When I was much younger, and my interest in chemistry was just beginning to influence my thoughts of post-secondary school and (a lifetime away) a career, there were hints of my destiny that came in the form of somewhat perverse interests.
One of these was my profound interest in the chemical constituents of coffee. A non-chemist simply sees a cup of coffee for what their nervous system and digestive see it: a black liquid that tastes bitter, inhibits your appetite, and gives you about a two hour energy boost. However, being the child of two coffee snobs who also happened to be career scientists, I had different notions.
What makes a good cup of coffee? People have their preferred brands, countries of origin, or even vintages, if you have that much money to throw away. There are even those who eat an obscenely expensive bean that has been digested and excreted by the Asian palm civet. But can you actually taste the difference? And more importantly, what makes up the difference in flavour?
Most coffees are simply described in terms of acidity, roast, and some vague notes about other flavours (vanilla, caramel, berries, chocolate). This, to me, was completely unsatisfactory. Little did I know that coffee flavour chemistry is a legitimate field of study, as is the study of flavours and scents in general.
In my slightly-more-recent searches on the topic, I stumbled across a book entitled Coffee Flavour Chemistry; a better match for my childhood fascination could not have been conceived. Though now somewhat out of date, the book represented a major achievement in the field in 2002 when it was written by Ivon Flament, and contains some very interesting information and a comprehensive review of the work done in identifying coffee’s chemical components. It identifies over one thousand compounds present in both green and roasted coffee beans, describing their aromas and their significance in the overall flavour of the coffee. Intuitively, the most important constituents of coffee flavour are the ones with the highest “signal-to-concentration” ratio, or how easily they can be detected at a certain concentration. Some of the most important constituents can be seen below, with their described flavours:
The majority of these compounds are of course formed via the well-known Strecker degradation and the Maillard reaction. These, as well as other important aspects of the roasting of coffee beans, are discussed within the book.
Perhaps what I found most interesting are the techniques used to determine what are called “odour activity values”, which essentially amount to a quantification of how strong a compound’s odour is at a given concentration.
The methods used for finding these values are actually gas-chromatographic olfactometry, which is exactly what you think it is: they run a GC, and have someone sniffing the end of the column, who presses a button each time they smell a compound. I laughed at this when I read it, not because it’s unreliable (quite the contrary), but because imagining running a column with a panel of “sniffers” at the end instead of a mass spectrometer is quite an image indeed.
The original method (GC-olfactometry) was purely qualitative in terms of odours, but later methods known as CHARM (a proprietary technique involving dilution-to-threshold) and AEDA (aroma extract dilution analysis, a similar method) have overshadowed it in recent years. Yet another method, named with a classic, groan-inducing acronym is “GC-SNIF”, which stands for “gas chromatography-surface of nasal impact frequency”, in which panels of test subjects are used to determine how “smellable” a compound is at a single concentration, producing averaged and normalized curves for the detection of smells.
Apart from being a funny image, these methods of analysis (which are almost always used in conjunction with purely analytical methods) illustrate an interesting point about flavour chemistry. While GC or LC data, as well as mathematical methods such as canonical analysis or principal component analysis are useful for predicting simple properties, the use of human organoleptic testing is essential to actually understanding the results.
Another salient point that was mentioned was the perception of quality. In a 1986 paper, Liardon and Spadone discovered that while the degree of coffee roasting was correlated to a large number of compounds and their concentrations, the quality of the coffee did not appear to be correlated to any of them. One of the few quantitative differences that could be established based on quality was that between green C. arabica and C. robusta beans. Robusta tended to have higher concentrations of methanol, acetone, pyridine, methylpyrazine, and furfural, and also seemed to be unique in that they contained methyl formate, t-butyl alcohol, and furfuryl alcohol, not found in Arabica strains. Robusta beans are widely perceived to be inferior in flavour and aroma to arabica beans, which is why many coffee packages will declare themselves to contain “100% arabica beans” to avoid confusion and distinguish themselves.
So what does this tell us? After skimming through the book I came away with two lessons: the first is that while computers and statistical analyses become more and more powerful every day, it seems there is usually a place for subjective human-generated data. Without olfactory analyses from panelists, much of the work on coffee and its constituents would have been completely useless. The second is that while this data is essential to understanding the importance of certain compounds in generating a specific flavour, it is almost worthless when trying to establish a causal link between specific compounds and the perceived quality of the coffee (edit: this is not strictly true for identifying fundamental flaws in the bean due to parasites, mould, or poor growing conditions, which can all be identified by screening for certain compounds). For the most part, as everyone has heard so many times in their life: there’s just no explaining some people’s tastes.
However, I think for most of us in grad school, the fact remains that the most important compound in coffee is one which contributes almost nothing to its odour profile: