Coffee Roasting: From Green To Delicious
At Everyday Joe’s, we proudly and gladly and exuberantly brew the beans roasted by Intelligentsia Coffee & Tea out of Chi-town (Chicago, for those not in the “know”). Each week or so their VP of Coffee, Geoff Watts, sends out a message concerning something in his coffee-obsessed mind. This week, it is chemistry.
Hola:
I’ve been thinking a lot about roast chemistry lately. As you may or may not already know, green coffee contains hundreds of volatile organic compounds that are responsible for its many flavors and aromas. To complicate things further, the roasting process involves a massive amount of chemical change and transformation…polysaccharides breaking down, sucrose reducing to simpler sugars, caramelization and Maillard reactions, acid chains breaking and reforming…my goodness. It’s a lot to wrap your head around. Small variations in the application of heat during the roast will lead to profound differences in the cup.
There are some things that are very well known with others crying out for further study. A lot of the research into coffee quality is fairly recent, and compared to other beverages, our collective understanding of what gives coffee its unique tastes is still in its infancy.
But let’s begin at the top. Stand back lest ye become blinded by the science. (Just kidding.) I’m no chemist, maybe in another life. Still, I’ve had to learn a lot about what is going on in during the roasts, and while there is admittedly plenty that the industry has yet to discover and quantify, there is some fundamental knowledge that guides our efforts…and it’s about to drop on you.
Step One:
We take the green coffee, which has been selected based on its positive organoleptic traits. (For the laymen in the audience, this is a fancy way of saying that we bought it because it tastes delicious.) Before roasting we need to establish some basic understandings about the physical properties of the coffee that will guide our approach to it. So we take measurements of the bulk density. We measure the moisture content of the bean. We can run it through screens to determine the size of the beans and their uniformity. Those things give us some basic ideas about how the coffee will respond once it gets put into the roaster.
Beyond the physicals, we also need to have some targets with regard to roast degree. Every coffee is different, and a singular approach to roast profiling will not give each individual coffee the kind of articulation it needs to reach its true potential in the cup. So upon receipt of a coffee we test it in our sample roaster and spend time evaluating and discussing it before deciding what to do in the production roast.
Some coffees (like a great Kenya) excel on the basis of their profound cane-like sweetness, lively and confident acids that yield various tropical fruit flavors, their captivating and powerful aromatics, and graceful floral traits. Others (like a great El Salvador) succeed on the basis of deep and slightly syrupy ripe cherry notes, chocolatey undertones, and brown sugar or molasses-style sweetness. So the goal must be to identify what it is that makes a certain coffee tick (to see what is under the hood, so to speak) and then figure out how to maximize and accentuate those traits through the roast. Not an easy task, believe me.
Step Two:
Once the targets are acquired, the roaster takes on the challenge of executing the perfect transformation of acids and sugars that bring the coffee to life. It can take a while to get it right. Since every coffee responds a bit differently to heat, the roaster must “get to know” the coffee over time and learn about its particular idiosyncrasies. As mentioned, some things can be anticipated based on knowledge of the physical specs, but many things cannot be known until the bean is in the machine.
First, we weigh out the appropriate amount of coffee. Every machine has an “ideal” batch size, usually about 70% of its listed capacity. Once the coffee is weighed out, the machine is brought up to pre-selected temperature. When it hits temp, the coffee is loaded into the drum and the roast begins.
During the first 4 minutes or so the bean doesn’t change color very visibly. It retains its green color and looks relatively similar to how it started out. Beans start at about 11% humidity, and in this stage, the moisture in the bean is heated and driven out. Once the free water is released, the color of the bean begins to change as the chlorophyll degrades and the sugar browning processes kick into gear.
Around 5 or 6 minutes the beans take on a deep yellow hue. Now the chemistry is heating up. Two processes modify the sugar, and the first is caramelization, which is essentially the melting of sugars which begins to occur at around 160 degrees C. The compounds in the sugar change, leading to changes in aroma and color as well as the formation of various organic acids.
Meanwhile, the Maillard reaction is going on full-bore. Any foodies out there are certainly well familiar with this as it is essential to the creation of many key aromas and flavors in chocolate, caramels, toasted breads, roasted meats, malted barley in beer, and many other culinary delights. It is a bit complex, but here’s a quick explanation, courtesy of my friend Joseph:
1. Amino acids combine with sugars to form several N-glucosamine compounds during roasting.
2. These unstable compounds undergo Amadori or Heynes reactions to form several other intermediates.
3. Strecker Degradation sets in, where N-glucosamine with double bonds react with other amino acids to form Pyrazines and Pyridines, along with other compounds
4. All remaining intermediates combine to form long chains of brown-colored melanoidins, which are responsible for coffee color.
Yipes. Lots of stuff going on here. Keep in mind that, as mentioned, there are hundreds of compounds in coffee that are all doing somersaults during the roasting process. Some land on their feet, some do not. Some become conjoined with others, some disappear into the atmosphere.
But that’s not all. Lipids (fats in the coffee bean) are degrading, there is on-going pyrolysis (the breakdown of organic materials that, if continued indefinitely, would leave nothing but carbon), and oils are migrating. It’s a massive circus of activity. And it is happening at a frantic pace, every second that goes by bringing substantial change to the coffee. It takes an experienced hand to monitor this process and guide the coffee to its crescendo.
Since a drum roast applies heat to the coffee in several different ways simultaneously via conductive, radiant, and convective thermal transfer, it becomes a challenge to get the optimal heat transfer environment. Drum speed, airflow velocity, drum materials, and the type of heat source all have an impact and small changes can easily change the way in which the coffees absorb heat and transform. Since we use atmospheric burners, the conditions on a given day can have an impact. Barometric pressure and ambient air temperature modify the heat transfer equations, so roasting in July is dramatically different than roasting in November. And since the raw coffees themselves change over time as a result of natural aging and oxidation, the roaster must always be adjusting to compensate.
Even the cooling is critical. Coffee cooled too slowly can lead to lots of problems, including the cross-linking of sugar molecules that makes them less soluble in water, leading to brewed coffees with less sweetness.
It is a lot to think about. I think that’s enough for now.
I would invite you to enjoy a cup of our Rwanda Zirikana, easily my favorite coffee in our current lineup. As you sip, imagine all the chemistry collaborating to yield the tastes and aromas you are experiencing. And if you get dizzy, just take a deep breath, clear your head, and allow the seductive sweetness of the Zirikana ease you into serenity.
As always, find our Nods at:
http://www.intelligentsiacoffee.com/origin/offerings.
Good luck to all of us,
Geoff Watts
VP of Coffee
Intelligentsia Coffee & Tea
Further:
I don’t know much about the chemistry of roasting but I sure do love roasting my own coffee at home. It taste so great too and is a wonderful hobby.