Chemical composition of coffee

The chemical compounds that affect the aroma of coffee

Thechemical compounds of the individual coffee products that accompany thearoma of coffee depend on several factors. In particular ,the type of green coffee bean, the geographical origin and the processing conditions. The perception of coffee aroma depends on the concentration of compounds and their smell.

Astatistical approach was used to select only six flavour markersfrom six alternative compounds in coffee . Acetic acid, furfural, 2-methylpyrazine, 2-furfuryl alcohol, 2,6 dimethylpyrazine and 5-methylfurfural. These 6 compounds cover more than 80% of the average relative amount of volatile compounds that were detected in the coffee samples. The aromatic compounds listed above are important for distinguishing the coffee variety or geographical origin

Coffee roasting from a chemical point of view

During roasting, amino acids and sugars combine to form a large number of reactions. This creates the aroma, taste and colour of the coffee. This is called the Maillard reaction. Thecoffee bean itself is made up of polysaccharides or sugars. But it also contains proteins, lipids and minerals. The role of the seed, in order to germinate, is therefore to provide some nutrition for the coffee embryo. The sugars, proteins, lipids and minerals are then the basis for theroasting process .

Coffee beans lose water in the first stage of roasting up to a temperature of 150°C. Actual roasting begins above 160°C. Chemical reactions, the number of which is incalculable, take place immediately afterwards. and the composition of the beans changes immediately. The main product is carbon dioxide.

During roasting, 50-80% of the trigonelline decomposes. Trigonelline is the source of some of the aromatic nitrogenous compounds found in the coffee aroma. The sulphur compounds of coffee oxidize easily in air and this is the reason why coffee stored in air loses its aroma.

The large amount of pyridine present in roasted coffee is produced by pyrolysis of the alkaloid trigonelline. This, however, causes an unpleasant aroma and, above all, reduces its quality. The darker the coffee is roasted, the more pyridine it contains.

Chemical reaction during espresso extraction

The coffee brewing process is not about a chemical change. It is about extracting compounds from the roasted coffee beans. How well different molecules can be extracted depends on their solubility, which in turn depends on their polarity.

Different types of atoms exert more pull on the electrons in chemical bonds than others. For example, oxygen exerts more pull on bonding electrons than carbon. And the bond between a carbon atom and an oxygen atom is what we call a polar bond because the bonding electrons are pulled closer to the oxygen atom, giving it a slight negative charge.

Polar molecules are more soluble inwater thanhydrogen atoms. This means that it surrounds other polar molecules, allowing them to dissolve. Polar molecules in coffee beans are extracted at higher percentages during the coffee brewing process than non-polar molecules.

Which compounds contribute to the aroma of coffee?

Thearoma contains heterocyclic compounds ( furans, indoles, pyrroles, etc.). The formation of these compounds is due to the Maillard reaction and caramelisation. Green coffee contains almost no aromatic substances. These are only formed during roasting, thanks to the Maillard reaction mentioned above.

You can also find aliphatic compounds (hydrocarbons, alcohols, esters, etc.) in the aroma of coffee. As specific examples I would mention:

• methional
• 3-methylbut
• 2-enthynol
• 3-methylbutanal
• 3-mercapto
• 3-methylbutyl
• formiate

Other substances affecting coffee flavourings:

• Furans
• Pyrans
• 5-ethyl
• 3-hydroxy
• 4-Methyl
• Maltol.

Among the phenols in coffee aroma we include:

• vanillin
• 4-vinylguaiacol
• 4-Ethyl-guaiacol

More than a thousand chemical entities have been identified in coffee beans, and a large number of them are extracted during coffee preparation. During studies, two main factors are often considered when assessing aroma:

• the concentration of the compound

• the aroma of the compound at the minimum concentration at which it can be detected. Total aroma is measured by the OAV - it gives a value for the strength of the aroma.

Important compounds that contribute to the aroma of coffee are mainly sulphur-containing compounds, including 2-furfurylthiol. Some compounds that are not pleasant to smell on their own may combine with other compounds to add nuance to the aroma.

For example, such a methanthiol has been likened to the smell that resembles rotten cabbage. Another sulfur-containing compound, 3-mercapto-3-methylbutylformate, is characterized as having a pungent odor.

Aldehydes have a fruity aroma. Furans contribute to the caramel aroma and pyrazines contribute to the earthy aroma. Guaiacol and other phenolic compounds are reminiscent of smoky, spicy notes. Pyrroles and thiophenes are also present in low concentrations .

Chlorogenic acids also influence the resulting aroma. Acetic acid, which is present in the green beans before roasting , has a major influence on the taste and acidity of the coffee. Maltol, which has a sweet, caramel-like taste and aroma, is also present in roasted coffee .