Roasting coffee: Physical changes during coffee roasting. In the previous article, we learned about the levels of coffee roasting as well as the typical flavor of each level. Then come to this article, let’s find out with Helena Coffee to see how, during the roasting process, the coffee beans will undergo physical changes.
The first stage of roasting is known as the “drying phase” of the beans, although throughout the roasting process the moisture content of the beans will decrease at a similar rate.
During the first few minutes of roasting, the loss of chlorophyll causes the seeds to change color from green to yellow. The seeds then gradually change from yellow to tan to light brown, mainly because of the Maillard reaction.
At the stage, after the seed is close to the first crack, the brown color will deepen due to caramelization. For dark roasts, the carbonization process can cause the beans to turn black.
The microstructure of unroasted coffee beans is relatively dense and organized, with oils encapsulating the cellulose matrix. When the coffee is roasted, the formation of steam and CO2 increases the pressure inside the bean, causing the texture of the bean to swell.
A few minutes before the first crack, the seed will swell to the point where it can release the silvery shell that is rolled into the core of the seed. When the grain’s cellulose can no longer expand, gas and water vapor violently escape from the fissures inside the grain and on the grain surface, thereby creating the crackle of the first crack.
Specialty roasters who often want to create a light or medium roast will usually stop roasting at the midpoint of the end of the first crack and the beginning of the second crack. Between these two stages, gas generation continues, regenerating pressure within the grain.
Meanwhile, the texture of the seeds will become more brittle, preparing for the second crack stage. At this point, the oil will flow to the surface of the beans and this is a signal of the beginning of a dark roast.
3. WEIGHT AND VOLUME
Water makes up about 10 to 12% of the processed and dried green coffee beans, but after roasting, the moisture content drops to less than 5%.
Loss of moisture and the conversion of some dry matter to gas is why green coffees have a reduced overall mass after roasting, losing about 12 to 20% of their weight on average after roasting.
It would be remiss to mention only the participation of water in the roasting process and forget that 16% of the dry mass of green coffee is lipids (mainly triglycerides). And because they are difficult to evaporate, during roasting, under high pressure in the seed, these compounds move from the center of the cell toward the surface of the grain.
Coincidence is that it is thanks to the thin layer of lipids that cover the beans that volatile compounds are trapped inside the grain structure, most of which are essential to creating the aroma of the coffee, without oil they cannot. dispersed in the extract.
In some cases, coffee beans can contain up to 18% lipids. Lipids are present in the cytoplasm of the coffee bean and are protected by a separate membrane along the cell wall. Changes in the structure of coffee bean tissue during roasting will destroy the biological organization of the bean cells and release lipids.
Along with the increase in gas pressure inside the bean, the coffee oil is pushed through the small microvessels in the cell wall to the surface of the bean. Since then, coffee has grown in size but reduced in volume. The roasting process also increases porosity, making the beans less dense and more soluble.