The following factors are essential:

• Blend - Without a good blend you cannot have a good espresso. Coffees must be blended to achieve the sweetness, aromatics and smoothness desired in espresso. The blend must also be fresh. It is recommended to use espresso within four days of roasting.
  
  
• Roast - Too often you will find espresso roasted very dark. This results in a bitter, charcoal tasting brew. People that know how to blend espresso will roast light to preserve the aroma and sugars.
  
  
• Grind - 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.
  
  
• 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 aroma of the coffee will be diminished. A conical/parallel hybrid blade is considered the best design by many coffee professionals.
  
  
• Dosing - 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.
  
  
• Distribution - Distribute the coffee evenly after dosing in the porta-filter 
  before tamping.
  
  
• Tamping - 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.
  
  
• Water mineral content - The water used for espresso should be filtered.
  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. The choice of the espresso machine is very important to both water temperature and temperature stability.
  
  
• Temperature stabilizing - A stable temperature means consistent espresso.
  
  
• Water pressure - 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 you may want to experiment with different boiler pressures, but this 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.
  
  
• 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.
  
  
• Time - 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 - This 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 - The importance of practicing and experimenting cannot be overstressed. The key to espresso is to realize that it always has further potential. By changing any one of these factors you can improve or diminish it. Espresso preparation is an art that demands the precision and dedication of science. A perfect espresso is more of a concept than an actuality. The beauty of espresso is that it is volatile and difficult. If it were easy, anyone could develop a machine that could brew a perfect espresso every time. There are so many factors involved in espresso preparation that only a great mind and a passionate heart can begin to understand and control its complexity.