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Changes in coffee beans during roasting you need to know


Coffee roasting is a complex process in which substances in the beans react with each other due to increased temperature. They gradually change their properties and produce new compounds. This leads to changes in coffee beans during the roasting process in terms of color, weight, moisture, and fat creating aroma and flavor. If you are experimenting with roasting coffee at home, please read this article to better understand this step so you can have delicious roasts!


Coffee beans change color during roasting


Before roasting, green coffee beans have a dense structure and natural green color. During roasting, heat will break down the chlorophyll cells, causing the seeds to turn from green to yellow, light brown, and finally dark brown when burned. The husk or outer covering will also peel off. The brown color of coffee will change from light to dark when it passes the first crack due to the caramelization process. Because at this time the temperature is higher, sugar and amino acids react to form melanoidin – a non-enzymatic brown polymer color compound. When the temperature increases too high or the roasting time is too long, the beans can carbonize and become black.

Light brown, lightly roasted coffees will have clear fruity, floral aromas and a sour taste. Dark roasted coffee usually has a dark brown color and a smoky, bitter, and carbonated aroma. You can control the temperature and time based on the color level to create a batch that suits your own preferences.

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The weight and moisture of coffee beans during roasting decrease sharply


Coffee bean weight can be reduced by 12 – 25% after roasting. Because in green coffee, water accounts for about 10 – 12%. After roasting, most of this water will evaporate from the beans, causing the coffee to lose moisture and become lighter. In addition, in addition to the available water, the amount of water created through chemical reactions is also evaporated due to temperature. Some compounds change into gases, making the overall mass lighter.

Different roasting modes will affect the progress of the dehydration process (evaporation rate, amount of reaction water,…). Changes in water activity at different points of the roasting process can lead to differences in chemical reactions. This may change the characteristics of the cup of coffee. So roasters often track weight loss percentages to help determine which batches may need closer monitoring to ensure quality.

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The porosity of coffee beans during roasting increases rapidly


The roasting process makes coffee beans more porous and more soluble in water. The density of green coffee beans is very dense, consisting of Cellulose cells tightly bonded together. When roasting, steam and C02 in the oven will increase the pressure inside the grain, causing the Cellulose to expand. When Cellulose cannot expand any more, the grain structure reaches its peak, cracks will develop inside and on the grain surface. Broken seeds allow steam and strong gases to escape, leading to cracking sounds. At this time, the grain begins to separate from the husk. The contents push toward the cell wall, the inside is filled with gas, causing the grain to become brittle and spongy.

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Oil of coffee beans during roasting


Most of the flavor of coffee comes from the oils contained within the beans. Under normal conditions, volatile aromatic compounds such as Aldehyde (creating fruity scent), Furans (caramel scent), Guaiacol (smoky and spicy smell),… are retained in the oil or lipid layer. Without this layer of oil, aromatic compounds can disperse and lose flavor. During roasting, high internal pressure causes these compounds to move from the center toward the surface of the bean. When pressure breaks the seeds, lipids are released, causing oil to secrete and cover the entire seed.

Every change the coffee bean makes during the roasting process affects the flavor. You can observe their changes in each stage to design a suitable roast profile right at home.

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If you want to enjoy specialty coffee beans roasted using an extremely light method, with a sour taste and gentle fruit flavor, please visit XLIII Coffee – A brand developed from the predecessor 43 Factory Coffee Roaster.

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