Tuesday, January 15, 2008

How espresso machines work

Introduction to How Espresso Machines Work

Espresso consumption in the United States has grown tremendously in the last decade or two. In Seattle, Washington, you can't walk more than a couple of blocks without seeing a cafe or espresso cart -- they're in bookstores, grocery stores, laundromats, gas stations and movie theaters. There are drive-through espresso shops in parking lots. Sometimes an espresso cart will just park on the sidewalk, like the hot-dog vendors in New York City.

People have been buying espresso machines for their homes, too. These machines are smaller than the commercial machines found in cafes, but they work on the same principles. In this article, we'll learn how these household espresso machines work. But first, let's see what espresso is.

What is Espresso?

If you go to a cafe or espresso bar and ask for an espresso, what you will get is a shot-sized glass holding a small amount of very strong coffee. There are many different types of espresso drinks (see iVillage: Coffee Glossary for a good list), including cappuccino, cafe latte and cafe mocha. All are made with one or more shots of espresso.

A shot of espresso is made by forcing about 1.5 ounces of hot water through tightly packed, finely ground espresso coffee. If everything goes well, what comes out is a dark brown, slightly thick liquid with a small amount of crema (a foam, sort of like the head on a beer) on top.

There are many variables in the process of making a shot of espresso. The temperature of the water, the pressure of the water, the fineness of the ground coffee and how tightly the coffee is packed are just a few. The skilled espresso maker, or barista, controls all of these variables to produce a quality shot of espresso. Let's start with one of the most crucial variables: the coffee.

The Coffee
Espresso coffee is a blend of several different types of coffee beans from different countries. The beans are roasted until they are dark and oily-looking.

Coffee beans (left) and ground espresso coffee (right)

The beans are ground very finely -- much finer than for drip coffee makers. The consistency is almost like powdered sugar. The more finely the coffee is ground, the slower the espresso comes out. Generally, for the best shot of espresso, it should take about 25 seconds for the water to pass through the coffee. Sometimes the grind is adjusted to control the brewing time.

Let's take a look at what happens to the coffee in an espresso machine.

A Simple Machine

To force the water through the coffee, the simplest espresso machines use pressure that comes from heating water inside a sealed vessel. These types of machines can be bought for around $50, and there's even one that is made especially to take on a camping trip. They all work on the same principle, so we'll take a look at one of the camping-style machines.

In this type of machine, the coffee is packed into a funnel-shaped piece of metal that has a tube extending to the bottom of the reservoir. A few ounces of water are put into the reservoir and the top is screwed on.

When the water is heated over a fire, pressure builds inside the vessel, and the only way for it to escape is up the tube, through the coffee and out of the tube in the top. Since the end of the tube is under water, the pressure forces the hot water up through the tube.

There are some disadvantages to a machine like this. The pressure in the system depends on the temperature of the water. The temperature required to build up enough pressure to force the water through the coffee might exceed the ideal brewing temperature.

This is why some home machines incorporate a pump. Let's take a look at one of these.

Pump-style Espresso Machines

This type of machine is a little fancier, but is still fairly simple to operate.

The whole pump machine

Let's start by seeing how the machine is put together.

The reservoir holds the cold water used in the espresso machine. It is not pressure-tight or heated, and it is removable.

The pump draws water out of the reservoir and pumps it into the heating chamber at high pressure.

Heating Chamber
The heating chamber is a sturdy, stainless-steel structure with a heating element built into a groove in the bottom. The resistive heating element is simply a coiled wire, very similar to the filament of a light bulb or the element in an electric toaster, which gets hot when you run electricity through it. In a resistive element like this, the coil is embedded in plaster to make it more rugged.

The heating chamber also contains a one-way valve that lets water into the chamber from the pump, but not back into the pump from the chamber.

The porta-filter is the removable part of the machine the holds the ground coffee. Inside the basket is a small removable screen into which the ground coffee is packed. On the bottom of the basket are two spouts where the espresso comes out.

Steam Wand
The steam wand is used to heat and froth milk for use in various espresso drinks. This wand is connected to the heating vessel. When the user puts the valve in the steam position, steam from the heating vessel is released out of the wand and into the milk.

Control Panel
The control panel in this machine contains the on/off switch, two indicator lights and a control valve. One of the lights indicates that the machine is on, and the other indicates if the heating chamber is up to the proper temperature. The valve is used to start the flow of water through the coffee in the porta-filter or to start the flow of steam from the steam wand. It also engages one of two micro-switches that control the pump and heating element.

Now let's take a look at what happens when you make a shot of espresso.

Making a Shot

Assuming that the machine already has water in it, you start by turning the machine on and waiting for the heater light to indicate that the heating vessel has heated the water to the ideal temperature (just below boiling).

Next, you put the ground espresso coffee into the basket and tamp it down.

You install the porta-filter by twisting it into the machine, and you place one small cup beneath each spout. You then turn the valve to the espresso position. This engages the micro-switch that starts the pump, which pressurizes the heating chamber and hot water to about 15 atmospheres (220 psi) of pressure. This forces the hot water through the ground coffee and out of the spouts. Ideally, it should take about 25 seconds for about 1.5 ounces of espresso to come out.

When the espresso has filled the cups, you put the switch back into the middle position. Most likely, you'll want to steam and froth some milk next.

To steam some milk for, say, a cafe latte, you place a container with some cold milk under the steam wand so that the wand is submerged. Then, you turn the valve to the steam position. This energizes the resistive heater, which quickly boils the water in the heating vessel and opens the valve, starting the flow of steam out of the nozzle. The pump runs intermittently to keep the heating vessel supplied with water. The steam quickly heats up the milk, and, if you hold the steam nozzle near the surface of the milk, can be used to make froth.

There are dozens of different espresso based drinks that you can make or order at an espresso bar. This page has descriptions of most of them.

For more information on espresso, espresso machines and related topics, check out the links on the next page.

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