Please welcome Erin Meister of The Nervous Cook who will be chiming in regularly with coffee dispatches. Take it away, Erin! —The Mgmt

Espresso sure is a complicated elixir for being so small. Like Barbara Stanwyck in Double Indemnity, a high-quality shot can be multilayered and complex: rich, sexy, sweet, but also fast and smooth—maybe even a little dangerous—with a hint of a bitter edge.

But have you ever wondered what gives your early morning cup that certain oomph? That coating, silky texture? That playful tang and sweet kick?

It's all about the extraction.

"Extraction" is what the water does when it fraternizes with its old pal ground coffee. When the two meet, the water pulls all kind of dissolvable stuff out of the coffee, like an old college buddy tapping you for $20 every time you see him. Some of what the water grabs on to is good. And some, well, not so much—which is part of what makes the truly great jolts so elusive.

An espresso extraction—which ideally lasts between 20 and 30 seconds for a double shot—can be broken down into several parts, making it possible for us to unravel some of tiny brew's many mysteries. The first few seconds of the water and coffee's commingling is where the espresso's body comes from—the thick, honey-like drips that languidly drop into the cup below. If you were to down this early part of the extraction on its own, it'd be almost chewy, coating your teeth and tongue.

The reason for this buttery mouth bomb is that when the water comes in contact with the finely ground coffee particles, it immediately begins to dissolve and absorb things from them, like delicious flavor oils—transporting the gooey, concentrated result from bean to mug.

Once the pressurized water from the espresso machine has started making inroads through the finely ground coffee, the flow of the liquid espresso turns from fat, lazy drips to a steadier stream, as more of the grounds' goodies have been picked up by the water that came before. After about 15 seconds, the shot begins to show a little color, the chestnut hue becoming a mix of brown and sultry blond. This part of the process contributes some of the coffee's more nuanced qualities—depending on the espresso blend's components, now's when nuttiness, fruit-like acidity and toasty flavors can start to shine.

Sweetness and balance are the keys to the extraction's final throes, though the barista must take care to stop the shot at the right moment—once the coffee trickling out of the espresso machine is fully blond and begins to look thin or watery, all the good stuff has been extracted from the ground beans.

What comes after? If the barista left the water running, he or she would be coaxing out more undesirable bitter flavors, while diluting the already-brewed espresso with a watery ickiness that may overpower any sweet or delicate characteristics.

Geeky dissection aside, though, all coffee lovers can agree on the single best part of the perfect coffee extraction: The happy and caffeinated mouth at the receiving end.

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