the desiccating fruit frequently to evenly heat and aerate it. The dried cher-
percent water content of the coffee cher-
electric cells detect duds, signaling for
eration is accomplished at a speed that no
for sorting and bagging. In the alternative
one starts with exceptional green beans.
most highly trained eye is incapable of.
pulped, washed, and finally dried and lib-
based in Trieste, Italy, use many sophis-
ticated process-control techniques to min-
thick walls: as much as five to seven mi-
R ERNESTO ILLY is chairman of illycaffè, a
beans, including ultraviolet fluorescence
dom. During roasting, these 30- to 40-mi-
analysis to spot moldy beans and trichro-
cron-diameter cells serve as tiny reactors
matic mapping to generate a color finger-
in which all the key heat-driven chemical
print (yellow-green, red and infrared) of
reactions occur that generate coffee’s se-
every day in Italy alone. Illy holds a doc-
each lot of beans. At illycaffè, a dichro-
ductive taste and fragrance. The cells of
matic sorting system developed in collab-
immature beans feature thinner walls.
advanced studies in molecular biology.
Unripe beans also lack the important aro-
is applied as a final control right before
roasting. As beans fall into bins, photo-
the last stages of the ripening process. 88 SCIENTIFIC AMERICAN
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. With more than 400 BILLION CUPS consumed every year, coffee is the world’s most popular beverage Coffee ranks SECOND ONLY TO PETROLEUM in terms
beans, forces all the off-flavors and fra-
grances to leave the beans. Sadly, the de-
(about $10.4 billion in exports in 2000).Roasting Coffee ACCORDING TO A SURVEY OF 3,000 AMERICANS CONDUCTED BY
r o a s t i n g is a pyrolytic (heat-driven)
THE NATIONAL COFFEE ASSOCIATION IN 2001:
process that greatly increases the chemi-
Fifty-two percent of adults (older than 18 years)
the roasting, the less desirable the aroma
in the U.S. drink coffee every day, representing
will be and the stronger the bitterness. 107 MILLION daily drinkers. Another 28 percent of adults (57 MILLION) drink coffee occasionally.
volatile molecular species, whereas roast-
American coffee drinkers consume, on average,
ed coffee gives rise to more than 800.
fail to develop fully the welcome aromas,
3.3 (NINE-OUNCE) CUPS OF COFFEE a day.
and acidity tends to come to the fore.
of a roasting machine (basically, a huge,
Controversy continues over whether SHADE-GROWN
hot rotating cylinder), residual water in-
Smelling Coffee coffee should be promoted to enhance bird habitats.
side each cell is converted to steam, which
a r o m a s c i e n c e is highly complex. Coffee and caffeine have been the subject ofextensive scientific study during the past quarter of
grances evolved during coffee bean roast-
a century, with 1,500 TO 2,000 PAPERS PUBLISHED EVERY
ars, proteins, lipids and minerals within
YEAR on the topic. Despite this close scrutiny, few negative health effects have been definitively linked
[see box on next page]. At high heat, from
olfactometry, in which skilled testers sniff
to the moderate consumption (TWO CUPS A DAY) of
and define the smell of each recognizable
caffeinated coffee. In fact, recent work indicates thatroasted coffee can be a good source of antioxidants.1000 B.C. TO A.D. 500 The nomadic Oromos tribe,
ization process called Maillard’s reaction.
composition of each odor. Sniffing roast-
living in the kingdom of Kefa (in modern-day
ed coffee aromas that have been fraction-
Ethiopia), eat coffee, crushed, mixed with fat andshaped into golf ball–size portions, as a pick-me-up.CIRCA 600 Coffee is brought by traders across the Red Sea into Arabia (modern-day Yemen).
the aromas of roses, Darjeeling tea, choco-
12 liters per kilogram of roasted coffee).
late, vanilla and violets, as well as truffles,
LATE 1400S TO EARLY 1500S Coffee beans, heretofore an Arabian monopoly, are brought to Turkey, Egyptand Syria by Muslim pilgrims returning from Mecca.
called cat scent, which, if diluted, smells
Arabian-influenced coffeehouses open in
volatile compounds give coffee its famil-
Constantinople, Damascus and other Near Easterncities, where European traders, particularly
iar fragrance. Pressure inside each cell in-
Venetians, are introduced to the beverage.
In the laboratories of illycaffè, techni-
CIRCA 1600 Calling it “the bitter invention of Satan,”
cians focus on the strongest odorants. advisers urge Pope Clement VIII to reject the favorite
try to escape but are sealed in by the thick,
Imagine listening to a recording of a choir
drink of the infidel Ottoman Turks. Instead hedecides to give papal authority to coffee, making it
oil. Some cells eventually burst, creating
1616 Dutch entrepreneurs start to cultivate coffee
the characteristic popping sound of roast-
commercially, beginning with a coffee plant
tend to dominate the ensemble. If the vol-
obtained from Yemen. By 1658 (according to some sources, the 1690S), the Dutch are growing coffee in Ceylon and their East Indian colony of Java.
stronger voices will still be recognizable
1714 The mayor of Amsterdam presents the French
even as the choir’s sound fades away. Di-
king, Louis XIV, with a coffee plant from Java. 1723 Gabriel Mathieu de Clieu, a French naval officer,
can last from 90 seconds to as long as 40
beyond a certain point, only the strongest
carries three coffee seedlings obtained underquestionable circumstances from the Royal BotanicalGardens on a perilous voyage to the Caribbeanisland of Martinique, where one of the plants thrives.
ics of the intracellular reactions differ ac-
1727 After having been called to arbitrate a border
cording to roasting time, and so does the
dispute between two coffee-growing colonies,
final result. A short roasting time, which
Dutch Guiana and French Guiana, Francisco de Melo
ethylglycolate, which are responsible for
Palheta, a Portuguese Brazilian official, smugglesseveral coffee seedlings to his home estates.1903 German coffee importer Ludwig Roselius
ruin a cup of coffee by their very presence. 1933 Francesco Illy patents the first
roasting periods, frequently used in poor-
characteristic earthy, chemical smell of ro-
1961 Ernesto Valente of Faema, an Italian coffee machine maker, designs the archetype of the
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. Preparing Coffee
Rio taste because it was first discovered in
drip coffee than when making espresso.
be found in corked wines as well. Its per-
system is shockingly low—six millionths
coffee by heated water. The interaction of
the four to six minutes of contact with the
of a billionth of a gram per milliliter.
boiling water, most of the soluble sub-stances present in the roasted coffee passinto solution. Thus, large quantities ofhighly soluble acids and caffeine dissolve
into the cup. In contrast, the much short-er percolation time of espresso allows less
acid and only 60 to 70 percent of the caf-
CREMA, the dense, reddish-brown foam that
tops an espresso, is shown in an enlarged cross
dioxide and water vapor bubbles (large circles)
surrounded by surfactant films, the crema also
and pressurize it to nine atmospheres.
aromatic compounds (particles with red
sistency, is placed in a perforated basket
borders) and dark fragments of the coffee bean Gas bubble
Cumulative Chemical Composition of Espresso with Increasing Extraction Time
grounds adhere to one another thanks toa thin coating of oil, which is as viscous as
THE OVEREXTRACTION of espresso (beyond the recommended 30 seconds) leads to the
honey. The oil binds the particles togeth-
incorporation of undesirable and less soluble aromatic compounds into the drink
er into a condensed maze of minuscule air
that the hydraulic resistance of this bed of
coffee grounds must be slightly less than
tion water, allowing it to flow through at
a rate of around a milliliter a second. CHOCOLATE
of percolation, a skilled barista (coffee bar
technician) produces about 30 milliliters
of dense coffee liquor covered by the all-
important crema. If the color of the foam
topping is light, it means that the espres-
Chemical Concentration (parts per million)
cause the grind was too coarse, the water
Extraction Time (seconds)
short. If the crema is very dark in hue andhas a “hole” in the middle, it is likely that
Chemical Composition of Raw and Roasted Arabica Coffee
the consistency of the coffee grounds was
(percentage of dry matter)
hibits either a white froth with large bub-
white spot in the center of the cup if the
the coffee grounds, including aroma-filled
oil and bits of the cellular structure. The
Green Beans Roasted Beans (average water content (average water content 8%–12%) 0%–5%)
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. FILTER DRIP METHODS (automatic drip, Melitta, Chemex pots).
the infusion, segregating the grounds at the bottom of the pot.
These popular techniques employ finely ground coffee in
Turkish method. Unlike other brewing processes, gentle boiling
receptacles lined with filter paper. A medium grind should be used
of the coffee is desirable when using this method. Mix equal
with a reusable gold filter. There are two keys to making superior
amounts of pulverized coffee, water and sugar in a special pot
coffee using these processes: first, rinse the paper filter with
called an ibrik, which sits directly over the heat. Stir the mixture as
boiling water to remove the papery smell; second, ensure that the
it comes to a slow boil. Stop stirring when the powdered coffee no
near-boiling-hot brewing water takes no more than four to six
longer sticks to a spoon. As the brew just begins to boil and foam
minutes to pass through the grounds, thereby producing optimal
up, remove the ibrik from the heat. Tap the ibrik to reduce the foam
extraction levels. The brewing time of an automatic drip machine
somewhat. Repeat the process at least two additional times. The
can be controlled by tailoring the quantity of water so that it flows
result is a uniquely thick, sweet brew.
for the recommended four to six minutes. French press or plunger pot. This apparatus steeps the coffee Adapted from The Great Coffee Book, by Timothy J. Castle and
in the hot water before the grounds are filtered out. Combine
Joan Nielsen (Ten Speed Press, Berkeley, Calif., 1999).
heated water and coarsely ground coffee in the pot and allow it to infuse from two to five minutes, depending on the desiredstrength. Then press the wire-mesh filter/plunger slowly through
aromatic volatiles dissolved in the emul-
oils, about 0.1 gram a cup. Intact cells in
sified oils as long as it remains there. These
oily flavor/fragrance carriers mean that the
Channel. Please check your local listings.
beverage, along with cell wall fragments,
complex chemistry of coffee to enjoy it. SA
which endow the foamy crema with whatis called the tiger-skin look. M O R E T O E X P L O R E Coffee, Vols. 1–6. R. J. Clarke and R. Macrae. Elsevier Applied Science, 1985. Coffee: Botany, Biochemistry and Production of Beans and Beverage. M. N. Clifford and
are bound to the dispersed gas bubbles, oil
K. C. Willson. Croom Helm, London, 1985.
droplets and solid fragments, all of which
Caffeine, Coffee and Health. Edited by S. Garattini. Raven Press, 1993.
are less than five microns in size. The col-
Espresso Coffee: The Chemistry of Quality. A. Illy and R. Viani. Academic Press, 1995.
loidal character of the dispersion gives the
Coffee: Recent Developments. R. J. Clarke and O. Vitzthum. Blackwell Science, 2001.
International Coffee Organization: www.ico.org
surface tension. Espresso thus visibly coats
International Scientific Association of Coffee: www.asic-cafe.org
Coffee industry–supplied information on coffee, caffeine and health: www.coffeescience.org
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.
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