Effect of different pretreatment methods on drying characteristics and quality of wolfberry (Lycium barbarum) by radio frequency-hot air combined segmented drying

In this study, fresh wolfberry in the current season was used as the experimental material and a radio frequency (RF)-hot air combined segmented drying process was adopted for berry preservation. Ultrasonic treatment, hot water blanching, sucrose in�ltration, ultrasonic & blanching, ultrasonic & in�ltration, and NaOH and NaCl solution impregnation were used for pre-treatment of combined drying. Taking natural drying and conventional Na 2 CO 3 dewaxing treatment as the control group, the effects of different pretreatment methods on drying characteristics, quality, and microstructure of wolfberry during combined drying were investigated. The drying time after ultrasonic and blanching pretreatment was the shortest(13.5h), which effectively retained total �avonoids(1.79mg/g) and enhanced their antioxidant activity(I = 60.78%). NaCl impregnation treatment signi�cantly increased the total phenol content(8.77 mg/g). The retention rate of soluble sugar (0.73 g/g), ascorbic acid (3.96mg/100g), betaine(2.72%), and other nutrients increased signi�cantly after ultrasonic and in�ltration treatment, while the color, rehydration rate and microstructure were better than other treatment methods. The Weibull distribution function can accurately describe the RF-hot air combined drying process of Lycium barbarum after different pretreatment, and simulated results were consistent with the results of the combined drying characteristic curve of Lycium barbarum. The aim of this study was explored suitable pretreatment methods to improve the drying rate and ensure the quality of dried products of wolfberry.


Introduction
As a perennial deciduous shrub of Solanaceae.Wolfberry (Lycium barbarum) has orange-red ellipsoid berries.It is an edible and medicinal plant with a long history in China and other Asian countries.Rich in polysaccharides, avonoids, betaine, and other bioactive substances, it has multiple healthy and pharmacological functions (Montesano et al., 2018).Given that the moisture content of ripe wolfberry fruit is more than 80%, it is very easy to decay owing to the action of microorganisms and enzymes at room temperature, which makes it di cult to transport and store.Meanwhile, drying is one of the most effective and widespread food preservation methods, which can prolong the shelf life and improve the retention rate and bioavailability of active substances.Therefore, apart from a small amount of wolfberry production being used for fresh food and as raw materials for foodstuff processing, most of them are preserved through controlled desiccation (Kafkas et al., 2021).
At present, the drying methods of wolfberry mainly include natural drying and mechanical drying.Natural drying is simple in operation and low in cost, but its drying time is long, and the drying conditions and quality are di cult to control, so the wolfberry is easy to get damp, caking and mildew.Mechanized drying methods mainly include hot air, microwave vacuum, far infrared, and vacuum freezing.
The color, rehydration and quality of wolfberry are typically quite good which dried by hot air, but longer drying time resulting in serious shrinkage of dried wolfberry fruit, loss of nutrients and quality reduction(Dandan Zhao,2018).Microwave vacuum drying technology can signi cantly shorten the drying cycle of wolfberry and has higher energy utilization rate, but it is easy to cause surface expansion and rupture, seed out, sugar over ow, and other phenomena, which makes it more suitable for the later drying of semi-dry products(Yue Duan, 2017).After vacuum far-infrared drying, the polysaccharide content and color differences of wolfberry are increased while the rehydration rate does not change signi cantly, but the surface is prone to crust hardening and forms small bubbles, which affects fruit quality (Lijuan Zhao et al., 2017).Vacuum freeze-drying technology can effectively retain the bioactive components of dried wolfberry, but the energy consumption is large, the output is small and the equipment is expensive.
(Mengze Wang, 2017).Therefore, it is necessary to seek a more e cient, suitable, and cost-effective drying technology for wolfberry.
In recent years, radio frequency (RF)-hot air combined drying has been introduced as a new environmental drying technology.It generates heat inside the material by oscillating charged ions and rotating polar molecules and has a good penetration ability, which can achieve rapid heating of the whole material (Mao Y & Wang S, 2021).Hot air-assisted RF heating can effectively avoid thermal deviation and glow discharge caused by excessive heating or uneven heating, reduce the in uence of corner effect and arcing effect in RF drying, and greatly improve the drying e ciency.At present, this technique has been successfully applied in the drying of red-jujube (Yao X et al., 2022), apricots (Topcam H et al., 2022), kiwifruit (Zhou X et al., 2018), apple (Peng Jing et al., 2019), mango (Zhang H et al., 2019), carrot (Ozbek, Hatice Neval, 2021), and other fruit slices.
The wax layer on the surface of wolfberry will hinder the evaporation of internal water during drying, so a pretreatment is needed before drying.Alkali immersion is the most common treatment method that can dissolve the long-chain fatty acid hydrocarbon compounds on the wolfberry surface, thus accelerating the drying rate (Zhou Yu-Hao et al., 2020).However, relevant studies have shown that ultrasonic pretreatment can effectively strengthen the internal heat and mass transfer process of fruit and vegetable slices, and improve the drying e ciency and quality of dried fruit (Zhiyuan Cao et al., 2021).For example, appropriate blanching treatment can maintain the quality and color of agaricus bisporus slices (Jiang Ning et al., 2015).Permeation pretreatment can enhance color protection and brittleness of dried carrot products (Souza et al., 2022).Impregnation is bene cial to maintain product shape and color, improve hardness and reduce brittleness( (Alipoorfard, F et al., 2020).Therefore, ultrasonic pretreatment, hot water blanching, sucrose in ltration, ultrasonic & blanching, ultrasonic & in ltration, and NaOH and NaCl solution impregnation treatments were used to pretreat wolfberry before RF-hot air combined drying in this study.Then, the combined drying characteristics of wolfberry under different pretreatments were explored and the quality and microstructure of dried wolfberry were compared and analyzed.The purpose of the study was found suitable pretreatment methods and provided a theoretical and practical basis for the postharvest processing of wolfberry, and provided certain technical guidance for the wolfberry rapidly drying with high quality. .

Experimental Materials
The variety of wolfberry used in this experiment was the fresh fruit of Ningqi No.1 planted in the Zhongning L.Barbarum Plantation in Ningxia, China.Fresh wolfberries was spread on the material plate of the rapid moisture meter (HKSF-2, Moisture Resolution 0.01%, Precision 0.1%; Wuxi Huake Instrument Co., Ltd., Wuxi, China); the measured average initial moisture content was 79 ± 2%, and fresh wolfberry was purchased and stored in a temperature-controlled cabinet at (2 ± 1°C) prior to testing.

Experimental Method
A 3 kW radio frequency heating system at 27.12 MHz (GJS-3-27-JY, Hebei Huashi Jiyuan High Frequency Equipment Co., Ltd., Langfang, China) and an electrically heated drum dryer (YQ101-0A-4A, voltage 220V/50 Hz, power 1.2kW, wind speed 3 m/s; Beijing Yuqin Tengda Pharmaceutical Equipment Co., Ltd., Beijing, China) was used for drying of wolfberry (Xu Y et al., 2022).This study was conducted on the basis of previous RF-hot air combined segmented drying study of wolfberry.The suitable parameters of the combined drying test were plate spacing of 90 mm, vacuum degree of 0.025 MPa, and hot air temperature of 55°C ( Xu Y et al., 2022).Wolfberries with an undamaged surface, high moisture content, comparable shape and size were chosen as the raw material, and they were subjected to the following different pretreatments after being removed from the temporary storage 1 hour before the experiment and waiting for the temperature to reach room temperature (22 ± 1°C).
The combined segmented drying process of wolfberry was shown as Fig. 1. 120 ± 0.5 g of quali ed fresh wolfberry were selected for each group, among which some were directly soaked when pretreated with NaOH and NaCl solution while the others were placed in 2% Na 2 CO 3 solution for 3 min before different pretreatment.Then, the wolfberries were evenly spread on a polypropylene plate (500 × 300 mm) and placed into the RF vacuum tank for the rst drying stage.The previous test results showed that the wolfberry surface would crust, harden, or even scorch when the moisture ratio is decreased to 0.4-0.5 g/g (Xu Y et al., 2022).This is because the dielectric properties of RF whole heating lead to faster evaporation of free water in the epidermis than in the interior, and the viscosity of water molecules increases and becomes di cult to move and diffuse.Therefore, 0.4-0.5 g/g was used as the moisture ratio node of the conversion drying method in the combined drying.The wolfberries were transferred to the electric hot-blast drying box for the second stage of drying when the moisture ratio reached the node, aiming to continue to evaporate the internal moisture of wolfberries..The total operation time was less than 1 min, and the temperature loss of the sample was less than 5°C during the transfer process.The sample was weighed every 60 min until a constant weight was reached.The natural drying and conventional Na 2 CO 3 solution dewaxing pretreatment were regarded as the control group, and each pretreat test group was repeated for three times.

Selection of the model
The Weibull distribution function was used to t the RF-hot air combined drying curves of Lycium barbarum after different pretreatments.The variation of moisture ratio in the drying process was analyzed, and the moisture ratio was expressed by Weibull distribution function as follows (Dai J W et al., 2015): (1) where α is the scale parameter and represents the drying rate constant, which is approximately equal to the time required for the material to remove 63.2% moisture during the drying process; β is the shape parameter and its value is related to the drying rate at the beginning of the drying process; t represents the drying time ; The multiple linear regression method was adopted to t the mathematical model equation to the experimental data, and the tting accuracy of the model was evaluated by the coe cient of determination(R 2 ), root mean square error(RMSE) and chi-square value (χ 2 ).
The tting accuracy of the mathematical model is higher when R 2 is closer to 1, and RMSE and χ 2 are closer to 0, which indicated that the Weibull distribution function can accurately describe the RF-hot air combined drying process of Lycium barbarum after different pretreatments.The R , RMSE and χ were calculated as follows (Ju H Y et al., 2016): Where MR exp,i is the value of the moisture ratio obtained from drying experiments, MR pre,i is the value of the model-predicted moisture ratio, is the average value of the experimental moisture ratio, N is the number of observations and n is the number of constants.

Quality analysis
According to the methods of our previous experiment (Xu Y et al., 2022), the drying parameters including moisture content, dry basis moisture content and moisture ratio of wolfberry were calculated after dried by RF-hot air, and the color, rehydration rate, sugar, total phenol, avonoid, antioxidant activity, ascorbic acid, betaine content of dried wolfberry were also determined utilized the exact same testing methods as the previous study (Xu Y et al., 2022), and then representative views under different drying conditions were selected for microscopic photography.

Statistical Analysis
The single-factor test method was adopted, each group was repeated for three times, and all data were expressed as mean ± standard error (SE).Treatment comparisons were performed by a one-way analysis of variance (ANOVA) using SPSS 26.0 (IBM, Armonk, North Castle, NY, USA) with α = 0.05.Origin 2021(OriginLab, Northampton, MA, USA) was used for mapping.

Effects of Different Pretreatments on Combined Drying Characteristics
Moisture ratio curves under different pretreatments as a whole showed a descending trend with the extension of drying time (Fig. 2(a)).This is because the material is in a warming state during the combined drying process and the internal moisture is continuously evaporated.Moreover, after the material was pretreated, under the in uence of high-voltage electric eld generated by RF and hot air convection drying after pretreatment, the energy of its internal drying medium increased, which led to the free water to continuously migrate from the inside to the surface and to complete the drying process with a rapid gasi cation.
After wolfberries were pretreated with ultrasonic and blanching, ultrasonic and in ltration, ultrasonic, impregnated with NaOH solution, heat blanching, impregnated with Na 2 CO 3 and NaCl solution, and sucrose in ltration, the total drying time required to achieve constant weight was 13.5, 14.5, 15, 15.5, 16, 17, 17.5, 18 hours, respectively.The segmented drying moisture ratio node (0.4-0.5 g/g) was rst reached after 6 h of RF drying when ultrasonic and blanching combined pretreatment was adopted, and the required time was 7.69%, 21.05%, 18.37%, 5.52%, 4.02%, 17.81%, 19.46% shorter than aforesaid pretreatment methods, respectively.The reason of U&B pretreatment could rst reached moisture ratio node was that the simultaneous strong thermophysical effect of ultrasonic and blanching pretreatment softened the internal tissue of wolfberry and enhanced the permeability, which was bene cial to rapid internal moisture diffusion (Wang C et al., 2021).
Since the material would rapidly warm in the heating environment of combined drying and then enter the constant speed and deceleration heating stage.It can be found the materials were all in a constant speed drying stage when the drying process was completed to 5 hours, and they entered a deceleration drying stage after 10 hours, that's meaning that the drying rate from all pretreatments remained similar consistent throughout 5-10 hours.The drying rate of the material under different pretreatments increased rst and then decreased (Fig. 2(b)).
The overall drying rate after in ltration treatment was slow, and the drying time was the longest (18 h), which was 5.56% higher than conventional Na 2 CO 3 dewaxing treatment.It's consistent with the nding of Jian Lyu et al (Lyu Jian et al., 2017) studied on the drying characteristics of peach chips that pretreated by osmotic dehydration in sucrose solution.This is because the sucrose molecules in the medium entered the surface cells of wolfberry during in ltration pretreatment, thus reducing the permeability and hindering the effective diffusion of water molecules.The drying rate was signi cantly accelerated, and the total drying time was the shortest (13.5 h) after ultrasonic and blanching pretreatment, which was reduced by 20.6% compared with the Na 2 CO 3 dewaxing treatment.It's consistent with the result of Qian Zhang et al (Qian Zhang et al., 2022) that the drying rate of wolfberry will accelerated which preteated by ultrasonic and blanching.
The main reason is that blanching treatment led to the plasmolysis of surface cells of wolfberry, which increased the permeability of cell wall.In addition, the mechanical effect and cavitation effect of ultrasound increased the number of micropores on the surface of wolfberry, which was conducive to the rapid vaporization and evaporation of moisture inside the material and accelerated the drying rate(Mehta et al., 2017).The simulation results of Weibull distribution function are shown in Table 1.The average value of R 2 is 0.99457, the average value of RMSE is 0.00363, and the average value of χ 2 is 2.0466 × 10 − 4 , which indicated that the Weibull distribution function could better simulate the RF-hot air combined drying process of Lycium barbarum after different pretreatments.

.1.Simulation analysis of drying process
The size parameter α was the smallest (8.532) when pretreated by Ultrasound & Blanchingand, while it was the largest when pretreated by In ltration (11.376), as shown in Table 1.The simulated results explained that the drying time of Lycium barbarum was the shortest and the drying rate was the fastest when pretreated by Ultrasound & Blanchingand, while the drying time was the longest and the drying rate was the slowest when pretreated by In ltration, which was consistent with the results of the combined drying characteristic curve of Lycium barbarum.
The shape parameter β represents the drying rate of the material at the initial stage of drying, which increased rst and then decreased when β > 1, and the drying process is controlled by both surface and internal moisture.The value of β was 1.55169 ~ 1.85551 as is shown in Table 1, so theoretically there is a short ascending drying stage in the combined drying process, and then a descending drying process controlled by internal moisture.

Veri cation of the model
The drying time of Lycium barbarum was the shortest and the drying rate was the fastest when pretreated by Ultrasound & Blanchingand, as shown in Table1.Therefore, the process parameters of the veri cation experiment were as follows: plate spacing was 90 mm, vacuum degree was 0.025 MPa and the hot air temperature was 55°C, the Lycium barbarum was pretreated by Ultrasound & Blanchingand before dried.
The comparison between the experimental value of MR and the simulated value of Weibull distribution function was shown in Fig. 3.The determination coe cient R 2 of the validation model was 0.99835 obtained by tting analysis, indicating that the experimental values are in good agreement with the predicted value of the model, and the weibull model could better re ect the variation of moisture ratio in the process of Lycium barbarum RF-hot air combined drying after different pretreatments.Color difference and rehydration rate of dried wolfberry under different pretreatments differed signi cantly (P < 0.05; Table 2).The L* values of all dried wolfberry were less than those of fresh samples, and the L* values of pretreated wolfberry were generally compared with the natural drying, indicating that RF-hot air combined drying could reduce the brightness of wolfberry epidermis and darken the color.This is because the Maillard reaction was intensi ed when the material was heated during combined drying, resulting in different degrees of browning and color degradation on the sample surface(Malaikritsanachalee et al, 2020).However, the surface brightness value L* of dried wolfberry treated by ultrasonic and blanching was higher than that by other pretreatment methods, which indicated that the pretreatment method had a better color protection effect on the samples.

Effect on Color and Rehydration
The b* values of dried wolfberry under different pretreatments were different.In addition to natural drying, the b* value of ultrasonic and blanching pretreatment was signi cantly higher than that of other methods, and the surface color of the sample was yellow and showed a good appearance.Judging from the total chromatic aberration ΔE, the color change of ultrasonic and in ltration was the smallest compared with natural drying, followed by ultrasonic pretreatment, and the change of NaCl solution immersion was the largest, which increased by 15.4%, 20.7%, and 35.4%, respectively, compared with the natural drying.This may result from the fact that sucrose in ltration reduced the activity of water molecules in wolfberry, inhibited the browning reaction, and thus protected color effectively (Mehta et al., 2017).In addition, NaCl solution has high permeability, which accelerated dehydration and led to expansion of materials, thus affecting the color quality of wolfberry (Onal et al., 2019).
From the perspective of rehydration performance, the rehydration rate of dried wolfberry was signi cantly reduced compared to natural drying (P < 0.05) after RF-hot air combined drying.The reason is that the combined drying made the material quickly warm while destroying its cell structure, which increased the elasticity of cell structure, prevented water from penetrating into the kernel of wolfberry and thus reduced the recovery performance of cells.The rehydration rate of dried wolfberry decreased by 24.2% after ultrasonic and in ltration pretreatment compared with natural drying, but signi cantly increased compared with other pretreatment methods (P < 0.05).This is because sucrose in ltration increased the content of soluble solids in the material, enriched cells and strengthened tissue structure, which combined with the mechanical effect and cavitation effect of ultrasonic, could make the internal structure of dried wolfberry become uffy and porous, and improved its rehydration performance (Bozkir Hamza et al., 2019).The rehydration rate of the material is the lowest after blanching pretreatment, which is 36.8%lower than natural drying.This may be the result of the short-term high temperature of blanching treatment which caused the solidi cation of protoplasts in wolfberry cells, destroyed their integrity, and further affected the rehydration performance of materials(Doymaz Ibrahim, 2010).

Effects on Total Sugar
RF-hot air combined drying can signi cantly improve the retention rate of polysaccharides in dried wolfberry compared with natural drying (Fig. 4(a)).This is because the combined drying can enhance the drying rate effectively, shorten the drying cycle of the material, better retain its effective components, and reduce the loss of polysaccharides.The polysaccharide content of dried wolfberry under combined treatment by ultrasonic and in ltration was the highest (0.73 g/g), which was 54.8% and 32.9% higher than that of natural drying and Na 2 CO 3 dewaxing, respectively.The main reason is that the medium molecules of sucrose solution in ltrated into the wolfberry and thus increased the polysaccharide content during the combined pretreatment.Moreover, the cavitation effect of ultrasound could increase the number of micropores on the surface structure, which was conducive to more sucrose molecules entering into the material to improve its polysaccharide content (Doymaz Ibrahim, 2010).It was also found that the polysaccharide content decreased by 14.3% after NaCl impregnation compared with Na 2 CO 3 dewaxing treatment.The reason may be that because of the high permeability of NaCl solution, the evaporation of water inside wolfberry during drying increased the concentration of salt solution, and the in ltration and dehydration inside the material resulted in the separation of cytoplasmic wall, which increased the loss of sugars.(Bozkir Hamza et al., 2019).

Effect on Total Flavonoids
The total avonoid content of dry wolfberry obtained by combined drying was signi cantly higher than natural drying (Fig. 4(b)).Given the broad activity of avonoids, rapid warming of combined drying resulted in a large release of avonoids from the wolfberry matrix, which in turn increased the total avonoid content of the material.. Total avonoid content of the sample was the highest (1.79 mg/g) after ultrasonic and blanching treatment, which was 58.7% and 16.2% higher than that of natural drying and conventional Na 2 CO 3 dewaxing treatment, respectively.This is because the ultrasonic and blanching treatment are strong thermodynamic and physical effects, and the rapid drying after combined pretreatment inactivated oxidases of phenolic substances, which is conducive to the accumulation of avonoids(Ahmed, Maruf & Eun, Jong-Bang, 2018).Then, the mechanical effect of ultrasonic treatment would break the cell wall inside the material, allowing avonoids to be more easily extracted (Wiktor, A et al., 2019).In addition, the total avonoid content of the sample pretreated with Na 2 CO 3 dewaxing was 3.34% higher than that of NaCl impregnation.This may be because the increase of the mass concentration of NaCl impregnation medium in the material during the drying process resulted in the separation of cytoplasm wall and the decomposition of avonoids during the heating process(Ben Abdallah, S et al., 2016).

Effect on Total
The phenol content in dried wolfberry increased obviously compared with natural drying after RF-hot air combined drying (Fig. 4(c)).The reason is that phenols are highly active and the heating rate of combined drying is relatively fast, which promoteed the polymerization of phenols in the thermal eld and thus increased the total phenol content.Moreover, combined drying greatly shortened the drying cycle and reduced the loss of phenols in the sample.It can be seen from the gure that the total phenol content of dried wolfberry treated by NaCl impregnation was the highest (8.77 mg/g), which was 51.8% and 11.8% higher than that of natural drying and Na 2 CO 3 wax removal dewaxing treatment, respectively.This is due to the fact that all the other pretreatment methods can lead to different degrees of wax layer loss on the surface of wolfberry except for NaCl impregnation treatment, Which resulted in the loss of active substances and the dissolution and destruction of phenolic substances while increasing the drying rate(Augusto Batista de Medeiros et al., 2022)..The total phenol content (7.74 mg/g) of the sample by ultrasonic treatment was 3.62% higher than that of Na 2 CO 3 dewaxing treatment.This was mainly due to the short drying time after ultrasonic pretreatment, which reduced the oxygen contact time of phenols and improved the retention rate.The total phenol content of samples treated by blanching and in ltration decreased by 15.8% and 11.9%, respectively, compared with Na 2 CO 3 dewaxing treatment.This is because the oxidation of phenolic compounds was intensi ed after the cell structure was damaged by blanching pretreatment, and a large number of phenolic compounds are degraded because the drying rate of in ltration treatment is relatively low(Araújo, A.C et al., 2016).

Effect on Antioxidant Activity
Antioxidant activity of plants is derived from the synergistic inhibition of oxidative chain reaction by antioxidants such as phenolic compounds and is re ected by the phenol content in the sample and the scavenging capacity of free radical 1, 1-diphenyl-2-trinitrophenylhydrazine.In this study, the antioxidant activity is expressed by inhibition rate, and a greater inhibition rate means a stronger oxidation resistance (Nencini C et al., 2011).The antioxidant activity of dried wolfberry obtained by RF-hot air combined drying was signi cantly higher than that of natural drying, and the degradation of avonoids, phenols, and other antioxidants in the material was inhibited effectively because the combined drying has a fast heating rate and short drying time, thus improving antioxidant activity (Fig. 4(d)).Moreover, it was found that the inhibition rate of dried wolfberry treated by ultrasonic and blanching was the highest (60.78%) compared with other pretreatment methods, and the rate was 43.7% and 22.9% higher than that of natural drying and conventional Na 2 CO 3 dewaxing, respectively.A fast drying rate after ultrasonic and blanching combined pretreatment with rapid blanching can inactivate the oxidase in wolfberry and inhibit the degradation of phenolic compounds, and then release antioxidants through non-enzymatic reactions, thus increasing the inhibition rate (Thuwapanichayanan, R et al., 2014).In addition, the inhibition rates of NaCl and NaOH impregnation were 10.1% and 16.8%, lower than those of conventional dewaxing treatments, respectively, indicating that these two pretreatment methods may reduce the antioxidant activity of dried wolfberry.

on VC
V C of the combined drying wolfberry was signi cantly higher than that of natural drying (Fig. 4(e)), which may be because the chemical instability of vitamin C makes it easy to be oxidized and decomposed in the presence of heating, luminescence, and aerobic environment.In addition, the content of ascorbic acid would also be reduced if the drying temperature is too high or the time is too long(Sheng-Jiao Lei, et al., 2022).The RF vacuum drying stage was performed under the conditions of proper temperature, light avoidance, and vacuum.Moreover, combined drying shortened the drying time and effectively avoided the degradation of V C .The V C content of dried wolfberry obtained by ultrasonic and in ltration was the highest (3.96 mg/100 g) compared with other pretreatment methods, and the content was increased by 40.4% and 18.9% compared with natural drying and conventional Na 2 CO 3 dewaxing treatments, respectively.The reason is that the combined pretreatment enhanced the drying rate of materials, and that after the sugary medium in ltrated into the wolfberry cells by the effect of ultrasound, the cells were enriched and the density of cell tissue and structure were strengthened, thus greatly improving the retention rate of ascorbic acid greatly (Lyu, Ying et al., 2022).The ascorbic acid content of dried wolfberry decreased by 17.2% and 7.41% compared with Na 2 CO 3 dewaxing treatment, which was pretreated by ultrasonic and blanching, and blanching.The main reason is that V C is extremely sensitive to temperature, and blanching at a high temperature led to softening of wolfberry cell tissue, which destroyed the structure of V C and degrades it.

Effect on Betaine
RF-hot air combined drying can improve the betaine content of dried wolfberry effectively compared with natural drying (Fig. 4(f)).This is mainly because as an osmotic regulator similar to amino acid amphoteric ions, betaine has strong hygroscopicity and its content decreases with the increase of drying temperature and time(Sadowska-Bartosz Izabela & Grzegorz Bartosz, 2021).In this context, the proper temperature and combined drying can accelerate the drying rate and shorten the drying time, thus retaining the betaine components better in the material.The betaine content of dried wolfberry obtained by ultrasonic and in ltration pretreatment was the highest (2.72%), which was 46.7% and 13.2% higher than that of natural drying and conventional Na 2 CO 3 dewaxing, respectively.Since betaine is an internal quaternary ammonium compound, its content is closely related to the environmental conditions and betaine aldehyde dehydrogenase activity.It is possible that ultrasound and in ltration treatment signi cantly stimulated and accumulated betaine aldehyde dehydrogenase under the adversity conditions of RF electric eld, thus increasing the betaine content of materials (Luengo et al., 2016).The betaine content of dried wolfberry decreased by 14.4% and 7.63% compared with Na 2 CO 3 dewaxing treatment, which was pretreated by sucrose in ltration and NaCl impregnation, respectively.The main reason is that the drying rate of this pretreatment method is slower and more time-consuming compared with combined drying, resulting in the decrease of betaine retention rate.

Analysis
Figure 5(a) Fig. 5(b) show that the surface of fresh wolfberry is smooth and orderly arranged in bundles, and the wax layer is attached obviously, while the epidermis of wolfberry is arranged in a cordlike pattern after natural drying, with clear cellular tissue structure and some waxy layer lm fragments..The microstructure of wolfberry under different pretreatment varied, as cells on the material surface were arranged in a regular pattern but presented with dense micropores and bubbles after ultrasonic pretreatment (Fig. 5(c)).This is because the cavitation effect expands the cell gap when ultrasonic sound impacts the material repeatedly(Bengang Wu, et al., 2021).The surface of wolfberry was smooth and relatively stretched when pretreated by blanching (Fig. 5(d)).This is due to the thermal effect of shortterm blanching, which made the material tissue cells soften and helped to mitigate the shrinkage caused by drying (An NN et al., 2022).The surface structure of wolfberry was arranged closely, and there were colloidal substances attached when pretreated by sucrose in ltration (Fig. 5(e)), which is mainly due to sucrose molecules adsorbed on the surface tissue cells(R.Lemus-Mondaca et al., 2009).When pretreated by ultrasonic and blanching (Fig. 5(f)), the surface of the dried wolfberry was arranged loosely in bundles, and the local surface became rough and concave.
It may be that blanching can destroy the structure of the cell bio lm, resulting in the adhesion of wolfberry epidermal cells, while the mechanical effect of ultrasonic treatment makes the softened internal organizational structure produce internal crack and collapse during drying (Ren, F et al., 2018).By contrast, ultrasonic and in ltration pretreatment caused wolfberry surface tissue cells to be arranged regularly, yet the number of micropores and bubbles increased (Fig. 5(g)).This may be because sucrose molecules enrich tissue cells, strengthen tissue structure, and consolidate micropore channels (Xu, B et al., 2022).The surface microstructure of dried wolfberry treated with Na 2 CO 3 solution (Fig. 5(i)) was more regular than that treated with NaOH (Fig. 5(h)) or NaCl (Fig. 5(j)) and the wax layer particles were decreased.This is due to the residual salts on the wolfberry surface after pretreatment by NaCl impregnation and wax layer removal by Na 2 CO 3 solution.

Conclusions
In this study, fresh wolfberry was dried by RF-hot air combined segmented drying, and effects of different pretreatment methods on its drying characteristics and quality were explored.The results showed that the drying time was the shortest (13.5 h) when ultrasonic and blanching pretreatment was adopted before the combined drying of wolfberry, which was 20.6% shorter than that of conventional Na 2 CO 3 dewaxing treatment.The total avonoid content of dried wolfberry was the highest (1.79 mg/g), and the antioxidant activity was the strongest (I = 60.78%).Those two quality indexes increased by 58.7% and 43.7%, respectively, compared with natural drying, and increased by 16.2% and 22.9%, respectively, compared with Na 2 CO 3 .It was showed that U&B pretreatment could accelerated the drying rate and retained avonoids of wolfberry effectively.When ultrasonic and in ltration pretreatment was used, the dried wolfberry had the smallest change of total color difference ΔE, the highest rehydration rate (48.30%), the highest polysaccharide content (0.73 g/g), the highest betaine content (2.72%), and the highest Vc content (3.96 mg/100 g).The three nutritional ingredients increased by 54.8%, 46.7%, and 40.4%, respectively, compared with natural drying, and increased by 32.9%, 13.2% and 18.9%, respectively, compared with Na 2 CO 3 dewaxing treatment.This indicated that U&I pretreatment could protected color and retained nutrients such as sugars and Vc effectively during the drying process of wolfberry.The total phenol content (8.77 mg/g) of dried wolfberry pretreated by NaCl impregnation was the highest, which was 51.8% and 11.8% higher than that of natural drying and Na 2 CO 3 dewaxing drying, respectively.It showed that NaCl impregnation treatment was bene cial to retained phenolic substances in the drying process of Lycium barbarum.
The Weibull distribution function was used to t the RF-hot air combined drying curves of Lycium barbarum after different pretreatments.The average value of R 2 , RMSE and χ 2 was 0.99457, 0.00363 and 2.0466 × 10 − 4 respectively, and the variation described by the size parameter α and the shape parameter β is consistent with the drying characteristics in the drying characteristic curve.
In terms of microstructure, the dried wolfberry surface structure was arranged relatively neatly after natural drying, but there were bubbles generated after ultrasonic pretreatment.The surface was more stretched after heat blanching pretreatment and the colloidal substances were produced on dried wolfberry surface after sucrose in ltration pretreatment.Ultrasonic and blanching pretreatment resulted in epidermal tissue collapse of dried wolfberry.The wax layer particles on the dried wolfberry surface pretreated with Na 2 CO 3 solution were reduced more signi cantly than pretreated with NaOH solution, and there were salt residues on the wolfberry surface.After pretreatment by NaCl impregnation, the epidermal tissue of dried wolfberry was arranged more regularly and the number of micropores and bubbles increased.This ensured energy conservation and e ciency while improving its economic value.Effects of different pretreatment conditions on the quality of L. barbarum.Abbreviations: U&B, ultrasound and heat blanching; U&I, ultrasound and in ltration in Figure 4(a)-(f).Signi cant differences are represented by different letters (P < 0.05).

Figures
Figures

Figure 2 Combined
Figure 2

Table 1 The
Simulation results of Weibull distribution function that Lycium barbarum was combined drying under different pretreatment conditions

Table 2
Color and rehydration rate of dried L. barbarum under different pretreatments Note: Different letters in the same column indicate signi cant differences (P < 0.05).