The Science of Coffee

By Mark Trodden | August 7, 2006 7:48 pm

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, illycaffè, 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 melanoidins— which give rise to coffee’s dominant taste—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
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.

CATEGORIZED UNDER: Food and Drink, Science
  • Pete

    One of the secrets of roasting coffee is being sure to trap as many of those aromatic compounds as possible in the rosted bean – being aromatic, they’re likely to be driven off by the high heat involved. Cold water baths are used by at least some professional roasters, but how about collecting the vapors from roasting coffee in a distillation appartus or cold trap, and reinfusing the beans with those vapors? Coffee has so many aspects related to modern science – like intellectual property rights; there was a famous Dutch trader who smuggled live plants out of Arabia, risking death I believe, and went on to establish the initial South American plantings.

  • Paul Frampton

    Mark: Thank you for the interesting post about espresso which is likewise my addiction. Over the last ten years I had bought at least five large and expensive machines for home. Recently I found an attractive alternative. The smallest “Nespresso” machine is startlingly inexpensive (I have one at home, another at work) and makes great espresso with consistent crema. All the products can easily be purchased on the web with a reliable 48 hour turnaround. The coffee arrives in expensive sealed single-cup capsules. The results are, in my opinion, of comparable quality to using ground coffee in a big machine. Google “Nespresso”. Cheers.

  • Cynthia

    Mark, I empathize with you…I’m always striving for the perfect crema.

  • Phil Plait, aka The Bad Astronomer

    Carbon dioxide? We contribute to global warming by drinking coffee??


  • Plato

    So perfection in this case is not like, “Life is like a cup of coffee” you never know what your going to get?:)

    Maybe “bubble universes” take on a whole new meaning?

  • Mark

    Thanks for the interesting comment Pete.

    Hi Paul. I’ve heard those are pretty good. I have an Illy Francis Francis X5 (all the Francis Francis X? models have the same interior I think, just different exterior styles) and I love it. It really makes tremendous espresso when I’m careful.

    Hi Phil – I’ll drive something small with great mileage, use energy saving devices, and bike to work when it makes sense. But if it’s a choice between the planet and my espresso, then I’m taking the world down with me.

  • Sean

    I can’t help but notice that many separate steps go into the production of a good cup of espresso — the right vapor pressure, temperature, fineness of the grind, degree of roasting of the beans. Without any one of these, the coffee just wouldn’t be acceptable. What use, in other words, would espresso be that was made from the right beans but with cold water? None whatsoever.

    I conclude that espresso is Irreducibly Complex, and the existence of good coffee is evidence for an Intelligent Designer.

  • Urbano

    I conclude that espresso is Irreducibly Complex, and the existence of good coffee is evidence for an Intelligent Designer.

    So… Ernesto Illy = God?? :-) I always thought that Trieste was a special place, but not that much!!

  • Steve

    Phil – no need to worry. The carbon dioxide released from your coffee was in the air quite recently. Now, if you were making espresso from million-year old fossilized beans (sounds like something from Charlie Stross’ “Tales of the Dangerous Coffee Club”) there’d be problems….

  • citrine

    Thanks for the link, Mark!

    I ended my Physics thesis acknowledgment by thanking the person who “discovered” coffee, because if not for this beverage “… this thesis and most of my graduate education would not be possible”.

  • scerir

    How to get the famous ‘cremina’.

    A limited quantity of espresso on a modest quantity of sugar:

    Mix the above, very fast, for a while, let us say a couple of minutes? (There are people saying that the anticlockwise fast mixing is much better, but they do not support that with any experimental evidence).

    The outcome of the experiment: the cremina.

    The cremina into the cup. Then add the espresso

  • spyder

    Back in some of my formative early adult years i hung out with a family of italians who happened to be in the espresso-machine importing business, and a few other enterprises (and yes they were from Palermo, what of it?). I had never really liked coffee, nor had i drunk much espresso; and that was in the days before we had so many boutique and gourmet coffee roasters in the world (long before Starbucks). These people drank espresso all the time, particularly after meals, and without the mess of sugars and cremes (usually some figs and citrus fruits). I learned that espresso was all about flavor, not about caffeine or using coffee to sustain energy and effort. It was ritual and taste; it is all about the flavor. I have been an addict ever since.

  • Pingback: Nomadic - Technomadic, to be more precise » The Science of Coffee()

  • thera

    Nice spiel there from Paul Frampton. That wouldn’t be the Paul Frampton who is ‘Head of Digital’ at ‘web search marketing’ agency media contacts, would it?

    Surely Nestlé wouldn’t stoop so low?

  • Mark

    No thera, it is not that Paul Frampton – I know this one.

  • Sean

    I thought he was great as Billy Shears in the Sgt. Pepper movie.

  • thm

    Much like citrine, I included in my dissertation acknowledgements:

    “I would like to thank the coffee farmers of the world, without whose harvest none of this would have been possible.”

    I borrowed this line, more or less, from David Grier (now at NYU) who used it in a talk I saw once.

    One of the other Illys, Andrea, has written an extensive book about the chemistry of espresso. Unfortunately it’s priced like a science book.

  • citrine


    Wow! That’s interesting info. I hadn’t heard of Grier or his comment when I wrote mine.

  • Jerry


    You should search the Internet on coffee draining the adrenals, etc.

  • beajerry

    Best Posting On Coffee, Ever!

  • Mark

    Jerry, the only links I found in the first couple of google pages searching for your suggestion were “alternative” medicine sites. Are there any actual medical links that you can point me to? Cheers,

  • Stephen Uitti

    While coffee is indeed complex, it is an aquired taste. I went far enough to be barely able to tolerate coffee, but not far enough to enjoy it. My caffeine intake was mostly Coke and Mt. Dew. You get the caffeine rush. You tell yourself it keeps you awake and alert at meetings. After all, it really does keep you awake when you really want to sleep, and otherwise disrupts your sleep schedule. Caffeine interferes with the body’s uptake of calcium, and is implicated in arthritis and osteoperosis. It’s highly addictive, so years after going cold turkey, i smell coffee and want it. I have some control over getting it, but no control over wanting it. Only the memories of severe arthritic pain allows me any leeway. Maybe having choice was always an illusiion, but i want it back, and know it won’t happen. At least the arthritis is better now.

  • David Harmon

    I usually put espresso-ground coffee in a drip maker, and toss a few spices in. (Any combination of cinnamon, cloves, cardamom, anise, etc). Does that make me an irredeemable heretic?

  • Christine

    Brazil produces almost a third of all the coffee in the world.

    Certainly, the best grains do not stay here. In any case, Brazilians who do not drink coffee (our “cafezinho”, as we call it) are somewhat rare indeed. It is also relatively common for young Brazilians to drink coffee with milk in their breakfasts. At work, there is always the so-called “cafezinho”´s time. A time to drink coffee and for informal talks and gossips of course.

    I myself didn´t like coffee too much. Today, I simply can´t drink at least about 3 or more cafezinhos every day. I agree it is an aquired taste. It´s funny, because I generally do like more its smell than its taste. On the other hand, I love cappuccino.

    Best wishes
    Christine (from Brazil)

  • Wim L

    It is interesting how different drugs are temporarily gven the status of social or industrial necessity and are eventually supplanted by other drugs. Lately it’s coffee. Tobacco has also played that role, as has alcohol (in many forms: beer, wine, distilled liquor), and opium, and tea, and arguably cocaine, and probably others I’m not thinking of right now. At least coffee is not as harmful to our health as some of the others.

  • Pingback: Betablokker » A few links()

  • Pingback: Nonoscience / The Scian Melt - Edition Twenty()

  • coturnix

    Esspresso – it’s so artificial and machine-made. I fix my own Turkish coffee at home. Will anyone do chemical analysis of that?

  • glenn

    My best coffee ever was a freshly roasted coffee bean and grounded by a wooden mortar and pestle and just plain boiled in a very hot tin cup over a fire produced by wooden logs. No modern technology just plain third world methods. :)

  • James Hoffmann

    Roasters don’t use water baths, instead some use a quenching mist of water sprayed over the beans as they spill from the roaster to try and arrest the chemistry within the bean driven by heat. Many roasters who rank quality above ease and speed prefer to use air to cool the coffee because it doesn’t shorten the shelf life the way that water does.

    Illy’s article is an interesting, and the research they have done is excellent. I do, however, dislike the way that the crema of an espresso is treated in a romantic and mystical way. It is merely a foam of the coffee beneath, caused by bubbles of CO2 being trapped by a compound called a melanoidin that acts as a surfactant. The reason that it fades quickly is down to drainage.

    The most common reason that most people don’t get a crema on their espresso at home is that they are using an incorrect grind of coffee. The grind of the coffee is the most important thing in espresso as it determines the resistance to the machine’s pump which in turn affects the brew time/contact time of the coffee. The finer the grind, the slower the brew, the more flavour we extract. Most, if not all, preground coffee bought in a supermarket or store is too coarse, no matter what the label tells you. The correct grind should allow 2oz of liquid through a bed of 14-18g of coffee in around 25 seconds (quite slow).
    The other reason is that coffee stales very quickly once ground and not only are aromatics lost but much of the CO2, so necessary for crema, is freed in the process of grinding.

    Espresso is a wonderful thing.

  • Nick

    Esspresso is an artificial way to prepare coffee.


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Cosmic Variance

Random samplings from a universe of ideas.

About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


See More

Collapse bottom bar