The principle of the direct-fire coffee roaster is to directly transfer heat from the flame to the coffee beans. The following is a detailed explanation of its principle:
Heating method
The boiler surface of the direct-fire roaster is designed with perforations. The holes have a certain size and interval, allowing the furnace fire to directly reach the surface of the beans. Its heat sources are diverse. Generally speaking, the heat sources of direct fire include gas, electric heating tubes, infrared rays, far-infrared rays, charcoal fire, etc. By stirring the drum, the coffee beans are heated evenly, but the heat is hard to control, and it is easy to cause the coffee beans to be burnt or undercooked.
Heat transfer and control
Direct fire baking regulates heat through exhaust valves, which is a great test of the baker’s skills. Because the entire boiler is designed with holes, once the furnace fire provides the first heat at the bottom of the boiler, the heat energy will immediately dissipate from other holes, resulting in insufficient heat. However, if the temperature is raised to replenish heat, when the heat exceeds the tolerance of the amount of green beans put in, they will be burnt. Therefore, it is necessary to carefully adjust the heat according to the amount of green beans added. It is very important to confirm the time when the “yellow spot” appears and the time of the first burst.
Flavor expression
The process of direct fire roasting is to transfer heat directly to the green beans through the furnace fire, that is, the heat energy gradually penetrates from the external tissue of the green beans to the internal tissue. If roasted properly, direct roasting can bring out the flavor and aroma of coffee, and the effect is better than hot air or semi-hot air roasting, and the coffee taste is more mellow and fragrant. For instance, if you drop the beans 20 seconds before the end of the first burst, you can clearly smell the floral and citrus scents during the cooling stage. When you bite into the beans themselves, the aroma is revealed, and the aftertaste has the unique cocoa flavor of mocha beans, which is extremely charming. Let it stand for about 30 days before brewing. The aroma will be more expressed, and the floral fragrance will be more diverse and unrestrained. Cold shrinkage treatment can achieve a brighter flavor performance. From the complete end of the first roasting to the early stage of the second roasting, the direct-fired coffee beans have a balanced flavor, with a sweetness and aroma like ripe fruits. The caramel sweetness and honey sweetness that are displayed during the coffee bean pyrolysis process are stable. The roasting degree at this stage is relatively easy for the brewer to bring out the original flavor of the coffee, and it is extremely suitable for those who are not obsessed with the flavors of very light or very dark roasting.