Physical Changes During Coffee Roasting: The desired aroma and taste of coffee are developed during roasting. Coffee beans undergo a series of complex chemical reactions that lead to physical and chemical properties.
In the first part of this two-part series, we’ll learn more about the material transformations of coffee beans inside the roaster, starting with the change in grain structure, the process of releasing steam that leads to cracks, and particle color change.
Let’s learn about Physical Changes During Coffee Roasting with Helena.
The basis of physical transformations when roasted
Coffee beans have the highest density and cell diversity of all family plants, which is a prerequisite for any variation in the taste of coffee beans that we desire. Chemical processes essential for flavor and scent will not occur unless a precise physical structure is present.
Particle Structure – Physical Structure
Every plant cell (including mammals) has a cell membrane, which is made up of cellulose and a few additional carbohydrates, in the case of coffee beans. These cell components create a ‘plant skeleton,’ resulting in a highly robust structure.
If we extract around 20% of the solids in each coffee bean into espresso, the leftover cellulose frame makes up the remaining 80%. Green coffee beans have a cell wall about 10 micrometers thick, 20 to 30 micrometers in diameter, and a partition that does not make up half of the cell volume.
Second, most plant cells take the form of an empty ‘sac’ in the middle called a permanent vacuole. The coffee nucleus’s cell space is vast, accounting for more than half the cell volume.
Their main task (when the seeds are alive) is to retain water or nutrients such as protein, fat, and carbohydrates. After undergoing the roasting process, these ingredients become the main flavor compounds.
A change in senses
The most obvious change that occurs during roasting is color. Before roasting, the coffee beans are blue. Then a few minutes, the temperature will make the chlorophyll in the seeds turn yellow before triggering a subsequent sequence of chemical reactions so that the coffee beans turn brown – to dark brown and finally black. This process is responsible for two main groups of chemical reactions:
- First: The formation of melanoidins – the product of the Maillard reaction
- Second: The response to caramelize the natural sugars in the seeds.
Although the color of the coffee beans will constantly switch from blue to yellow, brown, and dark brown to black.
But since the transition from gold to brown takes place at the end of the roasting process with a short time, the classification on the color scale is quite complex, so we will summarize the five primary levels as follows:
Color and taste development
The first issue to pay attention to is that the development of flavor inside coffee beans always progresses later than understanding the color outside the hat. As a result, an astute artisan must oversee the process and guarantee that the interior flavor is entirely established when the color reaches its particle.
It should also be noted that while changes in color and particle development are minimal in the first half of the operation (Light roast), the difference in the second half (Darkroast) is significant.
Second, it’s worth noting that dark roasting does not cause coffee to grow faster. On the other hand, light roasting does not imply that the coffee has reached its full potential. Taste is expressed in several ways, including color. As a result, every color of the roasting process may produce a rich and rich flavor with the correct development.
You can find coffee roasted light or medium at will in the convenience store, but you will find it challenging to use these words to describe coffee at a mainstream specialty coffee shop. They work with insights into the origin of each coffee bean to find a Roast Profile that shows the quality potential of that nut.
Structure, humidity, and volume of coffee beans
As mentioned above, the microstructure of a coffee bean is the matrix of cells made up of cellulose. The roasting process destroys the structure of the coffee cell. Although the frame of the cell members remains intact, the diminished cytometer is pushed towards the cell wall, giving way to a gas-filled void in the central region.
This cell wall layer becomes thinner as it roasts, and the gases and steam increase the pressure to increase the cell size. Therefore, in parallel with the increase in volume, the measured porosity increases gradually during roasting.
During roasting, the grain’s free water evaporates and creates high pressure. This expansion will change the cell wall structure from rigid to more flexible. The particle will gradually weaken, become softer, and more porous. Due to the pressure of intracellular steam, the inner matter will be squeezed out into the cell wall leaving an intracellular void.
Most of these gaps will be filled with CO.2. When water penetrates these cavities during preparation, the soluble compounds will follow the water into the extract and give us a sense of coffee flavor.
Loss of moisture during coffee roasting
Coffee will lose 12%-24% of its weight throughout the roasting process, depending on the initial humidity (and the roasting level at the end). Since the first crack appeared, coffee has lost 11%-13% of its volume.
About 30 seconds after the first crack ends, it is about 14%-16%. And at the start of the second crack, the coffee lost 17%-18% of its mass. In the specialty coffee industry, with a preference for preserving the native flavors of coffee beans, most roasters keep coffee beans “losing weight” at an average of 14%-16% of their original weight.
The loss of weight above – water accounts for up to 90% of the lost weight. The rest is organic matter emitted in the form of CO2 gas and small amounts of carbon monoxide, nitrogen, aromatic compounds, and volatile acids. Organic losses increased significantly from the medium roasts phase, with organic matter wear and tear of 5-8% and up to 12% from the dark roasts onwards.
* Scott Rao calculates these estimates from the initial humidity of green coffee to be 10%-12%, and the roasting time is 11 to 12 minutes, which can vary significantly.
The first crack & the second crack
When the steam pressure in the coffee beans is continually increased between 100 and 160°C, the grain structure is broken down, resulting in the first crack. Coffee beans can double in bulk at this time, and the humidity in the seeds will drop from 10-12 percent to around 3-5 percent, depending on the extent of roasting.
The effects coffee bean structure becomes increasingly brittle after the first crack, setting the way for the second fracture (about 225-230°C). However, unlike fracture firsts, the second explosion did not result from the liberation of free water in the particle. This is since the remaining water in the county was used in the breakdown of carbohydrates and proteins.
Fat during roasting
It would be flawed to refer only to the participation of water during coffee bean roasting and forget that 16% of the dry mass of green coffee is lipids (mainly triglycerides). And since they are difficult to evaporate, during roasting, under high pressure in the particle, these compounds move from the center of the cell towards the particle’s surface.
The coincidence is that thanks to the thin layer of lipids that cover the seeds, volatile compounds are retained inside the grain structure, much of which is essential to produce the scent of coffee. Without the oil, they cannot disperse in the extract.
Coffee beans can contain up to 18 percent lipids in rare situations. The cytospin of the coffee nucleus, which is protected by a distinct membrane placed along the cell wall, contains lipids.
The biological organization of particle cells will be destroyed, and lipids will be released when the structure of coffee bean roasted tissue changes during roasting.
Coffee oil is pumped via tiny microchips in the cell wall that go up to the grain surface, and gas pressure rises inside the grain. In moving to the grain surface, many drops of oil appear. They will gradually re-conclude and become thicker, eventually covering the entire grain with a shiny oil film.
The physical changes in the structure of coffee beans go one way to explain the shift from green coffee – to roasted coffee beans. Still, in one respect (or more precisely on the other front), the chemical changes during roasting are the “molecular basis” that forms the flavors.
Promoting physical transformation is also one area that lacks the most understanding of the coffee industry.
Reference source:
- The Coffee Roaster’s Companion (Scott Rao, 2014)
- www.perfectdailygrind.com/ What Happens During Coffee Roasting: The Physical Changes
- www.beanscenemag.com.au/ The affect of roasting and grinding on cell structure