Acidic Electrolyzed Water Activates Disease Resistance and Sustains Postharvest Quality of Yellow Passion Fruit

Abstract

Passion fruit deteriorates rapidly after harvest owing to fungal decay and quality loss. This study examined whether acidic electrolyzed water (AEW, pH 2.5) could strengthen host defense responses and thereby prolong the marketable storage period of passion fruit. Freshly harvested yellow passion fruits (without any prior storage) were immersed for 20 min in AEW containing 0 (control), 30, 60 or 90 mg/L available chlorine concentration (ACC) and then packaged in polyethylene film bags and stored at 25 °C for 15 days to simulate typical ambient handling/marketing conditions, where polyethylene packaging is commonly used to maintain a high-humidity microenvironment and reduce moisture loss; physicochemical attributes, decay parameters and disease-resistance-related enzyme activities were then monitored. AEW—particularly at 60 mg/L ACC—significantly lowered decay incidence, disease index and cell membrane permeability while preserving pericarp color (hue angle h, L*) and pulp titratable acidity, vitamin C, total soluble solids, and total soluble sugars. The same treatment elevated the concentrations of disease-resistant metabolites (total polyphenolics, flavonoids and lignin) and up-regulated the activities of peroxidase, cinnamate-4-hydroxylase, 4-coumarate CoA ligase, phenylalanine ammonia-lyase, cinnamyl alcohol dehydrogenase, chitinase, and β-1,3-glucanase. These findings demonstrate that AEW mitigates postharvest deterioration of passion fruit by activating the metabolism of disease-resistant substances, highlighting its potential as an eco-friendly technology for maintaining quality during ambient handling/marketing conditions.

1. Introduction

Passion fruit, an ovoid fruit found in subtropical and tropical regions, contains a central cavity packed with fragrant, juicy pulp surrounding numerous black seeds [1]. Although native to Brazil, it is now extensively cultivated and increasingly popular in China, particularly in Guangxi, Fujian, Guangdong, Hainan, and Yunnan provinces [1,2]. Nationally, the cultivated area expanded from <700 ha in 2011 to >30,000 ha by 2019, with annual production reaching approximately 600,000 tons. This rising demand is attributed to the abundant nutrients present in fresh passion fruit, including vitamins, phenols, aromatic substances, crude fiber, and organic acids [1,3]. Nevertheless, being a climacteric fruit, passion fruit is particularly susceptible to decay, pathogen infection, quality deterioration, and loss of storability during high-temperature harvesting seasons. As a result, its marketability is compromised and its shelf life is markedly reduced [3,4,5]. Therefore, elucidating the mechanisms underlying postharvest decay and quality degradation in passion fruit is imperative.

Accumulating evidence suggests that postharvest quality in fresh produce is closely associated with its inherent disease resistance [6,7,8]. This resistance is largely determined by enhanced levels of defensive metabolites, including phenolic constituents and lignin, together with coordinated regulation of disease-resistance-related enzymes (DREs). Key DREs include 4-coumarate CoA ligase (4-CL), peroxidase (POD), β-1,3-glucanase (GLU), cinnamate-4-hydroxylase (C4H), phenylalanine ammonia-lyase (PAL), and chitinase (CHI) as well as cinnamyl alcohol dehydrogenase (CAD) [9,10,11,12]. Recent evidence suggests that limited buildup of protective metabolites, coupled with reduced DRE function, can compromise host defense and consequently hasten the progression of postharvest decay in newly harvested produce [13]. Conversely, higher levels of these metabolites and enhanced DRE activities are often linked to strengthened resistance, improved quality retention, and extended storability [14,15,16]. Consequently, the levels of defense-related metabolites and DRE activities—two core determinants of disease resistance—has a pivotal influence on postharvest quality and storage performance in fresh produce.

Acidic electrolyzed water (AEW) is produced by electrolysis of a dilute sodium chloride solution in an electrolytic cell and is widely regarded as a reliable, food-safe disinfection technique [17]. The chamber contains an anode and cathode, with a diaphragm membrane serving as a separator between them. During the electrolysis of a diluted sodium chloride solution, hydrochloric acid (HCl), chlorine gas and hypochlorous acid are generated on the anode side. The solution produced through this process is then referred to as AEW. Furthermore, AEW typically exhibits elevated oxidation–reduction potential (ORP) and an acidic pH while maintaining comparatively high levels of available chlorine (ACC) [18]. These physicochemical properties confer strong antimicrobial efficacy, enabling AEW to inactivate a broad spectrum of microorganisms, such as some foodborne pathogens (Listeria innocua, Bacillus cereus, Escherichia coli, Salmonella typhimurium, or Cronobacter sakazakii, etc.) [17,19]. Moreover, AEW is considered a safe, novel, cost-effective, convenient, and highly efficient application for treating postharvest fresh produce. Earlier studies suggest that AEW treatment helps maintain postharvest quality in fresh produce, including wampee [2] and jujube [17], in part by strengthening defense capacity and slowing the onset and progression of decay.

In the Chinese market, yellow passion fruit and purple passion fruit are the two predominant commercial types that are most commonly marketed and consumed. Our previous study revealed that yellow passion fruit is more nutritious than purple passion fruit because yellow passion fruit has higher levels of pulp nutrient substances, including vitamin C, total soluble sugar, and total soluble solids (TSS) [20], whereas yellow passion fruit is more perishable than purple fruit within storage, with a higher weight loss rate and serious shrinkage symptoms and a lower commercially acceptable fruit rate [20]. Accordingly, elucidating the mechanisms underlying postharvest quality deterioration in yellow passion fruit is of clear practical relevance. To date, little documentation exists on the postharvest application of AEW in stabilizing storage quality in fresh yellow passion fruit, and the potential mechanism for enhancing the disease resistance level is inadequately understood. In this study, we assessed how varying AEW concentrations influence storage performance and key quality parameters of freshly harvested yellow passion fruit with the aim of determining the most effective treatment condition. We then investigated how the optimal AEW treatment influenced decay development and postharvest disease progression as well as overall storage quality. To our knowledge, the current research might potentially be the first to study the mechanisms by which AEW retains storage quality in fresh yellow passion fruit though influencing its disease resistance. Overall, this work aimed to (i) assess how AEW influences postharvest quality and storage performance, (ii) optimize AEW treatment conditions, and (iii) characterize the contribution of disease resistance to AEW-mediated quality retention. In practical production applications, the results of the present work can offer a useful method for slowing down the development of disease and decay in yellow passion fruit and retaining its quality.

2. Materials and Methods

2.1. Preparation of AEW

AEW (pH 2.5) with target ACC concentrations of 30, 60, and 90 mg/L was produced using an electrolyzed-water generator (Beijing Huo-Ren-Jing-Chuang Medical Equipment Co., Ltd., Beijing, China; HRW-1500). These AEWs with respective ACC concentrations were then defined as ACC30, ACC60, and ACC90. Regarding the preparation of AEW with different ACC concentrations, the high ACC concentration of 90 mg/L was prepared first by the AEW machine, and then the low ACC concentrations of 30 and 60 mg/L were obtained by dilution with distilled water. The ORP or ACC values were measured using an ORP analyzer (Guangdong JIA-BEI Water Treatment Co., Ltd., Guangzhou, China; A57-B) or a high-density chlorine tester (Saitama Kasahara Chemical Instruments Corp, Saitama, Japan; RC-3F).

2.2. Passion Fruit and Treatment

Yellow passion fruit (Passiflora edulis Sims cv. Fujian Baixiangguo NO. 3) was harvested from a passion fruit orchard in Nan’an, Fujian, China during the commercial maturity stage (chromaticity L* = 47.60 ± 1.14; hue angle h = 116.37 ± 0.70). The fruit was transported to the laboratory, where samples of comparable size, shape, and color with no visible physical damage or pathological symptoms were selected. These chosen fruits were rinsed with the distilled water and subsequently utilized for the following treatment. All treatments were initiated on the day of harvest, and the fruits were not subjected to any pre-storage period prior to AEW application.

Thirty passion fruits in total were adopted to measure the fruit attributes on the day of harvest. The other 600 fruits were allocated to 4 groups of 150 each: (i) control, fruit immersed in 250 L of distilled water; and (ii) AEW treatments, fruit immersed in 250 L of AEW (pH 2.5) with ACC of 30, 60, or 90 mg/L (ACC30, ACC60, and ACC90). After a 20-min dip, fruits were drained and allowed to air-dry at 25 °C for ~1 h. Each group was then packaged in non-perforated polyethylene film bags (30 cm × 40 cm, 20 μm thickness; 10 fruits per bag). After packaging, each bag was closed by tying a knot (i.e., not heat-sealed). Furthermore, the permeability of moisture, O₂, and CO₂ in the polyethylene film bag was approximately 18 g m⁻² d⁻¹, 4000 cm³ m⁻² d⁻¹, and 1400 cm³ m⁻² d⁻¹, respectively. The packaged fruits were stored at 85% relative humidity and 25 °C for 15 d. The storage temperature (25 °C) was selected to simulate typical ambient marketing conditions for yellow passion fruit in producing areas during the harvest season, where the fruit often experiences rapid quality loss and decay. This setting allowed us to evaluate the efficacy of AEW under a practically relevant ambient (non-refrigerated) scenario. Within storage, sampling was conducted at 3 d intervals; thirty fruits (three bags) per treatment were taken to evaluate storage- and quality-related indices. The AEW condition that most effectively maintained fruit quality was identified by comparing the three ACC treatments. On this basis, additional analyses were performed to elucidate whether the optimal AEW treatment delayed disease and decay development and stabilized storage quality by modulating disease resistance in yellow passion fruit.

2.3. Evaluation of Fruit Disease, Decay Incidence and Pericarp Cell Membrane Permeability (CMP)

Ten independent fresh passion fruits were chosen for disease assessment using the protocol described by Lin et al. [21]. The proportion of diseased surface area was scored on a five-level scale: 0, symptom-free; 1, 1–24% affected; 2, 25–49% affected; 3, 50–74% affected; and 4, ≥75% affected. Then, the disease index was computed as Σ[(disease grade/maximum grade) × (number of fruits at that grade/total number of fruits)].

Decay incidence was evaluated according to Li et al. [22] and reported as 100 (%) minus the proportion of commercially acceptable fruit (%). Fruits without visible disease symptoms or pericarp browning were considered commercially acceptable.

Relative leakage rate (%), as an indicator of CMP, was determined following Lin et al. [23]. Briefly, twenty round pieces (0.2 cm² each) were excised from the equatorial pericarp of five fruits and incubated in 25 mL of distilled water for 3 h. The electrical conductivity of the bathing solution was measured using a conductivity meter and recorded as C1. Subsequently, the solution containing the pericarp discs was subjected to a boiling water bath for 30 min. After cooling to room temperature, distilled water was added to restore the volume to 25 mL, and the total electrolyte conductivity was measured as C2. Relative electrolyte leakage was expressed as the percentage of the initial electrical conductivity (C1) relative to the total conductivity (C2).

2.4. Determination of Pericarp Color

Five fresh independent passion fruits were employed to determine the pericarp color. For each fruit, chromaticity (L*) and hue angle (h) were measured at four equidistant points of the equatorial region, according to You et al. [1] and Xu et al. [5], using a CR-400 chroma meter (Tokyo Konica Minolta Inc., Tokyo, Japan).

2.5. Assay of Pulp Nutrient Substance Contents

Based on the methods of Xu et al. [5] and Lin et al. [21], tissue of a fresh fruit pulp was removed and ground, followed by the centrifugation, and then the supernatant (fruit juice) was collected. The drop of supernatant was obtained by a dropper for assaying the TSS level in the passion fruit using a PAL-1 pocket refractometer (Tokyo Atago Corp., Tokyo, Japan). Five fruits were tested in duplicate for each treatment.

Additionally, the pulp tissue of five fresh passion fruits (per treatment) was removed, thoroughly ground, and then centrifuged to remove seeds. The supernatant (fruit juice) was collected for measuring the levels of other nutrient substances in the passion fruit pulp.

An aliquot of the supernatant (5 g) was diluted with distilled water, and the resulting extract (10 mL) was used to determine titratable acidity (TA) following You et al. [1] and Lin et al. [23] using an automatic potentiometric titrator (Shanghai Mettler Toledo Instrument Co., Ltd., Shanghai, China; ET18) with sodium hydroxide (0.02 M) as the titrant.

Vitamin C was quantified according to You et al. [1] by mixing the supernatant (1 g) with 10 mL of 15% (w/v) trichloroacetic acid; a 1 mL aliquot of the resulting mixture was then used for analysis.

Total soluble sugars were determined as described by Lin et al. [22]. In brief, the supernatant (1 g) was combined with 6 M HCl (10 mL) and distilled water (20 mL), boiled for 30 min, and then neutralized with 6 M NaOH using phenolphthalein as the endpoint indicator. The neutralized solution was subsequently used for sugar determination.

Vitamin C and total soluble sugar contents were expressed as g/kg fresh weight, whereas TSS and TA were reported on a percentage basis.

2.6. Determination of Pericarp Phenol Contents

Total polyphenolics and total flavonoids were quantified following Wang et al. [12]. In brief, 1 g of pericarp tissue from five fruits was homogenized in HCl–methanol (100 mL, 1% v/v) and centrifuged. For total polyphenolics, the supernatant (50 μL) was reacted with Folin–Ciocalteu reagent (1 mL, 1 M), Na₂CO₃ (0.8 mL, 7.5% w/v) and distilled water (1.15 mL). For total flavonoids, the supernatant (1 mL) was combined with AlCl₃ (1 mL, 3% w/v) and ethanol (0.5 mL, 30% v/v). Data were reported as g/kg fresh weight.

2.7. Determination of Pericarp Lignin Content

Pericarp tissue (10 g) collected from ten fruits was oven-dried at 100 °C until a constant mass was reached. Lignin content was then quantified using 2 g of the dried material following Sun et al. [24], with the content expressed as g/kg dry weight.

2.8. Determination of Pericarp DRE Activities

Following Lin et al. [23] and Sun et al. [24], the pericarp DRE activities, containing POD, 4-CL, PAL, C4H, GLU, CHI, and CAD, were measured.

For PAL and POD, pericarp tissue (1 g) from five fruits was homogenized in phosphate-buffered saline (PBS; 50 mM, 10 mL, pH 5.5) containing polyvinylpyrrolidone (PVP; 2% w/v) and centrifuged. The collected supernatant served as the enzyme extract. For PAL activity determination, 0.5 mL of enzyme extract was mixed with 3.0 mL of 50 mM PBS and 0.5 mL of 20 mM L-phenylalanine. The reaction mixture was incubated at 37 °C for 1 h and subsequently terminated by the addition of 0.1 mL of 6 mM HCl. For POD activity determination, 1.0 mL of enzyme extract was mixed with 1.0 mL of 2% (v/v) H₂O₂, 3.0 mL of 50 mM PBS (pH 5.5), and 1.0 mL of 50 mM o-methoxy-phenol. The reaction mixture was incubated at 35 °C for 10 min and terminated by the addition of 2.0 mL of 20% (w/v) trichloroacetic acid. One activity unit was considered as the enzyme quantity that resulted in an absorbance increment of 0.01 min⁻¹ at 290 nm for PAL or 0.01 min⁻¹ at 470 nm for POD. The activities of both enzymes were expressed as U/mg protein.

For C4H, pericarp tissue (1 g) from five fruits was extracted with trismetyl aminomethane (Tris)–HCl buffer (50 mM, 6 mL, pH 8.9) containing PVP (0.15% w/v), phenylmethylsulfonyl fluoride (1 mM), β-mercaptoethanol (15 mM), leupeptin hemisulfate salt (10 μM), MgCl₂ (4 mM), glycerol (10% v/v), and ascorbic acid (5 mM). After centrifugation, an aliquot of 0.5 mL of enzyme extract was mixed with 2.5 mL of 50 mM Tris–HCl buffer (pH 8.9) containing 2 μM nicotinamide adenine dinucleotide phosphate disodium salt, 5 μM D-glucose-6-phosphate monosodium salt, and 2 μM trans-cinnamic acid. The reaction mixture was incubated at 25 °C for 30 min and subsequently terminated by the addition of 0.1 mL of 6 M HCl. One activity unit was considered as an absorbance increase of 0.01 min⁻¹ at 340 nm. C4H activity was expressed as U/mg protein.

For 4-CL, pericarp tissue (1 g) from five fruits was homogenized in Tris–HCl (50 mM, 6 mL, pH 8.0) containing 100 mM dithiothreitol and 25% (v/v) glycerol. After centrifugation, an aliquot of 0.5 mL of enzyme extract was mixed with 0.15 mL of 15 mM p-coumaric acid, 0.05 M ATP, 0.15 mL of 1 mM coenzyme A, and 0.45 mL of 0.015 mM magnesium chloride. The reaction mixture was incubated at 40 °C for 10 min, after which the absorbance was measured. One activity unit was taken as the amount of enzyme causing an absorbance change of 0.01 min⁻¹ at 333 nm. The activity was expressed as U/mg protein.

For CAD, pericarp tissue (1 g) from five fruits was extracted with PBS (100 mM, 6 mL, pH 6.25) supplemented with ethylene diamine tetraacetic acid (EDTA; 1 mM), β-mercaptoethanol (15 mM), and 0.1% (w/v) PVP. After centrifugation, 1.0 mL of enzyme extract was mixed with 1.0 mL of 0.1 M PBS (pH 6.25), 1.0 mL of 2 mM NADP, and 1.0 mL of 1 mM trans-cinnamic acid. The reaction mixture was incubated at 37 °C for 30 min, and the absorbance was subsequently measured. One activity unit was considered as an absorbance change of 0.01 min⁻¹ at 340 nm. CAD activity was expressed as U/mg protein.

For CHI and GLU, pericarp tissue (1 g) from five fruits was homogenized in PBS (0.2 M, 10 mL, pH 5.2) containing PVP (8% w/v), β-mercaptoethanol (5 mM), and EDTA (1 mM). After centrifugation, the collected supernatant served as the enzyme extract. For CHI activity determination, 3.0 mL of enzyme extract was mixed with 0.5 mL of 0.5% (m/v) chitin solution. The reaction mixture was incubated at 37 °C for 60 min, and the absorbance was subsequently measured at 585 nm. For GLU activity determination, 0.3 mL of enzyme extract was mixed with 0.4 mL of 0.1% (m/v) kombucha polysaccharide solution and incubated in a water bath at 37 °C for 60 min. The sugar content of the reaction mixture was assayed by the anthrone colorimetric protocol. One unit of GLU or CHI activity was taken as the quantity of enzyme producing 1 mg glucose h⁻¹ or 1 μg N-acetyl-D-glucosamine min⁻¹, respectively. The activities of both enzymes were expressed as U/mg protein.

The soluble protein concentration of the enzyme extract was determined using the Bradford method [25]. Briefly, 0.2 mL of enzyme solution was mixed with 10 mL of Coomassie Brilliant Blue G-250 reagent. After 2 min of color development, the absorbance was measured at 595 nm. A standard curve was prepared using bovine serum albumin solutions. Enzyme activities were subsequently expressed on a protein basis, i.e., U/mg protein, where U represents the activity unit defined for each enzyme assay and mg protein refers to the total soluble protein contained in the volume of enzyme extract used for the corresponding assay.

2.9. Statistic Analyses

The TSS content was determined with ten biological replicates, while other indicators were measured the three times. Each data point in figures was revealed as the mean ± standard error (n = 3). Statistical analyses were carried out using the 22.0 SPSS software (IBM Corp., Armonk, NY, USA) with one-way analysis of variance followed by Duncan’s multiple range test. Values with a p < 0.05 were taken as statistically significant, and values with a p < 0.01 were regarded as highly significant.

3. Results

3.1. Changes in the Fruit Appearance, Disease Index, Decay Incidence and Pericarp CMP

On the day of harvest, the appearance of the passion fruit exhibited a yellow–green color (Figure 1). Over time, the pericarp of the control fruit displayed a gradual shift towards a yellow hue. During storage (days 6–15), the control fruit displayed brown disease spots and severe decay symptoms, accompanied by a rising fruit disease index (Figure 1B) and decay incidence (Figure 1C). Compared to the control samples, all the AEW-treated treatments displayed less severe diseases and decay symptoms and better appearance quality, with a lower disease index and decay incidence on 6–15 d. Additionally, the AEW (ACC60)-treated samples showed a significantly lower disease index and decay incidence values than the control fruit on days 9–15.

Pericarp CMP increased in all groups over the 15-day storage period (Figure 1D). However, the AEW-treated fruit kept a lower pericarp CMP than the control fruit, with the best AEW condition being ACC60. In addition, clear differences between days 3 and 15 were observed in the AEW (ACC60)-treated passion fruit.

Hence, AEW (particularly with ACC60) effectively suppressed fruit disease and decay and reduced the pericarp CMP of the passion fruit.

3.2. Changes in Pericarp Color

Figure 2A shows that the chromaticity L* value of the pericarp in all groups ascended rapidly during the first 9 days, followed by a decline afterward. However, the fruit treated by AEW, including ACC60, retained a higher value than the other groups and showed conspicuous differences between days 6 and 15 compared to the control group.

Figure 2B indicates that a reduced level of the pericarp hue angle h during storage was displayed in all groups, whereas the AEW-treated groups revealed a greater value of the hue angle h in the pericarp during storage, with the best condition being ACC60. Furthermore, between days 6 and 15, the AEW-treated samples, including ACC60, exhibited a conspicuously lower pericarp hue angle h than the control passion fruit.

Hence, AEW treatment (particularly with ACC60) effectively maintained greater pericarp chromaticity L* as well as hue angle h values in the fresh passion fruit during storage.

3.3. Changes in the Pulp Nutrient Substance Contents

There was an uptrend in the values of pulp TSS (Figure 3A) and TA (Figure 3B) in all groups during the first 3 days, followed by a decline until day 15. Regarding the TSS content, all the AEW-treated samples retained greater levels between days 0 and 15 than the control samples, with the optimal ACC condition being ACC60. In addition, the passion fruits treated at this concentration had a prominently higher pulp TSS level than the control group between days 3 and 15. Regarding the TA content, the AEW (ACC60)-treated fruit had a higher pulp TA content than the control fruit, with large differences within days 6–15.

Figure 3C demonstrates that the pulp vitamin C content of all samples declined speedily from day 0 to the last day, whereas the AEW-treated fruit kept higher pulp vitamin C value, especially in the AEW treatment with ACC60. Additionally, the AEW (ACC60)-treated fruit showed a notably greater level than the control fruit between days 6 and 12.

Figure 3D demonstrates that the pulp total soluble sugar amount of all groups revealed an overall downward trend during storage, whereas the AEW-treated group, particularly the ACC60 group, displayed a greater amount compared to the control group during storage. Moreover, the fruit treated by AEW containing ACC60 consistently retained a higher pulp total soluble sugar content than the control group between days 6 and 15.

So, the treatment with AEW (especially with ACC60) effectively slowed down the decline in various pulp nutrient substances, including TSS, vitamin C, and TA as well as total soluble sugar, in the fresh passion fruit during storage.

The above results showed that the AEW treatment with ACC60 had the optimal effect in stabilizing the postharvest quality of the fresh passion fruit. Accordingly, subsequent experiments focused on the mechanisms by which AEW suppresses disease and decay and preserves storage performance, with a particular emphasis on its regulation of disease resistance.

3.4. Changes in the Pericarp Phenols and Lignin Contents

Based on the screening results of AEW available chlorine concentrations (ACC30, ACC60, and ACC90) on postharvest quality and decay-related parameters (Figure 1, Figure 2 and Figure 3), ACC60 was identified as the optimal condition and was therefore selected for subsequent mechanistic analyses. Accordingly, Figure 4 and Figure 5 compare the control and the ACC60-treated fruit to elucidate whether AEW enhances the accumulation of defense-related metabolites and activates defense-related enzymes during storage.

Figure 4A indicates that the pericarp total polyphenolic level of the control group increased swiftly during the first 9 days but reduced quickly afterward, whereas the pericarp total polyphenolic content of the AEW-treated samples rose rapidly until day 9 and then decreased more gradually until day 15. Additionally, the AEW-treated group maintained a significantly higher pericarp total polyphenolic content from days 12 to 15 than the control samples. On day 15, the pericarp total polyphenolic value of the AEW-treated samples was 1.08 times that of the control samples.

Figure 4B shows that the pericarp total flavonoid content of the two groups rose speedily within the first 6 days but descended quickly after day 6. By contrast, between days 0 and 15, the pericarp total flavonoid content in the AEW-treated samples was greater than the control samples, with a conspicuously higher level between days 6 to 15. The pericarp total flavonoid value of the AEW-treated samples was 1.10 times that of the control samples on day 15.

Figure 4C indicates that the pericarp lignin level in the control samples rose swiftly during 0–6 d and reached the highest value of 539.43 g/kg on day 6, and thereafter, it decreased quickly until 15 d. However, the pericarp lignin level of the AEW-treated samples raised appreciably within 0–6 d and reached the highest level of 620.50 g/kg on 6 d but fell rapidly after day 6. In addition, compared to the control samples, the AEW-treated group kept an obviously higher level throughout days 6–15.

Therefore, the AEW treatment raised the levels of pericarp phenol compounds, such as total polyphenolics, total flavonoids, and lignin, in the fresh passion fruit during storage.

3.5. Changes in the Pericarp PAL, C4H, 4-CL, CAD, POD, CHI, and GLU Activities

Figure 5A demonstrates that, in the control passion fruit, the pericarp PAL activity ascended swiftly during 0–12 d but dropped quickly afterward. However, an uptrend in pericarp PAL activity was measured in the AEW-treated passion fruit during storage. Additionally, during days 3–15, compared with the control group, the AEW-treated fruit revealed a significantly greater level than control fruit. Regarding pericarp PAL activity, the activity of the AEW-treated fruit was 1.51 times that of the control samples on 15 d.

Figure 5B demonstrates that the C4H activity in the pericarp of both groups increased quickly during 0–12 d but fell rapidly until day 15. In contrast to the control group, greater pericarp C4H activity was revealed in the AEW-treated passion fruit, with remarkable differences on day 3 and within 9–15 d. The pericarp C4H level of the AEW-treated samples was 1.74 times that of the control samples on day 15.

Figure 5C revealed that the pericarp 4-CL activity in both groups showed an escalating trend on 0–9 d, while a downtrend was observed on days 9–15. Contrasting the control group, the AEW-treated passion fruit had a higher value, with clear differences at day 9 and day 15. For instance, 4-CL activity in the pericarp of the AEW-treated fruit was 1.11 times that of the control samples on day 15.

Figure 5D demonstrates that the pericarp CAD value of the control fruit changed from 2.98 U/mg protein on 0 d to 4.13 U/mg protein on 15 d, while the pericarp CAD level in the AEW-treated passion fruit increased from 2.98 U/mg protein on 0 d to 5.74 U/mg protein on 15 d. In addition, compared with the control fruit, the AEW-treated fruit clearly maintained a greater value between days 12 and 15.

Figure 5E demonstrates that the pericarp POD activity of the control samples rose slowly on 0–3 d and then raised quickly on 3–6 d, and thereafter, it increased slowly on 6–9 d but fell slowly during 9–12 d and finally reduced sharply after 12 d. Meanwhile, the pericarp POD level of the AEW-treated samples ascended quickly during days 0–9 but fell sharply until day 15. On day 15, pericarp POD activity of the AEW-treated samples was 1.22 times that of the control group.

Figure 5F indicates that the pericarp CHI activity in both groups raised sharply throughout storage, whereas the AEW-treated samples maintained a greater pericarp CHI activity than the control samples, with conspicuous differences on 9–15 d. On 15 d, the pericarp CHI level of the AEW-treated samples was 1.08 times that of the control group.

Figure 5G indicates that the pericarp GLU activity of the control samples increased from 3.03 U/mg protein on 0 d to 10.05 U/mg protein on 15 d, while the GLU activity in the pericarp of the AEW-treated fruit increased from 3.03 U/mg protein at day 0 to 11.94 U/mg protein at 15 d. In addition, during days 6–15, compared to the pericarp GLU activity of the control samples, the AEW-treated fruit maintained a significantly higher value.

Therefore, AEW application raised the PAL, C4H, 4-CL, CAD, POD, CHI, and GLU levels in the pericarp of the fresh passion fruit during storage.

4. Discussion

4.1. Influences of Different AEW Conditions on the Storage Behaviors and Quality Properties of Passion Fruit

Fresh passion fruit is perishable to decay, disease, and quality deterioration, leading to a loss of storability during storage. Some reports have expounded that AEW acts as a novel and safe postharvest handling method for retaining storage quality and suppressing decay development in fresh produce [2,8]. Moreover, postharvest quality is influenced by changes in storage behaviors and quality properties like pericarp appearance level as well as pulp nutrient amounts. The vital parameters of storage behaviors, including CMP and commercially acceptable fruit rate, can affect postharvest quality in fresh produce [23]. Indices of pericarp appearance quality, such as hue angle h and chromaticity L*, can be used to evaluate the appearance quality of fresh produce [26]. In addition, the quantities of pulp nutrients are influenced by the levels of vitamin C, total soluble sugar, TSS, or TA [22,23]. It should be noted that the fruits were packaged in non-perforated polyethylene film bags and closed by knotting, which may have influenced the storage microenvironment. However, this packaging condition was identical for the control and all AEW treatments, and thus it was treated as a controlled constant in our comparative analysis. The in-package O₂ and CO₂ concentrations were not monitored in the present study. Future work incorporating gas composition measurements will help to quantify the packaging atmosphere dynamics and further distinguish their contribution from AEW-induced effects.

Throughout storage, in contrast to the control group, better appearance quality (Figure 1A), lower levels of disease index (Figure 1B) and decay incidence (Figure 1C), lower CMP (Figure 1D), greater values of L* (Figure 2A) and hue angle h (Figure 2B), and higher amounts of TSS (Figure 3A), TA (Figure 3B), vitamin C (Figure 3C), and total soluble sugar (Figure 3D) were measured in the AEW-treated group. So, AEW treatment could improve the storage behavior and levels of pericarp appearance and pulp nutrients in fresh passion fruit. These findings were as a result of the boosted disease resistance caused by AEW, thus maintaining the higher quality and storability of the fresh fruit. Further comparison showed that AEW with ACC60 was the optimal condition for enhancing the passion fruit quality during storage. A plausible explanation for the superior performance of ACC60 is a dose-dependent response to AEW. At the lower ACC (ACC30), the antimicrobial/oxidative capacity of AEW may be insufficient to effectively suppress epiphytic microorganisms and/or to elicit a strong defense response, resulting in a weaker preservation effect. In contrast, an excessively high ACC (ACC90) may impose stronger oxidative/chlorine stress on the pericarp, potentially causing subtle tissue injury (e.g., disruption of surface integrity and membrane function) and thereby offsetting part of the benefit from microbial suppression. Therefore, ACC60 may represent a balanced level, providing adequate antimicrobial action while avoiding excessive oxidative stress. Hence, the above concentration was chosen for further study of mitigated disease and decay developments in fresh passion fruit by mediating its disease resistance. Similarly, our prior research reported that AEW-enhanced wampee fruit quality was due to AEW’s impact on the storability and quality properties [23]. Additionally, Hui et al. [27] reported that short-time partial dehydration could be an effective method for the preservation of Actinidia arguta, which resulted in a reduced fruit decay rate but higher levels of TSS, TA and vitamin C.

4.2. AEW Stabilized the Storage Behaviors and Quality Properties of Passion Fruit by Accumulating Phenols and Lignin

When plants undergo biological stresses, they can active defense systems, leading to accumulation of disease-resistant substances and increased DRE activities, thereby strengthening overall resistance to infection [8]. Among the defense-associated metabolic pathways, the phenylpropanoid pathway is widely recognized as a major contributor to plant resistance [15,24]. In this pathway, phenolics, flavonoids, or lignin act as the key products [15]. Phenolics and flavonoids play key roles in the fruit stress response that can suppress decay development in fresh produce [28]. In addition, lignin, a key ingredient in plant tissues, consists of sinapyl alcohol, p-coumaryl alcohol, and coniferyl alcohol [6]. Its polymerization occurs via ester, ether, and C-C bonds through the action of POD [29]. Increased lignification has been associated with enhanced resistance by reinforcing tissue integrity and forming a physical barrier that limits pathogen invasion and restricts access to water and nutrients required for pathogen colonization [6,7,24]. Collectively, lignin-associated strengthening is an important determinant of disease resistance and can therefore influence postharvest fruit quality.

The literature confirmed that some applications of burdock fructooligosaccharide [12], chitosan [13], ε-poly- L-lysine [24], or melatonin [29] can increase the contents of phenols or lignin. Consequently, this enhanced the disease resistance level, suppressing disease occurrence, retarding decay symptoms, and improving quality in fresh produce. Therefore, a higher level of disease-resistant substances might boost disease resistance and reduce disease while increasing the quality of fresh produce.

In the present work, at the early and middle stages of storage, raised total polyphenolic, total flavonoid and lignin contents were shown in the control samples (Figure 4A–C), which was due to pathogen-infection-induced disease and decay development (Figure 1A–C). These results hinted that phenols and lignin biosynthesis were key for enhancing disease resistance and inhibiting pathogen infection. However, in the late stage of storage, the control passion fruit revealed dropped phenols and lignin levels (Figure 4A–C), thus reducing disease resistance but expediting disease and decay development (Figure 1A–C). By contrast, throughout storage, compared with the control samples, higher values of pericarp phenols, including total polyphenolic (Figure 4A), total flavonoid (Figure 4B), and lignin (Figure 4C), were accompanied by less serious development of fruit disease and decay, a lower disease index and decay incidence, and a lower pericarp CMP but higher levels of pericarp L* or hue angle h and greater amounts of pulp vitamin C, TSS, and TA as well as total soluble sugar (Figure 1, Figure 2 and Figure 3) in the AEW-treated fruit. Hence, these results implied that AEW boosted the postharvest disease resistance and reduced the development of disease and decay so as to retain better storage performance and higher quality features of the fresh passion fruit. These results were attributed to AEW-increased levels of lignin and phenols such as total polyphenolic and total flavonoid contents, thus enhancing the disease resistance of fresh passion fruit. Similarly, methyl jasmonate can accumulate lignin and phenolic substances to induce disease resistance, resulting in better storage quality of kiwifruits [15]. Furthermore, Yu et al. [30] found that treatments with chitosan or chitooligosaccharide enhanced lignin content to boost disease resistance, eventually reducing the disease index to retain quality in pears.

4.3. AEW Stabilized the Storage Behaviors and Quality Properties of Passion Fruit by Increasing the DRE Activities

The production of phenylpropanoid-derived metabolites, including phenolic compounds, flavonoids, and lignin, is a complex, multistep process governed by a network of enzyme-catalyzed reactions [12,31]. Some of these critical enzymes, like PAL, C4H, POD, 4-CL, and CAD, may influence the process of phenylpropane synthesis [6,21]. PAL is an initial enzyme in this process, which improves lignin and phenol biosynthesis [15,32]. It catalyzes the deamination of phenylalanine to form cinnamic acid [6,21]. C4H catalyzes the hydroxylation of cinnamic acid to yield p-coumaric acid [29,33]. Then, 4-CL converts various hydroxycinnamic acids into corresponding coenzyme A (CoA) esters, supplying some precursors for phenol or lignin biosynthesis [15]. These CoA esters can further generate corresponding aldehydes through cinnamoyl CoA reductase [30]. These aldehydes are catalyzed to monolignols by CAD [12,30]. POD, the crucial enzyme in the terminal phase of lignification, promotes the oxidative coupling of monolignols, thereby driving lignin polymer formation [33]. In addition, increased activities of these enzymes can enhance the contents of phenols or lignin, thus boosting the plant’s resistance to some abiotic or biotic stressors [12]. Beyond these five enzymes, CHI and GLU are also pivotal components of defense responses in harvested produce [34,35]. CHI decomposes chitin, while GLU hydrolyzes β-1,3-glucan—two key components of fungal cell walls [36,37].

Some documents revealed that high DREs are related to better quality in fresh produce. For instance, treatments with serine protease [8], ultraviolet-C irradiation [14], burdock fructooligosaccharide [12] or antimicrobial peptide CB-M [37] can increase DRE activities. These heightened activities enhance defense responses, boost disease resistance, and eventually slow down disease and decay development, retaining fresh product quality.

In the present research, in contrast to the control samples, throughout storage, the AEW-treated fruit revealed greater pericarp DRE activities, such as PAL (Figure 5A), C4H (Figure 5B), 4-CL (Figure 5C), CAD (Figure 5D), POD (Figure 5E), CHI (Figure 5F), and GLU (Figure 5G), and higher contents of pericarp phenols and lignin (Figure 4). Furthermore, the AEW-treated fruit also displayed better storage quality than the control fruit throughout storage. This was evident in the less severe disease and decay symptoms and lower disease index and decay incidence of the fresh passion fruit (Figure 1). Additionally, there were lower pericarp CMP and higher pericarp L* and hue angle h levels and pulp TA, vitamin C, and TSS as well as total soluble sugar (Figure 2 and Figure 3). Therefore, these findings hinted that AEW application might increase the action of PAL, C4H, POD, 4-CL, and CAD to boost levels of lignin or phenols as well as increase the activities of CHI and GLU in fresh passion fruit. These results would enhance the level of disease resistance, resulting in delayed disease and decay developments, thereby retaining the better storage quality of fresh passion fruit. Such data was consistent with some documented cases. For example, ε-poly-L-lysine application might increase the activities of 4-CL, CAD, PAL and C4H, which are lignin-synthesis-related enzymes, resulting in a raised lignin content, thereby boosting resistance and reducing disease severity in wampee fruit [2]. The joint applications of sodium alginate and chlorogenic acid retained better quality and storability in pears as a result of increased activities of POD, 4CL, C4H, PAL and CAD [16]. Moreover, ultraviolet irradiation induced higher PAL, POD, CHI, and GLU activities to enhance disease resistance, subsequently reducing mangosteen fruit disease [14].

Therefore, the aforementioned results offer novel insights into the mechanism through which AEW application delayed disease, reduced decay incidence, and stabilized storage quality of fresh passion fruit by enhancing its disease resistance. A summary of the mechanism is presented in Figure 6. Given that AEW is widely used as a postharvest sanitizer with strong antimicrobial efficacy, it is plausible that AEW treatment could also reduce the surface microbial load of passion fruit and thereby contribute to the lower decay observed in this study. However, in the present work, we did not directly quantify surface microbial populations (e.g., by plate counts/CFU enumeration or molecular approaches). Therefore, although our results support that AEW is associated with enhanced host defense responses, the relative contribution of direct microbial reduction versus host resistance induction warrants further investigation. Future studies will include microbial enumeration and/or community profiling to better resolve these contributions.

5. Conclusions

AEW treatment markedly suppressed the progression of disease and decay and helped maintain the storage quality of fresh passion fruit. This protection was accompanied by increased levels of phenolic metabolites (total polyphenolics and flavonoids) and lignin, which coincided with higher activities of PAL, CAD, C4H, 4-CL, and POD as well as elevated GLU and CHI activities. Collectively, these biochemical changes strengthened disease resistance and thereby maintained postharvest quality. These findings demonstrate the efficacy of AEW in delaying disease onset and preserving fruit quality. Moreover, the present results may provide a useful reference for evaluating AEW-based strategies to mitigate decay and maintain quality in other tropical and subtropical fruits; however, the efficacy and optimal treatment conditions should be validated and optimized in a commodity-specific manner under different storage and packaging systems. Future studies employing proteomic, transcriptomic, and metabolomic approaches should elucidate the molecular mechanisms by which AEW retards disease progression and preserves postharvest quality in passion fruit.