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Journal of Mechanical Engineering Science and Technology (JMEST)

Abstract

Indonesia is the third-largest coffee producer globally, facing challenges including rising raw material costs, increased energy consumption, and unpredictable weather conditions. One potential solution is optimizing energy efficiency during roasting. This study investigates the effect of preheating on time and energy efficiency in heat and mass transfer by utilizing waste heat to preheat green Robusta coffee beans. The experiment was conducted on a laboratory scale using a rotating drum roaster with a double-layer drum made of stainless steel and clay, at 200 °C under atmospheric pressure. Two treatments were applied: roasting with a preheater and without, each repeated three times for data reliability. Results showed that the preheater reduced total energy consumption by 62.33%, from 39.081 kJ to 15.364 kJ, and total power usage by 50.83%, from 43.424 kJ/s to 21.339 kJ/s. LPG consumption decreased significantly by 60.65%, from 0.829 kg to 0.326 kg. The preheater also shortened roasting time from 900 s to 720 s. Coffee beans roasted with a preheater exhibited greater mass reduction (0.580 kg) compared to those roasted without (0.642 kg), indicating more efficient moisture and volatile compound removal. Beans with a preheater achieved a medium to dark roast level (Agtron #45 to #55) within 12 minutes, while those without preheating required 15 minutes to reach a similar range. These results demonstrate that preheating significantly improves both energy and time efficiency without compromising roast quality, offering a promising approach to enhancing the sustainability and performance of coffee roasting processes.

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