Analysis of Physical Changes in Coffee Roasting

“Green coffee powder exposed to the air cannot emit the same flavor compounds as roasted coffee powder at a similar temperature and time. Beans with a complete structure are necessary “microreactors” for chemical reactions. Structure Intact means that without its specific physical structure, the coffee bean cannot undergo the chemical reactions required to produce flavor and aroma.

Roasting Physical Changes:


Before roasting, coffee beans are green. They turn brown due to the production of melanin. This is due to the combination of sugars and amino acids upon heating to form polymers. The bean hull, or silverskin (which is the papery outer layer of the coffee bean), also falls off during the roasting process.

yellowing period
yellowing period

Here is our video showing the color changes of coffee beans when roasting.

Moisture and Weight

After processing and drying green coffee beans, the moisture content is about 10-12%, but after roasting, it will be reduced to about 2.5%. Chemical reactions during the roasting process create additional moisture in addition to the moisture already present in the green coffee beans. However, this part of the moisture will evaporate during the roasting process. The loss of moisture and the conversion of some dry matter to gas is responsible for the overall mass loss of beans after roasting. Coffee beans are on average 12-20% lighter in weight.

Different roast profiles will affect when dehydration occurs. Changes in water activity at different roast points mean differences in chemical reactions.

Volume and Porosity

Coffee beans are the plants with the strongest cell walls in the plant kingdom. Their outer rings reinforce the cellular structure, increasing its strength and toughness. During roasting, the increased temperature and vaporized water create high pressure inside the beans. In this case, the structure of the cell wall changes from a rigid structure to a rubbery one due to the presence of polysaccharides (synthetic sugar molecules). The inner matter pushes the cell wall outward, leaving a gas-filled void in the center. This means that as coffee beans lose weight, the beans expand in volume. Most of the gas gathered together is carbon dioxide released after roasting. Roasting also increases the porosity of the cell walls, reducing the bean’s density and enhancing solubility. Of course, this is crucial for subsequent brewing and extraction.


Coffee beans contain lipids or oils. During roasting, the high pressure inside causes these compounds to migrate from the center of the cell to the surface. Lipids can retain volatile compounds inside cells. Volatile compounds are chemicals with a high vapor pressure at room temperature. Some of them are essential to create the aroma and aroma of coffee. Without oils, they may evaporate.

The longer the roasting time, the more pronounced the structural transformation becomes. Coffee beans become less dense, and more gassed over time, and in extremely dark roasts, oils can be seen migrating to the surface of the bean.

coffee bean
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