In the coffee processing chain, roasting represents an essential step that can either bring out the aromatic characteristics of the coffee or mask them with burnt aromas and acidic undertones. The quality of roasting, along with the quality of the raw coffee, significantly impacts the market price of the final product. Green coffee beans undergo numerous physical and chemical changes during the roasting process. The most significant changes include color transformation, doubling in size, halving in density, gaining sweetness, increasing acidity levels, and the development of up to 800 new aromatic compounds.
The roasted coffee's characteristics are strongly influenced by the process parameters during roasting, specifically the time-temperature conditions inside the coffee bean as a function of heat transfer. Heat is transferred to the coffee beans during roasting through conduction, convection, and radiation mechanisms. Among these, convection is the most effective and suitable for even roasting. Roasters based on fluidized bed technology achieve almost purely convective heat transfer, enabling fast roasting and resulting in coffee with low density and high yield. In comparison, traditional roasters with horizontally rotating drums, which primarily operate through conductive heat transfer, are slower, produce a product with a higher defect percentage due to uneven roasting, and require more workspace. Faster roasting retains more soluble solids, a lower proportion of degraded chlorogenic acids, preserves more volatile substances, and acquires less burnt taste compared to coffee roasted using the slower method. Therefore, the faster roasting method produces a more delicious and aromatic final product.
The goal of this thesis was to create a coffee roaster based on the technology of swirling hot air. A high flow of hot air suspends the coffee beans above a perforated bottom in the roasting chamber, rotating and convectively heating them. Chaff separated from the beans during the process, along with other lighter debris, progresses with the outgoing air into a cyclone separator and collects in a dedicated container. After achieving the desired roasting level, the coffee beans need to be rapidly cooled to prevent further unwanted roasting. This is accomplished by blowing room-temperature air into the roasting chamber. The control of the device is handled by an Arduino Uno microcontroller equipped with the TC4+ expansion shield. A thermocouple for measuring the temperature of the incoming air to the roasting chamber and the temperature of the coffee beans are connected to the microcontroller via a digital-analog converter with cold junction compensation located on the expansion shield. A heating element, connected via a semiconductor relay, is responsible for heating the air, and a high airflow is facilitated by using an air pump. The personal computer and Artisan software automate the roasting process with PID regulation, monitor the real-time temperature graph, and provide useful information to the roaster for process improvement.
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