For sheer sensory enjoyment, few everyday experiences can compete with a good cup of coffee. The alluring aroma of steaming hot coffee just brewed from freshly roasted beans can drag sleepers from bed and pedestrians into cafés. And many millions worldwide would find getting through the day difficult without the jolt of mental clarity imparted by the caffeine in coffee. But underlying this seemingly commonplace beverage is a profound chemical complexity. Without a deep understanding of how the vagaries of bean production, roasting and preparation minutely affect the hundreds of compounds that define coffee’s flavor, aroma and body, a quality cup would be an infrequent and random occurrence.
Connoisseurs agree that the quintessential expression of coffee is espresso: that diminutive, heavy china cup half-filled with a dark, opaque brew topped by a velvety thick, reddish-brown froth called crema. Composed of tiny gas bubbles encased in thin films, the surprisingly persistent crema locks in the coffee’s distinctive flavors and aromas and much of its heat as well. Espresso—the word refers to a serving made on request expressly for the occasion—is brewed by rapidly percolating a small quantity of pressurized, heated water through a compressed cake of finely ground roasted coffee. The resulting concentrated liquor contains not only soluble solids but also a diverse array of aromatic substances in a dispersed emulsion of tiny oil droplets, which together give espresso its uniquely rich taste, smell and “mouthfeel.”