Ever wonder why espresso costs much more per cup than regular drip coffee?
Part of the reason is the expensive equipment needed to brew a proper
espresso. A high-powered burr grinder first shears the coffee beans to a fine
powder without producing too much heat. (Heating the coffee in the grinding
stage prematurely releases the volatile oils that give espresso its rich
flavor and aroma.) The ground coffee is put into a cylindrical container
called a gruppa and tamped down firmly to provide an even flow of water
through it. An electrically heated boiler inside the espresso machine
maintains water in a reservoir at 1.4 bar and \(109^{\circ} \mathrm{C}\). An
electric pump takes cold water at \(15^{\circ} \mathrm{C}\) and 1 bar, raises
its pressure to slightly above 9 bar, and feeds it into a heating coil that
passes through the reservoir. Heat transferred from the reservoir through the
coil wall raises the water temperature to \(96^{\circ} \mathrm{C}\). The heated
water flows into the top of the gruppa at \(96^{\circ} \mathrm{C}\) and 9 bar,
passes slowly through the tightly packed ground beans, and dissolves the oils
and some of the solids in the beans to become espresso, which decompresses to
1 atm as it exits the machine. The water temperature and uniform flow through
the bed of packed coffee in the gruppa lead to the more intense flavor of
espresso relative to normal drip coffee. Water drawn directly from the
reservoir is expanded to atmospheric pressure where it forms steam, which is
used to heat and froth milk for lattes and cappuccinos.
(a) Sketch this process, using blocks to represent the pump, reservoir, and
gruppa. Label all heat and work flows in the process, including electrical
energy.
(b) To make a 14-oz latte, you would steam 12 ounces of cold milk (3^'C) until
it reaches 71 ^ C and pour it over 2 ounces of espresso. Assume that the steam
cools but none of it condenses as it bubbles through the milk. For each latte
made, the heating element that maintains the reservoir temperature must supply
enough energy to heat the espresso water plus enough to heat the milk, plus
additional energy. Assuming
$$
\left(C_{p}\right)_{\operatorname{milk}}=3.93 \frac{\mathrm{J}}{\mathrm{g}
\cdot^{\circ} \mathrm{C}}, \quad \mathrm{SG}_{\mathrm{milk}}=1.03
$$
calculate the quantity of electrical energy that must be provided to the
heating element to accomplish those two functions. Why would more energy than
what you calculate be required? (There are several reasons.)
(c) Coffee beans contain a considerable amount of trapped carbon dioxide, not
all of which is released when the beans are ground. When the hot pressurized
water percolates through the ground beans, some of the carbon dioxide is
absorbed in the liquid. When the liquid is then dispensed at atmospheric
pressure, fine \(\mathrm{CO}_{2}\) bubbles come out of solution. In addition,
one of the chemical compounds formed when the coffee beans are roasted and
extracted into the espresso is melanoidin, a surfactant. Surfactant molecules
are asymmetrical, with one end being hydrophilic (drawn to water) and the
other end hydrophobic (repelled by water). When the bubbles (thin water films
containing \(\mathrm{CO}_{2}\) ) pass through the espresso liquid, the
hydrophilic ends of the melanoidin molecules attach to the bubbles and the
dissolved bean oils in turn attach to the hydrophobic ends. The result is that
the bubbles emerge coated with the oils to form the crema , the familiar
reddish brown stable foam at the surface of good espresso. Speculate on why
you don't see crema in normal drip coffee. (Hint: Henry's law should show up
in your explanation.) Note: All soaps and shampoos contain at least one
surfactant species. (A common one is sodium lauryl sulfate.) Its presence
explains why if you have greasy hands, washing with plain water may leave the
grease untouched but washing with soap removes the grease.
(d) Explain in your own words (i) how espresso is made, (ii) why espresso has
a more intense flavor than regular drip coffee, (iii) what the crema in
espresso is, how it forms, and why it doesn't appear in regular drip coffee,
and (iv) why washing with plain water does not remove grease but washing with
soap does. (Note: Many people automatically assume that all chemical engineers
are extraordinarily intelligent. If you can explain those four things, you can
help perpetuate that belief.)
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