Abstract
Nowadays, the application of vacuum infrared dryers has expanded considerably due to the high quality of the final product. This fact is noteworthy despite the challenges associated with their high energy consumption and the resulting environmental issues. The application of new approaches in the operation of this equipment can substantially contribute to addressing existing problems. In this regard, the present study introduces a novel innovative intermittent method combined with the specific application of microwave waves in a vacuum infrared dryer. The quality attributes of the final product along with energy consumption were evaluated, and ultimately the optimal operating point was proposed. For this purpose, 4-mm-thick slices of red beetroot (as a case study) were investigated during the drying process. The independent variables included temperatures of 45, 60, and 75 °C; vacuum pressures of 20, 50, and 80 kPa; applied microwave powers of 150, 300, and 450 W; and microwave application times of 0, 10, and 20 s. Response surface methodology and a central composite design were employed to analyze the obtained data and optimize the process. Physical, mechanical, bio-chemical, and energy indices were examined as dependent variables. The optimal drying conditions, determined using response surface methodology, were a temperature of 50 °C, a vacuum pressure of 20 kPa, a microwave power of 339 W, and a microwave application time of 10 s. Under these optimal conditions, antioxidant activity of 96%, vitamin C content of 6.30 mg ml⁻¹, color changes ΔL*, Δa*, and Δb* of 18.15, 5.24, and 2.62, respectively, a shrinkage percentage of 40.75%, and a specific energy consumption of 1585.45 MJ kg⁻¹ water were obtained. Achieving superior results, except for shrinkage percentage, compared with the conventional drying method, in addition to no increase in capital investment cost, indicates the success of this approach.
EXTENDED ABSTRACT
Introduction
The root part of red beetroot (Beta vulgaris) is a widely valued vegetable around the globe, recognized for its rich nutritional composition. It serves as an excellent source of essential nutrients, including dietary fiber, natural pigments such as betalains, various vitamins (A, B1, B2, B6, and C), antioxidants, and carbohydrates. The continuous growth of the world population consequences in the increasing demand for nutritious foods. Therefore, the dehydration (especially vacuum drying method) of fruits and vegetables has emerged as an essential engineering approach within the food industry. This process aims to extend product shelf life, ensure year-round market availability, minimize packaging and transportation costs, and improve the overall quality of the final product. However, drying operations are among the most energy-intensive processes in food production. Consequently, considerable research efforts have been directed toward minimizing energy consumption and reducing production costs while maintaining product quality. These efforts include the development of innovative drying technologies and the optimization of existing systems. In this study, the feasibility of a novel drying approach based on periodic temperature strategy under vacuum conditions was investigated. This tactic was considered  as a potential alternative technique for developing advanced drying methods that can substantially decrease processing time and associated costs for the manufactured apparatus, while the quality attributes of the final product were improved.
Material and Methods 
In this study, the performance of an infrared–vacuum dryer in normal conditions and a newly developed drying technique involving periodic temperature conditions was examined. The novel approach was further supported by microwave application. The red beetroot samples of 60×40×4 mm in dimension were exposed under periodically applied temperatures (45, 60, and 75 °C), vacuum pressure (20, 50, and 80 kPa), microwave power (150, 300, and 450 W), and microwave exposure times (0, 10, and 20 s). The study investigated the influence of drying methods on response variables, including ΔL*, Δa*, Δb*, shrinkage, vitamin C, antioxidant activity, Deff, and SEC. In the periodic drying method, a fixed temperature was maintained during each run, while microwave power was applied every 10 minutes until the samples reached the target moisture content. Experimental design, data analysis, and process optimization were carried out using the software of Design-Expert 13.0. 
Results and Discussion  
The lack of fit (P-value) for all studied variables in periodic technique method was found to be non-significant between 0.09 to 0.94, indicating that the developed RSM models exhibited a strong capability in predicting the variations of the response variables. The coefficient of determination (R²) values for all proposed models ranged between 95.19 and 99.70, confirming a high degree of correlation between experimental and predicted data. Furthermore, the low coefficients of variation (C.V. < 6.93) for all models demonstrate good reproducibility and reliability of the experiments. The optimal drying conditions were determined to be a temperature of 50 °C, a vacuum pressure of 20 kPa, a microwave power level of 339 W, and a microwave application time of 10 s. The selected desirability value obtained under the optimized drying conditions was 0.816, indicating a good level of optimization. The found results of vitamin C content compared to the fresh sample value of 7.53 mg/ml, indicates relatively good retention of this valuable compound. To compare the response variable values at the optimized points, an additional experiment was conducted using the vacuum dryer under normal drying conditions at 50 °C and 20 kPa, with three replications. The obtained results for vitamin C content (6.09 and 6.30 mg/ml), antioxidant activity (85% and 96%), color change parameters ΔL* (21.49 and 18.15), Δa* (6.74 and 5.24), Δb* (3.38 and 2.62), shrinkage percentage (37.57% and 40.75%), effective moisture diffusivity (2.43 × 10⁻¹⁰ and 3.43 × 10⁻¹⁰ m²/s), and specific energy consumption (2138.48 and 1585.45 MJ/kg water) were obtained for the normal and periodic drying methods, respectively. The comparative analysis of these response variables showed changes of 34.48%, 12.94%, 15.54%, 22.26%, 22.48%, 8.46%, 41.15%, and 25.86%, respectively. Except for shrinkage percentage with 8.46% value, all parameters showed improvement under the periodic drying method. This enhancement can be attributed to the synergistic advantageous effect of microwave and vacuum drying, which combines the efficient energy transfer of microwaves with the gentle drying conditions of vacuum processing. Whereas, conventional vacuum drying is relatively slow due to the absence of convective heat transfer, the incorporation of microwave energy significantly reduces the drying time while simultaneously preserving product quality. 
Conclusion
The periodic drying technique in the infrared–vacuum drying process, when combined with specific microwave pre-treatment, demonstrated remarkable superiority over normal vacuum drying method. This periodic technique achieved a great improvement in effective moisture diffusivity and reduction in specific energy consumption, while providing excellent preservation of color parameters (closer to the fresh conditions) and bio–chemical quality attributes. These findings confirm the significant advantages of the periodic infrared–vacuum drying approach assisted by microwave application over the conventional vacuum drying method. The results of present study revealed a great achievement due to simultaneous enhancement in both product properties (physical, biochemical and thermal) and drying device efficiency (SEC). Among these attributes, vitamin C content is particularly significant, as its preservation generally indicates that other quality characteristics of the sample are also maintained. However, the results are specific to red beetroot, and further investigations are required to validate the applicability of this technique to other agricultural products.
Author Contributions
Nazat Hasan Jeejo: PhD student
Reza Amiri Chayjan: Supervisor
Hassan Sarikhani: Advisor
Jafar Amiri Parian: Advisor
Data Availability Statement
All data and results have been presented within the text of the article.
Ethical Considerations
The authors have adhered to ethical principles in conducting and publishing this scientific work, and this has been confirmed by all of them.
Conflict of Interest
There are no contributions of interest in this article, and this has been confirmed by all authors.