Principle of coffe roasting machine:
How can roasting machines be able to “ripen” coffee beans evenly? How do the principles of these machines work?
Understanding the principles of coffee roasting machine will help coffee roasters understand and be more proactive in their coffee making process. And the heat transfer mechanism is the most important, most special mechanism for any coffee roaster. Each coffee roaster will have different heat transfer mechanisms.
Convection heat, heat transmission and radiant heat
- Traditional roasting machine is a roasting machine with roasted drum that is in direct contact with fire, first roasting coffee beans with convection heat and then heat transmission. The heat radiated from roasting machines and coffee beans is also part of the coffee roasting process.
- Indirect roasting machine is a roasting machine with roasted drum far away from the heat source, ensuring that the roasting cage is not too hot. Convection heat plays a major role in the heat transfer process for roasting coffee.
- The roasting machine is roasting machine without roasted cage, coffee is roasted by hot air flow blowing into roasting tube with high speed making coffee beans hovering, absorb heat and “ripen”.
- The type of roasting machine that absorbs and recovers difficult heat generated during roasting. Both lines are designed to roast completely with convection heat.
In the first stage of the roasting process, the temperature of the roasted cage will be dramatically reduced when the green coffee has room temperature (about 37oC) is put into the machine. In the first few minutes of the batch with traditional cage roasting machines, the role of heat transfer into coffee beans from roasted cage clearly shows. Whereas the temperature in the roasting cage will start to increase gradually after falling down in the first phase, convection heat will begin to play its role. For this type of machine, roasted cage acts as a "heat-bearing cage" to ripen coffee in the early stages of roasting. Machines that only roast coffee with convection heat often need to measure larger heat to get the necessary heat in the early stages of the roasting process as well as compensate for the heating temperature of the roasting cage.
Heat transfer and temperature variation
About two-thirds of the first time of roasting is the process of heat recovery, which means that the beans will absorb energy, and the heat energy will be transmitted from outside to inside the beans. The temperature variation, or "temperature difference" inside the coffee bean depends entirely on the heat transfer efficiency. Understand simple, the greater the temperature amplitude, the faster the temperature inside the coffee bean. The temperature amplitude in the first phase is estimated to be about 50 ° C, reaching a maximum at that point or maybe a little higher, this amplitude will decrease gradually as the next roll. In other words, after the first few minutes of the roasting, the temperature in the core of the coffee bean is gradually equal to the surface temperature of the grain. In general, the temperature amplitude is usually higher when roasting faster and lower when roasting slowly.
Thermal energy and heat transfer in coffee beans
The mechanism of heat transfer and heat transfer in coffee roasting machine plays an important role in determining the quality of coffee beans after roasting. Therefore, coffee roasters often carefully consider the selection of roasting machines with suitable heat.
In the first phase of the outer layer of coffee beans, steam begins to evaporate to form a "vapor layer" entering the inner core. The cellulose structure in coffee beans is cooled, retaining the moisture inside the core. When heated cavity structures are turned into steam, the pressure increases making the grain expand.
Heat transfer and humidity
The moisture content of roasting media and moisture in coffee beans affects the heat transfer during roasting. After the initial stagnation of the temperature, the moisture inside the roasting cage will increase the heat transfer process to release the moisture in the coffee beans. The amount of water vapor contained in the grain directly affects the roasting process. Large amounts of steam for heat transmission will occur three main effects on green beans.
- Increased heat transfer ability because moisture increases the heat transmission in coffee beans
- Increasing the amount of heat, means more heat must be used for the same weight.
- The amount of steam released from coffee beans is more, which will limit the ability to transfer heat into the core.
The final effect is that the temperature in the moist grain will increase more slowly in dry grains. Therefore, the roaster must calibrate the temperature appropriately, by heating it larger when roasting wet seeds and adjusting the temperature appropriately when roasting dry beans.
Each coffee roaster will be designed according to its own specific operating principles, and in order to achieve the best quality when roasting, roasted workers need to know the roasting mechanism. coffee of the machine I use to master the operation, control it.