How does the structure of specialty coffee beans change through each roasting stage?

The drying stage begins as soon as the green coffee beans are put into the roaster. The drying process takes about 4 – 6 minutes. At the end of the drying process, the temperature will reach 160°C. During that time, the water content begins to evaporate and pressure begins to build inside the coffee beans. After a few minutes, the moisture will quickly decrease, making the coffee lighter. The seeds begin to change from green to yellow, expanding and cracking the husk surrounding the seeds. Different aroma compounds begin to form, giving the coffee its flavor.

The Maillard reaction appears after 7-10 minutes of drying. The temperature in the roaster has now reached about 150°C/302°F. Coffee begins to turn brown due to Melanoides produced from the reaction between amino acids and sugar. In addition to changing the color of the bean, the Maillard reaction also contributes to the flavor and body of coffee. Depending on the heating rate, the temperature drives the pre-coffee into a Strecker decomposition state. Amino acids will react with carbonyl molecules to form aldehydes and ketones – an important compound that creates flavor and aroma. Besides, the temperature during this process also causes gases (carbon dioxide, water vapor) and some volatile compounds to be created, causing the volume of coffee to swell. The pressure and the amount of carbon dioxide and monoxide gas increase rapidly enough to break down the cell walls of the coffee bean, causing cracks to appear.

The development stage is the time when the coffee cracks, causing the first explosion. During this stage the color of coffee changes from light brown to medium brown or dark brown. The roasting process changes from an endothermic reaction (beans absorb heat) to an exothermic reaction (beans release heat). Roasting oven temperature up to 195°c. The exothermic reaction takes place, causing the pressure within the particle to increase Cracks.

become apparent causing the coffee to burst and aroma compounds to flourish. The grain increases in porosity, and lipids from inside the grain gradually move to the cell wall. Amino acids, sugars, and aromatic compounds react to create fruity odors and sweetness due to caramelization of sugar. If the development time is slow, organic acids have more time to decompose, causing a more pleasant cup of coffee to have a milder acidity. However, if roasted for too long, organic substances will enter the carbonization process, making the coffee taste bitter and harsh.

According to each level of heat, coffee will gradually change and create compounds that form different aromas. Good control of the roasting process will help you influence these compounds and thereby roast coffee batches with the delicious flavor you desire.

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