Delaying browning of fresh cut taro root via carbohydrate metabolism and AsA-GSH cycle under high voltage electrostatic field treatment

doi:10.21203/rs.3.rs-2815800/v1

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Research Article

Delaying browning of fresh cut taro root via carbohydrate metabolism and AsA-GSH cycle under high voltage electrostatic field treatment

https://doi.org/10.21203/rs.3.rs-2815800/v1

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Taro (Colocasia esculenta) root is regarded as an important food crop in China, which can provide starch and vitamins and other nutrients. Taro root industry has brought good economic value to China. However, fresh cut taro root is easy to brown in a short time, resulting in lower acceptance and shorter shelf life, which greatly reduce the economic benefits of taro industry. This research was aimed to study the antioxidant effects of High voltage electrostatic field (HVEF) on taro root through carbohydrate metabolism and reactive oxygen species (ROS) scavenging system. The results showed that HVEF could retain starch and sucrose contents, enhanced activities of sucrose phosphate synthase (SPS), neutral invertase (NI), hexokinase (HK), glucose-6-phosphate dehydrogenase (G6PDH), and inhibited activities of α-amylase and sucrose synthase (SS) to maintain nutrients. Moreover, HVEF enhanced ascorbic acid (AsA) and glutathione (GSH) contents, promoted the activities of ascorbate peroxidase (APX), catalase (CAT) and glutathione reductases (GR) to resist browning and prolong shelf life. Overall results indicate that HVEF is an effectively physical preservation method delaying browning via carbohydrate metabolism and ROS scavenging system for fresh cut taro root.

Anti-browning

fresh cut taro root

ascorbic acid

high voltage electrostatic field

Taro (Colocasia esculenta) root is taken as a major economic crop in China, which can provide starch and vitamins and other nutrients. China had the third production level and the highest export volume of taro root in the world (Otekunrin et al., 2021). Because of alkaline mucus in the peel of taro root, it can irritate the skin and cause physiological symptoms such as allergy and itching. The pre-processing of taro is commonly conducted prior to its sale. (Pereira et al., 2018). Pre-process contain cleaning, peeling, cutting and packaging which will cause browning, senescence and result in reducing acceptance, shorter shelf life and economic loss (Bußler et al., 2016). Thus, developing effective methods to prevent fresh cut taro root from browning during storage period is necessary.

Previous studies have reported some preservation technologies for fresh cut taro root. The application of rose hydrosol and citronella hydrosol at a concentration of 500 ml L^− 1 on fresh cut taro root effectively suppressed enzyme activity involved in the enzymatic browning process, such as polyphenol oxidase (PPO), leading to inhibition of browning (Xiao et al., 2020). Wang et al. (2022) reported that 10 mM ferulic acid (FA) treatment could decrease total phenolic content, soluble quinone content and expression of related gene in benzene propane metabolic pathways such as 4-Coumarate-CoA ligase (4CL) to slow down browning of taro root. The recent study also found that 0.1 g L^− 1 cinnamic acid suppressed the related gene expression, enzymatic activities of chalcone synthase (CHS) and phenylalanine ammonia lyase (PAL), as well as decreased the total flavonoid content in the flavonoid biosynthesis pathway resulting in inhibition of browning in fresh cut taro root (Xiao et al., 2022). While the aforementioned chemical methods can partially delay the browning of fresh cut taro roots. The utilization of chemical preservatives in fresh cut taro root may result in the presence of residual chemicals on the surface, potentially posing food safety concerns. The implementation of physical preservation technology not only circumvents the aforementioned issues while preserving the nutrition of vegetables and fruit, but also presents a cost-effective approach that aligns with the contemporary values of eco-friendliness, economic viability, and food safety in modern lifestyles.

There have been many studies about preservation effects of physical fields. Studies have indicated that physical fields can prolong shelf life of fruit and vegetables by inhibiting respiratory metabolism, enzyme deactivation, sterilization and other ways. High voltage electrostatic field (HVEF) belongs to one kind of physical fields. HVEF not only suppress the pectin degradation and softening tissue by inhibiting the activity of pectin esterase in persimmon, but also inhibits the rate of respiratory metabolism and delays the oxidation and tissue degradation (Liu et al., 2016). Another research reported that HVEF can affect the enzyme conformation and inhibit the respiratory metabolism rate contributing, thereby delaying senescence, maintaining high content of chlorophyll and ascorbic acid resulting in improved quality of pakchoi during postharvest storage (Zhang et al., 2022). In our previous study, it was discovered that HVEF induced rearrangement of secondary structure and irreversible disruption of tertiary structure, leading to the burial of the active site of polyphenol oxidase (PPO). This resulted in the inactivation of PPO and subsequent delay in the enzymatic browning of mushrooms. (Yan et al., 2020). Previous studies have demonstrated that HVEF treatment holds significant promise in the field of fruit and vegetables preservation.

The browning of fresh cut fruit and vegetables could be caused by senescence and stress conditions including reactive oxygen species (ROS), energy deficiency and mechanical damage (Chen et al., 2018; Lin et al., 2016; Moggia et al., 2017). Previous studies revealed that ROS accumulation is the main cause of senescence in fresh cut vegetables and fruit (Davalli et al., 2016). With the aggravation of aging, a series of irreversible changes will occur in the color, texture, and nutrition of fresh cut vegetables and fruit, leading to the corruption of quality and the reduction of commercial rate. Together, ROS scavenging enzymes including but not limited to ascorbate peroxidase (APX) and endogenous antioxidants such as reduced ascorbic acid (AsA). (Ahmad et al., 2010). Under usual circumstances, under the joint action of ROS scavenging enzymes and endogenous antioxidants, ROS produced in fruit and vegetables can be timely removed, so that ROS generation and removal are in a balanced state, and their damage to fruit and vegetables can be avoided (Mittler et al., 2022). However, under adverse stresses such as aging and mechanical damage, the scavenging ability of ROS in fresh cut vegetables and fruit decreases, and the imbalance between ROS generation and removal leads to excessive ROS accumulation, which accelerates the occurrence of senescence, browning, softening of pulp (Lin et al., 2020).

Furthermore, AsA can be synthesized through the carbohydrate metabolic pathway, and carbohydrates can be divided into structural carbohydrates and non-structural carbohydrates according to their presence in plants. Structural carbohydrates, including pectin, cellulose and hemicellulose, mainly participated in plant morphogenesis (Martinez-Vilalta et al., 2016). Most studies suggested that with the enhancement of cellulase and other cell wall-degrading enzyme activities, protopectin, cellulose, and other cell wall substances are degraded and destroyed, which gives rise to softening of fresh cut foods, resistance to storage, short shelf life, and decreases economic value (Prasanna et al., 2007). Non-structural carbohydrates, including starch and soluble sugars, are important reactants for plant life activities and participate in plant metabolic processes. Starch, as one of the main long-term energy storage substances in plants, is the most important carbohydrate in plant storage organs. Soluble sugars, such as glucose, fructose, sucrose, and others, are the predominant types of sugars found in many biological systems. Fruit and vegetables consume non-structural carbohydrates to produce energy through respiratory metabolism to meet their basic physiological metabolism (Gao et al., 2020). However, with the continuous respiration of fruit and vegetables after harvest, the large consumption of carbohydrates in fruit and vegetables leads to the reduction of nutrient contents and the deterioration of taste. It has been reported that Nano-packaging material could down-regulate carbohydrate metabolism, suppress energy metabolism, participate in the generation and metabolism of amino acids, activate antioxidant system and enhance resistance to stress thus contribute to long shelf life of F. velutipes (Fang et al., 2017). And Wang et al. reported that high sucrose content in peach fruit could suppress severe internal browning caused by cold damage (Wang et al., 2013).

However, there are few information available about the influence of HVEF on the browning of fresh cut taro root. Therefore, this study aimed to investigate the influence of HVEF about browning of fresh cut taro root through carbohydrate metabolism and ROS scavenging system.

2.1 taro materials

Taro roots with uniform maturity, no physical and mechanical damage were bought from local market then delivered to the laboratory. Rinse the taro roots with tap water and dry the surface then peel using a paring knife and slice them into 1.0-cm × 1.0-cm ×3.0-cm shape. 50 taros slices were randomly placed into a container for fresh fruit and vegetables (Xiao et al, 2022).

2.2 High voltage electrostatic field

The HVEF device in use in this research was purchased from Jingcai Infinite Environmental Technology (Suzhou, China) Co., Ltd. The device consisted of a controller and two electrode plates, each with positive and negative charges, with a standard voltage of 3000 V, a standard operating current of 5.0 mA, and a frequency of 50–60 Hz. And the device was set at the refrigerator. Fresh cut taro roots were randomly divided into two containers (HVEF treatment and untreated group), each container contained 50 fresh cut taro root slices. Every 12 hours, fresh cut taro roots were treated by HVEF. The storage conditions of two groups were 8 ± 1°C with 85–90% relative humidity. From 0 h to 48 h, every 12 hours 10 slices were taken for measurement of browning index and flesh firmness, and other slices were frozen, ground and stored at -80 ℃ for other experiments. All experiments were measured in parallel for three times.

2.3 Measurement of firmness

The texture analyzer TA-XT (Stable Micro System Ltd., England) was used to determine the firmness of fresh cut taro root. The probe was P/2 probe. Before the test, the descending velocity is 2.00 mm s^− 1. When the test is going the descending velocity is 1.00 mm s^− 1. The test depth of probe was 8 mm. Data were expressed in Newton (N) (Yan et al., 2020).

2.4 Determination of browning

The color was determined to quantify the degree of browning with the CS-280 colorimeter (CHN Spec Technology Co., Ltd, Hangzhou, China). The values for L*, a* and b* were determined from 5 different places of each fresh cut taro root slice. The browning index (BI) was calculated according to the following formula (Hsieh et al., 2020):

$$BI=588.32(X-0.31)$$

where:

$$X=\frac{(0.57{a}^{*}+{L}^{*})}{(3.23{L}^{*}+0.57{a}^{*}-1.72{b}^{*})}$$

2.5 Cellulose, hemicellulose, total pectin and protopectin contents determination

2.5.1 Cellulose content determination

Based on a previously reported method with modifications (Liu et al., 2021), cellulose content of sample was determined using the anthrone-sulfuric acid method. Briefly, 0.50 g of sample powder was weighed (${W}_{1}$) and mixed with 1.0 mL of 80% ethanol at 25 ℃. The mixture was homogenized and then bathed in 90 ℃ water for 20 min. After centrifugation at 6,000 × g at 25 ℃ for 10 min, the sediment was cleaned with 80% ethanol and acetone three times to obtain the rough cell wall. Subsequently, 1.0 mL of extraction solution was added to the rough cell wall, which was then soaked for 15 hours and centrifuged at 6,000 × g at 25 ℃ for 10 min. The sediment was washed three times with distilled water and dried at 60 ℃ to obtain the cell wall (${W}_{2}$). A 0.0050 g cell wall sample (${W}_{3}$) was then mixed with 0.5 mL of distilled water and homogenized, with additional distilled water added to make a final volume of 1.0 mL. Then, 0.75 mL of concentrated sulfuric acid was slowly added and mixed into the homogenate. The mixture was then allowed to stand in an ice bath for 30 min before being centrifuged at 8,000 × g at 4 ℃ for 15 min to obtain the supernatant. After dilution twenty times, the cellulose content was determined using the anthrone-sulfuric acid method and the sample concentration ($X$) was obtained according to the standard curve of glucose.

Cellulose content (gžkg ^− 1 ) =$22.52\times X\times {W}_{2}÷{W}_{3}÷{W}_{1} \left(3\right)$

2.5.2 Hemicellulose content determination

Samples were dried at 40 ℃ then thoroughly ground with a mortar, and sifted. Follow the BC4440-50T/48S kit (Solarbio Co., Ltd., Beijing, China) instructions for content determination with Enzyme-labeled Instrument (Thermo Fisher Scientific, USA).

2.5.3 Protopectin and pectin contents determination

In this study, we followed a modified method (Liu et al., 2021) to determine the pectin content in the sample. A 1.0 g sample was homogenized with 25 mL 95% ethanol and boiled in a water bath for 30 min. After cooling to 25 ℃, the mixture was centrifuged at 8,000 × g for 15 min. This step was repeated three times. The sediment was dissolved in 20 mL aqua destillata and kept in a 50 ℃ water bath for 30 min to dissolve the pectin. After cooling and centrifugation, the supernatant was transferred to a 100 mL volumetric flask and made up to the mark with distilled water. This was the soluble pectin solution. The sediment was left in the test tube and 25 mL 0.5 mol L^− 1 sulfuric acid was added and boiled for 1 h to hydrolyze the protopectin. After cooling and centrifugation, the supernatant was transferred to a 100 mL volumetric flask and made up to the mark with distilled water. This was the protopectin solution. The total pectin content was the sum of the protopectin and soluble pectin content, which were determined using the Carbazol-sulfuric acid colorimetric method and expressed as mmol kg^− 1.

2.6 Starch, glucose, sucrose and fructose contents determination

Contents of starch, glucose, sucrose and fructose were measured with the A148-1-1, F006-1-1, A099-1-1, and A085-1-1 kits (Jiancheng Bioengineering Institute, Nanjing, China). The experimental operation and content calculation were carried out according to the instructions, and the absorbances were determined.

2.7 Measurement of AsA, GSH, H₂O₂ and total phenolic contents

2.7.1 AsA and GSH content determination

The AsA content was measured with the A009-1-1 kit (Jiancheng Bioengineering Institute, Nanjing, China), and denoted as g kg^− 1. Slight modifications of method reported (Li et al., 2021) was used to determine glutathione (GSH) content. Briefly, 1.0 mL of pH 7.7 phosphate buffer solution and 0.5 mL of 4 mmol L^− 1 5,5-dithiobis-2-nitrobenzoic acid solution were added to GSH solutions of varying concentrations. The absorbance of the reaction solutions was then measured at a wavelength of 412 nm to generate a standard curve. Next, 1.0 g of the sample was weighed and placed in a mortar, followed by the addition of 5.0 mL of 5% (w/v) trichloroacetic acid solution (containing 5 mmol/L EDTA-Na₂) that had been pre-cooled to 4 ℃. The samples were homogenized in an ice bath and subsequently centrifuged at 4 ℃, and the supernatant was collected for GSH content determination. The content was calculated based on the standard curve.

2.7.2 H₂O₂ content determination

According to A064-1-1 kit (Jiancheng Bioengineering Institute, Nanjing, China) draw standard curve. Weigh 1.0 g sample, add 5.0 mL pre-cooled acetone, grind into homogenate in laboratory fume cupboard under the condition of ice bath, centrifuge at low temperature and high speed for 20 min. collect the supernatant as sample extraction solution. The content of hydrogen peroxide (H₂O₂) in the sample was determined and calculated according to the experimental method of the instructions of kit.

2.7.3 Total phenols determination

Base on the method reported determine total phenolic content (Matic et al., 2017). 1.0 g sample was extracted through 5.0 mL of 60% ethanol, then centrifuge at high speed and low temperature for 10 min. Take out 1.0 mL supernatant, react with 1.0 mL 50% (v/v) Folin-Ciocalteu reagent and saturate 2.0 mL 15% Na₂CO₃ solution. Reaction time was 30 min. Then reaction solution was taken for the measure of optical density at 750 nm. Total phenolic content was calculated according to gallic acid standard curve and denoted as mg kg^− 1.

2.8 Enzyme activities

2.8.1 α-amylase and total amylase activity

α-amylase and total amylase activity was assayed with the YX-2-C402 kit (sinobestbio Co., Ltd, Shanghai, China). Weigh 0.10 g sample, add 1.0 mL distilled water, then grind and homogenize. The homogenate was poured into the centrifuge tube, and the homogenate was placed at indoor temperature for 15 min in order to extract enzyme (oscillation every 5 min) to make it fully extracted. Then centrifuge at 6000 × g at 25 ℃ for 10 min. The supernatant was added with distilled water to 10 mL and then shaken well to determine the activity of α-amylase. Absorb 1.0 mL of the above amylase solution, add 4.0 mL distilled water to dilute it, and mix it well to form the amylase dilution solution, which is used to determine the total activity of (α + β) amylase. The activities of α-amylase and total amylase were measured according to the operation table of the kit with Ultraviolet-Visible spectrophotometer (Shimadzu, Co., Ltd., Japan). The calculation formulas are as follows.

α- amylase activity (Užkg ^− 1 ) = $2\times {X}_{1}÷W \left(4\right)$

total amylase activity (Užkg ^− 1 ) =$10\times {X}_{2}÷W \left(5\right)$

Where enzyme activity unit (U) represents for the production of 1 mg reducing sugar per g tissue per minute; ${X}_{1}$ and ${X}_{2}$ are solution concentration of α-amylase and total amylase, respectively, mg mL^− 1; $W$ represents for the sample mass, g.

2.8.2 SPS and SS activity

Sucrose phosphate synthase (SPS) activity and Sucrose Synthase (SS) activty were measured with the A098-1-1 and A097-1-1 kits (Jiancheng Bioengineering Institute, Nanjing, China), respectively. Both of them were determined at 480 nm. The calculation equations of SPS and SS activity are as follows:

SPS activity (Užkg ^− 1 ) =$\frac{\varDelta A+0.009}{0.004}÷T÷\left(\frac{W}{{V}_{T}}\times {V}_{S}\right) \left(6\right)$

SS activity (Užkg ^− 1 ) =$\frac{\varDelta A-0.021}{0.004}÷T÷\left(\frac{W}{{V}_{T}}\times {V}_{S}\right) \left(7\right)$

Where U represents for the production of 1 µg fructose per g tissue per minute. $T$ represents for reaction time, min; ${V}_{T}$ represents for total volume of extract solution added during pre-treatment, mL; ${V}_{S}$ represents for volume of sample addition, mL.

2.8.3 NI activity

Neutral invertase (NI) activity was determined with the YX-C-B507 kit (Sinobestbio Co., Ltd, Shanghai, China). Weigh 0.10 g tissue and add 1.0 mL extracting solution for homogenization in ice bath. Then centrifuge at low temperature and high speed for 10 min, and the supernatant was mixed with other agents to react evenly. After boiling for 10 min, the absorbances were measured at 540 nm by water cooling. The sample concentrations were calculated according to the glucose standard curve. NI activity was calculated based on the instructions, where U represents for the production of 1 µg reducing sugar at 37 ℃ g per g tissue per minute.

2.8.4 HK activity

Hexokinase (HK) activity was determined according to YX-C-B200 kit (Sinobestbio Co., Ltd, Shanghai, China). The extraction method of enzyme solution was consistent with 2.8.3. The reaction solutions in accordance with the reagent order to add to cuvette, immediately mixed, add enzyme solution at the same time start timing, at 340 nm wavelength with ultraviolet-visible spectrophotometer (Shimadzu, Co., Ltd., Japan). The value at 20 s value was ${A}_{1}$. Quickly take out the cuvette and put it into 37 ℃ water bath, accurately react for 5 min, then take out the cuvette and wipe it dry, and measure the absorbance ${A}_{2}$ at 5 min 20 s. HK activity was calculate as follows:

HK activity (Užkg ^-1 ) =$1113\times ({A}_{2}-{A}_{1})÷W \left(8\right)$

Where U represents for the production of 1 nmol NADPH per g tissue per minute.

2.8.5 G6PDH and GalLDH activities

Glucose-6-phosphate dihydrogenase (G6PDH) activity was measured with YX-W-A102 kit (Sinobestbio Co., Ltd, Shanghai, China). The extraction method of enzyme solution was consistent with 2.8.1.3. The determination process based on the determination method in 2.8.4 with slight modifications. Prepare the reaction solution in advance. The change of absorbance within 5 min was measured at 340 nm. The calculation formula was according instruction book, where U represents for the production of 1 nmol NAPDH per g tissue per minute.

L-galactono-1,4-lactone dehydrogenase (GalLDH) activity was determined according to the method (Dong et al., 2016). 0.50 g ($W$) sample was homogenized in 5.0 mL ($V)$ of 100 mmol L^− 1 phosphate buffered saline (pH 7.4) containing 400 mmol L^− 1 sucrose, and then filtered with two layers of gauze. The extracts were centrifuged at low temperature and high speed for 20 min, and the supernatant was taken for determination. The reaction solution consisted of 1.0 mg mL^− 1 cytochrome C and L-galactono-1,4-lactone. Before the assay, 2.0 mL cytochrome C and 200 µL L-galactono-1,4-lactone were mixed and reacted at 25 ℃ for 1 min ($T$). The reaction lasted for 1min from the addition of 200 µL ($\upsilon )$ enzyme solution. Absorbance of first end at 550 nm were recorded, and the difference was denoted as $\varDelta A$. Activity was calculated as follows:

GalLDH activity (Užkg ^-1 ) =$\varDelta A\times V\times {10}^{6}÷\left(\epsilon \times \upsilon \times T\times L\times W\right) \left(9\right)$

where U represents for the reduction of 2.0 nmol min^-1 Cyt c; $\epsilon$ represents for the extinction coefficient which is 17.3 mol L^-1 m^-1 in this experiment; $L$ represents for the light diameter of cutette, cm.

2.8.6 APX, CAT, and GR activities

Ascorbate peroxidase (APX) activity, catalase (CAT) activity, and glutathione reductases (GR) activity were measured according to A123-1-1, A007-1-1, A062-1-1 kits (Jiancheng Bioengineering Institute, Nanjing, China). The methods of enzyme extraction were similar to 2.8.1.3, and referred to 2.8.1.5 for determination methods, where U of APX activity represents for the reduction of 1.0 µmol AsA per g tissue per minute; U of CAT activity represents for the reduction of 1.0 µmol H₂O₂ per g tissue per minute; U of GR activity represents for the reduction of 1.0 mmol NADPH per g tissue per minute.

2.8.7 PPO activity

The enzyme extraction method referred to 2.8.1.3. The reagents used was from the BC0195-100T/48S kit (Solarbio Co., Ltd., Beijing, China). The reaction solution was added in sequence of reagents, with the sample solution being added last in the experimental group and the boiled sample solution in the control group. The whole reaction system in 25 ℃ water bath for 10 min, and then quickly put it into boiling water for 10 min. Centrifuge at 6,000 × g for 10min at indoor temperature. Collect the supernatant. The absorbance of experimental tubes and control tubes were detected at 410 nm. Each experimental tube needs to be set up with a control tube. U represents for the change of 0.05 absorption per g tissue per minute.

2.9 Statistical analysis

Data in this article are expressed as means ± standard deviation. SPSS 20.0 software was used for one-way analysis of variance, and Duncan test was used for significance analysis. P < 0.05 indicated significant difference.

3.1 Effect of HVEF on visual appearance, browning index and firmness

The apparent changes of fresh cut taro roots during storage are showed (Fig. 1a). As the storage time become longer, the surface color of fresh cut taro root in control group began to deepen and the water loss was serious. Browning degree and water loss of HVEF group was not obvious compared with control group. The firmness of fresh cut taro roots was 11.99 N at first. Both of the firmness declined during storage period, but the firmness of HVEF treatment group was higher than control group (Fig. 1b). Apparent color of control group gradually turned to brown during the storage time. HVEF treatment alleviated the browning degree of fresh cut taro roots. However, BI of control group increased from 24h. And at 48 h, the browning degree was significant (Fig. 1c).

3.2 Cellulose, hemicellulose, protopectin and total pectin contents

Cellulose content of untreated group reached a maximum of 622.04 g kg^-1 at 24 h, and that of HVEF group reached a maximum of 713.69 g kg^-1 at 36 h (Fig. 2a). Hemicellulose content in HVEF group showed an increasing tendency, but showed a single peak change, reaching a maximum of 181.61 g kg^-1 at 36 h (Fig. 2b). Protopectin content showed a slightly decreasing tendency in HVEF group from 43.62 mmol kg^-1 to 33.69 mmol kg^-1, but showed a significantly decreasing tendency in control group from 43.62 mmol kg^-1 to 32.00 mmol kg^-1. And content of control group exhibited significantly lower than HVEF group (Fig. 2c). Total pectin content showed a similar tendency with protopectin content (Fig. 2d). From 24 h to 48 h, total pectin content of control group was from 160.41 mmol kg^-1 to 143.31 mmol kg^-1, obviously lower than HVEF group from 181.73 mmol kg^-1 to 164.24 mmol kg^-1.

3.3 Starch, glucose, sucrose and fructose contents

Starch content of both of groups showed a decreasing tendency. Starch content of HVEF group was from 702.40 g kg^-1 to 385.43 g kg^-1, and that of control group was from 702.40 g kg^-1 to 433.33 g kg^-1 (Fig. 3a). Glucose content of control group was from 9.94 g kg^-1 to 5.89 g kg^-1 at the time of 12 h to 24 h, then increased to 9.31 g kg^-1 at the time from 24 h to 48 h. And content of HVEF group was 4.67 g kg^-1 which showed a single peak change at 24 h (Fig. 3b). Sucrose content of control group was from 0.89 g kg^-1 to 1.43 g kg^-1, then turned to 1.20 g kg^-1 which showed a tendency of first rising then falling. Sucrose content of HVEF group was from 1.03 g kg^-1 to 1.53 g kg^-1, then turned to 1.33 g kg^-1 which was first increasing then decreasing (Fig. 3c). And fructose content of treated group and control group had similar trend, but fructose content of control group which was from 4.40 g kg^-1 to 3.71 g kg^-1 more than HVEF group which was 3.38 g kg^-1 to 3.37 g kg^-1 from 12 h to 48 h (Fig. 3d).

3.4 AsA, GSH, H₂O₂ and total phenolic contents

HVEF treatment significantly increased the AsA content compared with control group (Fig. 4a). AsA content of HVEF group was from 2.86 g kg^− 1 to 3.22 g kg^− 1, and that of control group was decreasing from 2.86 g kg^− 1to 2.49 g kg^− 1. GSH content in HVEF group reached a maximum of 196.97 mmol kg^− 1 (Fig. 4b). H₂O₂ content in control group remained basically unchanged from 12 h to 48 h of 9.30 and 9.67 mol kg^− 1, in treated group showed an increasing tendency from 3.16 to 9.49 mol kg^− 1 (Fig. 4c). And as shown (Fig. 4d), total phenolic content in HVEF group showed a decreasing tendency from 12h to 24 h which was from 12.17 mg kg^− 1 to 10.93 mg kg^− 1, then increasing from 24h to 48h which was from 10.93 mg kg^− 1 to 14.58 mg kg^− 1. Meanwhile, control group showed a decreasing tendency which was from 14.53 mg kg^− 1 to 12.18 mg kg^− 1 at the time from 12 h to 48 h. Taro roots exhibited significantly lower total phenols accumulation than that of HVEF treatment at 36 h from 48 h.

3.5 Enzyme activities

3.5.1 Enzyme activities in the carbohydrate metabolism

At the time of 12 h to 36 h, α- amylase activity of control group was from 1.01$\times$10³ U kg^-1 to 1.38$\times$10³ U kg^-1 which was significantly more than that of HVEF group (Fig. 5a). Total amylase activity of control group was from 2.625$\times$10⁵ U kg^-1 to 2.583$\times$10⁵ U kg^-1 which was more than that of HVEF group which was from 2.510 $\times$10⁵ U kg^-1 to 2.442$\times$10⁵ U kg^-1 at the time of 24 h to 36 h, then less than HEVF group at 48 h with 2.615$\times$10⁵ U kg^-1 compared with 2.854$\times$10⁵ U kg^-1 (Fig. 5b). SPS activity of both groups showed an increasing trend. SPS activity of control group were 4.051$\times$10⁵ U kg^-1 at first and 8.435$\times$10⁵ U kg^-1 at 48 h, and that of HVEF group was 8.875$\times$10⁵ U kg^-1 at 48 h. SPS activity of HVEF group was significantly more than control group (Fig. 5c). SS activity of both groups showed a decreasing trend. SS activity of control group were 3.533$\times$10⁶ U kg^-1 at first and 2.375$\times$10⁶ U kg^-1 at 48 h, and that of HVEF group was 1.648$\times$10⁶ U kg^-1 at 48 h. SS activity of control group was significantly more than HVEF group (Fig. 5d). NI activity of control group were 38 U kg^-1 at first and 28 U kg^-1 at 48 h, and that of HVEF group was 32 U kg^-1 at 48 h. NI activity of HVEF group was significantly more than control group from 24 h to 48 h (Fig. 5e). HK activity of control group were 1.726$\times$10⁴ U kg^-1 at 0 h and 7.46$\times$10³ U kg^-1 at 48 h, and that of HVEF group was 1.120$\times$10⁴ U kg^-1 at 48 h. HK activity of HVEF group was more than control group from 24 h to 48 h (Fig. 5f). G6PDH activity of control group were 1.400$\times$10⁶ U kg^-1 at first and 6.343$\times$10⁵ U kg^-1 in the end, and that of HVEF group was 9.409$\times$10⁶ U kg^-1 at 48 h G6PDH activity of HVEF group was more than control group from 24 h to 48 h (Fig. 5g). GalLDH activity of control group were 3.384$\times$10⁴ U kg^-1 at first and 2.945 $\times$10⁴ U kg^-1 in the end, and that of HVEF group was 3.499$\times$10⁴ U kg^-1 at 48 h. GalLDH activity of HVEF group was significantly more than control group (Fig. 5h).

3.5.2 Enzyme activities in the ROS scavenging system

APX activity of HVEF group were 250 U kg^-1 at 36 h and 270 U kg^-1 at 48 h, and that of control group were 210 U kg^-1 at 36 h and 220 U kg^-1 at 48 h. APX activity of HVEF group was significantly more than control group from 36 h to 48 h (Fig. 6a). CAT activity of control group was 5.6$\times$10³ U kg^-1 at 12 h and 1.86 $\times$10⁴ U kg^-1 at 48 h, and showed a tendency which was first increasing from 12 h to 36 h, then decreasing from 36 h to 48 h. CAT activity of HVEF group were 1.300$\times$10⁴ U kg^-1 at 12 h and 1.900$\times$10⁴ U kg^-1 at 48 h, and showed a growth trend. CAT activity of HVEF group was significantly more than control group at 48 h (Fig. 6b). GR activity of control group reached a maximum of 1.103$\times$10⁴ U kg^-1 at 12 h. GR activity of HVEF group showed a growth trend and were 1.292$\times$10⁴ U kg^-1 at 36 h and 1.235$\times$10⁴ U kg^-1 at 48 h, which were significantly more than control group (Fig. 6c).

3.5.3 PPO activity

PPO activity of control group showed a decreasing trend which were 2.096$\times$10⁵ U kg^− 1 at 12 h and 8.740$\times$10⁴ U kg^− 1 at 48 h. PPO activity of HVEF group showed a growth trend which were 4.480$\times$10⁴ U kg^− 1 at 12 h and 8.040$\times$10⁴ U kg^− 1 at 36 h, then turned to 7.580$\times$10⁴ U kg^− 1 at 48 h. And PPO activity of control group was significantly more than HVEF group from 12 h to 48 h (Fig. 6d).

The softening of fresh cut taro roots tissue is a distinguishing feature of senescence. Firmness gradually decreased during storage. A close relationship between firmness and cellulose and protopectin contents (Koziol et al., 2017). Previous study has reported the effect of cell wall polysaccharides on fruit softening, showing that higher protopectin, cellulose and hemicellulose contents can maintain higher firmness of persimmon fruit (Wang et al., 2020). We also found similar significant increase in cellulose and protopectin contents of fresh cut taro root in HVEF group compared with control group (Fig. 2a, c). The reason of cellulose content at 48 h showed a decreasing tendency could be cellulose is degraded to hemicellulose. These results showed that HVEF treatment could maintain firmness of fresh cut taro root by increasing cellulose and protopectin contents.

BI is an another apparent character to evaluate the economic value of fresh cut taro root and affect consumers acceptance (Fu et al., 2022). In this research, we explore the effect of HVEF on the BI value of fresh cut taro root. We found that HVEF treatment reduced BI value in contrast to control group, and HVEF treatment alleviated deterioration of fresh cut taro root during storage. ROS plays a critical role in browning of fresh cut fruit and vegetables. Therefore, we postulate that browning of fresh cut taro root is in connection with ROS, and we further investigated the impact of HVEF treatment on the browning process by examining its effects on the ROS scavenging system. The result showed that HVEF treatment elevated the activities of APX, CAT and GR of fresh cut taro roots (Fig. 6a, b c), as well as AsA and GSH contents (Fig. 4a, b). AsA-GSH cycle occupies an important position in ROS scavenging system. APX is one of the important antioxidant enzymes and one of the key enzymes in AsA metabolism, not only a key enzyme in removing H₂O₂, but also can oxidize AsA to produce monodehydroascorbate reductase. GR participates in the removal of ROS in oxidative stress responses, and is also involved in the AsA-GSH cycle (Yao et al., 2021). CAT is widely found in plants, and it is the most important H₂O₂ scavenging enzyme and plays a vital role in ROS scavenging system. Above results were similar to the researches in which HVEF treatment enhanced AsA content and APX activity in pomegranate fruit, CAT activity and GR activity in wheatgrass seed to delay browning (Leong et al., 2016; Lotfi et al., 2022).

Total phenols have antioxidant activity and are the main natural antioxidants, which can resist the external oxidative stress. Therefore, total phenolic content is also a crucial indicator to evaluate the quality of fresh cut taro roots (Arıcı et al., 2016). In this research, HVEF treatment promote the accumulation of total phenols from 36 h to 48 h. The results revealed that HVEF treatment effectively preserved the total phenolic content of fresh cut taro roots, thereby maintaining robust antioxidant capacity. PPO is an oxidase widely existing in plants. It can oxidize monophenol and diophenol to produce quinone, which can cause browning (Xiao et al., 2021). HVEF treatment inhibited PPO activity which could explain the increased accumulation of total phenolic content and retardation of browning in fresh cut taro root treated by HVEF during storage. These results in our study showed that HVEF treatment could activate ROS scavenging system to inhibit browning of fresh cut taro root, and we found that HVEF treatment can also inhibit PPO activity, which may play a synergistic role in delaying browning of fresh cut taro root. The findings of our study are consistent with previous research that reported the beneficial effects of hot air pretreatment, which include the suppression of polyphenol oxidase (PPO) activity associated with oxidative browning, activation of the AsA-GSH cycle, and inhibition of wound-induced ROS accumulation, ultimately leading to the alleviation of browning and retention of quality in fresh-cut pitaya fruit (Li et al., 2022).

In addition, L-galactose pathway is one of the crucial pathways for AsA synthesis (Ivanov Kavkova et al., 2018). L-galactose could be converted to AsA by GalLDH through carbohydrate metabolic pathway. Nonstructural carbohydrates are important nutrients in fresh cut taro root including starches and soluble sugar. HVEF treatment inhibited α-amylase activity which can degrade starch to UDP-G. The results showed that HVEF treatment could enhance SPS activity, and basically maintain NI and HK activity and sucrose content (Fig. 7). Moreover, the results indicated that HVEF treatment could enhance the activity of G6PDH and GalLDH at 48 h. In many plants, SPS is involved in sucrose synthesis, which can catalyze the formation of sucrose from UDP-G (Lunn and MacRae, 2003). SS and NI are known to be involved in sucrose cleavage and cleave to UDP-G and fructose, respectively (Wan et al., 2018). Fructose can be converted to G-6-P by HK, then G-6-P convert to L-galactose by G6PDH (Stein and Granot, 2018). Results in this research are similar to the research which proved that the sucrose contents in abscisic acid-treated peach fruit increased or were maintained for a longer period, increased SS and SPS activities, and resulted in delaying internal browning of flesh (Zhao et al., 2022). Seo et al. (2020) reported that methyl jasmonate treatment could delay browning of pepper fruit caused during storage through promoting contents of antioxidant metabolites including sucrose, glutamate, and galactinol. Additionally, it has been reported that blue light can up-regulate the gene expression of carbohydrate-active enzymes to promote the conversion of pentose and glucuronic acid, as well as the metabolism of starch and sucrose, ultimately leading to the formation of brown film on mushroom surface (Huang et al., 2020). In conclusion, HVEF treatment could retain starch and sucrose contents to resist browning of fresh cut taro roots and reduction of nutrient contents.``

HVEF treatment promoted the production of cellulose and protopectin to significantly maintained better firmness of fresh cut taro root. In addition, HVEF treatment inhibited α-amylase and SS activities to retain starch and sucrose contents which are the main nutrient in fresh cut taro root. Moreover, HVEF treatment promoted the generation of antioxidant substances ASA and GSH by promoting the enzyme activities of SPS, NI, HK and G6PDH in carbohydrate metabolism pathway and APX, CAT and GR in ROS scavenging system, thus inhibiting the browning of fresh cut taro root and maintaining its good storage quality. In conclusion, as a low budget physical method, HVEF treatment could be used to preserve apparent quality and nutrient, resist senescence, and extend shelf life of fresh cut taro roots by being involved in carbohydrate metabolism and ROS scavenging system. These findings could expand the application of HVEF in the field of fresh food preservation, especially root and tuber vegetables, and promote the economic development of the taro industry.

CRediT authorship contribution statement

Cai Yue and Chongjiang Cao searched literature, designed and finished experiments, analyzed data and wrote this article.

Zhu Jie, Mengjun Wang and Linyu Nian participate in experimental design, data analysis and article revision.

Chongjiang Cao and Shujie Cheng provided financial support for this research.

Acknowledgement

Funding: This work was supported by Agricultural Science and Technology Innovation Fund of Jiangsu Province [CX (21)3171], Modern Agricultural Machinery Equipment and Technology Demonstration and Promotion Project of Jiangsu Province [NJ2021-20], Key Research and Development Program of Jiangsu Province [SBE2022310082].

Conflict of interest: The authors declare that they have no conflict of interest.

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Delaying browning of fresh cut taro root via carbohydrate metabolism and AsA-GSH cycle under high voltage electrostatic field treatment

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 taro materials

2.2 High voltage electrostatic field

2.3 Measurement of firmness

2.4 Determination of browning

2.5 Cellulose, hemicellulose, total pectin and protopectin contents determination

2.5.1 Cellulose content determination

2.5.2 Hemicellulose content determination

2.5.3 Protopectin and pectin contents determination

2.6 Starch, glucose, sucrose and fructose contents determination

2.7 Measurement of AsA, GSH, H2O2 and total phenolic contents

2.7.1 AsA and GSH content determination

2.7.2 H2O2 content determination

2.7.3 Total phenols determination

2.8 Enzyme activities

2.8.1 α-amylase and total amylase activity

2.8.2 SPS and SS activity

2.8.3 NI activity

2.8.4 HK activity

2.8.5 G6PDH and GalLDH activities

2.8.6 APX, CAT, and GR activities

2.8.7 PPO activity

2.9 Statistical analysis

3. Results

3.1 Effect of HVEF on visual appearance, browning index and firmness

3.2 Cellulose, hemicellulose, protopectin and total pectin contents

3.3 Starch, glucose, sucrose and fructose contents

3.4 AsA, GSH, H2O2 and total phenolic contents

3.5 Enzyme activities

3.5.1 Enzyme activities in the carbohydrate metabolism

3.5.2 Enzyme activities in the ROS scavenging system

3.5.3 PPO activity

4. Discussion

5. Conclusions

Declarations

References

Additional Declarations

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2.7 Measurement of AsA, GSH, H₂O₂ and total phenolic contents

2.7.2 H₂O₂ content determination

3.4 AsA, GSH, H₂O₂ and total phenolic contents