The key to analyzing espresso extraction for quality is to go beyond the fact that crema is present. We must look at variables of the crema that are more complex and that better reflect the extraction process, giving true indications of what the resulting flavors may be.
The first task in determining whether a shot of espresso is indeed excellent is to look at the very first drop of the espresso extraction. Is the liquid dark, or is it crema? The first drop of a truly high-quality espresso shot should be crema. More drops of crema should follow the first drop and within two to four seconds of the first drop we should see a steady flow of crema. This flow should be consistent and continuous in both speed and in volume. The width of the flow should never be wider than one-eighth of an inch. The speed of the flow should be constant (it should not speed up as the volume of espresso extracted increases) all the way through the espresso extraction.
Next, we must look at color and consistency. The color of the crema should be medium brown. At this point, it should not be yellow, white, or tan to white. The flow should not have a lot of air bubbles, and should be as viscous as possible. As the crema flow starts layering itself on the bottom of the demitasse cup, it should not disintegrate or separate. It should simply remain a syrupy consistency, and should begin layering.
After about one-half to three-quarters of an inch of crema has accumulated in the bottom of the cup, you should see the crema change to a lighter, off-white color. It is just before this point (which should occur at 25 to 30 seconds from the push of the button for espresso extraction) that the flow should be stopped, since any more liquid allowed into the demitasse equals more acidity and bitterness.