Wednesday, November 13, 2019

Chemical reactions during coffee roasting

A coffee bean has a porous structure consisting of cellulose, arabinogalactans, lignin, oils,and other organic compounds. Within this porous structure, there are biological cells containing aromatic compounds, water, and various gases.

Green coffee beans lack the colour and characteristicaroma of roasted coffee; both are formed during the roasting process.

During roasting process, the coffee beans change colour from green to a dark brown, and the main coffee aroma compounds are developed. Additionally, the coffee beans increase in size, as well as lose most of their moisture content.

Maillard reaction
A key reaction for the development of roasted coffee flavor and color is the Maillard reaction. At temperatures from 150-200°C, carbonyl groups (from sugars) and amino groups in proteins react to form aroma and desirable flavor compounds. Heat speeds up the reaction.

The change in color is due to the production of melanoidins. Humic acids or melanoidins, as final products of the Maillard reaction between amino acids and monosaccharides; they are the brown-colored substances that impart to roasted coffee its characteristic colour.

Strecker Degradation
The reaction is dependent on other compounds created during the Maillard reaction. Strecker degradation is a reaction between an amino acid and an α-dicarbonyl with the formation of an aminoketone that condenses to form nitrogen hetero-cyclic compounds or reacts with formaldehyde to form oxazoles. Strecker degradation also contributes to the brown color of the coffee.

Carbon dioxide is quantitatively the most important non-aroma-contributing volatile compound in roasted coffee. It is generated by pyrolysis and the Strecker degradationreaction. The amount is dependent on the degree of roast and can be up to 10 ml /g coffee.

Caramelization of Sugars
From 170-200°C the sugars in coffee start caramelizing, which browns the sugar and releases aromatic and acidic compounds. This process converts complex sugars into more simple sugars. This is a non-enzymatic reaction, meaning that it takes place only in the presence of heat. This reaction continues until the end of the roast and it also contributes to the sweet notes in the coffee’s aroma, such as caramel and almond ones.
Chemical reactions during coffee roasting

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