Commercial Coffee Roasting Process
Ever wonder what happens after you press the charge button on your roaster? Yes, the beans get roasted, but what steps does the roaster go through to get them to your desired color, flavor profile, and moisture content? While the specific process varies depending on the roaster type and options this is a general overview of the entire Industrial Coffee Roasting Process.
Pressing the charge button
The Roaster has been preheated, and there is a batch in the Charge (Feed) hopper. When you press the charge button the roaster checks a few things before allowing the beans to be dropped (Charged) into the drum. These processes all happen simultaneously.
The Roaster first checks it has a place to discharge the batch when complete. The roaster then begins to bring the temperature up to Charge Temperature. If you have an exhaust abatement system, the roaster may also check to ensure it is running and at temperature before charging. Once all the parameters are met, the gate opens, and the beans begin their journey.
Roasting: Curve Tracking and Recipe Control
Curve Tracking– This process allows for a much tighter control of the roasting process and can help ensure consistent quality of your roasts. Before using Curve Tracking you must have developed a recipe that meets the quality requirements for that “Recipe”. Curve Tracking uses a record (Master Batch) of the actual temperatures and settings of the roaster throughout an initial (Master Batch) roast to control the roast process. This allows for consistent roasts even during different ambient conditions.
During the roasting process it is important to keep a negative pressure inside the drum, this helps to ensure a good flow of air is continually moving through the beans in the roaster (as seen below), and any smoke and CO is exhausted out of the building. This is achieved by the speed of the roast fan, and control of various flaps. If you have ever noticed a roaster that seems to release a lot of smoke into the building, that is a sign that something could be causing the pressure inside the drum to be positive. This is what allows the smoke to escape from the roaster into the building.
Recipe Control – This process simply follows the recipe you have selected. The Roaster monitors the current product (Bean) temperature and adjusts various settings per the recipe. This is changed per “stage” as each stage is triggered by the temperature set in the recipe. These settings include various settings such as burner percentage, fan speed, etc. The roaster continues through the stages until it reaches to the final bean temperature.
Finishing the roast: Quenching, Drying, Discharging, and Cooling.
Quenching– When the bean temperature has reached the final temperature set in the recipe or the curve profile, the roaster prepares to discharge the beans. The beans are very hot and will continue to roast even if they are removed from the heat. The roasting process also removes quite a bit of moisture from the beans, quenching helps to not only cool the beans, but add some moisture back to them. You may have an option to set a “Quench Delay”, this is a delay after the beans reach their final temperature until the quenching process starts. Once this time has elapsed, the roaster will quench the beans with a specified amount of water. This value is controlled via the recipe. Depending on roaster type, you may have the ability to do a “First Quench”, or “Pre-Quench” This limits the amount of water introduced to the beans at first to reduce the amount of steam that is created. Once the time or quantity of First Quench has been reached, it will then continue to quench until the desired total quench amount has been released. If you experience water pressure issues at your facility, you may get a quench fault (Roaster Quench Overtime Fault). The roaster monitors the total time the quench process takes, if it exceeds a calculated time, this fault will occur. When the quenching occurs, steam is created when the cold water hits the hot beans and drum, this causes the drum pressure to increase so it needs to be able to pull that extra volume out and release it. Depending on the roaster, it may open the exhaust flap or increase the roast fan speed to help remove the excess pressure.
Drying– Once the quench process is complete, the roaster is in its Drying stage. This process is as simple as it sounds. The roaster does not release the beans until the Drying Time has elapsed. This allows the beans to dry enough so they are not wet when released into the cooler.
Discharging and Cooling – Just before the drying process ends, the cooler/destoning fan along with the stirrer will start. This closes a flap in the duct work causing air to be pulled up, or down through the cooling tray. At this time, the roaster will open the discharge door to release the beans into the cooling tray. The stirrer motor helps move the beans from the roaster door. The cooling air is often sent through an abatement system for a period after cooling begins, then switches to exhaust into the atmosphere when the beans have stopped smoking. This begins the cooling process; the stirrer moves the coffee away from the discharge door and distributes the coffee evenly over the cooling tray. In addition, the stirrer continually moves the beans around, so they all get cooled evenly. Some roasters can adjust the amount of time the stirrer stays on during the cooling process (also set through a parameter). This helps to reduce breakage for darker roast profiles. How long the total cooling process takes is adjustable to ensure the coffee is cooled enough for destoning or other transport.
Destoning – This process is relatively simple but can cause increased bean breakage if not setup correctly. This step switches an air flap causing the cooling/destoning fan to draw a vacuum into the destoner bin. Dual Fan systems are also which uses separate fans for the cooling and destoning process. Once the destoning system has started, the cooling tray discharge opens allowing the beans to drop into the destoner boot, the stirrer is designed to push the beans to the chute to clean it out completely. The bottom of the destoner boot is what allows the air to be pulled up into the destoner creating a vacuum. The beans, which are now much lighter than they were when they are green get sucked up into the destoner hopper. Because the beans are lighter any stones that were in the mix will fall to the bottom of the boot into a removable tray so they can be discarded. The speed at which the beans are pulled into the destoner hopper depends on how much vacuum is created. It is important that this be checked as too high of speed will cause the beans to break when entering the destoner. Some roasters use VFD (variable frequency drive), and the fan speed is adjustable, other systems have manual gates and flaps on the destoner to allow more fresh air in thereby reducing the vacuum. Finally, once the destoning time has completed, the cooling tray flap is closed and the destoning fan has stopped, the last step in the industrial coffee roasting process is to get an accurate weight of the roasted beans. This weight is added to the batch report along with all the other data from the roast and stored into the historical batch files, and or transferred along with the batch to a Handling system.
While this is a simple overview of the industrial coffee roasting process, it helps to understand what is going on after you press the Charge Button. The roaster has several more tasks it handles while roasting which were not discussed, the most important of which is ensuring both a good roast, but also a safe roast. Monitoring temperatures along with gas and water pressure, along with the monitoring of CO levels, chaff removal systems, bean handling interaction, inline bean moisture analyzers, and automatic samplers are all essential for both roast quality and safety.