For me, espresso is an integral part of every day. I typically start the day with a regular coffee, but then move on to an espresso mid-morning at work, followed by another either mid-afternoon or when I get home from work. I wouldn't call myself a real coffee connoisseur, but I certainly know what I like, and for my money you just can't beat a perfect cup of espresso with coffee in any other form. I'm certainly not alone in this, and many of my colleagues and friends are more knowledgeable about coffee and are even more devoted to it than I am. But, as scientists, we are seldom happy with a gut reaction, and you'll always find us seeking the why and how. For example, you'll notice that I used the phrase "a perfect cup of espresso" above. What does that mean? What constitutes a "perfect" espresso, and how can one ensure getting it every time? A real answer begs for experimentation, a healthy dose of hypotheses, more experiments, refined hypotheses, ..., - you know what I'm talking about. Most of us coffee lovers focus, understandably, on the (uncontrolled) experimental part of this process, find what we like, and just live with the fact that we don't really know what's behind it. But if you've got enough scientist in you, you'll never be completely happy with this, and will yearn for a more complete understanding. Luckily, such a scientific analysis exists! Ernesto Illy is a fascinating character. If you know coffee, you'll recognize his name from the highly successful Trieste-based coffee company, illycaffÃ¨, of which he is the Chairman. However, equally relevant to the topic at hand is that Illy holds a doctorate in chemistry and a background in molecular biology. He is fascinated with the science of coffee, and in June 2002 he wrote a wonderful article for Scientific American, titled The Complexity of Coffee (The article requires a subscription, but is also available on Illy's website). Since coffee comes in many forms, Illy focuses on espresso as a specific example. He discusses the importance of the perfect beans, what that means, and the role that modern technology is playing in improving speed and quality control in attaining them. He then talks about roasting, in terms that are music to a scientist's ear
... residual water inside each cell is converted to steam, which promotes diverse, complicated chemical reactions among the cornucopia of sugars, proteins, lipids and minerals within [...]. At high heat, from 185 to 240 degrees Celsius, sugars combine with amino acids, peptides and proteins according to a well-known caramelization process called Maillard's reaction. The end products are brownish, bittersweet glycosylamine and melanoidinsâ€" which give rise to coffee's dominant tasteâ€"along with carbon dioxide (up to 12 liters per kilogram of roasted coffee)
One part I particularly enjoy is the chart titled Cumulative Chemical Composition of Espresso with Increasing Extraction Time, which simultaneously tracks the concentrations of multiple compounds as a function of extraction time, side by side with a key that explains their role
Compound : Aroma 2,4-decadienal : RANCID ethylgujacol : SMOKE 2-ethyl-3,5-dimethylpyrazine : CHOCOLATE 2-ethyl-3,6-dimethylpyrazine : CHOCOLATE 2,4-nonadienal : RANCID methylsalicilate : CINNAMON b-damascenone : TEA DMTS : SULFUR isovaleraldehyde : SWEET a-ionone : FLOWERS linalool : FLOWERS
But what I learned the most from was the discussion of the crema. When I make espresso at home, I'm deeply disappointed if I can't achieve a wonderfully oily golden foam that I know, from experience, will correspond to a delicious cup.
Referring to this image, Illy explains -
the dense, reddish-brown foam that tops an espresso, is shown in an enlarged cross section. Composed mainly of tiny carbon dioxide and water vapor bubbles (large circles) surrounded by surfactant films, the crema also includes emulsified oils containing key aromatic compounds (particles with red borders) and dark fragments of the coffee bean cell structure.
and goes on to explain why the color, bubble size and thickness of the crema are all indicators from which one can discern the quality of the coffee. The complete article is an absolute joy, and, although I brew his coffee and use one of his machines, I am most thankful to Ernesto Illy for revealing the science behind my favorite daily drug.