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 Important things to notice for Espresso making:

1. Blend - Coffees for espresso making must be blended to achieve the balance, sweetness, aroma, and smoothness desired in espresso.  The blend must also be fresh.
2. Roast - Full city+ to Vienna roasts are suitable for espresso, as they increase body and sweetness without eliminating the varietal characteristics of the coffees constituting the blend. Too often you will find espresso roasted very dark.  This results in a bitter, charcoal tasting brew.
3. Grind - The grind must be continuously monitored to achieve an extraction time of 25+-2.5 seconds.
4. Grinder - A high quality burr grinder with slow burr rotation speed is essential for espresso. If the burrs rotate very quickly they can overheat the ground coffee and cause diminishing of the aroma.
5. Dosing - Coffee must be freshly ground to achieve peak flavors and dosed on demand before espresso making. Discard any old espresso grounds.
6. Distribution - Distribute the coffee evenly after dosing in the porta-filter before tamping.
7. Tamping - Tamp the coffee once very evenly with 15-25kg of pressure, and polish with very low pressure.
8. Water - The water used for espresso must be filtered.  Hard or very soft water reduces the life of your espresso machine, and also produces a worse taste espresso. 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 water tank and clean your machine.
9. Water temperature - The water temperature should be stable and somewhere between 90-96°C during espresso making. The choice of the espresso machine is very important to both water temperature and temperature stability.
10. 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. Most professional machines ease the process of quickly altering the water pressure.
11. Boiler pressure - The boiler pressure determines the amount of water to be incorporated in the steam. You can check your boiler pressure by looking at the boiler pressure gauge on the front of most espresso machines.
12. Extraction time - Extraction time to fill two 1-oz cups should be 25 ± 2.5 seconds.  Despite the time the extraction should be stopped if the espresso becomes slightly lighter in color (blonde). Use fresh coffee in order to avoid quick blonding.
13. 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.
14. Espresso machine cleanliness -  If the machine, basket, and porta-filter are not cleaned regularly the espresso will always taste rancid.
15. 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.
16. 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 ± 2.5 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.
17. Espresso cup - The espresso cup should be pre-heated and also have thick walls and a narrow mouth to retain heat and aroma, respectively.