Abstract
In this study, considering the importance of medicinal plants in the food and pharmaceutical industries and the critical role of drying in their processing, a solar–vacuum drying system was developed and evaluated for drying medicinal plants. The system consisted of a drying chamber, cold-water chamber, vacuum pump, temperature sensors, pressure gauge, Arduino-based monitoring circuit, water pump, condenser, and control valves. To evaluate dryer performance and drying kinetics, experiments were conducted on lavender (Lavandula angustifolia) at a pressure of 30 kPa using the solar–vacuum dryer and compared with sun drying, shade drying, and oven drying. The drying kinetics results showed that the shortest and longest drying times were obtained with the solar–vacuum dryer (188 min) and shade drying (1620 min), respectively. The drying behavior in the developed dryer was modeled, and the modified Henderson and Pabis model, with an R2R^2R2 greater than 0.96 and RMSE less than 0.05, was selected as the best-fitting model. Gas chromatography–mass spectrometry (GC–MS) was used to evaluate the quality of essential oils obtained under different drying conditions. Eucalyptol and borneol were identified as the main compounds, with the highest eucalyptol content (30.9%) observed in sun-dried samples and the highest borneol content (24.6%) in shade-dried samples.
Introduction
Medicinal plants constitute a cornerstone of both pharmaceutical and food industries, serving as primary sources of bioactive compounds essential for human health and wellness. The preservation of these valuable botanical resources through appropriate processing techniques is paramount to maintaining their therapeutic efficacy and commercial viability. Among various preservation methods, drying represents the most critical post-harvest processing step, directly influencing the quality, shelf-life, and bioactive compound retention of medicinal plants.
Traditional drying methods, including sun drying and shade drying, while cost-effective, often result in prolonged processing times, potential contamination risks, and significant losses of heat-sensitive bioactive compounds. Conventional hot-air drying methods, though faster, frequently expose plant materials to high temperatures that can degrade essential oils, phenolic compounds, and other thermolabile constituents. Consequently, there exists an urgent need for innovative drying technologies that can effectively preserve the quality characteristics of medicinal plants while ensuring efficient moisture removal.
Solar drying technology has emerged as a sustainable and energy-efficient alternative, harnessing renewable solar energy while providing controlled drying conditions. When combined with vacuum technology, solar drying offers enhanced advantages including reduced processing temperatures, accelerated moisture removal, and superior preservation of heat-sensitive compounds. This integrated approach addresses the dual challenges of energy sustainability and product quality preservation in medicinal plant processing.
Material and Methods
Lavender (Lavandula) was selected as the model medicinal plant for comprehensive evaluation due to its significant commercial importance and well-documented essential oil composition. Comparative drying experiments were conducted under controlled conditions at a reduced pressure of 30 kilopascals using the developed solar-vacuum system. Parallel experiments included traditional sun drying, shade drying, and conventional oven drying to establish performance benchmarks and quality comparisons.
The experimental protocol involved careful preparation of plant materials, determination of initial moisture content, and systematic monitoring of moisture loss over time. Environmental conditions including ambient temperature and relative humidity were continuously recorded to correlate external factors with drying performance.
This comprehensive research focused on the design, implementation, and systematic evaluation of an innovative solar-vacuum drying system specifically engineered for medicinal plant processing. The primary objectives encompassed the development of an efficient drying system, characterization of drying kinetics, mathematical modeling of the drying process, and comprehensive quality assessment of the dried products.
The experimental drying system comprised several interconnected components designed for optimal performance and reliability. The main drying chamber was engineered to accommodate various plant materials while maintaining uniform temperature and pressure distributions. The vacuum pump system enabled precise pressure control, creating sub-atmospheric conditions that facilitated enhanced moisture removal at reduced temperatures. Temperature sensors strategically positioned throughout the system provided real-time monitoring of thermal conditions, while pressure gauges ensured accurate vacuum level maintenance.
 An Arduino-based control board served as the system's intelligent hub, coordinating various operational parameters and enabling automated control sequences. the condenser system efficiently removed water vapor from the vacuum stream. 
Results and Discussion   
The experimental results demonstrated remarkable performance advantages of the solar-vacuum drying system. The most significant finding was the substantial reduction in drying time achieved by the innovative system. The solar-vacuum dryer completed the drying process in just 188 minutes, representing a dramatic improvement over traditional methods. In stark contrast, shade drying required 1,620 minutes (27 hours) to achieve comparable moisture levels, highlighting the exceptional efficiency of the vacuum-assisted solar drying approach.
This impressive time reduction can be attributed to the synergistic effects of solar heating and vacuum application. The reduced pressure environment lowered the boiling point of water, facilitating rapid moisture evaporation at relatively low temperatures. Meanwhile, solar energy provided the necessary thermal input for sustaining the evaporation process without exposing plant materials to excessive heat.
The drying kinetics were successfully characterized through mathematical modeling approaches. Multiple established thin-layer drying models were evaluated for their ability to predict moisture loss patterns during the solar-vacuum drying process. The modified Henderson and Pabis model emerged as the most accurate predictor of drying behavior, demonstrating exceptional statistical performance with correlation coefficients (R²) exceeding 0.96 and root mean square error (RMSE) values below 0.05. This high-fidelity mathematical model provides valuable insights into the underlying mass transfer mechanisms and enables prediction of drying times under various operational conditions. The model's accuracy facilitates process optimization and scale-up considerations for commercial applications.
Gas chromatography-mass spectrometry (GC-MS) analysis provided comprehensive characterization of essential oil composition across different drying treatments. The analysis revealed that eucalyptol and borneol represented the predominant compounds in lavender essential oil, consistent with literature reports for this species.
Quantitative analysis showed treatment-specific variations in compound concentrations. Sun drying yielded the highest eucalyptol concentration at 30.9%, while shade drying produced the maximum borneol content at 24.6%. The solar-vacuum drying treatment achieved balanced preservation of both major compounds, suggesting effective retention of essential oil quality while providing superior drying efficiency.
Conclusions
This research successfully demonstrated the technical feasibility and performance advantages of solar-vacuum drying technology for medicinal plant processing. The developed system achieved remarkable efficiency improvements while maintaining product quality, positioning it as a promising sustainable alternative to conventional drying methods. The comprehensive mathematical modeling provides a foundation for process optimization and commercial scale-up, while the quality analysis confirms the system's ability to preserve valuable bioactive compounds.
The findings contribute significantly to the advancement of sustainable processing technologies for the medicinal plant industry, offering potential solutions for improved product quality, reduced processing costs, and enhanced environmental sustainability.
Author Contributions
N. Hajivaise: Methodology and Writing; N. Behroozi-Khazaei: Conceptualization and Supervision; H. Samimi Akhijahani: Conceptualization and Cosupervision.
Data Availability Statement
Data is "Not applicable" 
Acknowledgements
This research was conducted with the support of the University of Kurdistan as part of a Master's thesis. Therefore, we hereby express our sincere gratitude and appreciation to the University of Kurdistan for their financial support.