Preparing Coffee

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The next major step in the transformation of roasted beans into a cup of espresso is the extraction of the active components in the roasted and ground coffee by heated water. The interaction of hot water and coffee grounds is, however, subtly different when making common drip coffee than when making espresso.

When filter drip coffee is prepared, hot water passes through a loose aggregation of medium-size coffee grounds. During the four to six minutes of contact with the boiling water, most of the soluble substances present in the roasted coffee pass into solution. Thus, large quantities of highly soluble acids and caffeine dissolve into the cup. In contrast, the much shorter percolation time of espresso allows less acid and only 60 to 70 percent of the caffeine to dissolve into the brew.

Brewing espresso requires specialized equipment that can heat water to a temperature of between 92 and 94 degrees C and pressurize it to nine atmospheres. Coffee, ground to a fine to medium consistency, is placed in a perforated basket and firmly tamped down to create a compacted bed of particles. The compressed grounds adhere to one another thanks to a thin coating of oil, which is as viscous as honey. The oil binds the particles together into a condensed maze of minuscule air passages. Experimentation has shown that the hydraulic resistance of this bed of coffee grounds must be slightly less than the pressure of the steaming-hot extraction water, allowing it to flow through at a rate of around a milliliter a second.

Using the recommended 30 seconds of percolation, a skilled barista (coffee bar technician) produces about 30 milliliters of dense coffee liquor covered by the all-important crema. If the color of the foam topping is light, it means that the espresso has been underextracted, probably because the grind was too coarse, the water temperature too low or the time too short. If the crema is very dark in hue and has a “hole” in the middle, it is likely that the consistency of the coffee grounds was too fine or the quantity of grounds was too large. An overextracted espresso exhibits either a white froth with large bubbles if the water was too hot or just a white spot in the center of the cup if the brewing time was too long.

The percolation process also washes out components present on the surface of the coffee grounds, including aroma-filled oil and bits of the cellular structure. The high pressure generated by the espresso machine emulsifies a small amount of the oils, about 0.1 gram a cup. Intact cells in the grounds create a fine effervescence, which is derived from gases (especially carbon dioxide) passing through tiny pores in the cell walls. Some very fine grounds can also find their way into the beverage, along with cell wall fragments, which endow the foamy crema with what is called the tiger-skin look.

The final result is a polyphasic colloidal system, in which water molecules are bound to the dispersed gas bubbles, oil droplets and solid fragments, all of which are less than five microns in size. The colloidal character of the dispersion gives the drink high body, high viscosity and low surface tension. Espresso thus visibly coats our tongues and continues to release the aromatic volatiles dissolved in the emulsified oils as long as it remains there. These oily flavor/fragrance carriers mean that the great taste and aroma of a good espresso can be savored for as long as 20 minutes after it has been drunk. Luckily, the drinker need not know anything about the complex chemistry of coffee to enjoy it.

pp. 90-91 Scientific American Magazine The Complexity of Coffee by Ernesto Illy (June 2002)

Image retrieved from Floyd & Blackies on August 17, 2014.

espresso, expresso, crema, coffee, caffeine