While the moisture content and physical characteristics of the bean undoubtedly have the biggest impact on the way a coffee roasts, what’s inside the bean also has an important effect.
The chemical reactions in roasting progress differently in beans from different origins. One study measured in real time the volatile compounds released while roasting different coffees. The researchers found that certain compounds were released at different stages of the roast, and in different amounts, depending on the origin of the coffee (Gloess et al 2014).
Coffees from different origins have different roasting chemistries. Certain volatile compounds are released at different rates from different coffees. The researchers compared arabica coffees from Colombia, Guatemala, Yirgacheffe, and Djimmah, as well as a robusta from Indonesia, under identical roasting conditions. Adapted from Gloess et al (2014): some data are extrapolated from the figures presented in the paper.
It’s not certain whether these differences are due to the chemical composition of the beans or to physical characteristics such as size and density. What this experiment does show, however, is that differences in roasting chemistry support the idea that it’s important to create different roast profiles for different coffees.
While we don’t know exactly what precursor compounds cause coffees to develop different flavours, the mixture of amino acids present certainly plays a large part in determining the various aromas in coffee.
Individual amino acids result in very different aromas when they take part in the Maillard reactions. For example, proline, phenylalanine, and tyrosine result in floral aroma compounds; glycine, lysine, and valine give rise to caramel odours; and the sulphur-containing amino acids cysteine and methionine react to form savoury, meaty aromas (Wong et al 2008).
One particularly striking example of how the amino acid composition affects flavour is tryptophan. Robusta has large amounts of this amino acid, which gives rise to 3-methylindol (also known as skatole),