We are putting 18g into an 20g basket.
Next step is distribution, and this is too often neglected, which is bad, as it is very important. It is important because by doing this you are essentially creating an even space. A space free of clump, weak spots, hard places, etc. You are making it impossible for the water, that will soon go through it, to find a weak spot and create what is called a 'channel'.
After the coffee has been distributed properly we tamp. Contrary to popular belief - you do not need to tamp hard. It is a waste of energy, and also will eventually give you arthritis or joint pain if you do it enough. Barista wrist is a thing, and it sucks!
Tamp lightly, but flat - this is what is actually important. What you are trying to acheive is a perfectly flat coffee bed, so that when the water from the machine hits it the water is evenly distributed over the top before it sinks into and through the coffee. You are trying to get an even extraction.
After you have tamped your coffee you need to run a little hot water through your machine. This is called purging, and it is to get the water in the grouphead as hot as possible again and also to clean the grouphead a little from the previous shot.
Once the machine is purged, clean your mess. You want everything to be clean, dry and organised always. You cannot make clean coffee in a messy environment.
Put the portafilter into the grouphead softly. Old school baristas bang it in and that will definitely create a channel, as the coffee puck that was just tamped flat gets broken with the banging. Old school baristas do a lot of things that are bad practice and should be avoided..
The yield, or weight out, is calculated against your dose (weight in). This is done by means of a ratio.
If you want a stronger espresso you need to have a lower ration - 1:1.5. This means 1 part ground coffee (18g) to 1.5 parts Espresso (18g x 1.5 = 27g).
If you want a sweeter, and more transparent espresso then it must be weaker and you would take a ratio of say 1:2.5 (18g x 2.5 = 45g).
This weaker, but cleaner espresso comes about because it allows us to grind a little coarser, pull more water through your coffee puck, and also have a little more time (contact time) which all result in more sugar being dissolved and in our cup.
Acidity comes out first, and continues to come out throughout the entire shot. Sugar and sweetness take a little longer to dissolve, and so it is important to ensure you get there.. You get to a point where the sugar is dissolved. The easiest way to do this is to use a light roasted coffee, and have a higher ratio when making espresso.
Ristretto's from lightly roasted speciality coffee taste so mind bendingly horrible. If you want a short strong shot it is a better idea to get a medium or darker roasted coffee.
Once our espresso's are finished extracting, it is time to clean the rim of the spots that form during the extraction process. Remember, always clean.