When it comes to espresso it all comes down to variables. Consistency is the key, without this every shot that you pull on your espresso machine will be different and theres nothing more frustrating than finally getting that great shot but not being able to replicate it. You can almost drive yourself mad trying to achieve this perfection, but armed with a little knowledge you will be able to replicate that liquid gold time after time. There are four main variables to be aware of which all tell a story about the shot of espresso you have just pulled.
Crema | Pour | Timing | Biscuit/Puck
Crema is the tan coloured almost foamy looking liquid that forms on an espresso extraction. It is generated when oils from within the coffee grounds are extracted under high pressure - the result is espresso. Crema will tell you a lot about the espresso you have just made, a good crema will look caramel in colour, thick in consistency, smell aromatic and should not dissipate quickly. If it is lacking in any of these it could mean that the coffee may be stale, or that the extraction was too quick or too slow.
Pour refers to the way the coffee comes out of the group handle. The coffee should pour out like honey and should appear viscous. Under-extraction occurs when the pour is too quick, this will mean that the water is passing through the grounds too quickly and you are not extracting the oils and flavour from the beans. Over-extraction occurs when the pour is too slow, the pour will be very slow and may even be dripping out rather than an even pour. This means there is too much resistance in the grounds and you will be extracting too much out of the coffee. This is going to relate in unsavoury flavours.
Fixes for this may include lighter/harder tamping, ensuring tamp is even or using finer/coarser grind.
Timing is all about how long the espresso extraction takes. Ultimately an extraction for an espresso shot of 60mls should take 23-27 seconds. This is going to be the correct amount of time to ensure the best flavours are extracted but the bad flavours are left behind.
Fixes for this are generally going to be the same as for correcting the pour
Biscuit/Puck is a term used to refer to the spent grounds in your group handle after the extraction is complete. If you keep this term in mind it will tell you a lot about the result you are looking for, the grounds should represent that of a hockey puck or biscuit. It should be firmly compressed and dry to touch, if the grounds are sloppy and wet your extraction will not be the best it could be.
Fixes for this may be checking on quantity of grounds being used, you may need more or less. Checking for even tamping as well as the coarseness of the grind.