The best espresso should be extraordinarily sweet, have a potent aroma, and flavor similar to freshly ground coffee. The crema should be dark reddish-brown and smooth, yet thick. A perfect espresso should be enjoyable straight with no additives, yet bold enough to not disappear in milk. A pleasant and aromatic aftertaste should linger on the palate for several minutes after consumption.
最好的意式浓缩应该具有特别的甜味、浓郁的芳香、类似于新鲜泥土气息的咖啡。 克丽玛（意大利一城市名，这里代指咖啡表面漂浮的小层物质。译者注）应该是暗红褐色、平滑、薄薄的。 一杯好咖啡直接任人愉悦而不会有任何添加剂，且足够浓郁味道也不会在牛奶中消失。Espresso那种愉悦且带有芳香的味道应该在你的上颚部分停留那么几秒，你可以好好去感觉那种浓郁的芳香。

The following steps describe in detail how to make espresso. You will also learn about the various factors and problems with espresso that limit its perfection. If any of these factors are off, you will not achieve a high quality espresso.
以下步骤详细介绍如何制作咖啡。你也将学习影响咖啡的多种因素及问题。假如不重视其中的影响因素，或许你没法获得一杯高质量的咖啡。

Making Perfect Espresso

Blending Espresso

Without a good espresso coffee blend you cannot have a good espresso. The best espresso coffee beans are blended to achieve the sweetness, aromatics, and smoothness desired in espresso. The espresso blend must also be fresh. We recommend using espresso within four days of roasting. Please see the espresso blending section for help on creating your own blends or order some excellent espresso blends from roasted coffee suppliers, such as Caffe D’arte or Espresso Vivace.

Roasting Espresso

Too often you will find espresso roasted very dark. This results in a bitter, charcoal tasting brew. People that know how to make an espresso will roast light to preserve the aroma and sugars. For more information, read the section about espresso roasting.

Grinding Espresso

The grind must be continuously monitored throughout the day to achieve an extraction time of 25-30 seconds. Do not change the pressure you tamp with to compensate for a grind that has become too large or small. For more information, read the section about espresso grinding.

Espresso Grinder

A high quality burr grinder is essential for espresso. A conical burr grinder is preferred to flat burrs since the particle size is more even, they last longer, and the coffee is not heated during the grinding process. If the burrs become hot the coffee aroma will be diminished. A conical/parallel hybrid blade is considered the best design by many coffee professionals.

Dosing Espresso

Coffee must be freshly ground to achieve peak flavors. Grind and dose on demand. When someone orders an espresso grind only what is necessary for one shot, dose properly, tamp, and brew. Discard any espresso grounds that are not used within 30 seconds. For more information, read the section about espresso grinding and dosing.

Distribution in Portafilter

Distribute the coffee evenly after dosing in the porta-filter before tamping.

Tamping Espresso

Tamp the coffee once very evenly with 5 lbs of pressure, then once with 30 lbs of pressure, and polish 720° with 20 lbs of pressure. For more information, read the section about espresso tamping.

Water mineral content

The water used for espresso must be filtered. Some cities must even compensate for the mineral content of their water. Over time oxygen will be forced out of the water in the espresso machine leading to off tasting water. Try filling a small glass with water, letting it cool, and tasting it for off flavors. If the water tastes strange you may want to dump the tanks daily and begin with fresh water.

Water temperature

The water temperature should be stable and somewhere between 92-96°C. Choosing the best espresso machine is very important to both water temperature and temperature stability.

Temperature stabilizing

A stable temperature helps ensure that you prepare excellent espresso. For more information about stabilizing the temperature of your espresso machine, read about espresso temperature stabilizing.

Water pressure for Espresso

The pressure of the water forced through the espresso should be between 9 and 10 atm. This pressure is responsible for the development of the crema.

Boiler pressure

The boiler pressure determines the amount of water to be incorporated in the steam. If your milk is not foaming correctly as described in the section on latte art, you may want to experiment with different boiler pressures. Boiler pressure, however, should only be altered by professionals. You can check your boiler pressure by looking at the boiler pressure gauge on the front of most espresso machines.

Extraction time

Extraction time to fill two 1-oz cups should be between 25-30 seconds. Despite the time the pump should be turned off if the espresso becomes slightly lighter in color. The goal is to have a dark red espresso take approximately 25-30 seconds to brew with no change in color. Fore more information, read the section on extracting espresso.

Porta-filter and basket

The porta-filter should always remain the same temperature as the water used to brew the espresso. Therefore it should always remain in the group head. The basket should hold 16-18 grams of coffee and must be straight walled. Curvatures in the basket will lead to uneven extraction.

Timeliness

Act quickly, but carefully. You should spend no longer than 30 seconds for the time it takes to dose, distribute, tamp, pre-heat, and brew the espresso.

Espresso machine cleanliness

Coffee machine cleaning is probably the biggest problem with espresso today. If the machine, basket, and porta-filter are not cleaned regularly, the espresso will always taste rancid.

Espresso grinder maintenance

Everyday the burr blades should be swept clean. Between shots you may want to brush out the excess espresso that gets stuck between the burrs and the dosing chamber. The burrs must be replaced at least yearly so that they continue to produce coffee granules with a maximal surface area.

Environmental Factors

The humidity and temperature will change throughout the day. Since coffee is hydroscopic (absorbs moisture), the grind size must be changed throughout the day to achieve a brew time of 25-30 seconds. The temperature will not affect the espresso like the humidity, but it is important to avoid exposing the coffee to any high temperatures until brewing.

Espresso cup

The espresso cup should be pre-heated from a source other than the espresso machine. Filling a cup with water from the espresso machine prior to brewing the espresso will lower the temperature of the water in the boiler and the espresso extraction will be uneven. The espresso cup should have thick walls and a narrow mouth to retain heat and aroma, respectively.

Practice makes perfect

If you want to learn to make espresso, it is essential to practice and experiment. The key to making espresso is to realize that it always has further potential. By changing any one of these factors you can improve or diminish its potential. Espresso preparation is an art that demands the precision and dedication of science. I have never achieved, nor have ever seen anyone make a perfect espresso. A perfect espresso is more of a concept than an actuality. The beauty is that espresso is volatile and difficult. If it were easy, we would develop a machine that knows how to make a perfect espresso every time. There are so many factors involved in espresso preparation that only a human mind and a passionate heart can begin to understand and control its complexity.

via: http://www.coffeeresearch.org/espresso/potential.htm