One espresso requires 7 to 9 grams (about ½ tablespoon) of ground coffee beans per 2 ounces (4 tablespoons) of water. This produces a 1.5 oz. shot of dark coffee concentrate possessing a strong, slightly bitter-sweet flavor and an almost syrupy consistency. Once ground, the oils in the coffee beans are exposed to air, beginning to oxidize and lose flavor almost immediately. This also affects crema. To ensure maximum flavor and aroma, beans should be ground as close to brewing as possible.
Espresso grind is much finer than regular brew coffee. Almost powder-like, yet slightly gritty, like the consistency of superfine sugar. This provides the proper resistance to the water being forced through it. It is generally agreed that blade grinders (the kind that, as a button is depressed, a blade rotates) produce a very poor quality grind, because they chop and pulverize the beans, rather than grind them. Burr grinders are better. Whether hand-crank or electric, a burr grinder slices the beans into controlled, sized pieces, optimizing the flavor extraction.
After the proper amount of ground beans is measured or dosed, the grind is gently, but firmly packed into the gruppa, a metal cup with holes in the bottom. Next, the grounds should be tamped to evenly interlock the bean granules. Unless they are compacted, water will flow too quickly through the grounds making the resulting coffee too weak. There are quite a variety of tamping tools available. While most people prefer the flat bottom type, there are a few others that like the rounded bottom. Like so many other components to the fine art of espresso making, there is an optimum density of compactness to strive for; not too hard, not too soft, but just right. Finally, the key to proper grind is extraction time. With proper dose and tamp, one shot of espresso should optimally be extracted in 25-30 seconds.