A Cupful Of Coffee Science – Part Two

Coffee aficionados consider perfectly-brewed espresso to be the ultimate in coffee because its special preparation enhances the inherent characteristics of the beans. To know espresso is to know coffee in all its forms.

Having addressed the coffee plant in my previous post, let’s continue with the next step: turning the coffee beans into a cup of espresso coffee.

The ultimate quality of coffee beans depends on the genetics of the plant, the soil in which it grows, and the microclimate. After the beans have been harvested, the roasting processes which are applied are also responsible for the taste differences among the many coffee beans that suppliers combine to produce the various distinctive blends.Roasting is a heat-driven process that greatly increases the chemical complexity of coffee. The aroma of green coffee contains some 250 different volatile molecular species, whereas roasted coffee gives rise to over 1,000. Depending on the temperatures and procedures applied, the roasting process can last from 90 seconds to 40 minutes. Twelve minutes is the traditional duration. The higher the final temperature of the roasting, the less desirable the aroma will be, and the stronger the bitterness. Conversely, low roasting temperatures fail to fully develop the welcome aromas, and acidity tends to come to the fore.

Coffee beanThe 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. Brewing espresso requires specialized equipment that can heat water to a temperature of 92 to 94 degrees C and pressurize it to nine atmospheres. Using the recommended 30 seconds of percolation, a skilled barista 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. Some very fine grounds can also find their way into the beverage, along with fragments of the coffee beans’ cell walls; they endow the foamy crema with what is called the tiger-skin look.

Espresso visibly coats the tongue and continues to release its aromatic volatiles as long as it remains there. These 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, prolonging the pleasure of a truly unique taste experience.

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