Espresso is a small, made to order, concentrated coffee consisting of liquid topped
by foam, or crema. The liquid and crema are each multiphasic systems consisting
of an emulsion, a suspension, and a solution.
Crema is composed primarily of CO2 and water vapor bubbles wrapped in liquid
films made up of an aqueous solution of surfactants. Crema also contains suspended
coffee bean cell wall fragments, or fines (responsible for "tiger striping," or
mottling), and emulsified oils containing aromatics.
The liquid phase of an espresso consists of dissolved solids, emulsified oils, suspended
fines, and an effervescence of gas bubbles.
Espresso Percolation: a Primer
What follows is a general overview of espresso percolation. This section is not intended
to be comprehensive, but rather to introduce the fundamentals.
Espresso is produced by the percolation of pressurized hot water through a tightly
packed bed of finely ground coffee. The water erodes solids and oils from the surfaces
of the coffee particles as it flows through the coffee bed and deposits the solids
and oils in the cup.
The flow rate of the water through the grounds is determined primarily by the
amount of pressure applied by the machine, the mass of the grounds, and the fineness
of the grind. Higher pressure, up to a point, increases the flow rate; beyond
that pressure, flow rate decreases. A larger dose or a finer grind produce greater
flow resistance and a slower flow rate.
Water always follows the path of least resistance through the coffee bed; it is
the barista's job to create not only the proper amount of flow resistance, but also to
form the coffee bed such that it provides uniform resistance to the water. A poorly
formed coffee bed is vulnerable to the creation of a channel, an area of high-velocity
flow through the coffee bed.
Channels are detrimental to brew strength and flavor. The large volume of water
flowing through a channel dilutes the shot and causes the grounds along the channel
to overextract, increasing bitterness. Because less water passes through the
denser areas of the coffee bed, those areas underextract, resulting in underdeveloped
flavors and lower brew strength.
Note:The terms "overextract" and "underextract" are subjective; by using them I do not mean to imply
there is a universally agreed-upon ideal level of extraction for coffee, tea, or espresso. Instead, the
reader should interpret overextraction as a general reference to extracting more than the intended
amount, usually to the point of excessive bitterness or astringency. Underextraction is meant to
indicate less extraction than intended, usually such that the resulting beverage has insufficient
To minimize channeling, a barista should
prepare a bed of grounds so it has a smooth and level surface, forms a tight seal with
the wall of the portafilter basket, and is of uniform density.
Evidence of channeling can sometimes, but not always, be seen when using a
bottomless portafilter. Channeling is indicated when extract flows more rapidly or
yellows more quickly from some areas of the basket than others.
The Role of the Barista, Grinder, and Espresso Machine
The yellow extract on the left indicates channeling
The Barista's Role: When preparing an espresso, a barista's basic goals should be to:
- Create a dose of consistent mass every shot.
- Choose the grind setting that will provide the desired flow resistance.
- Distribute the dose evenly to provide uniform resistance to the water.
- Tamp with enough pressure to eliminate void spaces within the coffee bed and
to seal the surface of the bed.
- Ensure the brewing water is of the desired temperature.
- Complete all of these tasks efficiently.
The Grinder's Role: The grinder is the most important piece of equipment in an espresso bar. Grinders
are usually overshadowed by more expensive, flashier espresso machines, but
grinder quality is arguably the single most important factor in preparing a great
A quality grinder must:
- Produce the proper particle sizes to provide adequate flow resistance.
- Create a bimodal or trimodal distribution of particle sizes. (See "Grinding for
- Cause minimal heating of the grounds during grinding.
- Limit the production of fines.
Fines play many important roles in espresso percolation; these will be discussed
in detail in Chapter 3. For now it is important to know that the brewing water can
transport and deposit fines lower in the coffee bed during percolation, a phenomenon
known as fines migration. When fines and large insoluble protein molecules
are deposited at the bottom of the coffee bed they can form a compact layer, or
densely packed solid mass. A compact layer clogs holes at the bottom of the filter
basket and can result in obstruction of flow paths, uneven resistance to flow, and
channeling. It is desirable to have some fines, but too many fines or too much fines
migration can damage espresso quality.
The Espresso Machine's Role: The espresso machine's task is to deliver water to the grounds in a predetermined
pattern of temperatures and pressures. These patterns are known as temperature
profiles and pressure profiles. A quality espresso machine should be able to produce consistent temperature
and pressure profiles every shot, even under heavy use.
Additional Sections in the Printed Edition:
The Phases of Espresso Percolation
Brew Strength and Yield: Espresso