Working principle of coffee roaster

Working principle of coffee roaster

Please read our series before you make decision to buy any coffee roaste brand.

How can  coffee roasters roast green coffee beans evenly? How do these machines work?

Understanding the operating principle of a coffee roaster will help coffee roasters around the world have better understanding and be more proactive in their coffee roasting process. And the heat transfer mechanism is the most important mechanism, especially for any coffee roaster. Each coffee roaster will have different heat transfer mechanisms. Follow us on www.danielmachine.com for more tips of roasting coffee.

Convection, conduction and radiant heat

- A traditional roaster is a roaster with a roasting cage in direct contact with fire, first roasting coffee beans by convection heat and then by conduction heat. Radiant heat from the roaster and from the beans themselves is also part of the coffee roasting process.

- An indirect roaster is a roaster with a roasting cage away from the heat source, ensuring that the roasting cage is maintained without overheating. Convection heat plays a major role in the heat transfer process for roasting coffee.

- Tube roaster is a roasting machine without a roasting cage, coffee is roasted by hot air blowing into the roasting tube at high speed, causing the coffee beans to float, absorb heat and ripen.

- Types of circulating roasting machines that suck and recover difficult to hot discharge during the roasting process. Both of these roasters are designed to roast coffee entirely by convection heat.

In the first stage of the roasting process, the temperature of the roasting cage will drop suddenly when room temperature green coffee (about 37oC) is put into the machine. In the first few minutes of roasting with a traditional cage roaster, the role of heat conduction into the beans from the roasting cage is evident. While the temperature in the roasting cage will begin to slowly rise after it has dropped in the first stage, convection heat will begin to play its role. For this type of machine, the roasting cage acts as a “heater cage” to ripen the coffee in the early stages of the roast. Machines that only roast coffee with convection heat often need to measure more heat to get enough heat in the early stages of the roasting process as well as compensate for the heat that heats the roasting cage.

Heat transfer and temperature variation

About 2/3 of the first time of roasting is endothermic, which means that the coffee beans will absorb energy, and the heat will be transferred from the outside to the inside of the coffee bean. The temperature variation, or “temperature difference” inside the coffee bean depends entirely on the efficiency of heat transfer. Simply put, the larger the temperature amplitude, the faster the temperature inside the coffee bean increases. The temperature range in the first stage is estimated to be around 50oC, peaking at that point or maybe slightly higher, this amplitude will gradually decrease with further roasting. In other words, after the first few minutes of roasting, the temperature in the core of the coffee beans slowly equals the surface temperature of the beans. In general, the temperature range is usually higher for fast roasting and lower for slow roasting.

Heat energy and heat transfer in coffee beans

The mechanism of heat transfer and heat transfer in a coffee roaster plays an important role in determining the quality of coffee beans after roasting. Therefore, coffee roasters often consider very carefully choosing a roaster with the right heat capacity.

In the early stage of the outermost layer of coffee beans, steam begins to evaporate forming a "steam layer" that penetrates the inner core of the bean. The cellulosic structure in the beans is cooled, trapping moisture inside the bean core. When the structure containing the steam is heated, it turns into steam, the pressure increases causing the particle structure to expand.

Heat and moisture transfer

The humidity of the roasting environment and the moisture in the coffee beans affect the heat conduction when roasting coffee. After the initial stagnation of temperature, the humidity 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 beans directly affects the roasting process. The large amount of steam that transfers heat will have three main effects on the green coffee beans.

- Increases heat conduction as moisture increases heat conduction in coffee beans

- Increasing the amount of heat, which means more heat must be used for the same grain weight.

- More steam is released from the coffee beans, which will limit the ability to transfer heat into the bean core.

The final effect is that the temperature in the wet grain will increase more slowly than in the dry seed. Therefore, roasters must adjust the temperature appropriately, by heating larger when roasting moist beans and adjusting the appropriate temperature 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 coffee, roasters need to understand the roasting mechanism. Origin of coffee that you use to master the process of operating and controlling it. Hope you enjoy roasting with Danielmachine.