Caffeine averages 1.2% of the total green (unroasted) bean weight of arabica coffee and it tastes bitter. Caffeine is very resistant to the roasting process and undergoes minimal degradation.
“There are only negligible losses of caffeine in the roasting process” (W. Heilmann, Coffee: Recent Developments)
So considering a typical weight loss of a roast might be around 14% for speciality coffee, that means many substances are breaking down in addition to moisture loss whilst caffeine is remaining more or less stable. Caffeine will, therefore, have a higher percentage of mass the further you roast. This also means that for darker roasts, where other chemicals and water have been evaporated or degraded further still, caffeine can be assumed to make up a slightly larger percentage of the weight of each bean.
Despite caffeine’s well-known bitterness, it typically contributes less than 10% of the bitterness of coffee (Clarke & Macrae, 1988). Decaffeinated coffee is still bitter, and spiking decaf with caffeine doesn’t change consumers’ perception of the coffee, so other compounds are considered more important in creating the bitterness of coffee.
Trigonelline is classed in the same category of chemicals as caffeine: an alkaloid. In plant biology, alkaloids are frequently used by the plant as a natural pesticide. This chemical is almost as abundant in green coffee as caffeine, at just under 1% on average (Stennert & Maier, 1994), but its final mass in the cup is reduced by roasting (see fig 3.1). This research found a mean reduction after roasting of 90% (Vignoli et al., 2014). It produces many aromatic substances with positive flavour associations such as pyrazine, furans, alkyl-pyridines, and pyrroles, while also imparting bitterness (Wei & Tanokura, 2015). It also forms niacin, an important dietary B vitamin (Trugo, 2003). While trigonelline’s bitter taste is also well established, it also contributes only a small part of coffee’s bitter taste — as little as 1% (Frank et al, 2007).
Chlorogenic Acids (CGA) are not one singular acid but a collection of around 30 polyphenols. They have such complex chemistry, food scientists are still working out how they all fit together and contribute to coffee flavour. The CGA content of coffee has been extensively studied thanks to its strong antioxidant capacity and therefore its potential health benefits.
“Of the commodities commonly consumed, probably only maté and globe artichoke have CGA contents approaching those found in coffee.” (M.N. Clifford, 1997)
Chlorogenic acids range from 5.5–8% in green coffee and will reduce by around 50% at a medium roast. There are associations with a metallic taste and a drying mouthfeel, but the impact of CGA on the flavour of brewed coffee is not well established. While early studies showed CGA to have a bitter taste, adding a buffer to mask its acidity makes the bitterness undetectable (Herrick & Graber, 1987), suggesting that it is the acidity of CGA that tasters are experiencing, rather than the bitterness.
Chlorogenic acids are however precursors to two of the most important compounds responsible for bitterness in coffee, formed during roasting: chlorogenic acid lactones, also known as quinides, and phenylindanes.
A photo showing two different roast degrees, the left side is very light, the right side is a little darker.
Quinides are “the key bitter constituents of a medium roasted coffee,” contributing to the pleasant bitterness expected in coffee (Hoffmann, 2009). Like the chlorogenic acids from which they are derived, these are a large group of related molecules, formed when the quinic acid moiety in the CGA undergoes lactonisation, reacting with itself to form a secondary ring structure.
Lactonisation of 4-CQA (Source: Wikimedia commons)
A number of quinides were isolated from brewed coffee by ultrafiltration, progressively separating larger molecules from the brew and determining which parts of the brew had the strongest bitter taste (Frank et al., 2005).
Phenylindanes are formed when coffee is roasted darker, from the breakdown of caffeic acid, itself formed by the breakdown of CGA into quinic and caffeic acids. Caffeic acid breaks down on heating into 4-vinylcatechol molecules, which then join together in short chains to form a family of molecules called phenylindanes. These compounds, tasted in isolation, have a lingering and harsh bitter taste, reminiscent of “espresso-type coffee” (Frank et al., 2007), and are thought to contribute to the increased bitterness of dark roasted coffee.
These compounds also extract in water much more slowly than the quinides, which in turn extract more slowly than the parent CGAs, indicating that controlling extraction can determine how much of these strongly bitter compounds make it into a brew (Blumberg et al., 2010).
Quinic acid has been reported to have a bitter, aspirin-like taste (Frank et al, 2007) as well as contributing to coffee’s acidity and astringency. The roasting process increases the overall quinic acid in coffee beans brought about by the breakdown of chlorogenic acids. As roasting progresses, Weers et al., (1995) found that quinic acid increases from 6.87g/kg unroasted to 9.12 g/kg at 6.5% weight loss.
Because of the name, quinic acid is often confused with quinine, the bitter compound responsible for the flavour of tonic water. In fact these are two unrelated molecules, but share a similar name because they were both originally extracted from the bark of the chinchona tree.