How to extract a good espresso?

The principle of espresso is to extract the particles present in the coffee with water and under pressure. A machine needs 9 bar to extract a good espresso. The main variables to play with are: water temperature, ratio and grind size. To extract neither too much nor too little, we use water between 92 and 95 degrees. What is the ratio? In espresso, most of the time a ratio of 2 or 3 is used. Let's take the example of a ratio of 2, for one gram of coffee you will have 2 grams of final drink in the cup. So if you use 18g of ground coffee for your recipe, you will have 36 grams in the cup. The higher your ratio the more diluted the drink will be, because the more water you add. The variable that has the most influence on the taste is the size of the grind. For espresso, the grind is very fine compared to filter coffee. If your coffee is very slow and difficult to brew, your espresso will most likely be over-extracted and too bitter, so you will have to change the grind size and grind more coarsely. On the other hand, if your coffee flows very quickly and thickly, your drink will most likely be under-extracted and contain little flavour. You will need to grind more finely. As a general rule, an espresso should flow for 20-30 seconds.