Under-extracted espresso tastes excessively sharp and acidic. Properly
extracted espresso has the sweetness to balance the acids and
bitters. Over-extracted espresso tastes dull and tarry, or just
bitter-sweet.
[...]
* Under-extracted: Do the fruit acids taste sour, the maillard
compounds sharply bitter? It's under-extracted.
* Properly extracted: Do the fruit acids taste crisp, and the
maillard compounds warm and round? It's properly extracted.
* Over-extracted: Are dull or tarry tastes overwhelming everything
else? Is it mostly bitter-sweet caramels and molasses with
nothing else? It's over-extracted.
(1)
Dose, temperature, pressure, and flow rate all affect the way the the coffee is extracted. Varying these has only a slight effect on the aromatics, but a large effect on mouthfeel, and the taste balance, that is, the balance of bitter, sour, and sweet flavors. There is a good deal of overlap to the effect of changing some of these variables. For instance, both more lungo extractions and cooler temperatures tend towards acidity, while both more ristretto extractions and higher temperatures tend towards bitterness. Every barista must develop his or her own competence in diagnosing the taste imbalances of a shot and changing the shot variables to correct them. Depending on machine and grinder, they'll pick one or two "go to" variables from the quartet of the dose, flow, temperature, and pressure. Currently I use dose and flow, and leave temperature and pressure be. You can use another set of variables, but my advice is to focus on these two first, since they are 'Mano" variable, available on any machine, whereas temperature and pressure may only be adjustable on some machines.
http://www.home-barista.com/tips/best-technique-for-finding-best-flavor-t9889.html