RE-ROASTING: WHY COFFEE SHOULD NOT BE ROASTED TWICE AT HOME IN CASE IT UNDERCOOKED
There is not enough moisture in roasted grains for new chemical reactions to occur.
Water makes up about 10-12% of the green grain. Each roasting begins with the evaporation of the free moisture in the grain. Only after most of the water is removed do the chemical reactions that affect the flavor, aroma, and body of a cup of coffee.
Most of the processes important for coffee flavor occur in the middle stages of the roasting process, when the water content of the bean drops to 2-7%. For example, moisture can interact with sugars and amino acids to produce various flavor compounds. Depending on the degree of roasting, the moisture level of the grain drops to about 3-5%. Therefore, if the process is restarted, no new reactions will take place.
During the roasting process, the water also serves to transfer heat. When the grain is heated, the water inside it evaporates and takes up the heat, which helps to equalize the temperature. With a small amount of moisture, there is a high risk of overheating the grain.
The flavor compounds are destroyed by repeated roasting.
Most important flavor compounds, such as pyrazines, begin to decompose with prolonged exposure to high temperatures. The concentration of these volatile substances decreases with increasing degree of roasting. They are formed when the moisture content of the coffee falls below 5%. Flavor levels will be highest at light and medium roasts. The longer the roast is roasted, the more substances are destroyed by the temperature. In addition, the cell walls become more porous and lose their ability to retain flavor.
The longer the roasting time, the more flavor compounds are destroyed.
Acids play an important role in creating the flavor of coffee. They are numerous, but the most important are citric, malic, tartaric, cinnamic, acetic and chlorogenic acids. They are sensitive to heat - roasting leads to the destruction of some and the formation of others. For example, citric, tartaric and malic acids, which give fruity and berry notes to the drink, are destroyed during roasting. Therefore, prolonged roasting or too high a temperature can significantly reduce the acidity of the coffee. Conversely, acetic acid is formed during the roasting process by the breakdown of certain sugars - its concentration can increase to three times the original concentration, which will negatively affect the flavor of the cup.
The bitterness increases sharply with repeated roasting
As the roasting time increases, so does the bitterness of the coffee. This is due to several factors.
Chlorogenic acids are broken down. This is a family of compounds that are present in the bean in large quantities. During roasting, they slowly decompose to form caffeic and cinnamic acids, with about 50% of the original substances being destroyed already at medium. Sensorially, cinnamic and caffeic acids are associated with increased levels of astringency, bitterness and body intensity, which are typically seen in darker roasted coffees.
Prolonged caramelization. If too many soluble sugars are burned off, the cup will be much less sweet and take on a bitterness and burnt sugar flavor.
Repeated roasting is difficult to control
The roaster monitors the roasting time, the current grain and air temperature and other parameters using sensors in the production plant.
The final stages of roasting gradually approach the conditions of the burning process. At this stage, control is crucial to ensure that the grain is not overcooked. The production roaster monitors four parameters: the temperature of the grain and the air coming out of the drum, as well as their rates of change. He does this with the help of sensors and the Cropster program, which displays the variables on a graph in real time. The roaster also makes sure the roast hits the control points for a particular profile. At home, there is no way to control the temperature and the rate of temperature change, so there is a high likelihood of simply burning the grain.
What's the bottom line:
The re-roasting process does not produce new compounds that improve the flavor of the coffee. On the contrary, many flavor compounds are destroyed by prolonged exposure to heat and bitterness increases. Without professional roasting equipment it is impossible to control the roasting parameters, so it will be very difficult to ensure its uniformity.
If you are not suitable for light roasting of grain, there is no sense to finish roasting it yourself. It is better to buy the right degree of roast right away.