Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
The Effect of Preheater on Heat and Mass Transfer Efficiency Using Rotating Drum Roaster Melisa Surya Andini.
Retno Wulandari*.
Heru Suryanto Department of Mechanical and Industrial Engineering.
Universitas Negeri Malang.
Jl Semarang 5.
Malang, 65145.
Indonesia *Corresponding author: retno.
ft@um.
Article history:
Received: 17 December 2024 / Received in revised form: 7 July 2025 / Accepted: 28 July 2025
Available online 15 September 2025
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 AC 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 83%, from 43.
424 kJ/s to 21.
339 kJ/s.
LPG consumption decreased significantly by 60.
65%, from 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 .
580 k.
compared to those roasted without .
642 k.
, indicating more efficient moisture and volatile compound removal.
Beans with a preheater achieved a medium to dark roast level (Agtron #45 to #.
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.
Copyright A 2025.
Journal of Mechanical Engineering Science and Technology.
Keywords: Coffee, heat transfer, mass transfer, preheater, roasting.
Introduction Indonesia is the third-largest coffee producer in the world, with production steadily increasing over the last five years, reaching approximately 794.
8 thousand tons in 2022 .
Coffee production in Indonesia was estimated to reach 794.
8 thousand tons in 2022, an increase of approximately 1.
1% compared to the previous year (YoY/Year-on-Yea.
Indonesia recorded its highest coffee production in 2021.
The lowest coffee production occurred in 2017, at 716.
1 thousand tons, while in 2018, it reached 756 thousand tons.
Subsequently, production increased to 752.
5 thousand tons and 762.
4 thousand tons in 2019 and 2020, respectively.
In 2021, it rose again to 786.
2 thousand tons .
This upward trend highlights Indonesia's potential to strengthen its position in the global coffee market, especially through enhancing added value via downstream processing .
Downstream processing refers to activities beyond primary production, involving transformation steps from raw coffee cherries into green beans .
rimary processin.
, and then further into roasted and ground coffee .
econdary processin.
Among these, roasting represents a crucial phase that directly affects the final product's aroma, flavor profile, color, and physical DOI: 10.
17977/um016v9i22025p404 Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
Roasting begins with an initial heating stage where moisture evaporates rapidly, followed by complex chemical reactions as the process progresses.
These reactions include the Maillard reaction and caramelization, which are key contributors to the development of characteristic coffee aromas and flavors .
Throughout the roasting process, the moisture content of the coffee beans decreases significantly, generally from an initial level of 11Ae12% to as low as 1Ae3% at the end of roasting .
In a rotating drum roaster system, heat transfer occurs through a combination of three primary mechanisms: radiation .
eat transferred from the burner to the outer drum surfac.
, conduction .
eat passed directly from the drum wall to the coffee bean.
, and convection .
eat carried by hot air circulating within the drum and between the bean.
Ensuring a stable and homogeneous thermal environment is essential to achieving uniform roasting results and avoiding defects such as scorched or underdeveloped flavors .
Support for coffee downstream processing can be achieved by optimizing products and utilizing other resources .
These efforts are crucial for achieving a sustainable coffee agroindustry by implementing a closed and cyclical production model in the production process .
Despite technological advancements, one of the main challenges in coffee roasting is energy efficiency.
The roasting process is energy-intensive, with substantial heat losses typically occurring through exhaust gases.
In industrial-scale systems, exhaust gases can reach temperatures of 320Ae860 AC and represent a potential source of reusable thermal energy .
To address this, several industrial solutions have been introduced.
For instance, the PROBAT RKV green coffee preheating system utilizes exhaust heat to preheat green coffee beans using 100 AC dry air, reportedly reducing energy consumption by 20% and shortening the roasting cycle time.
In furnace combustion systems, incorporating air preheaters has been shown to improve combustion efficiency from 83% to as high as 95%, demonstrating the effectiveness of preheating strategies in thermal processes .
However, there is still a limited number of studies that systematically evaluate the application of preheating on a laboratory scale, particularly in coffee roasting using smallcapacity drum roasters.
Most existing studies focus on large-scale industrial systems or other food drying processes.
Moreover, comprehensive data on how preheating influences heat and mass transfer dynamics, roasting time, energy consumption, and ultimately, coffee bean quality, remain scarce.
From a sustainability perspective, optimizing energy use in roasting is crucial.
The increasing costs of raw materials and energy, coupled with the need to minimize environmental impact, demand innovative approaches to improve process efficiency .
Preheating, by recovering and utilizing waste heat, offers a promising solution to reduce energy requirements and improve roasting consistency and quality.
Based on these considerations, this study aims to investigate the effect of preheating on heat and mass transfer efficiency, energy consumption, roasting time, and final coffee bean characteristics using a laboratory-scale rotating drum roaster with a double-layer drum .
tainless steel and cla.
The study was conducted at an initial roasting temperature of 200AC under atmospheric pressure for 15 minutes.
It is hypothesized that the use of a preheater will enhance heat transfer efficiency, reduce energy consumption and roasting time, and improve the uniformity of moisture loss and mass reduction, ultimately leading to better roasted bean quality compared to roasting without preheating.
By filling this research gap, the study aims to provide scientific evidence supporting the potential of preheating as a practical approach to enhancing the energy efficiency and quality of coffee roasting processes, particularly for small to medium-scale applications.
The findings are expected to Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
ISSN: 2580-0817 Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
contribute valuable insights for both researchers and coffee industry practitioners seeking more sustainable and efficient roasting solutions.
II.
Material and Methods This study used Robusta green coffee beans as the main raw material.
The experiment was conducted using an experimental method with parameters summarized in Table 1.
These parameters were selected based on laboratory-scale optimal conditions to simulate an actual roasting process while ensuring controlled and repeatable results.
Table 1.
Experimental Parameters Parameters Drum surface area
Average coffee bean surface area
Roasting time
Initial roasting temperature Airflow velocity Mass of green coffee beans Thermal conductivity of clay
Drum diameter Volumetric flow rate
Value 094285 m2
000081 m2
200 AC
1 m/s 75 Kg 7218 W/mK The roasting experiments were carried out using a rotating drum roaster designed with a double-layer drum: an outer layer made of stainless steel with a thickness of 3 mm, and an inner layer composed of clay with a thickness of 10 mm, as shown in Figure 1.
This configuration aims to improve thermal stability and uniform heat distribution inside the Fig.
Experimental research facility Two different treatments were applied: one using a preheater to preheat the green beans and the other without preheating, serving as a control to validate the effect of the preheater.
The preheating system utilized waste hot air to increase the initial temperature of the beans.
Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
thus potentially reducing energy consumption and roasting time.
The schematic diagram illustrating the heat and mass transfer processes is presented in Figure 2.
The experimental procedure started by calibrating all thermal sensors to ensure accurate Sensor calibration was performed using standard reference points at the melting point of ice .
AC) and the boiling point of water .
AC), and further validated with a certified digital thermometer to guarantee precision.
A total of 0.
75 kg of Robusta green beans was prepared for each trial.
The roasting machine was preheated until the combustion chamber reached 200 AC.
For the control treatment .
ithout preheatin.
, beans with an initial moisture content of 11.
4% were directly roasted for 15 minutes.
Meanwhile, for preheating treatment, beans initially had a moisture content of 13.
these were first subjected to preheating to reduce their moisture content to 11.
4% and raise their temperature to approximately 80 AC before roasting under the same conditions.
This standardization of initial moisture content aimed to eliminate potential bias and ensure that differences in results were solely attributed to the preheating effect.
Fig.
Heat and mass transfer scheme The experiments were conducted with three replications for each treatment to ensure reliability and consistency.
Controlled variables included the mass of coffee beans .
75 k.
, initial roasting temperature .
AC), roasting duration .
, and airflow velocity .
1 m/.
Energy efficiency was analyzed by comparing input and output energy using the heat energy equation, where m represents the mass of the beans, c is the specific heat capacity, and iT is the change in temperature.
Heat transfer is analyzed in the roasting process from each cycle, calculated with several equations below:
Heat transfer from the hot gas to the coffee bean surface In the initial phase of the roasting process, heat is transferred from the surrounding hot air or combustion gas to the surface of the coffee beans.
This external heat transfer plays a critical role in raising the surface temperature of the beans.
The amount of heat transferred is calculated using Eq.
This subsection focuses on external convection heat transfer from the hot air to the beans' surface, which serves as the foundation for subsequent internal heating and chemical transformations in the beans.
ycENyciyca = yayci y yaycyyci .
cNyci,ycn Oe ycNyci,ycu ) .
Heat generated from exothermic reactions during roasting Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
ISSN: 2580-0817 Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
During roasting, several exothermic chemical reactions occur within the coffee beans, including pyrolysis.
Maillard reactions, and degradation of organic compounds.
These reactions release heat internally.
The total heat generated is calculated using Eq.
This subsection explains that the beans themselves act as internal heat sources due to chemical activity, which accelerates the roasting process and influences the development of flavor, color, and aroma.
ycENyc = ycEyc y ycoyca.
Heat loss due to moisture evaporation in coffee beans As coffee beans are heated, the moisture contained within the beans evaporates.
This evaporation consumes a significant amount of heat energy known as the latent heat of The heat required for this process is calculated using Eq.
This subsection discusses the endothermic nature of moisture loss, which decreases the available heat for internal reactions and may affect the overall energy efficiency of the roasting process.
yccycu ycENyceyc = OIyayc (Oe ) ycoyca.
yccyc To evaluate the final product quality, an Agtron spectrometer was used to measure roast color level, while mass loss was recorded to assess moisture reduction efficiency.
Additionally, gas fuel (LPG) was used as the main heat source, and its consumption was monitored to support the energy analysis.
Limitations of this study include its laboratoryscale setup, which may differ from large-scale industrial roasting systems in terms of thermal dynamics and operational complexities.
Furthermore, the results are specific to Robusta beans with physical and chemical characteristics and might vary when applied to different coffee varieties or processing conditions.
Results and Discussions The temperature variation measurements of coffee beans and the roasting chamber over a roasting duration of 15 minutes .
at an initial heating temperature of 200 AC were analyzed to compare the effects of using a preheater versus no preheater.
The preheater significantly influenced the temperature change pattern of the beans and the roasting chamber during the roasting process.
Figure 3 presents the roasting profile of coffee beans over time, with roast levels determined using an Agtron spectrometer .
Beans roasted with a preheater achieved a dark to very dark roast (Agtron #35 to #.
within 15 minutes, while those without preheating only reached a medium to moderately dark roast (Agtron #55 to #.
Furthermore, the preheater reduced roasting time by approximately 2Ae3 minutes to achieve a medium roast, indicating that preheating significantly accelerates the roasting process and improves time efficiency without compromising roast quality.
Fig.
Coffee bean roasting profile preheater and without preheater Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
Table 2 provides detailed experimental data comparing the temperature evolution of coffee beans relative to the roasting chamber temperature.
The preheated beans consistently maintained higher temperatures, reaching 178 AC at 900 seconds, while beans without preheating only reached 172 AC.
During the initial phase .
Ae150 second.
, both methods showed a temperature drop due to heat absorption by the beans.
In the stabilization phase .
Ae450 second.
, preheated beans maintained slightly higher temperatures.
In the final heating phase .
Ae900 second.
, the preheater condition exhibited a clear advantage in temperature stability and heat absorption, demonstrating enhanced heating efficiency and more uniform roasting.
Table 2.
Experimental data on coffee bean temperature and roasting chamber temperature Time .
Bean temperature (EE) Preheater Without preheater Roasting chamber temperature (EE) Preheater Without preheater The experimental findings clearly indicate that the application of a preheater significantly improves the thermal efficiency of the coffee bean roasting process (Figure .
Specifically, without a preheater, the average thermal energy required to roast coffee beans 81 kJ (Figure 4.
, amounting to a total of 40.
002 kJ/s over 900 seconds.
In contrast, the treatment with a preheater required only an average of 1.
38 kJ, totaling 17.
93 kJ/s within 720 seconds.
The data suggests a considerable reduction in both average and total heat energy, which corresponds to faster and more uniform heat transfer.
This outcome aligns with the theory of convective and radiative heat transfer, where preheating enhances the temperature gradient between the air and the bean surface, thereby accelerating the heat penetration process.
Such improvements support the theory of convective heat transfer efficiency, as discussed by .
, who highlighted that preheating air in food drying systems can greatly increase overall heat transfer coefficients and system effectiveness.
Preheated systems exploit higher initial air temperatures, increasing the temperature gradient between the air and the food surface, thus speeding up internal heat absorption and reducing required external energy.
Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
ISSN: 2580-0817 Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
The exothermic heat released from the beans (Figure 4.
was greater in the preheated treatment .
17 J.
65 kJ/s in 720 second.
compared to the non-preheated condition .
12 J.
289 kJ/s in 900 second.
This suggests that the preheating treatment enhanced the internal chemical reactions, such as Maillard reactions and caramelization, critical for achieving the medium to dark roast profile (Agrton #45Ae.
Preheated processes improve exergy efficiency and boost energy self-reliance within the material by promoting internal heat generation.
Preheating or preheated fluid use in thermal systems can increase energy efficiency by 8.
66% and exergy efficiency by 7.
supporting the results of your coffee roasting system .
The moisture evaporation energy (Figure 4C) was higher with preheating .
verage 4 J.
12 kJ/s in 720 second.
, compared to 48.
3 J and 7.
72 J/s in 900 seconds without a preheater.
This shows faster and more effective dehydration, contributing to reduced roasting time and enhanced product uniformity.
Similar conclusions were drawn by .
, who analyzed energy and exergy performance of hybrid solar dryers and emphasized that preheating can significantly improve thermal efficiency and reduce total energy consumption in food processing.
Preheated green bean enhances the vapor pressure differential, increasing moisture migration and accelerating drying or roasting processes, which aligns directly with your results.
Furthermore, the overall energy savings .
33%),
LPG consumption reduction .
65%), and power savings .
83%) in your study illustrate strong economic and environmental benefits, reinforcing preheating as a crucial advancement in sustainable food processing technologies.
Preheater Without Preheater Preheater Without Preheater Time .
Time .
Preheater Without Preheater Time .
Fig.
Heat transfer: .
Hot gas to coffee bean, .
Exothermic, .
Moisture evaporation Preheating the green bean intake slightly enhances mechanical and thermal efficiency due to convective heat transfer .
The roasting process uses heat energy generated by Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
gases from combustion within the coffee roasting machine, which reaches high Before these gases are released into the environment, their heat is recovered to preheat the feed gas entering the preheater.
This process of using heat in the exhaust gas reduces the energy required to heat the feed gas separately, thereby increasing energy This experiment gives a result: coffee roasting equipped with a preheater can significantly reduce energy consumption 170.
8 kJ by up to 38% through efficient heat Chantasiriwan .
Investigate the air preheater is a key factor in enhancing the net efficiency of a power plant beyond its current limit.
This study repurposes waste energy by incorporating it into a preheater to reduce the moisture content of coffee beans by increasing their temperature, thus shortening the roasting process or reducing the turning point duration.
This clearly indicates that operating a solar dryer system in mixed mode with forced convection and the assistance of a preheater or backup heater can significantly enhance the efficiency of drying processes and extend food preservation .
These studies demonstrate that preheating can enhance heat transfer efficiency across various applications, although the optimal conditions vary.
The measurement results of coffee bean mass changes during a 15-minute roasting process with an initial heating temperature of 200EE is shown in Table 3.
Table 3.
Coffee bean mass measurement before and after roasting Test Preheater Without preheater Coffee bean mass (K.
After roasting Before roasting Table 3 shows changes in coffee bean mass before and after roasting.
Beans roasted with the preheater showed greater mass loss .
inal mass of 0.
580 k.
than those roasted without preheating .
642 k.
, reflecting more effective moisture reduction.
Table 4 and Figure 5 further detail moisture content dynamics during roasting.
At 300 seconds, beans with the preheater had a moisture content of 6%, while without preheating, it remained around 7%.
By 900 seconds, preheated beans reached 1.
6%, compared to 3.
1% without These findings demonstrate that preheating enhances moisture evaporation by improving heat transfer rates, thus supporting higher drying efficiency.
Figure 5 illustrates the reduction trend of coffee bean moisture content throughout the roasting process for both treatments.
At the beginning .
, both treatments started with the same initial moisture content of 11.
Over time, a continuous decrease in moisture content was observed.
however, the beans treated with a preheater exhibited a significantly faster rate of moisture reduction.
At approximately 300 seconds, the moisture content in the preheater-treated beans had already decreased to around 6.
9%, while beans without preheating remained higher at about 7.
By the end of the roasting process .
, the moisture content of beans with preheating dropped to 1.
6%, whereas the beans without preheating still had a moisture content of approximately 3.
These findings clearly demonstrate that the use of a preheater effectively accelerates moisture evaporation in coffee beans, thus enhancing the drying efficiency during roasting.
The faster reduction in moisture content can be attributed to the improved heat transfer efficiency provided by the preheater system.
From the perspective of mass and heat transfer theory, the presence of a higher initial bean temperature and a steeper thermal gradient increases the evaporation rate of internal water .
, .
Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
ISSN: 2580-0817 Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
Table 4.
Coffee bean moisture content during roasting Coffee bean moisture content (%) Preheater Without preheater Time .
Moisture Content (%) Without preheater Preheater Time .
Fig.
Comparison of moisture content during the roasting process with and without a Moreover, the significant decrease in moisture content using a preheater indicates a more energy-efficient roasting process, as less energy is wasted in overcoming latent heat This aligns with previous studies indicating that preheating can optimize drying and roasting processes across various materials by improving thermal penetration and reducing total energy consumption.
Ultimately, the application of preheating not only accelerates the roasting process but also supports more sustainable and cost-effective production practices.
Andini et al.
(The Effect of Preheater in Heat and Mass Transfer Efficiency Using Rotating Drum Roaste.
Journal of Mechanical Engineering Science and Technology Vol.
No.
November 2025, pp.
ISSN 2580-0817
IV.
Conclusions The application of a preheater in the coffee roasting process significantly enhanced the efficiency of heat transfer, resulting in marked reductions in energy consumption and fuel In this study, coffee beans roasted with a preheater required an average heat energy of 38 kJ, with a total of 17.
93 kJ/s over 720 seconds, compared to 2.
81 kJ and a total of 002 kJ/s over 900 seconds without a preheater.
The total power required was also significantly reduced: from 43.
424 kJ/s without a preheater .
equiring 15 minute.
to 339 kJ/s with a preheater .
equiring 12 minute.
, leading to a total power saving of Furthermore, total energy consumption decreased by 62.
33%, translating to substantial LPG fuel savings of 60.
65% .
educing from 0.
829 kg to 0.
326 k.
These quantitative improvements demonstrate that preheating not only shortens roasting time but also lowers operational costs and environmental impact.
The increased exothermic heat released by the coffee beans with a preheater .
17 J and total 2.
65 kJ/.
compared to the process without a preheater .
12 J and total 2.
289 kJ/.
further supports the effectiveness of preheating in promoting internal chemical reactions, such as Maillard reactions and caramelization, which are crucial for developing complex flavors and achieving medium-to-dark roast profiles (Agrton #45Ae.
The higher evaporation energy with preheating .
4 J and total 1.
12 kJ/.
also confirms better moisture removal, contributing to a more uniform and stable final product quality.
These results indicate that the use of a preheater optimizes the entire heat transfer mechanism, covering radiation, conduction, and convection, leading to improved thermal effectiveness and final bean However, it is important to acknowledge the limitations of this study.
The experiments were conducted at a laboratory scale using a specific batch of Robusta beans, which may not completely represent larger-scale industrial conditions.
Additionally, this research did not incorporate sensory analysis to evaluate aroma, flavor, and consumer-related organoleptic properties, which are crucial for final product acceptance.
Thus, future studies should validate these findings at industrial scales and include comprehensive sensory evaluations.
Such research will help ensure that preheater technology can be effectively implemented in large-scale coffee production, supporting more energy-efficient, cost-effective, and highquality roasting processes in practice.
Acknowledgment Thank you to the State University of Malang, facilitating the expedited running of this References