The humble coffee bean contains somewhere around one thousand volatile compounds, of which a few dozen control aroma, flavor, acidity, and bitterness. Reaching a perfect balance in your cup begins with precise temperature and moisture control during the roasting process.
Roasting Perfect Coffee Beans
The most critical stage in coffee production is roasting. Varying amounts of heat, pressure, and moisture affect the starches and volatiles in green beans differently, in turn altering the chemical composition in your cup. The perfect cup of coffee contains a finely tuned balance of at least twenty-eight aroma-producing compounds, each responsible for different aspects of its flavor profile.
The odd cellular structure of the bean – the cell membranes of which can withstand 16 bars at high temperatures without bursting – keeps its constituent chemicals under pressure throughout the roasting process. Over time, and at temperatures between 210 and 225 C, these break down along several reaction pathways to produce flavor-critical volatile compounds:
- Sugars caramelize (changing the color of the bean), fragment into lactic, formic, acetic, and glycolic acids, or combine with
- Strecker-active Amino Acids in a cascade of Maillard reactions, leading to rearrangements (and reactions between rearrangement products) producing dozens flavor-determining volatiles.
- Trigonelline transforms into nicotinic acid, pyridine, and pyridine derivatives.
- Chlorogenic acids yield several key lactones (chlorogenic and quinic acid lactones), along with quinic, ferulic, and caffeic acids.
- Lipids form carbonyls, such as vanillin and guaiacol, through interactions with caffeic and ferulic acid.
- Carotenoids yield β-damascenone, a volatile compound which smells a little like baked apples.
(Thank you, Dr. Jürg Baggenstoss, for writing your dissertation on this subject in 1977. We lift our mugs to you.)
A Quick Word on Caffeine…
One thing you’ll notice is the absence of the world’s most popular psychoactive drug, caffeine, from these breakdowns. Primarily, that’s because caffeine is already present in the beans (it’s a natural pesticide) and mostly unaffected by roasting. As far as affecting the taste of your coffee, caffeine is almost entirely irrelevant. While bitter, it is present in such low concentrations that is easily overshadowed.
… and Back to the Perfect Cup of Coffee
To return to roasting: the ideal process involves heating beans through convection to temperatures between 210 and 225 C for a period of twelve minutes – depending on the thickness of the beans.
Traditionally, coffee processors judged ideal roasting times by color. This is how we arrived at popular terminology such as dark roasts, blonde, and so on. As a rough gauge of where the beans are in a known roasting process, this will do – the breakdown of starches to caramelized sugars is reasonably predictable. However, it’s possible to manipulate temperatures and roasting times to produce beans of identical color with divergent chemical profiles.
In 2008, the inestimable Dr. Baggenstoss, et al, demonstrated that you can completely ruin a batch of coffee beans by rushing the process at high temperatures (260 C for 170 seconds), increasing concentrations of hexanal and pyridine – which do not taste good – while destroying key volatiles responsible for taste and aroma. The beans, however, were the same color as those which were roasted properly. Experiment with variations in time and temperature, by all means, but bear in mind that color is not the most reliable metric.
Water cooling is faster and simpler to engineer – drop the beans in water – but probably best avoided. Moisture content values in the finished bean affect your perception of their freshness, by increasing their concentration of dimethyl trisulfide. Now, what’s important in this case is not the dimethyl trisulfide itself, but that it forms from the oxidation of methanethiol, which is one of the flavor-critical volatiles responsible for that “fresh coffee” smell.
Air cooling usually involves agitating the coffee beans in a rotating, perforated drum in room temperature air. The apparatus is more expensive, certainly, but you’ll be rewarded with lower moisture content (1-2% is your target) and longer-lasting flavor in storage.
After roasting, you need to break up the coffee bean to access the cocktail of volatiles stored within its cell walls. You could, of course, drop whole beans in hot water and stir, but you would never have read this far if you found that even remotely appealing. Grinding increases the surface area of your bean and decreases the distance-to-center (how far water must penetrate before it’s leached out all roasting byproducts) of each resulting particle.
Moisture content and degree of roasting strongly effect grind consistency. Air-cooled beans, for example, are drier and more brittle than water-cooled beans, and will break rather than deform. Deformation can allow fragments to ‘bounce’ away from your grinding tool. Degree of roasting also corresponds to moisture levels, in addition to being an indicator of increased caramelization – yet more brittleness and finer particles.
For the perfect cup of coffee, start with the following average particle sizes:
Coarse 1.5 mm
Regular 1.0 mm
Drip 0.75 mm
Fine 0.38 mm
Espresso 0.20 mm
The reason you adjust your target particle size to brewing method is that each technique varies in the amount of time water is exposed to the ground coffee. A French press, for example, can steep as long as five minutes, while water passes through espresso grind coffee for less than thirty seconds.
Adjusting grind particle size gives you a tremendous amount of control over the taste and strength of your cup of coffee. For drip or coarser grinds, chopping with a blade grinder is best, while burr grinding is more effective for finer grinds.
Dr. Ernest E. Lockhart, former assistant research director at Coca Cola, former research director of the Coffee Brewing Institute, and founder of the International Life Sciences Institute, will forever be known for two things: there is a mountain named after him in Antarctica and he developed the ideal coffee brewing control chart.
Brew Formula is the ratio of ground coffee to water, in grams per liter. Solubles concentration, expressed as a percentage, is the amount of coffee brewed into your water. Espresso is off the chart, with an average solubles concentration of 2.2%.
Solubles yield is harder to measure without specialized equipment and refers to the amount of material successfully extracted from the ground coffee during brewing. This is the factor you adjust by altering the fineness of your chosen grind. Smaller particles will yield more soluble material in the same interval, compared to larger ones.
As with roasting, however, there’s a balance. Over-extraction will draw out more of the bitter-tasting compounds, compared to flavorful volatiles.
Experimentation with chemical composition (through roasting), extraction rate (through grinding), and strength (through brewing) will dial you in on your subjective ‘sweet spot’ on Lockhart’s brew control chart. Take careful notes, and you’ll be able to brew the perfect cup of coffee, every time.