A comparative study of drying methods on tomato peel residues: physicochemical and antioxidant properties

Abstract

This study aimed to systematically evaluate the effects of vacuum freeze drying (VFD), vacuum drying (VD), natural drying (ND), vacuum microwave drying (VMD), hot air drying (HAD) and microwave drying (MD) on the physicochemical properties, microstructure, and antioxidant capacity of tomato peel residues (TPs). Focusing on identifying the optimal approach that strikes a balance between product quality and industrial feasibility. The results indicated that VFD effectively retained the appearance color, microstructure, lycopene content (12.35 mg/100 g), reducing sugar content (4,024 mg/100 g), hydration capacity, and antioxidant capacity of TPs. VMD exhibited the highest vitamin C content (20.45 mg/100 g) and quality comparable to VFD, along with short drying time and low energy consumption. HAD resulted in the highest levels of total phenolic content (48.66 mg/100 g) and total flavonoid content (72.85 mg/100 g). VMD proved to be the most promising technology for achieving large-scale, high-value industrial drying of TPs due to its exceptional drying efficiency and excellent overall quality retention capabilities. This study provides key theoretical foundations and technical pathways for the value-added utilization of tomatoes processing by-products, holding significant importance for advancing the green, circular, and sustainable development of the food industry.

1 Introduction

Tomato (Solanum lycopersicum) is an annual herbaceous plant belonging to the genus Solanum in the Solanaceae family. It is noted for its distinct sweet–sour flavor and is one of the most widely cultivated fruits and vegetable crops globally. However, only a small portion of tomatoes can be consumed by consumers as fresh fruits, while the vast majority are processed by factories into products such as juice, jam, dices and powder (Farinon et al., 2024; Lu et al., 2019). The development of the tomato industry has produced a large amount of tomato peel residues (TPs). TPs, which consist mainly of peel, seeds, and residual pulp, accounts for approximately 5–30% of raw tomato material (Chabi et al., 2024). Currently, the disposal methods for TPs are limited to animal livestock feed, simple processing or direct disposal, failing to fully leverage their value (Farinon et al., 2024). This not only results in inefficiency within the food processing industry, but also causes resource waste and environmental pollution. In the context of a circular bioeconomy, reusing TPs to recover bioactive compounds is the key to reducing the disposal of organic waste in tomato processing (Selvaggi et al., 2021). TPs are rich in sugars, proteins, oils, lycopene, vitamin C, phenolic compounds, flavonoids and other minerals (Paniagua-García et al., 2024; Pouraghakouchak et al., 2022).

TPs offer potential health benefits for humans. The bioactive components in TPs, such as lycopene, vitamins, and phenolic compounds, mitigate the risk of chronic diseases including cancer and cardiovascular diseases by alleviating oxidative stress through their antioxidant activity (Kumar et al., 2021; Szabo et al., 2022). In the practice of enhancing the value of TPs, Emide et al. used enzymatic hydrolysis technology to convert tomato seeds in TPs into antifungal peptides, dedicated to protecting food crops from fungal damage. Other studies have also shown that adding dehydrated TPs to products such as tomato sauce, meat and bread can enhance the taste and texture of the products (Costa et al., 2023). Therefore, integrating tomato byproducts into food systems is an effective measure for enhancing resource efficiency, reducing waste, and improving public health in pursuit of sustainable development.

Fresh TPs are characterized by high moisture content and abundant nutrients, making them susceptible to microbial contamination. Drying can effectively reduce the moisture content of materials, extend the shelf life of food, and lower transportation and storage costs (Michalski et al., 2025). Nowadays, common drying methods including vacuum freeze drying (VFD), vacuum drying (VD), natural drying (ND), hot air drying (HAD), microwave drying (MD) and others. Different drying methods directly influence the final quality of the product. Research have reported that both VFD and HAD can effectively preserve the physicochemical properties and nutritional value of tomatoes (Tan et al., 2021). Raj and Dash have suggested that MD could reduce the color change of dragon fruit and increase the content of phenolic substances (Raj and Dash, 2022). However, there is an inherent contradiction among processing efficiency, product quality and energy consumption in traditional drying methods. Thermal efficiency of the thermal system is one of the important goals of renewable energy systems (El-Mesery et al., 2025a). Vacuum microwave drying (VMD) is a combined drying technology that integrates the microwave penetration mechanism with a vacuum environment (Zhu et al., 2024). It has emerged as a superior approach for processing agricultural products due to its high efficiency, energy conservation, and excellent product quality (Yan et al., 2025). Previous studies have shown that VMD could greatly improve the quality of apple pomace and the anthocyanin content of blueberry pomace (Bhat et al., 2023; Zhang et al., 2023). Moreover, Cin and Palazoglu have reported that rosehips dried by VMD produced high quality with an initial vitamin C content of over 75% (Cin and Palazoğlu, 2025).

Existing research on dried TPs is fragmented and usually limited to the comparison of several drying methods (Bayana and İçier, 2024; Chada et al., 2022). There are almost no reports on the application of VMD in TPs processing, and the impact of different drying methods on the quality of TPs remains unclear. There is still a lack of a systematic evaluation method in the literature that can simultaneously consider energy consumption, industrial scalability, and the comprehensive quality attributes of TPs after drying by multiple industrial-related methods. To bridge this gap, this study focused on balancing energy efficiency and product quality, provided the first systematic analysis of the effects of six drying methods (VFD, VD, ND, VMD, HAD, MD) on the energy consumption, physicochemical properties, microstructure, and antioxidant capacity of TPs. The aim of this study was to identify the optimal drying method for TPs, which was easy to scale up in an industrial level and could maintain product stability and retain valuable bioactive compounds.

2 Materials and methods

2.1 Materials

Fresh TPs (Chinese Vegetable 3311, Xinjiang Guannong Co., Ltd.), standard product (lycopene, vitamin C, rutin), ABTS and DPPH were purchased from Shanghai Yuanye Biotechnology Co. Folin–Ciocalteu and 2,4,6-tris(2-pyridyl)-1,3,5-triazine were purchased from Shanghai Eon Chemical Technology Co. Trolox (Shanghai McLean Biochemical Co.). All other reagents were analytically pure.

2.2 TPs drying process

Fresh TPs (100 g) were spread evenly and flat on trays. The specific operating parameters for the six drying methods were as follows. For more details, please refer to the supplementary documents.

VFD: TPs were freeze-dried (FD-1000, TOKYO RIKAKIKAI CO., LTD., Japan) with the condenser set to −60 °C, under a vacuum of 60 Pa, for 31 h.

VD: TPs were dried in a vacuum oven (DZF-6050A, Beijing Aerospace Keyu Testing Instrument Co., Ltd., China) at 60 °C and a pressure of 0.08 MPa for 26.5 h.

ND: TPs were dried naturally under direct sunlight. The process was carried out at an ambient temperature of 25–30 °C and 26–30% relative humidity for 24 h.

VMD: TPs were dried in a microwave vacuum dryer (HFWX-1, Shanghai Huaifeng Instrument & Equipment Co., Ltd., China) at 0.08 MPa, 60 °C, for 1.2 h.

HAD: TPs were dried in an electrically heated blast drying oven (HN101-3, Nantong Hu’nan Science Instrument Co Ltd., China) at 60 °C for 12 h.

MD: TPs were dried at high temperature in a microwave oven (P70J17L-V1, Guangdong Galanz Microwave & Electrical Appliances Manufacturing Co., Ltd., China) for 0.18 h.

All dried samples, with a final moisture content below 8%, were stored in sealed bags at low temperature for subsequent analysis. The moisture content was determined according to the Chinese national standard (GB/T 5009.3-2016).

2.3 Yield and specific energy consumption of TPs

The drying yield was calculated as the percentage of the final dry mass relative to the initial fresh mass, according to Equation 1 (Mondal et al., 2019).

where M₁ and M₂ were the weights of the TPs pre-and post-drying, respectively.

The specific energy consumption (SEC) is essential for assessing the performance of various drying techniques. The SEC of the dried TPs is calculated according to Equation 2 (Hu et al., 2023).

where SEC stands for specific energy consumption (kWh/kg), E represents the energy consumption measured by an electric meter (kWh), and M refers to the weight of fresh TPs (kg).

2.4 Color properties of TPs

A colorimeter (TS7820, Shenzhen Threenh Technology Co., Ltd., China) was employed to quantify the color parameters of both fresh and dried samples. Calculation of the color difference (ΔE) was conducted using Equation 3 (Bhat et al., 2022).

where L^*, a^*, and b^* represented brightness, red–green value and yellow-blue value from fresh samples, respectively. L, a, and b implied dried samples.

2.5 Hydration performance

The water binding capacity (WBC), water holding capacity (WHC) and oil holding capacity (OHC) were measured in accordance with the method proposed by Zhang et al. (2024).

2.6 Microstructure determination

The dried samples were immobilized on a metal sample stage through a conductive gel, and then sprayed with gold at an accelerating voltage of 3.0 kV, and the sample morphology was observed via scanning electron microscopy (SEM) (FlexSEM 1000 II, Hitachi High-Tech, Japan) at different magnifications.

2.7 Fourier transform infrared spectroscopy

The dried samples subjected to different drying methods were analyzed via Fourier transform infrared (FT-IR) spectrometer (Thermo Nicolet IS5, Thermofisher Scientific, United States) to detect the functional groups, samples were measured via the potassium bromide (KBr) solid compression method. KBr and samples were dried at 110 °C for 5 h, then mixed at a ratio of 1:100. The mixture was compressed into uniform transparent sheets using an infrared tablet press at 20 MPa pressure. Subsequently, FT-IR spectroscopy was collected within the wavelength range of 4,000–400 cm⁻¹.

2.8 Quantification of lycopene content in TPs

Lycopene was determined based on the previous method (Ge et al., 2023). The maximum absorption wavelength of ethyl acetate was 503 nm, and lycopene was extracted via ultrasonic extraction with ethyl acetate as the extractant. The liquid–solid ratio was set at 1:35, with the ultrasonic power adjusted to 100 W and the extraction time fixed at 35 min. Subsequently, the supernatant was obtained via filtration, absorbance values were acquired by detecting at a wavelength of 503 nm, and the lycopene content was figured out by the lycopene standard curve.

2.9 Determination of vitamin C content in TPs

The vitamin C content was measured via the molybdenum blue colorimetric method (Zhang et al., 2024).

2.10 Determination of reducing sugar content in TPs

Assessment of reducing sugar levels was performed via the DNS method (Jin et al., 2017).

2.11 Determination of total phenols content and total flavonoids content in TPs

A total of 1.0 g sample was put into centrifuge tube, following the incorporation of 30 mL 60% (v/v) ethanol. Ultrasonic extraction was performed on the mixture for a period of 1 h, standing at 4 °C away from light for a duration of 24 h, finally centrifuge (10,000 rpm, 4 °C, 15 min) to obtain the supernatant. The total phenols content (TPC) was quantified using the Folin–Ciocalteu method (Yao et al., 2020). Determination of total flavonoids content (TFC) was conducted using the AlCl₃ colorimetric method (Malakar and Arora, 2022).

2.12 Assessment of the antioxidant capacity in TPs

Assessment of the antioxidant capacity of TPs was conducted via three approaches - DPPH radical scavenging capacity, ABTS radical scavenging capacity and ferric reducing antioxidant power (FRAP), which were determined following previous research (Ma et al., 2021). For DPPH radical scavenging capacity, 2.0 mL diluted supernatant was added to 4.0 mL 0.1 mmol/L DPPH solution, followed by a 30 min reaction in darkness, measured the absorbance at 517 nm. For ABTS radical scavenging capacity, 0.4 mL diluted supernatant was combined with 4.0 mL ABTS solution, maintain the reaction in the dark for a total of 30 min, determined the absorbance at 734 nm. For FRAP, 0.1 mL diluted sample extract reacted with 4.0 mL FRAP solution at 37 °C for 30 min, the FRAP was ascertained by monitored the absorbance of the mixture at 593 nm. Trolox was applied to plot the standard curves for the three antioxidant capacities, the results were represented as Trolox equivalents per gram of samples dry weight (mg TE/g DW).

2.13 Data processing

In this study, statistical analysis was performed using IBM SPSS Statistics 27 (SPSS Inc., Chicago, United States). The significant differences among the different drying treatment groups were evaluated by one-way analysis of variance (ANOVA). Upon obtaining a significant F-test (p < 0.05), post-hoc comparisons were conducted using Fisher’s LSD and Tukey’s HSD test to identify specific differences between group means. The results were presented as the mean ± standard deviation of three technical replicates (n = 3). Graphical plotting was generated using Origin 2022 (OriginLab Corporation, Northampton, United States).

3 Results and discussion

3.1 Yield, drying time and SEC

In industrial production, the drying yield, processing time and SEC (Table 1) critically influence both production costs and operational efficiency. The yield comparison of different drying methods was as follows: MD > VFD=ND > HAD>VD=VMD. ND samples showed the highest yield, and the measured moisture content was 6.47%. VMD samples had the lowest yield, and the measured moisture content was 5.12%. This indicated that the yield differences under different drying methods were closely related to the moisture content. In terms of drying efficiency, MD samples required the shortest drying time (0.15 h) and had the lowest energy consumption (1.03 kWh/kg). This was because the moisture gradient formed by microwave heating within the material increased the rate of energy transfer, intensifying moisture migration and evaporation, which significantly enhanced drying efficiency (El-Mesery et al., 2025b; Li et al., 2025). VFD required the longest drying time (31 h). It can be explained by the slow evaporation of moisture under the low-temperature and low-pressure conditions (Silva-Espinoza et al., 2019). VD exhibited the highest energy consumption, requiring 26.5 h. This is because VD relies solely on radiant heat under vacuum without forced convection, leading to lower drying efficiency (Wu et al., 2025). ND consumed no energy but was susceptible to external environmental factors during drying. Compared to other drying methods, VFD, VD, and ND were less suitable for large-scale industrial production, they may impact overall production efficiency. VMD took 0.28 h and demonstrated the second-lowest energy consumption (3.35 kWh/kg). As a combined drying technology, VMD can achieve efficient and uniform drying under low-pressure conditions (An et al., 2024). Han and Yan’s modeling of VMD for pitaya indicated that during actual VMD processes, the material’s shrinkage behavior significantly accelerated internal moisture migration, thereby substantially enhancing drying rates in terms of kinetics (Han and Yan, 2025). The drying efficiency of VMD in this study surpassed that of the novel infrared drying technology reported by El-Mesery et al. (2023). This highlights the unique advantage of VMD in its internal moisture transport mechanism.

3.2 Color parameters

Color serves as a key indicator for evaluating the drying quality of fruits and vegetables, as it directly influences market value and consumer acceptance. The visual appearance and color parameters of TPs before and after drying are shown in Figure 1 and Table 2, respectively. Compared to fresh samples, VFD and VMD samples exhibited a brighter, light red color, whereas VD and ND samples appeared darker red. In contrast, HAD and MD samples showed the darkest coloration, appearing dark red and reddish-brown, respectively. This color darkening is likely attributable to pigment degradation, which are exacerbated at high temperatures (Barani et al., 2020). Most dried samples demonstrated significantly higher L* and a* values than the fresh samples. According to Nzimande et al., elevated L* and a* values correlate with an increased degree of browning (Nzimande et al., 2024). Consequently, browning occurred to varying extents across all six dried samples. Among all samples, VFD samples had the highest L* value and a relatively high a* value. This superior color preservation can be attributed to the low-temperature, oxygen-free environment in VFD, which effectively inhibits both the Maillard reaction and the oxidative degradation of carotenoids (Liao et al., 2025; Nzimande et al., 2024). No significant differences were observed in L* and a* values between ND and VFD samples. Although both VD and VMD samples had relatively high L* values, the a* value of VD samples was significantly lower than that of VMD samples, indicating a more pronounced loss of red color. This color deterioration is likely associated with the prolonged drying time required by VD. HAD and MD samples recorded the lowest L* and a* values among all samples. This pronounced darkening is attributed to the high temperature accelerating non-enzymatic browning in the samples (Li et al., 2025). This finding is consistent with previous studies linking color loss in tomato products to lycopene degradation (Costa et al., 2023). A low total color difference (ΔE) indicates close resemblance to the fresh reference material, whereas a high value signifies a substantial color deviation from the original state. VFD samples exhibited the smallest ΔE value, VMD samples showed a relatively low ΔE value, demonstrating effective color retention. MD samples presented the highest ΔE value, indicating the most severe color deterioration. The a*/b* ratio has been reported as a reliable indicator for evaluating the color of tomato products (Nzimande et al., 2024). In the present study, the highest a*/b* ratio was observed in fresh samples, followed by VFD and VMD samples. This result suggests that the color of VFD and VMD samples were the closest to that of the fresh samples.

Figure 1

3.3 SEM structure

Figure 2 showed the microstructure of TPs under different drying methods. The surface of VFD samples was smooth, free of cracks, and had a complete microstructure. SEM images (1,000×) revealed that the VFD samples exhibited a porous honeycomb structure, and other studies also observed a similar structure in VFD materials (Guo et al., 2024; Xu et al., 2022). The porous microstructure of VFD samples originated from the sublimation of ice under low-temperature vacuum conditions (Hu et al., 2023). VFD process created a rigid, porous network that prevented the collapse of the solid matrix, playing a positive role in maintaining the functional properties of plant tissues. It has been reported that the loose and porous structure of VFD samples is associated with their excellent hydration capacity (Yang et al., 2025). Moreover, VFD samples presented the strongest antioxidant capacity (DPPH, ABTS and FRAP) in this study, indicating that cell integrity can enhance bioactivity and improve antioxidant capacity (Maisaroh et al., 2025). A similar honeycomb-like structure was found in VMD samples, which was consistent with the results obtained from VMD of moutan cortex (Shang et al., 2024). This might be attributed to the expansion of the structure caused by moisture evaporation via microwave heating under low pressure. Cellular collapse and shrinkage were observed in the VD and HAD samples during the dehydration process. It resulted from the rapid migration of water at high temperatures and the accelerated depletion of substances such as polysaccharides, leading to the compaction of cellular structure (Wu et al., 2022). Cell disruption in HAD samples can be attributed to three main factors: loss of cell turgor pressure during prolonged drying, increased tissue dehydration, and severe deformation of cellular structure (Yao et al., 2020). Severe collapse and shrinkage deformation were observed in the cellular structures of ND and MD samples, with numerous cracks and irregular voids on the surface. This was detrimental to the migration and diffusion of moisture (Zang et al., 2023). During the MD drying process, internal temperature and moisture gradients caused structural damage, compromising cellular tissue integrity (Chumroenphat et al., 2021). In addition, rapid heating induced by electromagnetic waves and the subsequent increase in internal stress caused by high vapor pressure may also contribute to cell rupture (Dhara et al., 2023).

Figure 2

3.4 Hydration properties of TPs

WHC and WBC are important factors for measuring the physiological function of TPs. OHC is an essential index for evaluating the ability of TPs to lose oil during processing. As shown in Table 3, VFD samples exhibited the strongest WBC, WHC, and OHC. It could be attributed to the loose and porous structure formed during the VFD process, which increased the specific surface area and provided more binding sites for water and oil molecules (Qiu et al., 2022). There was no significant difference in the WBC between VMD and HAD samples. However, VMD samples exhibited markedly higher WHC and OHC than HAD. This was attributed to the rapid low-temperature drying characteristics of VMD, a process that effectively suppressed oxidative degradation, thereby preserving the polysaccharide and fiber structures more completely (Bhat et al., 2023). The low porosity of the HAD sample’s microstructure impeded moisture transport, compromising its hydration performance (Li et al., 2023). MD samples suffered severe damage to their cellular structure due to high temperatures and localized microwave overheating effects, accelerating polysaccharide degradation and reducing hydrophilic groups, resulting in the poorest hydration properties (Llavata et al., 2022). Research indicates that the cellular structure of samples was closely related to their hydration capacity (Zhang et al., 2024).

3.5 FT-IR spectra analysis

The TPs subjected to six drying methods were analyzed using FT-IR spectroscopy to characterize their functional groups. As shown in Figure 3, the FT-IR spectra of the six dried samples were fundamentally similar, exhibiting identical characteristic absorption peaks, with the exception of differences in their intensities. The broad peak appearing at 3,422 cm⁻¹ corresponded to the O-H stretching vibration. The absorption peak located at 2,925 cm⁻¹was assigned to the antisymmetric stretching of CH₂, which is typically associated with the peak at 2,850 cm⁻¹ (a symmetric stretching vibration of CH₂). The peak at 1,455 cm⁻¹ originated from the scissoring bending of CH₂. The broad absorption between 1,650 and 1,750 cm⁻¹ was attributed to C=O stretching vibrations. Peaks in the 1,100–1,300 cm⁻¹ range corresponded to C-O and C-O-C stretching vibrations, which are characteristic of glycosidic bonds. Absorption peaks located in these regions were recognized as the characteristic absorption peaks for carbohydrates (Chi et al., 2017). A prominent absorption peak appeared at 1,061 cm⁻¹ is attributed to the telescopic vibration of C-O and C-O-C (glycosidic bond), as well as the angular vibration of O-H and C-O-H in cyclic ethers. This peak is also characteristic of polysaccharides (Ma et al., 2018). FT-IR analysis revealed that although the basic chemical structures of the TPs were similar under different drying methods, the intensity of characteristic absorption peaks in VFD samples was lower than that in other dried samples. This further suggests that the other five drying methods degraded the long-chain macromolecules in the TPs powders during preparation, leading to an increase in short-chain structures and the rupture of cell walls. These changes can be primarily ascribed to discrepancies in drying processes and the interaction between water and TPs surfaces (Alarcón-Moyano et al., 2023).

Figure 3

3.6 Lycopene and vitamin C content

Lycopene and vitamin C, as key heat-sensitive bioactive compounds in tomato pomace, serve as crucial indicators for evaluating the effectiveness of drying processes. The results of this study indicated that different drying methods significantly affect the content of both components (Figures 4A,B). VFD and VMD demonstrated the best performance in lycopene retention, with no significant difference between the two methods. In terms of vitamin C retention, VMD demonstrated a significant advantage, achieving the highest content of 20.45 mg/100 g, followed by VFD. The high retention rate of lycopene in VFD samples could be due to its low-temperature and oxygen-free conditions, which effectively prevented the thermal degradation of heat- sensitive compounds and minimized oxygen exposure (Souza et al., 2018). Owing to the suppression of oxidation reactions by vacuum and microwave-driven rapid drying (Gul et al., 2024), VMD effectively preserved lycopene. This system ensured uniform energy distribution and lower drying temperatures (Yan et al., 2025), improving the issue of uneven heat distribution in conventional MD. VD exhibited a relatively high levels of lycopene and vitamin C, reaching 11.08 mg/100 g and 15.22 mg/100 g, respectively. In contrast, HAD and MD samples contained less lycopene and vitamin C, most likely because of isomerization and oxidative degradation that occurred during prolonged exposure to high temperatures (Bakir et al., 2023; Mieszczakowska-Frąc et al., 2021). ND samples had the lowest lycopene and vitamin C content, which can be attributed to their prolonged exposure to natural environmental factors such as light, heat, and atmospheric oxygen. Research has confirmed that drying temperature and time are the key factors responsible for vitamin C loss (Bhat et al., 2022). It is noteworthy that the vitamin C content in VFD samples was significantly lower than that in VMD, which might be due to the prolonged drying time of VFD. In this study, VFD and VMD were the most effective methods for lycopene and vitamin C preservation in TPs.

Figure 4

3.7 Reducing sugar content

Reducing sugars are a primary energy source in fruits and vegetables. Determining reducing sugar content aids in understanding the nutritional composition of TPs (Wang et al., 2023). Changes in reducing sugar content serve as a reliable indicator for measuring quality deterioration of TPs during processing. Statistical analysis revealed that the drying method had a significant effect on the reducing sugar content of the TPs (Figure 4C). VFD samples demonstrated the highest reducing sugar content (4,024 mg/100 g). Similar results had also been reported in tumorous stem mustard (Zheng et al., 2024). VMD exhibited the second-highest reducing sugar content (3719.53 mg/100 g). In contrast, VD and ND samples showed relatively lower reducing sugar content, while HAD and MD samples had the lowest levels. Research indicated that carbohydrate degradation caused by high-temperature dehydration leads to decreased reducing sugar content, while vacuum and low-temperature drying maintain a high level of reducing sugars (Guo et al., 2024). Prolonged exposure to high temperatures induced non-enzymatic browning reactions in reducing sugars, primarily including the Maillard reaction and caramelization reaction (Lei et al., 2025). Besides, the high temperature of HAD and MD not only directly disrupted sugar structures but also activated oxidative enzymes or accelerated browning reactions (Erdem et al., 2025). The low level of reducing sugars in MD samples also could be attributed to restricted water utilization efficiency during the final stage of microwave treatment (Zhang et al., 2024). This restriction caused rapid heating of the TPs, which accelerated the degradation of reducing sugars.

3.8 TPC and TFC

Total phenols and total flavonoids are crucial natural antioxidants, capable of effectively neutralizing free radicals and mitigating oxidative damage. The contents of these two components of TPs serve as indicators of the antioxidant capacity of TPs. Figure 5 illustrated the TPC and TFC of the six drying methods. HAD exhibited the highest TPC (48.66 mg/100 g) and TFC (72.85 mg/100 g), consistent with previous findings in tomatoes and Rosa roxburghii fruit (Luo et al., 2024; Nzimande et al., 2024). High temperatures can affect enzyme activity. When drying temperatures exceed a certain range, polyphenol oxidase (PPO) and peroxidase (POD) are prone to denaturation, leading to a decrease or loss of catalytic activity (Cheng et al., 2007; Lin et al., 2025), thereby effectively curbing the severe degradation of phenolic and flavonoid compounds caused by enzymatic browning. In addition, heat treatment releases phenolic compounds by cleaving ester bonds and glycosylation or generates Maillard reaction products to increase TFC (Ratseewo et al., 2025). Similarly, other studies have also confirmed that thermal exposure disrupts cellular structures and degrades cell wall constituents (pectin, protein, and fiber, etc.), releasing phenolic compounds (Ratseewo et al., 2020; Silva et al., 2020). These factors explain why HAD exhibits high levels of TPC and TFC. The TPC and TFC of VFD were inferior to those of HAD. This is because during the VFD process, ice crystals formation can destroy the cellular matrix. This damage may lead to the release of certain enzymes and activators (such as PPO and H₂O₂), which in turn induces the enzymatic oxidation of phenolics and flavonoids, thereby reducing their final content (Tan et al., 2021). It has been reported that after thawing, increased enzyme activities such as peroxidase activity may promote the degradation of certain phenolic compounds (Ma et al., 2021). Research indicated that drying temperatures below 70 °C and VFD processes can preserve phenolic compounds and flavonoids (Luo et al., 2024). No significant differences were found in TPC or TFC levels between VMD and VFD. VD and MD presented relatively low TPC and TFC. The high internal thermal energy generated by microwave radiation can give rise to the oxidative degradation of phenolic compounds (Aydar et al., 2020). Notably, the total phenolic retention rate of MD (68.20%) was significantly higher than that of VD (55.85%). The difference may be attributed to unique mechanism of MD. ND presented the minimum levels of TPC and TFC.

Figure 5

3.9 Antioxidant capacity

As shown in Figure 6, the overall trend in antioxidant capacity was similar to the results for TPC and TFC, suggesting a positive correlation between antioxidant capacity and the TPC and TFC (Ma et al., 2021). However, although HAD samples exhibited the highest TPC and TFC, VFD samples demonstrated the strongest antioxidant capacity. This apparent discrepancy indicates that the antioxidant capacity of samples is not solely determined by phenols and flavonoids, but rather by the synergistic effects of multiple bioactive compounds. The antioxidant capacity of HAD and VMD samples was second only to that of VFD samples, while ND samples presented the lowest antioxidant capacity. The superior antioxidant capacity of VFD samples can be due to low-temperature and oxygen-limited conditions of VFD, which inhibit the chemical reactions induced by high temperature (Luo et al., 2024), thereby mitigating the degradation of antioxidants cause by exposure to heat and oxygen. Previous study has shown that low-temperature drying processes can effectively retain antioxidants such as lycopene, Vc, and phenolic compounds. These antioxidants possess high redox potential, enabling them to function as both reducing agents and hydrogen donors, thereby enhancing the overall antioxidant capacity (Managa et al., 2020). Furthermore, the superior performance of VFD is likely due to the synergistic protective effects of specific antioxidants, particularly lycopene and vitamin C. These two components, acting as potent antioxidants, significantly enhance the antioxidant capacity of TPs. The experimental results in this paper confirmed that the lycopene and vitamin C retention rate in VFD samples were 13.50 and 26.58% higher than those in HAD samples, respectively. This finding was consistent with previous research (Bakir et al., 2023), which reported that VFD effectively preserved lycopene and vitamin C content in tomatoes, whereas HAD caused substantial degradation. Wu et al. noted that antioxidant capacity depended not only on the content of phenolic compounds but also on the composition of compounds and the antioxidant detection methods employed (Wu et al., 2025). The relatively strong antioxidant capacity of HAD samples might correlate with the high TPC and TFC. High temperatures can break certain covalent bonds within cells, releasing antioxidant substances from subcellular compartments (Calín-Sánchez et al., 2020). It was worth noting that the prominent antioxidant performance of VMD samples indicated that moderate microwave processing effectively preserved the antioxidant capacity of TPs (Guo et al., 2025).

Figure 6

3.10 Principal component analysis

To comprehensively evaluate the impact of different drying methods on the overall quality of TPs, principal component analysis (PCA) was performed (Figure 7) on key quality indicators in this study. This model not only revealed the overall differences between samples but also visually reflected their proximity to the quality of fresh samples through spatial relationships. The results indicated that the cumulative variance contribution of the first and second principal components was 95.21% (PC1: 86%; PC2: 9.21%). VFD and VMD samples exhibited the closest spatial distribution to fresh samples. This demonstrated that both methods were the most effective among all drying methods for preserving the comprehensive quality of TPs.

Figure 7

4 Conclusion

This study provided a novel and systematic comparison of six drying methods on the physicochemical properties, microstructure, and antioxidant capacity of TPs. Results demonstrated that VFD best preserved the color and microstructure of TPs, retained the highest levels of lycopene (73.39%) and reducing sugars (86.54%). Furthermore, it exhibited strong DPPH radical scavenging capacity (24.42 μmol TE/g), ABTS radical scavenging capacity (17.35 μmol TE/g) and ferric reducing antioxidant power (16.88 μmol TE/g). VMD demonstrated the highest vitamin C retention rate (71.67%), while its lycopene content, TPC and TFC were comparable to that of VFD. Although HAD had the highest retention ratios of TPC (76.51%) and TFC (88.96%), their overall performance was subpar. Compared with ND, HAD and MD, VD could effectively maintain the quality of TPs. The quality of ND and MD was the poorest. Considering drying efficiency and production costs, although VFD consumed less energy (5.67 kWh/kg), the drying time (31 h) was too long. Among different drying methods, VMD demonstrated outstanding comprehensive advantages. Not only did it achieve an extremely short drying time (0.28 h) and relatively low SEC (3.35 kWh/kg), but it also most closely matched the VFD sample in key quality indicators. VMD had been proven to be the most promising technology for the industrial drying of TPs. The findings of this work are expected to promote the high-value conversion and sustainable development of the tomato processing industry. However, this research was conducted in the laboratory, which presented a discrepancy with industrial continuous production. It needs to be verified on a future pilot scale. Additionally, a comprehensive industrial assessment must include detailed technical and economic analyses. It is recommended to combine VMD with other drying technologies or employ optimization tools such as response surface methodology to systematically optimize process parameters. In the future, we will undertake work related to drying kinetics modeling and predictive analysis to maximize the value of TPs. This study provided a systematic comparison of drying methods for TPs, offering critical data on their effects on product quality and energy efficiency.

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