Complex chemistry during coffee roasting
This blog is the continuation of the previous two blogs on green coffee composition and introduction to coffee roasting. In this blog, we’ll look at the chemical changes, namely carbohydrate, lipids, CGA, amino acids and alkaloids during roasting.
In the previous blog, we learned that the heat evaporates the water and releases carbon dioxide, accompanied by some carbon monoxide and organic volatiles. Water and CO2 reactions are produced by partly numerous pyrolitics reactions and mainly by Maillard reactions, which leads to the browning, the melanoidins and part of organic volatiles.
The chemical changes in green coffee composition in roasting is depicted in the diagram below (Maier, 1995).
Carbohydrates in coffee are mainly the complex composition of sugars (poly-, oligo- and monosaccharides.)
Sucrose is partially broken down by hydrolysis and the rest by pyrolysis (caramelisation). The Maillard reaction is favoured in breaking down sugars because of the reactivation temperature is lower (approximate 130 °C - 140 °C) than caramelisation (approximately 180 °C), and the presence of the nitrogen compound such as amino acids, free amino groups in protein and peptides. Further, the breaking down of sugars by Maillard reactions produces many volatile aroma compounds and acids, and non-volatiles melanoidins.
Carbohydrates in roasted coffee are both soluble and insoluble, with the sum is lower than in green coffee. The polysaccharides (except cellulose) are partly soluble. The insoluble carbohydrates (galactomannan and arabinogalactan) are responsible to stabilise the foam in espresso (Nunes et al, 1997).
Lipids account for approximately 10% (robusta) - 15% (arabica) of green coffee. In roasting, the quantity of lipids is changed slightly with the exception of sterols and most of the triglycerides, which remain the same. The amount of fatty acids increases. The diterpenes, cafestol and kahweol decompose to some extent and form dehydrocafestol, dehydrocafestol, dehydrokahweol, cafestal and kahwael, with the increase in roasting temperature.
Most of the CGAs are destroyed during roasting (hydrolysis), which varies depending on the roasting temperature and its genetic composition. The CGA’s losses are estimated to be approximately 45-54% in the light roast (Moon, Yoo, & Shibamoto, 2009), 60% in medium roast and 100% in the dark roast (Michael N. Clifford, 1979; Trugo & Macrae, 1984).
The CGAs break down to form caffeic and quinic acid. Studies show that CGA’s by-products such as 6 lactones of the mono-hydroxycinnamoyl-quinic acids are assumed to contribute to the bitter taste of roasted coffee (Ginz, 2001) and the 3 di-hydroxycinnamoyl-quinides are supposed to modulate the effect of caffeine (Martin et al., 2001). Moreover, the free quinic acids decompose further to form phenols like hydroquinone. However, little is known about the caffeic acid. The minor part degrades to simple phenols and the rest is assumed to be incorporated into the melanoidins.
Other acids in green coffee are partially decomposed during roasting. For example, citric acids form citraconic, itaconic, mesaconic, succinic, glutaric and other acids (such as malic acids to form fumaric and maleic acids). The phosphoric acid is stable and it’s content increases by hydrolysis of the inositol phosphates. Furthermore, acids such as formic and acetic are generated by Maillard reaction. Their contents reach the maximum at the medium roast and degrade in a darker roast.
Caffeine is mostly stable during roasting, with only a small portion of caffeine sublimated. Trigonelline is partially decomposed, amounting to 50% in light roasting and approaching 100% in the very dark roast (Viani and Horman, 1975; Stennert and Maier, 1996). Trigonelline decomposed to form pyridines and nicotinic acid. Nicotinic acid also is known as niacin (Vitamin B3), which is responsible for coffee's anti-cavity effect.