Achieving perfect espresso is easy if you follow the numbers. Here are the definitive requirements to achieving perfect espresso.
Coffee Dose
8 +/- 1 grams of freshly ground coffee per single shot of espresso
7 - 9 grams of freshly ground coffee evenly distributed in a single portafilter
14 - 18 grams of freshly ground coffee evenly distributed in a double portafilter
The definitive measure of coffee is 7 grams of coffee per single shot of espresso in order to achieve true Italian espresso. It is impossible to make espresso with less than 7 grams of coffee per single shot of espresso; however it is possible to use more than 7 grams of ground coffee. Any increase in coffee dose will have to be offset with a change (coarser) to the coffee grind in order to maintain the desired extraction pressure, time, and volume. Some novice users may find it easier to achieve a good extraction by "overdosing" to 8 or 9 grams of ground coffee per single shot, this works because basically there is more oil and flavour available to create the crema. For this reason, a number of coffee roasters recommend using more than 7 grams of coffee as it allows individual cafe operators and home users a greater margin for error in producing acceptable espresso.
Water temperature
90 +/- 2 degrees centigrade
88 - 92 degrees centigrade at the point of extraction
Playing around with water temperature is not recommended for the novice, home bartista, or regular cafe operator. In fact changing water temperature (if you have the facility to do so) can lead you astray very quickly. If you are not achieving a good result with your espresso, the problem is almost guaranteed to not be with the water temperature. Alot of design and technical expertise is injected into the design of quality espresso machines to ensure temperature stability. Choose a good quality Italian espresso machine and trust in the design that the water temperature will be stable enough to make great espresso time and again.
Extraction pressure
9 bar (130 psi)
Extraction pressure is dictated by the amount of "resistance" the coffee provides to the flow of water through the group head. Commercial espresso machines, home e61 style, and the top quality prosumer domestic espresso machines all contain technology to limit the maximum pump pressure. However, this is just a limit on the maximum pressure. Too little coffee, too course a grind, or too little tamping pressure will result in insufficient extraction pressure.
Extraction time
5 seconds pre-infusion + 25 seconds extraction
30 seconds maximum
As a general rule the extraction time will approximately correspond to the volume in each cup when the machine and grinder are correctly setup. For example a ristretto should take approximately 20 seconds to extract and an espresso should take 30 seconds. An extraction longer than 30 seconds will start to draw undesirable bitter compounds from the coffee. Equally an extraction that delivers a 30ml espresso in 20 seconds will tend to be under-extracted.
Volume in the cup
25 +/- 5 ml per per single shot of espresso
20 ml for a ristretto (restricted) espresso
30ml for a completely extracted espresso
2x 20ml for two ristrettos
2x 30ml for two espressos
The volume of coffee delivered is dictated by the extraction time and choice of filterholder/dose. The choice of the double or single portafilter dictates if you are making a double or single shot (2 shots of espresso or 1 shot of espresso). A single portafilter should be filled with 7-9 grams or ground coffee and a double portafilter should be filled with 14-18 grams of coffee. You cannot use a double portafilter to make a single shot of espresso, if only one shot of espresso is required then a double portafilter can be used and coffee captured from only one outlet or extract a ristretto.
Regardless of if a single or double espresso is being made the extraction volume per cup will be the same, 20ml for a ristretto and 30ml for a complete espresso.