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Overextraction and Underextraction
Brewing great espresso takes an understanding of what
actually winds up in the cup when coffee is exposed to
water. Roughly 30% of a roasted coffee bean is made of
water soluble compounds. 20% of those compounds
dissolve fairly easily, while the remaining 10% take a little
more work – which is a good thing, because that less-
soluble 10% is acidic, bitter, and generally unpleasant. The
goal of all coffee making is to extract the easily dissolved
oils and compounds while leaving the rest in the grounds.
If ground coffee steeps in water too long, all the soluble
compounds will be extracted, which makes for a very bitter
brew. This is called overextraction. The opposite of
overextraction is underextraction, which occurs when coffee
is not exposed to the brew water long enough, leaving the
essential flavors and aromas locked in the grounds.
Underextraction results in coffee that is weak in taste.
Whether brewed coffee is overextracted, underextracted,
or just right depends on several factors, including the ratio
of coffee to brew water, the fineness of the grind, the
brewing temperature, and the length of time the water is
in contact with the coffee. All these factors are either
directly or indirectly affected by the barista’s technique.
Before Brewing: The Elements of a Great Espresso
Before the espresso machine is even plugged in, you’ll need
several elements to produce great coffee.
Fresh Coffee Beans
Great coffee can only come from fresh coffee beans,
properly roasted. Many baristas recommend buying
beans roasted no darker than a medium roast, the color
of which appears as an even chocolate brown. This roast
preserves the natural sugars and flavor of the bean,
which sets the stage for excellent espresso. A medium
roast is the darkest a bean can be roasted without oils
developing on the surface.
Darkly roasted beans – which appear dark brown or
nearly black – look great, but the extra roasting
overwhelms the more delicate coffee flavors and
caramelizes any sugars. A heavy “roasted” coffee flavor,
often bitter and sharp, will predominate with a dark roast.
To preserve the freshness of
coffee beans:
Keep beans in an opaque, air-tight container and store
them in a cool, dry place. Refrigeration is not
recommended, as condensation tends to form on the
beans whenever the container is opened. Freezing can help
preserve beans stored for an extended period, but it will
also impair flavor.
Great Tasting Water
An often overlooked element of great espresso is the brew
water. If you don’t enjoy the flavor of your tap water, don’t
use it to brew espresso – use bottled, purified water
instead. Since it doesn’t take long for fresh water to
acquire a “flat” quality and taste, it’s also a good idea to
change the water in the tank often and refill the boilers
after a long period of non-use.
Do not use mineral water or distilled water – they can
damage the espresso machine.
The Right Grind – and Grinder
Espresso demands a very fine, very consistent grind. Blade
grinders and inexpensive burr grinders usually fall short
when it comes to producing the grinds needed for
outstanding espresso.
The best espresso requires a quality burr grinder, like the
Artisan™ Burr Grinder. A good burr grinder will maximize
the flavor and aroma of espresso by producing an
extremely consistent grind with very little frictional heating.
What is Espresso?
Espresso began as an attempt in the 1800s to quickly brew
coffee on demand, by the cup. The goal was to serve the
freshest, most flavorful coffee possible and avoid the
burned, stale taste of coffee kept warm on a stovetop. To
speed the brewing process, coffee pioneers struck upon the
idea of forcing water through the grounds under pressure.
Steam was initially used to supply the pressure, followed by
compressed air, lever operated pistons, and finally, the
electric water pump.
Through the decades, the elements of espresso brewing
were tested and refined to produce the standards we have
today: one cup (30 ml) of true espresso comes from
exposing 7 grams of finely ground and packed coffee to
90–96º C water under 9 bars of pressure. In a brief 25
seconds, most of the highly flavorful coffee aromas and oils
are extracted, while the more bitter compounds and off-
tastes are left behind.
When the ground coffee is fresh and the brewing is done
well, the pressurized brew water emulsifies the coffee oils
into the golden foam called crema, which crowns the
espresso with ultimate flavor and aroma.
