Orange peel, a major byproduct of the juice industry, presents a valuable opportunity for upcycling due to its rich content of bioactive compounds like fibers, vitamins, and polyphenols. However, its high moisture content makes it susceptible to spoilage, necessitating stabilization methods. Hot air drying is a common technique, but its high energy consumption and potential for degrading bioactive compounds necessitate improvements. This research explores the potential of pre-treatments to enhance the efficiency and quality of orange peel drying. The high energy demands of conventional hot air drying are a significant drawback, often resulting in undesirable structural changes and degradation of valuable bioactive compounds. Therefore, pre-treatments offer a promising approach to mitigate these issues. Various pretreatments have been explored, including conventional methods like osmotic treatments, blanching, and freeze-thawing, as well as advanced technologies such as cold plasma, ultrasound, electro-infrared treatments, and pulsed electric fields (PEF). These pretreatments aim to modify the food structure, facilitating moisture transport during subsequent drying. Freeze-thawing, for instance, leverages ice crystal formation to induce cell wall damage, while PEF technology utilizes high-voltage electric pulses to create pores in cell membranes, enhancing mass transfer. Another strategy involves enhancing the drying process itself, using airborne ultrasound (US) to reduce drying times through mechanical stress and improved moisture transport. This study focuses on investigating the combined effects of freeze-thaw and PEF pretreatments with conventional and ultrasound-assisted drying to optimize the drying process of orange peels while preserving their structural integrity and bioactive compounds. Specifically, it addresses the need for a more efficient and quality-preserving method for drying orange peels, a significant byproduct of the juice industry with significant potential for upcycling.

Numerous studies have examined the effects of various pretreatments on the drying kinetics and quality of different food products. Freeze-thaw pretreatments have shown varying results, with some studies reporting reductions in drying time and others showing no significant effect. The impact depends on factors like freezing rate, ice crystal size and distribution, and the specific food matrix. Similarly, the application of PEF technology has demonstrated both positive and negative effects on drying kinetics. Electropermeabilization, resulting from PEF treatment, can increase mass transfer; however, excessive treatment intensity can damage cell structure, hindering moisture transport. The integration of airborne ultrasound during drying has proven effective in shortening drying times across various food products. Ultrasound generates mechanical stress and micro-stirring at solid-gas interfaces, promoting moisture transport. However, the effectiveness of ultrasound is influenced by the product's internal structure. Combining pretreatments with ultrasound-assisted drying has been investigated in several studies. However, there is a limited understanding of the combined effect of freeze-thaw and PEF pretreatments on the kinetics and microstructure of ultrasound-assisted drying of orange peels. This gap in the literature highlights the need for this present study.

Valencia Late variety oranges were obtained from a local market, peeled, and sliced into 5.0 × 2.5 × 0.3 cm pieces. The initial moisture content was 2.9 ± 0.2 kg water/kg d.m. Two types of freeze-thaw pretreatments were used: fast freezing (−35.0 ± 0.3 °C for 2 h) and slow freezing (−18.0 ± 0.7 °C for 24 h). PEF pretreatments were performed using a PEF system at 1.25 kV/cm, 10 Hz frequency, and 25 μs pulse width. Two intensity levels were tested: low intensity (8 pulses, 0.33 kJ/kg) and high intensity (24 pulses, 0.98 kJ/kg). Unpretreated samples served as controls. Drying was conducted in an ultrasound-assisted convective dryer at 50 °C and 1 m/s. Two drying methods were employed: conventional air drying (AIR) and ultrasound-assisted drying (US). The ultrasound system operated at 21.8 ± 0.4 kHz with 50 W constant power. Drying kinetics were modeled using a diffusion-based model incorporating both internal and external resistance to moisture transport. The effective diffusivity (Deff) and mass transfer coefficient (k) were determined using the SIMPLEX method in Matlab. The percentage of variance explained (%Var) by the model was calculated. Microstructure analysis was performed using light microscopy after sample preparation involving fixation, dehydration, and embedding in epoxy resin. Thin sections were stained, and images were captured at 20x and 60x magnification.

Conventional drying of orange peels showed that slow freezing slightly accelerated drying, while fast freezing slowed it down. Low-intensity PEF pretreatment slightly, but significantly, shortened drying time compared to the unpretreated samples. Ultrasound-assisted drying significantly shortened drying time regardless of pretreatment. The combination of pretreatments and ultrasound-assisted drying yielded the most significant reductions in drying time, with the most significant improvements observed in the low intensity PEF combined with ultrasonic drying. Modeling of drying kinetics revealed that conventional drying of pretreated samples resulted in lower effective diffusivity (Deff) values, indicating increased internal resistance to mass transfer. However, the mass transfer coefficient (k) was higher in pretreated samples, except for fast-frozen samples. Ultrasound-assisted drying significantly increased both Deff and k, indicating reduced internal and external resistances. Slow freezing and low-intensity PEF pretreatments before ultrasound-assisted drying resulted in the highest Deff and k values. Microstructural analysis showed that conventional drying caused significant structural damage to the orange peel. Freeze-thaw and PEF pretreatments resulted in degraded structures with large intercellular spaces but better-defined cell walls compared to unpretreated samples. In contrast, ultrasound-assisted drying significantly preserved the original structure of the orange peel, maintaining distinct flavedo and albedo tissues. The best preservation was observed in unpretreated samples dried with ultrasound assistance. However, the low intensity PEF pretreatment samples showed significant improvements over the control sample in both structure preservation and drying time. The combined use of low intensity PEF and ultrasound-assisted drying resulted in the shortest drying times and maintained a relatively well-preserved structure compared to the other treatments.

The results demonstrate that the combined application of pretreatments and ultrasound-assisted drying offers a significant improvement over conventional drying for orange peels. The impact of freeze-thawing is linked to the ice crystal formation: slow freezing causes larger crystals that break cell walls, potentially accelerating drying, while fast freezing leads to smaller crystals causing less damage and slowing drying. PEF pretreatment modifies cell membranes, influencing mass transfer, but the effect is complex and depends on treatment intensity. The optimal PEF intensity appears to be moderate, as excessively high intensity may cause structural damage that hinders ultrasound's effectiveness. Ultrasound's positive effects stem from its ability to reduce both internal and external mass transfer resistances through mechanical effects, improving moisture transport. The positive effects of the pretreatments are greatly enhanced through the addition of ultrasound-assisted drying which helps to better preserve the structure of the final product. The results suggest that the synergy between pretreatments and ultrasound-assisted drying offers a method for reducing drying time and preserving the structure and quality of the orange peels. This method is particularly valuable for preserving the integrity of the cell structure, which is associated with maintaining higher levels of bioactive compounds and nutritional value. The optimal conditions found in this study present a pathway toward developing a more sustainable and value-added approach to processing orange peels.

This study successfully demonstrated that combining low-intensity PEF pretreatment and ultrasound-assisted drying provides the most efficient and structure-preserving method for drying orange peels. The optimized approach significantly reduced drying time while maintaining a relatively intact microstructure. Future research could explore a wider range of PEF parameters to further optimize the process and investigate the impact of these treatments on the retention of bioactive compounds and the overall quality of the dried orange peels. Exploring other pre-treatment techniques and their compatibility with ultrasound-assisted drying will provide further opportunities for optimizing the process.

This study focused on a specific orange variety and a limited range of drying parameters. The generalizability of the findings to other orange varieties and drying conditions should be investigated. Further research is necessary to evaluate the impact of the combined pretreatments and drying methods on the retention of specific bioactive compounds in the orange peels. A more in-depth analysis of the energy efficiency of the combined methods compared to traditional drying methods is warranted.