
==== Front
Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00289-X
10.1016/j.ultsonch.2024.107041
107041
Original Research Article
Influence of ultrasonic pretreatment on the quality attributes and pectin structure of chili peppers (Capsicum spp.)
Nian Xin a
Wang Jitao bc
Wang Mengze wangmengze@nxu.edu.cn
a⁎
Wang Yaqi a
Liu Shiwei c
Cao Yudan a
a School of Food Science and Technology, Ningxia University, Yinchuan, Ningxia 750000, China
b School of Civil and Hydrulic Engineering, Ningxia University, Yinchuan, Ningxia 750000, China
c Horticulture Technology Extension Center of Ningxia, Ningxia 750000, China
⁎ Corresponding author. wangmengze@nxu.edu.cn
28 8 2024
11 2024
28 8 2024
110 10704128 6 2024
11 8 2024
20 8 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Graphical abstract

Chili peppers (Capsicum spp.) exhibit a diverse range of quality characteristics and pectin structures, which are influenced by various factors. This study aimed to investigate the effects of ultrasound (US), ultrasonic combined hot blanching (US-BL), and ultrasonic combined freezing and thawing (US-FT) on the quality characteristics and pectin structure of vacuum pulsation-dried (VP) chili peppers. The results indicated that US-BL samples exhibited the highest L* and a* values, retained maximum capsorubin, and showed an increase in vitamin C, total phenols, and rehydration by 14.28 %, 40.87 %, and 8.66 %, respectively. In contrast, the US-FT samples exhibited the highest capsaicin and dihydrocapsaicin content, which increased by 54.97 % and 64.04 %, respectively. Pretreatment resulted in higher pectin linearity, a lower degree of branching, and a reduced molecular weight in the US-BL sample. Atomic force microscopy confirmed the degrading effect of pretreatment on the pectin structure. Pearson’s correlation analysis revealed that capsorubin, capsaicin analogs, vitamin C, and total phenols were highly correlated with pectin linearity and molecular weight. This study found that US-BL was the most effective pretreatment method for improving the quality of pulsatile chili peppers and provides theoretical support for the application of VP chili peppers.

Keywords

Chili peppers
Pretreatment
Pectin structure
Quality
==== Body
pmc1 Introduction

Capsicum, commonly known as pepper seed, belongs to the Solanaceae family and is one of the most widely grown cash crops worldwide. Capsicum is rich in capsorubin, capsaicin, vitamins, phenols, and other antioxidants, and is widely favored by consumers because of its color and flavor characteristics. Peppers contain 65–80 % water, making them prone to spoilage and have a short shelf life. To extend their shelf life, they are often dried [1]. Natural drying and hot air drying are the most commonly used drying methods for chili peppers. Natural drying is a low-cost method that is susceptible to climatic conditions, which makes it challenging to maintain quality control, whereas hot-air drying is simple to operate but has a long drying time, high temperatures, and contact with oxygen, resulting in severe deterioration of the quality of the dried product [2]. In recent years, vacuum freeze-drying technology has been applied to large-scale drying and processing of fruits and vegetables, which can effectively preserve the color, aroma, taste, shape, and active ingredients of fresh products [3]. However, this technology has low drying efficiency, high energy consumption, and product brittleness, limiting its application in the drying of chili peppers [4]. A new vacuum pulsation drying technology has been developed that has high drying efficiency and low energy consumption but produces slightly poor quality products. Moreover, the catalytic activity of enzymes during drying results in the loss of capsorubin, capsaicin, dihydrocapsaicin, and the degradation of vitamin C. Therefore, improving the drying efficiency while maintaining excellent color and good spiciness is a critical issue that must be addressed to popularize and apply this technology, which has become an urgent problem in the industry [5].

Conventional pretreatments such as ultrasonic, blanching, freezing and thawing, microwave, and osmosis have been found to significantly improve the quality of dried fruits and vegetables [6], [7]. For instance, US can alter the cellular structure of fruits and vegetables, form microchannels, and improve their texture and color [6], [8]. Similarly, BL can inhibit enzyme activity, prevent discoloration, and improve the antioxidant capacity of dried fruits and vegetables, which is directly related to softening of cellular tissues and increased permeability [9], [10]. During FT, the formation of ice crystals can cause cellular damage and reduce cell wall adhesion, thereby facilitating nutrient release [11], [12]. At present, a single pretreatment method is widely used for dried fruits and vegetables, and there is still a need to further explore the effect of combined pretreatment on the quality of dried fruits and vegetables.

Pectin plays a crucial role in determining the quality of fruits and vegetables. Studies have shown that there are interactions between cell-wall pectin polysaccharides and polyphenols. The higher the pectin content, the more soluble phenolics, the more linear the pectin structure, and the fewer the branching regions, the better they bind to proanthocyanidins Additionally, pectin polysaccharides can reduce the hardness of fruit and vegetable tissues by weakening cell wall adhesion and strength [13]. However, the effect of pretreatment on the properties of pulsatile chili peppers and the relationship between quality and pectin properties have rarely been reported.

The main objective of this study was to investigate the effects of US, US-BL, and US-FT on the quality characteristics (capsorubin, capsaicin, dihydrocapsaicin, vitamin C, polyphenols, color, hardness, water content, and rehydration), microstructure, and pectin structure of VP peppers. Additionally, this study aimed to elucidate the constitutive relationship between pectin characteristics and quality. The ultimate goal was to enhance the quality of chili peppers and provide theoretical and technical support for large-scale production of dried chili pepper products using vacuum pulsation drying (VP) technology.

2 Materials and methods

2.1 Raw materials

Chaotian pepper (JiaoYan 8) was obtained from the pepper planting base of Pengyang County, Ningxia, China. Chaotian peppers were carefully selected as raw materials because of their consistent size, color, maturity, and lack of mechanical damage. After harvest, the chili pepper were transported to the laboratory using a 5 ℃ refrigerated truck at 85 % humidity, and both washing and cutting were completed within 2 h. Subsequently, the peppers were cleaned, de-stemmed, and cut into 1.5 ± 0.5 cm segments.The initial moisture content of fresh peppers was measured to be 78 ± 1.28 % (wet basis) using a rapid moisture meter.

2.2 Pretreatment methods

2.2.1 Ultrasonic pretreatment

Chaotian pepper (200 ± 10 g) was weighed, placed in a beaker containing distilled water (material: liquid ratio of 1:5), and ultrasonicated at 30 °C for 20 min at 400 W, followed by vacuum pulse drying.

2.2.2 Ultrasonic combined hot blanching pretreatment

After ultrasonic pretreatment, the chili pepper segments were placed in a steamer, where they were boiled and preheated to 90 ± 4 °C for 90 s of steam blanching. The segments were cooled with cold water, drained, and dried using vacuum pulsation drying.

2.2.3 Ultrasonic combined freeze–thaw pretreatment

Following the ultrasonic pretreatment described in section 2.2.1, the chili pepper segments were frozen at −20 °C for 24 h and then thawed at 15 °C for 12 h. Subsequently, the segments were subjected to vacuum pulsation drying after completing the freeze–thaw procedure.

2.3 Vacuum pulsation drying

Chili peppers pretreated with ultrasound (US), ultrasonic combined hot blanching (US-BL), and ultrasonic combined freeze–thaw (US-FT) methods, as well as untreated (control) peppers, were dried using vacuum pulsation drying. The drying experiments were conducted at 50 °C with a pulsation ratio of 10:5, which means that the applied vacuum (7.5 kPa) and atmospheric pressure (101 kPa) were maintained for 10 and 5 min, respectively. Moisture content was measured every 2 min using a moisture rapid tester until the moisture content was below 13 %.

2.4 Determination of chemical quality of chili peppers

2.4.1 Determination of capsorubin

The metabolite content was profiled using an Agilent Liquid chromatograph equipped with a VWD detector (HPLC1200, Agilent Technologies, USA) by Norminkoda Biotechnology Co., Ltd. (Wuhan, China). HPLC separation was performed using an EC-C18 guard column (250 mm*4.6 mm, 4 μm) at 35 °C. The mobile phase consisted of solvents A (acetonitrile) and B (0.02 mol/L ammonium acetate). The flow rate was 1 ml min−1 and the injection volume was 20 μL. Capsorubin content was expressed as mg g−1 DW [9].

2.4.2 Determination of capsaicinoids

Extraction and determination of capsaicin and dihydrocapsaicin were performed following the method described in GB/T21266-2007 [10].

2.4.3 Determination of vitamin C content

The ascorbic acid content in chili peppers was determined using the 2,6--dichloroindophenol titration method detailed in GB 5009.86–2016 [11].

2.4.4 Determination of polyphenol content

Total phenol content (TPC）was determined using the Folin-Ciocalteu colorimetric method [14]. Briefly, 2.5 mL of Folin-Ciocalteu reagent (diluted 1:10 with distilled water) was added to 500 μL of the sample extract, and the mixture was placed in a glass tube. Subsequently, 2 mL of 20 % sodium carbonate was added, and the sample was vortexed for 10 min and allowed to react for 30 min at room temperature. The absorbance was measured at 760 nm using a UV spectrophotometer, with 80 % methanol serving as the blank. The TPC was expressed as milligrams of gallic acid equivalent per gram of dry weight of the sample (mg GAE/100 g).

2.5 Determination of physical quality of chili peppers

2.5.1 Color determination

Pass the paprika through a 40-mesh sieve and scan with a handheld colorimeter (CR-20, Haiqin Technology, Beijing, China)to determine the color of the paprika. The brightness indices L*, a*, b*, redness (+a*), greenness (−a*), yellowness (+b*), and blue (−b*) of the samples were measured according to the CIELAB system.ΔE=(L∗-L0∗)2+a∗-a0∗2+(b∗-b0∗)2

2.5.2 Moisture content determination

The moisture content (MC) of the dried samples was determined using a rapid moisture meter (LHS20-HR, Shanghai, China) with an accuracy of 0.005 %.The specific operations are performed as follows:A precise weight of 5 g of the chili pepper was introduced into the rapid moisture meter and measured at a temperature of 105 °C.

2.5.3 Hardness determination

Hardness testing of the chili peppers was performed using a texture analyzer (TAXT2i Ltd., Vienna 153Court, UK) [8]. The probe was placed horizontally on a heavy platform with a pre-test, mid-test, and post-test speed of 2 mm/s and trigger force of 10 g.

2.5.4 Determination of rehydration

The rehydration rate of the chili pepper segments was determined by immersing 2 g of chili pepper segments in 200 mL distilled water at 25 °C for 30 min. After rehydration, the samples were removed, wiped with absorbent paper for surface water, and weighed. The rehydration ratio was calculated as the ratio of the mass of the rehydrated sample to that of the initial sample [3].

2.6 Microstructure analysis

The microstructures of the chili pepper samples were examined using scanning electron microscopy (SEM). Dried samples were mounted on a sample holder, coated with a layer of gold, and observed. The magnifications were 100×, 300×, and 500× [15].

2.7 Pectin structural characterization

2.7.1 Extraction and purification of pectin

The procedure for extracting and purifying pectin from chili peppers involved pulverizing the dried chili peppers and passing them through a 40 mesh sieve to obtain chili powder. Capsicum powder (50 g) was extracted with 1 L of water (pH 2.0) at 90 °C for 1 h and the residue was then centrifuged. This process was repeated three times, and the extract was separated by centrifugation at 4100 r/min for 20 min and then concentrated under reduced pressure. An amount of ethanol equivalent to three volumes of sample was added to the concentrate to precipitate pectin. After storage at 4 °C for 12 h, pectin was collected and washed twice with 95 % ethanol. Finally, the pectin was freeze-dried for 24 h [16]. The pectin extracted from the Control, US, US-BL and US-FT pretreatments was labeled as CP, UP, UBP, and UFP, respectively.

2.7.2 Analysis of monosaccharide composition

The monosaccharide composition of pectin was analyzed using a Thermo ICS5000 ion chromatography system equipped with an electrochemical detector. The pectin sample (5 mg) was accurately weighed in a clean chromatographic vial and 1 mL of TFA (2 M) acid solution was added. The sample was heated at 121 °C for 2 h, blown dry by passing nitrogen gas, and cleaned with methanol. The same procedure was repeated three times. Sterile water was added to dissolve the sample, which was then transferred to a chromatographic vial for analysis. A Dionex™ CarboPac™ PA20 (150 × 3.0 mm, 10 µm) liquid chromatography column was used with an injection volume of 5 μL. Mobile phase A (0.1 M NaOH), mobile phase B (0.1 M NaOH, 0.2 M NaAc) were used at a flow rate of 0.5 mL/min, and the column temperature was maintained at 30 °C for on-board testing [13].

2.7.3 Determination of molecular weight

The molecular weights of the various fractions were measured using SEC-MALLS-RI, which measures the homogeneity of the fractions. The sample fractions were added to a 0.1 M NaNO3 aqueous solution containing 0.02 % NaN3 and DMSO solution containing 0.5 % LiBr. The weight, number-average molecular weight (Mw and Mn), and polydispersity index (Mw/Mn) were measured using a DAWN HELEOS-II laser photometer (Wyatt Technology Co., USA) equipped with two tandem columns (300 × 8 mm, Shodex OH-pak SB-805 and 803; Showa Denko K.K., Tokyo, Japan) and three tandem columns (300 × 8 mm, Shodex OH-pak SB-805, 804, and 803; Showa Denko K.K., Tokyo, Japan) at 45 °C and 60 °C using a model column heater (Sanshu Biotechnology). Co., LTD (Shanghai, China). The flow rate was 0.6 mL/min and 0.3 mL/min. A differential refractive index detector (Optilab T-rEX, Wyatt Technology Co., USA) was simultaneously connected to provide the concentration of fractions and dn/dc value. The dn/dc value of the fractions was determined to be 0.141 mL/g in a 0.1 M NaNO3 aqueous solution containing 0.02 % NaN3, and 0.07 mL/g in a DMSO solution containing 0.5 % LiBr [17].

2.7.4 Fourier transform infrared spectroscopy determination

The molecular structure of pectin was analyzed by Fourier transform infrared spectroscopy (FTIR). Dried samples (1 mg) and KBr (0.5 g) were weighed using a dry agate mortar. The samples were then crushed and pressed. The samples were then subjected to FTIR equipment with a frequency range of 500–4000 cm with 16 scans [18].

2.7.5 X-ray diffractometry

X-ray diffraction is a method that reflects the crystalline properties and crystallinity of a sample by the diffraction phenomenon generated by X-rays in the crystal. XRD analysis was performed on the chili pectin samples using an X'Pert PRO instrument [19]. The measurement parameters were as follows: copper target, scanning speed of 4°/min, and scanning range of 5-45°.

2.7.6 Atomic force microscopy determination

AFM was used to analyze the extracted pectin grade, which was diluted 100 times with ultrapure water. A 3 μL sample of the resulting solution was then placed on freshly cleaved mica. The specimens were dried overnight in a desiccator at 22 °C and stored until AFM observations. AFM imaging was conducted using Multimode 8 with a Nanoscope V controller (Bruker) in semi-automated knockout mode [20].

2.8 Data statistics and analysis

Data are presented as the mean ± standard deviation of three parallel measurements. Statistical analysis was performed using SPSS 22.0, and analysis of variance (ANOVA) was used to assess any significant differences between the various pretreatment methods at p < 0.05. Duncan's multivariate analysis was also employed to compare mean values of main effect and interactions between variables. Data processing and calculations were performed using Excel 2016 software and graphs were plotted using Origin 8.0. Pearson’s correlation analysis was used to examine the relationship between the quality and pectin structure of chili peppers.

3 Results and discussion

3.1 Effect of pretreatment on chemical quality of chili peppers

3.1.1 Determination of capsorubin content

Capsorubin, an important coloring substance in chili peppers, primarily exists in the skin of red chili peppers and affects their selling price. The impact of different pretreatments on capsorubin content is shown in Fig. 1-A. Compared to the Control, the capsorubin content in the US and US-FT samples decreased by 6.67 % and 18.75 %, respectively, whereas no significant difference was observed in US-BL (p > 0.05). This trend aligns with the redness value (a*) shown in Table 1.The loss of capsorubin in the US-BL samples was less due to hot blanching. Hot blanching inactivates biologically active enzymes, alleviates enzymatic oxidative degradation, disrupts pepper cell walls and cell membranes, and increases the dissociation of capsorubin from protein complexes, thereby enhancing their extractability [7], [21]. The most significant loss of capsorubin in the US-FT samples resulted from the severe disruption of the pepper epidermis due to freeze–thaw treatment, resulting in the loss of tissue protection from intracellular compounds, leading to the degradation of capsorubin [22]. Ordonez-Santos [23] found that the loss of bioactive compounds such as vitamin C, vitamin E, and phenols was detrimental to the maintenance of capsaicinoid stability, as confirmed by the sharp decrease in vitamin C (Fig. 1-C) and polyphenol content of the US-FT samples (Fig. 1-D).Fig. 1 Effect of different pretreatment methods on the quality of pepper.Control:un-pretreatment, US: ultrasonic pretreatment, US-BL: ultrasonic combined with hot blanching pretreatment, US-FT: ultrasonic combined with freeze–thaw pretreatment.A:capsanthin, B:Capsaicin and dihydrocapsaicin, C: Vitamin C, D: Total phenolic content.

Table 1 The moisture content, hardness and color of pepper change after different pretreatment.

	Control	US	US-BL	US-FT	
Apparent
characteristic					
MC (%)	9.88 ± 0.05a	7.78 ± 0.02b	6.69 ± 0.09c	5.91 ± 0.01d	
Drying time(min)	710 ± 12.00a	600 ± 10.00b	505 ± 4.00c	440 ± 8.00d	
Hardness (N)	631.11 ± 26.79a	442.34 ± 27.67b	401.97 ± 21.33c	359.12 ± 21.11d	
Rehydration (g/g)	3.81 ± 0.01c	3.99 ± 0.00b	4.14 ± 0.02a	4.00 ± 0.00b	
L*	42.35 ± 2.25a	41.26 ± 1.39a	48.16 ± 0.78b	46.49 ± 0.02c	
a*	34.22 ± 0.87a	31.91 ± 1.16b	34.70 ± 0.29a	29.77 ± 0.31c	
b*	27.96 ± 1.20b	27.35 ± 2.12b	31.98 ± 0.42a	29.27 ± 0.40b	
△E	---	3.17 ± 0.32c	7.11 ± 0.63a	5.81 ± 0.59b	
Different superscript letters in the same row indicate a significant difference (p < 0.05).L* is the whiteness value, a* is the redness value, b* is the yellowness value, and ΔE is the total color difference.

3.1.2 Determination of capsaicinoids content

Capsaicinoids are specific to the Capsicum genus, comprising capsaicin and dihydrocapsaicin, accounting for approximately 90 % of the total capsaicinoids, and are the main spicy substances in chili peppers [24]. As shown in Fig. 1-B, the capsaicin content in the US, US-BL, and US-FT samples increased by 29.04 %, 29.19 %, and 54.97 %, respectively, and dihydrocapsaicin increased by 31.46 %, 38.20 %, and 64.04 %, respectively, compared to the control (p < 0.05). The observed increase in capsaicin and dihydrocapsaicin content is attributed to the cavitation effect of US, which ruptured cell walls and facilitated the release of capsaicin-like compounds. BL and FT further destroyed the cell structure of chili pepper, weakening the cell structure of chili peppers. The results indicated that US-FT caused the most serious destruction of the cell structure (see Fig. 2), which was more conducive to the dissociation of spicy substances, resulting in an increased release of these compounds.Fig. 2 SEM images of the pepper after pretreatment. A1-A3: SEM image of control samples, B1-B3: SEM image of ultrasonic samples, C1-C3: SEM image of samples ultrasonic combined with hot blanching pretreatment, D1-D3: SEM image of samples ultrasonic combined with freeze–thaw. The magnification was set to 100 (1), 300 (2), and 500 (3).

3.1.3 Determination of vitamin C content

Fig. 1-C shows the effect of different pretreatment methods on the vitamin C content of the samples. The vitamin C content decreased by 19.13 % and 68.98 % in the US and US-FT samples, respectively, and increased by 9.93 % in the US-BL samples compared to the control. This variation in vitamin C content may be attributed to the accumulation of free radicals in peppers owing to US, which accelerates the oxidation of vitamin C [25]. The US-FT samples exhibited the lowest vitamin C content, which was attributed to the formation of large ice crystals in the cells of peppers during the freeze-thawing treatment. These ice crystals cause irreversible damage to cell walls, intermediate lamellae, and protoplasts, leading to the release of large amounts of vitamin C from the cells. During prolonged thawing, released vitamin C is oxidized [26]. In contrast, BL inactivates ascorbate oxidase, which inhibits the degradation of vitamin C, thus facilitating the retention of vitamin C.

3.1.4 Determination of total phenolic content

The effects of the different pretreatment methods on the total phenol content are shown in Fig. 1-D. Compared to the control sample, the US and US-BL samples displayed increases of 19.26 % and 40.87 %, respectively, whereas the US-FT samples showed a decrease of 3.70 %. The increase in total phenolic content after US pretreatment was attributed to the “sponge effect” induced by ultrasound, resulting in the breakdown of covalent bonds and ruptured cell membranes, facilitating the release of phenolics bound to starch and protein matrices [27]. This process is also associated with the generation of hydroxyl radicals (OH−) by sonication, as the generation of the second hydroxyl group enhances the antioxidant capacity of the phenolic molecule [28]. The highest total phenol content (11.89 mg GAE/100 g DW) was found in the US-BL sample, primarily due to the inhibition of α-helices in polyphenol oxidases at high temperatures. The structure undergoes irreversible changes, leading to weakened degradation reactions, disruption of the cellular structure, and loosening of the cell wall pectin network structure by hot blanching, further improving the phenolic extraction rate [29]. In contrast, US-FT severely damaged the cellular structure of chili peppers, resulting in decreased polyphenols due to the loss of juice during thawing [11]. Similar results were reported by Chen et al., who compared dried blueberries [30].

In summary, capsorubin, capsaicin, dihydrocapsaicin, vitamin C, and total phenolic content were optimally retained and released in US-BL samples.

3.2 Effect of pretreatment on physical quality of chili peppers

3.2.1 Color

Color is an important indicator of the quality of chili peppers. The effects of different pretreatments on the color of chili peppers are shown in Table 1. Compared to the Control, both the US-BL and US-FT samples exhibited higher L* values, indicating less browning and brighter colors in the combined pretreated chili peppers. This improvement was attributed to the high drying efficiency of the US-BL and US-FT samples along with the short oxidation reaction time during drying. However, the slightly lower L* value of the US samples was attributed to the disruption of the cellular structure caused by US pretreatment, resulting in browning owing to the oxidation of vitamin C and polyphenols [31]. The a* values were redder in color, and the US-FT samples had the smallest a* values among the three pretreatments, which may be attributed to the degradation of capsorubin and the release of intracellular compounds such as acids during slow thawing [32]. The ΔE values for the US, US-BL, and US-FT samples were 3.17, 7.11, and 5.81, respectively, compared to the control, and were attributed to the severe cellular deformation, membrane rupture, and texture changes caused by the combined pretreatments, which resulted in changes in the internal scattered light and surface-reflected light [33].

3.2.2 Water content and drying time

Table 1 shows the effects of different pretreatments on the water content and drying time of the chili pepper. Compared to the Control group, the water content of the US, US-BL, and US-FT samples decreased by 2.1 %, 3.19 %, and 3.97 %, respectively, whereas the drying time was shortened by 15.35 %, 29.15 %, and 37.89 %, respectively (p < 0.05). This reduction in water content and shortening of drying time are attributed to the mechanical fluctuation and cavitation effects induced by US treatment, which cause cell structure damage, increase cell gap size, weaken water molecules and intermolecular forces, and convert bound water and semi-bound water to free water, thus reducing water migration resistance and accelerating the rate of water diffusion [8]. US-BL treatment further accelerates water diffusion owing to the additional cavitation effect, which softens the cell wall and increases cell wall permeability. The US-BL samples exhibited a reduction in the moisture content with less drying time among the treated samples [1]. The US-FT samples were mainly due to the growth of ice crystals during the FT process, resulting in irreversible damage to the cell wall, leading to the deposition of some phospholipid molecules and an increase in the number of microchannels [34]. The US-FT samples showed the lowest moisture content and shortest drying time among all the samples.

3.2.3 Hardness

Table 1 shows the effect of different pretreatments on the hardness of the chili pepper. The hardness values were significantly (p < 0.05) reduced in all pretreatment groups compared to the control group. Specifically, the hardness values of the US, US-BL, and US-FT samples decreased by 29.91 %, 36.31 %, and 43.10 %, respectively. The hardness of chili peppers was reduced because of the disruption of the adhesion of cell wall pectin caused by US and weakened the polysaccharide polymer structure [12]. BL degraded pectin and hemicellulose in the cell wall and weakened the mechanical strength of the cell wall, leading to lower hardness in the US-BL samples [35]. The hardness value of the US-FT samples was the lowest because freeze-thawing exacerbated the degree of damage to the cell structure of chili peppers, resulting in weakening of the conduit and sieve tube structures, deformation of the dense structures, and increase in porosity (Fig. 2). In summary, the combined pretreatment exacerbated damage to the cellular structure, resulting in a significant decrease in hardness, which is consistent with Zhang's findings [36].

3.2.4 Rehydration properties

Rehydration is an important factor in determining the quality of dried products and reflects the degree of damage to fruit and vegetable tissues. As shown in Table 1, all pretreatments considerably enhanced the rehydration properties of chili peppers (p < 0.05). The rehydration properties of the US, US-BL, and US-FT samples increased by 4.72, 8.66, and 4.99 %, respectively, compared to the control. This was attributed to the US cavitation effect that resulted in the formation of microscopic channels in chili peppers, and BL further induced structural changes in the cell walls and cell membranes, promoting an increase in the porous structure, which contributed to water filling, resulting in the highest rehydration capacity of US-BL samples. This result was consistent with the findings of Chao et al. [9]. In contrast, in the US-FT samples, freeze–thaw damaged the internal structure of chili peppers to a higher degree and could not maintain the original water-holding capacity, resulting in a slightly lower rehydration capacity than the US-BL samples. In conclusion, the increase in porosity and the formation of microscopic channels contribute to the water rehydration of chili peppers, and whether changes in the pectin structure in the cell walls play a role in the increase in porosity and the formation of microscopic channels needs further investigation.

It can be observed that there are two aspects of the effects of pretreatment methods on chili peppers. US-FT, which had the highest drying rate, did not have as high a nutritional quality as that of US and US-BL. Among the three pretreatments, US-BL was effective in shortening the drying time, while increasing or releasing capsorubin, capsaicin, dihydrocapsaicin, and vitamin C.

3.3 Effect of pretreatment on microstructure of chili peppers

The microstructure of fruits and vegetables plays a crucial role in determining the quality attributes of dried products, such as rehydration, color, texture, and nutrition [37]. The SEM images of the chili pepper samples are shown in Fig. 2. The Control (Fig. 2A1-A3) samples have dense cells with small and intact pores, which is due to the fact that pectin and hemicellulose molecules in the cell wall are tightly linked together to maintain the shape of the cell [38]. The “sponging” and “cavitation” effects of ultrasound promoted the formation of intercellular pores and microchannels, which is consistent with the findings of XU et al. [11]. Ultrasound pretreatment (US, US-BL, and US-FT) increased the porosity of the chili pepper samples. The hardness of the Control and US-BL samples was maintained, whereas that of the US-FT samples decreased slightly. This decrease in hardness is due to the displacement of cell wall pectin and hemifibril molecules by hot ironing, leading to tissue softening [7]. In the US-FT (Fig. 2D1-D3) samples, an intact cell wall structure was almost absent, and some large pores and voids could be clearly observed. This was due to the growth of ice crystals during the freezing process, which affected the cell membrane, leading to the deposition of some phospholipid molecules and the appearance of micropores [39]. Scanning electron microscopy results showed that ultrasound pre-treatment decreased the hardness, increased rehydration, and accelerated the drying rate of the US, US-BL, and US-FT samples.

3.4 Effect of pretreatment on the structure of pepper pectin

3.4.1 Analysis of monosaccharide composition

The effect of different pretreatments on the composition of pectin monosaccharides is shown in Table 2. Gal-UA was the predominant structural unit among the four pectin fractions, with relative molar ratios ranging from 37.63 % to 54.29 %. In addition to Gal-UA, chili pectin contained some neutral sugars such as Rha, Ara, Gal, Glc, Xyl, Man, and Ara (13.59–17.13 %) was the major neutral sugar in the pectin fractions, suggesting that the four pectins were arabinose-rich pectic polysaccharides. Compared to CP, the Gal-UA content in UP, UBP, and UFP increased by 20.49 %, 30.48 %, and 44.27 %, respectively, and the Glc content decreased by 34.43 %, 43.65 %, and 55.74 %, respectively (p < 0.05). This suggests that pretreatment disrupted the branched structure of pectin and increased its purity, with UFP having the highest purity [40].Table 2 Composition and sugar ratios of pectic monosaccharides after different pretreatments.

Neutral sugars
(molar mass ratio%)	CP	UP	UBP	UFP	
Rha	8.40 ± 0.02b	11.87 ± 0.02a	8.22 ± 0.05b	7.63 ± 0.03c	
Ara	17.13 ± 0.10a	16.17 ± 0.11b	15.16 ± 0.10c	13.59 ± 0.04d	
Gal	15.39 ± 0.04a	15.40 ± 0.03a	15.09 ± 0.06b	13.23 ± 0.11c	
Glc	4.88 ± 0.01a	3.20 ± 0.01b	2.75 ± 0.01c	2.16 ± 0.05d	
Xyl	5.06 ± 0.03a	3.59 ± 0.03b	3.54 ± 0.01c	3.34 ± 0.03d	
Man	5.77 ± 0.02a	1.14 ± 0.05d	3.58 ± 0.00c	3.66 ± 0.01b	
Gal-UA	37.63 ± 0.09d	45.34 ± 0.12c	49.10 ± 0.12b	54.29 ± 0.14a	
Glc-UA	5.76 ± 0.01a	3.30 ± 0.01b	2.56 ± 0.02c	2.11 ± 0.02d	
Ratio1 = GalA/(Rha + Ara + Gal + Xyl)	0.82	0.96	1.16	1.44	
Ratio 2 = Rha/GalA	0.22	0.26	0.17	0.14	
Ratio 3 = (Ara + Gal)/Rha	3.87	3.75	3.68	3.52	
Different superscript letters in the same row indicate a significant difference (p < 0.05).

The pectin contents of Rha, Ara, Gal, and Glc in pectin varied due to different pretreatments, and ratios 1, 2, and 3 were utilized to assess the linearity of the pectin backbone, degree of branching, and extent of branching of the RG-I structural domains to gain a deeper understanding of the structural characteristics of the pectin fractions [41]. As shown in Table 2, ratio 1 increased in UP, UBP, and UFP pectins compared to CP, indicating that the linearity of pectins was improved in all cases, with UFP having the highest linearity, followed by UBP and UP, which may be attributed to the hydrolysis and cleavage of pectin homogalacturonic acid α-1,4 glycosidic bonds induced by ultrasonication treatment. The structure of the homoglucuronic acid chain in the hot blanching treatment was cleaved by β-elimination splitting and leaching of pectin from the middle layer, leading to deformation of the pectin structure. Ice crystals rupture cell membranes during freeze-thawing to form loose pectin reticulation [9], [10]. When Ratio 2 was between 0.05–1, the pectin structure was dominated by the RG-I-type structure [16], and the molar ratios of CP, UP, UBP, and UFP were in the range of 0.14 to 0.26 in the current study indicated that all four pectin fractions were predominantly rhamnogalacturonic acid (RG-I type). The Ratio 3 of UP, UBP, and UFP were reduced by 3.10 %, 4.90 %, and 9.04 %, respectively, compared to CP, indicating that pretreatment, especially co-pretreatment, enriched the pectin main chains and weakened the side chains [41].

3.4.2 Determination of molecular weight

The molecular weight distribution of pectin was evaluated for different pretreatments, as shown in Table 3. The Mw/Mn values of the pectin fractions obtained from all four pretreatments were above 1, indicating the successful column fractionation of pepper pectin. Comparing the treatment groups to CP, Mn and Mw showed different degrees of reduction from high to low, with UBP, UP, and UFP having lower Mn and Mw values than CP. This reduction was attributed to the ultrasonic “cavitation effect,” which caused glycosidic bond breakage in pectin molecules, resulting in low-Mw pectin molecules. The cavitation bubbles generated during ultrasonic treatment led to the formation of relative displacement, friction, and shear forces, which caused degradation and breakage of pectin chains. Hot blanching exacerbated the β-elimination reaction, leading to further breakage of the galacturonic acid skeleton of pectin and a decrease in the molecular weight. Freeze thawing disrupted the side chains of pectin, leading to breakage of the neutral chain and induced cellular debonding, which resulted in a decrease in molecular weight. Although the molecular weight of UFP was higher than that of UBP and UP, this was probably due to the loss of small molecular weight pectins during freeze-thawing, resulting in a larger molecular weight of pectins extracted from it [20], [40]. In comparison, the molecular weight of UBP was higher than those of UP and UFP, which suggests that UBP contains more high-molecular-weight pectins. Reducing the molecular weight of pectin is a prerequisite for achieving its desired bioactivity. Low-Mw pectins exhibit lower viscosity, enhanced solubility, and increased exposure to bioactive groups, resulting in higher activity levels. This explains the higher content of vitamin C, polyphenols, and capsaicin in US-BL samples compared to other pretreatments.Table 3 Molecular weights of different pretreatment pepper pectin.

	CP	UP	UBP	UFP	
Mn (kDa)	182.61 ± 1.53a	167.46 ± 1.09c	119.67 ± 0.97d	173.58 ± 1.10b	
Mw (kDa)	290.72 ± 1.89a	261.34 ± 1.77c	205.16 ± 1.32d	280.25 ± 1.21b	
Polydispersity (Mw/Mn)	1.59 ± 0.05c	1.56 ± 0.02d	1.71 ± 0.02a	1.62 ± 0.01b	
Different superscript letters in the same row indicate a significant difference (p < 0.05).

In conclusion, US-BL pretreatment proved to be more effective in producing low-molecular-weight pectin, thereby facilitating the release of antioxidants, including capsaicin, vitamin C, and polyphenols.

3.4.3 Fourier transform infrared spectroscopy analysis

In the Fourier Transform Infrared Spectroscopy (FTIR) analysis, strong and broad peaks were observed at 3478 cm−1, which were attributed to intramolecular or intermolecular O-H stretching vibrations, mainly due to the intermolecular hydrogen bonding of polygalacturonic acid. The peaks between 2950 cm−1-2750 cm−1 were due to the stretching vibrations of the methyl galacturonate residues O-CH3. The peak at 1750 cm−1 was attributed to the C=O stretching vibration of the carboxyl and ester carbonyl groups, indicating the presence of acetyl groups. The peak near 1670 cm−1 was attributed to the asymmetric vibrational stretching of free carboxyl functional groups, indicating that all four extracts were pectin polysaccharides [42].

The weak peaks at 918 cm−1 and 855 cm−1 are due to β and α glycosidic bonds, respectively. Compared to CP, the peaks at 3401 cm−1 for UP, UBP, and UFP showed a significant blue shift, indicating that the hydrogen bonding in these three pectins was significantly weakened. Conversely, the absorption peaks near 2940 cm−1 were significantly strengthened, which was induced by the stretching and vibration of C-H (mainly including CH, CH2, and CH3). This suggests that pretreatment led to the breakage of glycosidic bonds, causing the opening or degradation of the pectin molecular chain structure [16]. The absorption peak of UFP at 2940 cm−1 was the strongest, indicating that most of the H atoms were eliminated from the pectin side chains under this treatment, resulting in the most linear pectin chains. This finding is consistent with the monosaccharide composition results.

3.4.4 X-ray diffraction analysis

X-ray diffraction (XRD) was employed to characterize the crystallinity of pectin, and the intensity of the diffraction peaks reflects the degree of crystallinity, which significantly influences the mechanical and thermal properties of food materials [43]. Fig. 3-B illustrates the effects of different pretreatments on pectin crystallinity. The pectins extracted from different pretreated samples exhibited similar profiles, all exhibiting crystalline and non-crystalline regions with two diffraction peaks at 2θ 12.9° and 20.9°, respectively. The signal was sharper and more intense at 20.9°, indicating that all pretreated samples retained the structural characteristics of pectins, which are typically amorphous or semicrystalline structures. The increase in peak intensity confirmed that the pretreatment attenuated hydrogen bonding at 19.8°, as evidenced by the enhanced peak intensity for UP, UBP, and UFP.Fig. 3 Effect of different pretreatment on the pectin structure. CP: Pectin extracted from the un-pretreated samples, UP:Pectin extracted from the ultrasonic-pretreated samples, UBP:Pectin was extracted by ultrasonic combined with hot blanching, UFP:Pectin extracted from samples pretreatment by ultrasonic combined with freeze–thaw. A: Effect of different pretreatment methods on FTIR diffraction of pepper pectin, B: Effect of different pretreatment methods on XRD diffraction of pepper pectin.

3.4.5 Atomic force microscopy analysis

Atomic force microscopy (AFM) can provide high-resolution images of pectin, allowing the characterization of linearity, branching degree, and associative structure [44]. AFM plots of pectin under different pretreatments are shown in Fig. 4. CP exhibited large clusters and long-chain structures (Fig. 4-A), whereas ultrasonic pretreatment (UP) resulted in reduced clusters and long-chain structures, with the appearance of small dots and short straight-chain structures (Fig. 4-B). A small number of chain structures were observed in UBP, primarily consisting of dense dots and a few clusters (Fig. 4-C). In contrast, ultrafast pretreatment (UFP) led to the formation of curled chain structures that formed clusters (Fig. 4-D). This was due to the fact that the ultrasonic treatment impaired the rigidity of the UP chain and increased the content of terminal carboxyl groups (–COOH), which could form intramolecular hydrogen bonds with hydroxyl and carboxyl groups, leading to the pectin chain fracture. Hot blanching exacerbated pectin skeleton fracture, increased the number of rigid short chains, and promoted aggregation of pectin molecules via thermal movement of the short chains and intermolecular hydrogen bonding. Meanwhile, freeze–thaw treatment influences the ice crystals, leading to the fracture of pectin glycosidic bonds, degradation, aggregation, and curling of pectin chains, and the formation of a doughnut-like structure [43].Fig. 4 Effect of different pretreatment methods on AFM of pepper pectin. CP:Pectin extracted from un-pretreated chilli peppers, UP: pectin extracted from ultrasonically pretreated chilli peppers, UBP: pectin extracted from ultrasonically combined with hot blanching pretreated chilli peppers, UFP: pectin extracted from ultrasonically combined with freeze–thaw pretreated chilli peppers. A:CP;B:UP, C: UBP, D: UFP. (Lc: long chain; Ls: large-scale agglomerated, Sp: spot, Ss: short straight chain.).

In summary, different pretreatments resulted in increased pectin linearity, decreased branching, and decreased molecular weight. UBP had the lowest molecular weight and more complete degradation than UP and UFP.

3.5 Correlation analysis between quality and pectin structure of chili peppers

Pectin plays a significant role in the physicochemical properties and nutritional characteristics of fruit and vegetable products and is most abundant in the primary cell walls and intermediate lamellae of plants. Its molecular structure is often altered during fruit and vegetable processing, which affects the quality of the final product [45]. To explore the relationship between pulsatile pepper pectin and quality, Pearson’s correlation analysis was conducted, as shown in Fig. 5. The |R| values represent different linear relationships and can be categorized into three ranges: |R| = 0.0–0.39, |R| = 0.4–0.59, and |R| = 0.6–1.0, which represent weak, moderate, or strong linear relationships between the indicators, respectively [46]. The analysis revealed that the linearity of capsorubin with Ratio1 and Ratio3 was −0.81 and 0.85, respectively. The linearity of capsaicin with Ratio1 and Ratio3 was 0.93 and −0.98, respectively,The linearity of dihydrocapsaicin content with Ratio1 and Ratio3 was 0.96 and −0.99, respectively.Indicating that the higher the linearity of pectin,the lower the capsorubin content, the higher the capsaicin and dihydrocapsaicinoid content, which contributes to the release of spiciness, leading to the oxidative degradation of capsaicinoids due to glycosidic bond rupture during pretreatment. Mw was highly negatively correlated with vitamin C, total phenolic content, and rehydration, at −0.50, −0.93, and −0.72, respectively. This suggests that the lower the molecular weight of pectin, the higher the capsicum Vitamin C content, total phenol content, and rehydration. This finding is attributed to the low molecular weight pectin, which had low viscosity, making the vitamin C and total phenol of chili peppers more soluble and better rehydrated. Additionally, hardness was negatively correlated with Gal-UA content, which is consistent with a previous study conducted on pretreated yellow peach slices [47].Fig. 5 Pearson correlation analysis between quality attributes and pectin structure. Circles indicate the positive (green) or negative (red) correlations between the indicators. (For the explanation of the color reference in this legend, please see the web version of this article.

Changes in pectin structure and intracellular compounds during pretreatment are shown in Fig. 6. The results indicated that the impact of US-BL was more significant than that of the other two pre-treatments. The mechanical effect and β-elimination reaction induced by US-BL resulted in the rupture of the cell wall of chili peppers, with breakage of the pectin chain and weakening of glycosidic hydrogen bonding. This led to the formation of a loose cell-wall pectin network structure, which facilitated the release of intracellular compounds including total phenolics, vitamin C, and capsorubin. The increase in total phenols and vitamin C content following hot blanching treatment better explains the increase in the a* value of chili peppers by US-BL. The mechanical disruption of chili pepper cell walls by US is less extensive than that by US-BL. Thus, the depolymerization of pectin chains in the microstructure was lower than that in US-BL. The drastic disruption of pectin chains by US-FT leads to the oxidative degradation of intracellular compounds during the treatment process, resulting in a decreased nutritive quality compared to US-BL. According to the AFM results, the degree of pectin chain depolymerization in UPB was significantly greater than that in the other treatments, presenting a homogeneous dispersed system, which is consistent with the fact that US-BL had higher qualities (color, capsorubin, total phenols, and vitamin C) (Fig. 1, Table 1). Overall, these findings suggest that US-BL is the most effective method for improving the quality of dried chili peppers.Fig. 6 Schematic diagram of the relationship between pectin structure and intracellular compound changes after different pretreatments. Control:un-pretreatment; US:ultrasonic pretreatment, US-BL: ultrasonic combined with hot blanching pretreatment, US-FT: ultrasonic combined with freeze–thaw pretreatment.

4 Conclusion

The results showed that all three treatments, namely US, US-BL, and US-FT, were effective in increasing the content of capsaicin and dihydrocapsaicin, reducing the drying time and hardness, and improving the rehydration of chili peppers. The destruction of cellular structure and inactivation of enzymes enhanced the overall quality of US-BL chili peppers and facilitated the retention of capsorubin and the release of vitamin C and total phenols. However, US-FT was found to cause irreversible structural damage and deteriorate the quality of chili peppers. The formation of US-FT ice crystals causes irreversible damage to the structure by generating ice crystals, resulting in severe deterioration of the quality of the peppers and loss or degradation of the active substances (capsorubin, total phenols, and vitamin C) during the slow thawing process. Pectin structure analysis of the samples showed that pretreatment increased the linearity of pectin, decreased the molecular weight and degree of branching, and the molecular weight of UBP was the lowest. Furthermore, the degradation of pectin molecular structure by pretreatment was further confirmed by AFM, and Pearson correlation analysis showed that the higher the pectin linearity, the lower the content of capsorubin and the higher the content of capsaicin and dihydrocapsaicin. Additionally, the lower the molecular weight of pectin, the more easily the vitamin C and total phenols of the chili pepper are solubilized and the better the rehydration. Therefore, the improvement in the quality of chili peppers is closely related to changes in their microstructure and pectin structure. This study provides supporting data for the development of Ultrasonic combined hot blanching pretreatment to improve the quality and economic benefits of pepper. Further studies are needed to investigate whether the interaction of other macromolecules with pectin would affect pepper quality.

CRediT authorship contribution statement

Xin Nian: Writing – original draft, Methodology, Investigation, Conceptualization. Jitao Wang: Investigation, Formal analysis. Mengze Wang: Writing – review & editing, Supervision, Resources, Funding acquisition, Conceptualization. Yaqi Wang: Investigation. Shiwei Liu: Validation. Yudan Cao: Software.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Key Research and Development Program in China (2021YFD1600305 ), Ningxia Hui Autonomous Region Young Top Talent Program (030103032208 ). The authors are thankful for their support.
==== Refs
References

1 Grimaldi M. Cavazza A. Pitirollo O. Zoccali M. Mondello L. Giuffrida D. Analytical evaluation of carotenoids, apocarotenoids, capsaicinoids, and phenolics to assess the effect of a protective treatment on chili peppers dried at different temperatures Eur. Food Res. Technol. 248 2022 2339 2349
2 Kaveh M. Abbaspour-Gilandeh Y. Fatemi H. Chen G. Impact of different drying methods on the drying time, energy, and quality of green peas J. Food Process. Preserv. 45 2021
3 Xu B. Chen J. Sylvain Tiliwa E. Yan W. Roknul Azam S.M. Yuan J. Wei B. Zhou C. Ma H. Effect of multi-mode dual-frequency ultrasound pretreatment on the vacuum freeze-drying process and quality attributes of the strawberry slices Ultrason. Sonochem. 78 2021
4 Liu Y. Zhang Z. Hu L. High efficient freeze-drying technology in food industry Crit. Rev. Food Sci. Nutr. 62 2021 3370 3388 33393368
5 Xie L. Mujumdar A.S. Fang X.-M. Wang J. Dai J.-W. Du Z.-L. Xiao H.-W. Liu Y. Gao Z.-J. Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) of wolfberry (Lycium barbarum L.): effects on drying kinetics and quality attributes Food Bioprod. Process. 102 2017 320 331
6 Zhang X. Wang Y. Nian R. Li Q. Zhu D. Cao X. Effects of ultrasonic pretreatment on drying characteristics and water migration characteristics of freeze-dried strawberry Food Chem. 450 2024
7 Xu X. Zhang L. Feng Y. ElGasim A. Yagoub A. Sun Y. Ma H. Zhou C. Vacuum pulsation drying of okra (Abelmoschus esculentus L. Moench): better retention of the quality characteristics by flat sweep frequency and pulsed ultrasound pretreatment Food Chem. 326 2020
8 Zhang L. Liao L. Qiao Y. Wang C. Shi D. An K. Hu J. Effects of ultrahigh pressure and ultrasound pretreatments on properties of strawberry chips prepared by vacuum-freeze drying Food Chem. 303 2020
9 Chao E. Li J. Fan L. Enhancing drying efficiency and quality of seed-used pumpkin using ultrasound, freeze-thawing and blanching pretreatments Food Chem. 384 2022
10 Wang J. Chen Y. Wang H. Wang S. Lin Z. Zhao L. Xu H. Ethanol and blanching pretreatments change the moisture transfer and physicochemical properties of apple slices via microstructure and cell-wall polysaccharides nanostructure modification Food Chem. 381 2022
11 Xu X. Zhang L. Feng Y. Zhou C. Yagoub A.E.A. Wahia H. Ma H. Zhang J. Sun Y. Ultrasound freeze-thawing style pretreatment to improve the efficiency of the vacuum freeze-drying of okra (Abelmoschus esculentus (L.) Moench) and the quality characteristics of the dried product Ultrason. Sonochem. 70 2021
12 Ando Y. Maeda Y. Mizutani K. Wakatsuki N. Hagiwara S. Nabetani H. Impact of blanching and freeze-thaw pretreatment on drying rate of carrot roots in relation to changes in cell membrane function and cell wall structure LWT Food Sci. Technol. 71 2016 40 46
13 Liu X. Renard C.M.G.C. Bureau S. Le Bourvellec C. Interactions between heterogeneous cell walls and two procyanidins: insights from the effects of chemical composition and physical structure Food Hydrocoll. 121 2021
14 Obajemihi O.I. Esua O.J. Cheng J.-H. Sun D.-W. Effects of pretreatments using plasma functionalized water, osmodehydration and their combination on hot air drying efficiency and quality of tomato (Solanum lycopersicum L.) slices Food Chem. 406 2023
15 Zhang R. Chen G. Yang B. Wu Y. Du M. Kan J. Insights into the stability of carotenoids and capsaicinoids in water-based or oil-based chili systems at different processing treatments Food Chem. 342 2021
16 Qin Z. Liu H.-M. Cheng X.-C. Wang X.-D. Effect of drying pretreatment methods on structure and properties of pectins extracted from Chinese quince fruit Int. J. Biol. Macromol. 137 2019 801 808 31255624
17 Chen P. You Q. Li X. Chang Q. Zhang Y. Zheng B. Hu X. Zeng H. Polysaccharide fractions from Fortunella margarita affect proliferation of Bifidobacterium adolescentis ATCC 15703 and undergo structural changes following fermentation Int. J. Biol. Macromol. 123 2019 1070 1078 30465831
18 Ognyanov M. Remoroza C. Schols H.A. Georgiev Y.N. Petkova N.T. Krystyjan M. Structural, rheological and functional properties of galactose-rich pectic polysaccharide fraction from leek Carbohydr. Polym. 229 2020
19 Yang N. Wang D. Geng Y. Man J. Gao Y. Hang Y. Zheng H. Zhang M. Structure, physicochemical characterisation and properties of pectic polysaccharide from Premma puberula pamp Food Hydrocoll. 128 2022
20 Qiu W.-Y. Cai W.-D. Wang M. Yan J.-K. Effect of ultrasonic intensity on the conformational changes in citrus pectin under ultrasonic processing Food Chem. 297 2019
21 Kaseke T. Opara U.L. Fawole O.A. Effect of microwave pretreatment of seeds on the quality and antioxidant capacity of pomegranate seed oil Foods 9 2020
22 de Mendonça K.S. Corrêa J.L.G. Junqueira J.R.d.J. Cirillo M.A. Figueira F.V. Carvalho E.E.N. Influences of convective and vacuum drying on the quality attributes of osmo-dried pequi (Caryocar brasiliense Camb.) slices Food Chem. 224 2017 212 218 28159258
23 Suo G. Zhou C. Su W. Hu X. Effects of ultrasonic treatment on color, carotenoid content, enzyme activity, rheological properties, and microstructure of pumpkin juice during storage Ultrason. Sonochem. 84 2022
24 Palma J.M. Terán F. Contreras-Ruiz A. Rodríguez-Ruiz M. Corpas F.J. Antioxidant profile of pepper (Capsicum annuum L.) fruits containing diverse levels of capsaicinoids Antioxidants 9 2020
25 Cheng L. Soh C. Liew S. Teh F. Effects of sonication and carbonation on guava juice quality Food Chem. 104 2007 1396 1401
26 Holzwarth M. Korhummel S. Carle R. Kammerer D.R. Evaluation of the effects of different freezing and thawing methods on color, polyphenol and ascorbic acid retention in strawberries (Fragaria×ananassa Duch.) Food Res. Int. 48 2012 241 248
27 Erihemu M. Wang F. Zhang D. Wang M. Zhao N. Cui G. Gao J. Guo Q. Zhang, Optimization of the process parameters of ultrasound on inhibition of polyphenol oxidase activity in whole potato tuber by response surface methodology Lwt 144 2021
28 Wang J. Xiao H.-W. Ye J.-H. Wang J. Raghavan V. Ultrasound pretreatment to enhance drying kinetics of kiwifruit (Actinidia deliciosa) slices: pros and cons Food Bioprocess Technol. 12 2019 865 876
29 Feumba Dibanda R. Panyoo Akdowa E. Rani P.A. Metsatedem Tongwa Q. Mbofung C.M.F. Effect of microwave blanching on antioxidant activity, phenolic compounds and browning behaviour of some fruit peelings Food Chem. 302 2020
30 Chen F. Zhang M. Devahastin S. Yu D. Comparative evaluation of the properties of deep-Frozen blueberries dried by vacuum infrared freeze drying with the use of CO2 laser perforation, ultrasound, and freezing-thawing as pretreatments Food Bioprocess Technol. 14 2021 1805 1816
31 Xu B. Feng M. Chitrakar B. Cheng J. Wei B. Wang B. Zhou C. Ma H. Multi-frequency power thermosonication treatments of clear strawberry juice: impact on color, bioactive compounds, flavor volatiles, microbial and polyphenol oxidase inactivation Innov. Food Sci. Emerg. Technol. 84 2023
32 Xing Y. Ma Q. Wang K. Dong X. Wang S. He P. Wang J. Xu H. Non-thermal treatments of strawberry pulp: the relationship between quality attributes and microstructure Ultrason. Sonochem. 98 2023
33 Zhang Z. Wang J. Zhang X. Shi Q. Xin L. Fu H. Wang Y. Effects of radio frequency assisted blanching on polyphenol oxidase, weight loss, texture, color and microstructure of potato Food Chem. 248 2018 173 182 29329841
34 Liu J. Bi J. McClements D.J. Liu X. Yi J. Lyu J. Zhou M. Verkerk R. Dekker M. Wu X. Liu D. Impacts of thermal and non-thermal processing on structure and functionality of pectin in fruit- and vegetable- based products: a review Carbohydr. Polym. 250 2020
35 Xu H. Guan Y. Shan C. Xiao W. Wu M. Development of thermoultrasound assisted blanching to improve enzyme inactivation efficiency, drying characteristics, energy consumption, and physiochemical properties of sweet potatoes Ultrason. Sonochem. 101 2023
36 Zhang Z. Yao Y. Shi Q. Zhao J. Fu H. Wang Y. Effects of radio-frequency-assisted blanching on the polyphenol oxidase, microstructure, physical characteristics, and starch content of potato Lwt 125 2020
37 Niamnuy C. Devahastin S. Soponronnarit S. Some recent advances in microstructural modification and monitoring of foods during drying: a review J. Food Eng. 123 2014 148 156
38 Tan L. Eberhard S. Pattathil S. Warder C. Glushka J. Yuan C. Hao Z. Zhu X. Avci U. Miller J.S. Baldwin D. Pham C. Orlando R. Darvill A. Hahn M.G. Kieliszewski M.J. Mohnen D. An Arabidopsis cell wall proteoglycan consists of pectin and arabinoxylan covalently linked to an arabinogalactan protein Plant Cell 25 2013 270 287 23371948
39 Li D. Zhu Z. Sun D.-W. Effects of freezing on cell structure of fresh cellular food materials: a review Trends Food Sci. Technol. 75 2018 46 55
40 Bao X. Zhang S. Xiao Y. Jiang Y. Liu Z. Wang T. Hu X. Yi J. Effect of pasteurization processing and storage conditions on softening of acidified chili pepper: pectin and it related enzymes Int. J. Biol. Macromol. 253 2023
41 Yu Q. Li X. Hu J. Wang W. Bi J. The effect of three pectin fractions variation on the browning of different dried apple products Food Hydrocoll. 134 2023
42 Yu M. Xia Y. Zhou M. Guo Y. Zheng J. Zhang Y. Effects of different extraction methods on structural and physicochemical properties of pectins from finger citron pomace Carbohydr. Polym. 258 2021
43 Kazemi M. Khodaiyan F. Hosseini S.S. Eggplant peel as a high potential source of high methylated pectin: ultrasonic extraction optimization and characterization Lwt 105 2019 182 189
44 Sriamornsak P. Thirawong N. Nunthanid J. Puttipipatkhachorn S. Thongborisute J. Takeuchi H. Atomic force microscopy imaging of novel self-assembling pectin–liposome nanocomplexes Carbohydr. Polym. 71 2008 324 329
45 Shi Q. Zou M.-Y. Song M.-M. Wang J.-H. Zhao H.-W. Xiong S.-Q. Zhang H. Liu Y. Effects of ultrasonic on structure, chain conformation and morphology of pectin extracted from Premna microphylla Turcz Carbohydr. Polym. 296 2022
46 Oliveira S.M. Fasolin L.H. Vicente A.A. Fuciños P. Pastrana L.M. Printability, microstructure, and flow dynamics of phase-separated edible 3D inks Food Hydrocoll. 109 2020
47 Wang F. Lyu J. Xie J. Bi J. Texture formation of dehydrated yellow peach slices pretreated by osmotic dehydration with different sugars via cell wall pectin polymersmodification Food Hydrocolloids 134 2023 108080
