Timing is a vital part of extracting 'the god shot' (so good that it tastes heavenly in your mouth) and the expert barista is attentive to them all.
Every aspect of the process will affect the quality of the shot.
From the time that the coffee beans were ground,to the time it takes to engage the now filled portafiler with the espresso machine and begin the extraction process,to the time it takes after extraction to serve the espresso to the customer.
The expert barista is striving for the minimal time at each of these touch points to maximise the potential for the perfect espresso extraction.
But by far the most important timing is the length of the extraction time and identifying the optimal time to stop the process.
The perfect time to finish the extraction is when the total available flavours of the ground coffee have been extracted yet seconds before the tanic acid has had a chance to breakdown from coffee shell and leach into and destroy the cup with its bitter, burnt and astringent aftertaste.
You know you have gone too far when the rich dark caramel crème changes to a lighter straw colour extraction.
Solution – throw it away and start a new shot.