Postharvest Quality Exploration of “Crystal” Grapes in Karst Mountainous Area: Regulatory Effect of High Concentration 1-MCP Fumigation

Abstract

The comprehensive exploration and thorough understanding of the physicochemical characteristics of “Crystal” grapes in the Karst area of Southwest China can not only produce edible table grapes, but also offer significant theoretical backing for the management of rocky desertification in the Karst area and consequently generate enhanced social and ecological benefits. This study explored the regulatory effect of 1-MCP fumigation on the postharvest storage quality of "Crystal” grapes, aiming to provide some technical support for the development of the grape industry in the Karst mountainous areas. The results indicate that compared with the control group, both the 10.0 and 50.0 μL/L 1-MCP treatment groups effectively inhibited the increase in decay rate, browning rate, shedding rate, and color change of the grapes, significantly inhibited the enhancement of polyphenol oxidase (PPO), peroxidase (POD), and lipoxygenases (LOX) browning-related enzyme activities, improved the superoxide dismutase (SOD) activity of the fruits, hindered the decrease of ascorbic acid (AsA) and polyphenols, delayed the accumulation of malondialdehyde (MDA) content, and significantly postponed the peak period of polygalacturonase (PG) activity in the fruits and the degradation of protopectin to soluble pectin, thereby maintaining desirable texture characteristics. The utilization of 1-MCP for fruit fumigation yielded a heightened overall fragrance, thereby facilitating the emission of terpenes, alcohols, aldehydes, ketone aromatic compounds, and methyl aromatic substances within the fruit. Overall, both the M10 and M50 treatments are equally effective in preserving the storage quality of “Crystal” grapes. Under the condition of ensuring the excellent edible quality of crystal grapes, M2 can extend the shelf life of crystal grapes by about 5 days, and M10 and M50 can extend the storage period of crystal grapes by 10~15 days. Considering the cost, it is recommended that the industry employs a concentration of 10.0 μL/L of 1-MCP for “Crystal” grapes. Simultaneously, this study also provides theoretical support for the ecologically high-quality development of cultivated land in the Karst mountainous area of Guizhou and tree selection.

1. Introduction

Currently, the Karst Landform is extensively dispersed throughout southwest China, encompassing a total area exceeding 500,000 km². The prevalence of soil erosion in the Karst area has caused a rise in rocky desertification, thereby impeding the vegetation productivity of the Karst mountains. Consequently, the Karst area is confronted with the dual challenges of barren land and environmental deterioration [1]. Research has demonstrated that the Karst area in southern China is widely distributed, with a significant prevalence of rocky desertification, and rocky desertification control in various regions relies on different crops to reduce soil erosion and improve soil structure. Liana, as a pioneering plant in the forest community succession of rocky desertification areas, exhibits remarkable adaptability to environmental conditions. The utilization of liana for rocky desertification control can expedite the process of vegetation restoration [2].

The “Crystal” grape (Vitis vinifera L.) is a perennial plant in the liana genus, a new high-quality variety derived from the cross of Cassady and Concord, originating from the Americas and commonly referred to as the Niagara grape. “Crystal” grapes are highly praised by consumers for their appealing taste, nutritional richness, and functional compounds [3]. Furthermore, due to the thin skin and brittle pericarp tissues of “Crystal” grapes, the fumigation preservation technology, which employs gas penetration into the fruit, can effectively avoid secondary damage after harvest, and, moreover, decrease manpower and economic costs [4,5]. Sulphur dioxide (SO₂) fumigation currently represents the prevailing and potent treatment for preserving the quality of table grapes and mitigating storage diseases [6]. Nonetheless, the presence of SO₂ residues can result in tissue damage, such as peel cracking, bleaching, and berry detachment. Consequently, there is a pressing need to explore alternative, safe, and efficient preservation methods to SO₂, which holds significant implications for the storage and industrial advancement of “Crystal” grapes.

1-Methylcyclopropene (1-MCP) is a novel ethylene receptor inhibitor that effectively inhibits ethylene signaling, thereby delaying the post-ripening and aging process of fruits and vegetables [7]. Currently, 1-MCP is widely used in blueberries [8], sweet cherries [9], and other fruits to preserve their excellent quality. Furthermore, 1-MCP has been shown to effectively maintain the storage quality of “Crystal” grapes and other table grapes [10,11]. Zhu et al. [11] found that fumigating selenium-rich grapes with 1.0 μL/L 1-MCP can effectively maintain the selenium content, and improve the sugar, titratable acidity (TA), and amino acids content during storage, thus enhancing the edible value of the fruits. Moreover, the inhibition of various substances associated with fruit ripening, such as lycopene, chlorophyll, peel color, and polygalacturonic (PG) activity, has also been observed [12]. It is worth noting that the effectiveness of 1-MCP treatment depends on the fumigation concentration and time. Insufficient fumigation may fail to achieve the full fumigation effect, resulting in unsatisfactory preservation [13]. Some studies have demonstrated the beneficial impact of higher concentrations of 1-MCP on preserving the postharvest quality of fruits. For example, Inaba et al. [14] found that treatment with 20.0–40.0 μL/L 1-MCP prevented the ripening-inducing effect of propylene on bananas. Wills et al. [15] fumigated mature tomatoes with 5.0–100.0 μL/L 1-MCP, effectively extending their postharvest storage period, and exposure to 20.0 μL/L 1-MCP increased the postharvest life of tomatoes by 25%. It has also been found that a high-dose 1-MCP (30.0 μL/L) fumigation treatment enhances antioxidant capacity in newly harvested rose petals [16]. Furthermore, 1-MCP showcases the virtues of being non-toxic, remarkably efficacious, and devoid of residues [17]. It exerts its inhibitory or retarding impact on fruit maturation and senescence by modulating the physiological reactions of ethylene, thereby guaranteeing a heightened level of safety. Consequently, investigating the application of higher concentrations of 1-MCP treatment may offer novel insights into the postharvest storage technology for “Crystal” grapes. However, there are limited reports on the application of high-dose 1-MCP fumigation in the postharvest preservation of fruits and vegetables, and its regulatory mechanism remains unclear. Especially, the impact of high concentration 1-MCP treatment on the deterioration of “Crystal” grapes after harvest remains inadequately investigated.

The distinct aroma (alcohols, esters, aldehydes, ketones, and terpenes) of “Crystal” grapes serves as a pivotal factor in captivating consumers [18] The utilization of electronic nose technology can offer a comprehensive analysis of the volatile gases emitted by grapes. However, there is presently limited research on the deployment of electronic nose technology for the identification of stored grapes, necessitating further investigation. Moreover, fruit texture also represents a crucial factor influencing consumers’ acceptability. Nevertheless, achieving precise evaluations poses a challenging task [19]. The texture properties of grapes with different treatments during storage can be objectively evaluated through texture profile analysis (TPA).

Given the limited research on the high-concentration 1-MCP fumigation of fruits, there is a need to further explore the mode of action of high-concentration 1-MCP in inhibiting the degradation of “Crystal” grapes’ storage quality. The aroma difference between fruits treated with 1-MCP fumigation and the control group was analyzed using an electronic nose. To clarify the regulatory effects of postharvest quality of “Crystal” grapes and this study are of great significance for the development of effective preservation technology to guarantee the development of the grape industry in the Karst mountain area, and to accelerate the area of rocky desertification control.

2. Materials and Methods

2.1. Materials

The “Crystal” grapes utilized in this study were hybrid cultivars, with Concord as the male parent and Cassady as the female parent. These grapes were harvested on 25 August 2019, from Xingjin Planting Professional Cooperative, located in Dafengdong Township, Kaili City (Guizhou, China). The geographical coordinates of the harvest site were 107°52′22″ longitude and 26°43′21″ latitude. Only fruits of consistent size, ripeness, and without any mechanical damage or disease were gathered and promptly transported to the laboratory within a three-hour timeframe. The polyethylene modified atmosphere bags had a thickness of 0.02 mm (National Engineering Research Center for Preservation of Agricultural Products, Tianjin, China). All reagents were purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Treatment and Storage

In this study, the collected grape fruit samples were randomly divided into four groups. Each group was placed in tents with a volume of 1 m³ and covered with PE plastic film to isolate them from the outside air. The temperature inside the tents was maintained at 25 ± 1 °C. The fruit samples in each group were then fumigated with 1-MCP (Dow Chemical Company, Shanghai, China) at concentrations of 0, 2.0, 10.0, and 50.0 μL/L for a duration of 2 h. These groups were labeled as CK, M2, M10, and M50, respectively (Figure 1). After the fumigation treatment, all the grape fruits were individually packed into polyethylene bags with a thickness of 20.0 μm, weighing 2.5 ± 0.5 kg each. A total of 12 bags per treatment were prepared, resulting in a total weight of 30.0 kg. The packed samples were then transferred to a precooling room with a temperature of 1.0 ± 0.3 °C and a relative humidity of 90–95%. The fruits were stored under these environmental conditions for 40 d. During the storage period, the outbound index of the fruit was determined every 10 d. Additionally, for further analysis, parallel fruits from each treatment were minced and rapidly frozen using liquid nitrogen at a temperature of −80 °C. This process was carried out in triplicate for each treatment.

2.3. Changes in Appearance Quality during Storage

Firmness, Color Difference, Browning Rate, Shedding Rate, Decay Rate, Water Content

The fruit firmness was determined using the method described by Xin et al. [20]. A texture analyzer (Stable Micro Systems Co., London, UK) was used to assess the firmness of each complete fruit equator on both sides (n = 15 per group). The measurement protocol involved a downward speed of 30 mm/min before measurement, an upward speed of 30 mm/min after measurement, and a puncture depth of 8 mm.

The color of the fruit samples was determined using a handheld digital colorimeter (Konica Minolta Inc. Tokyo, Japan) and a CIELab colorimetric system, following the approach presented by Barbagallo et al. [18]. After calibrating the colorimeter on a whiteboard and a blackboard, the middle equator of the “Crystal” grape berries were measured with 15 replicates per treatment. The overall color change of the fruit samples under different treatment conditions was calculated using Formula (1), which is as follows:

$$\Delta E=\sqrt{(\Delta L*){}^2+(\Delta a*){}^2+(\Delta b*){}^2}$$

(1)

The overall color change is represented by ΔE, where brightness is denoted by L*, the red-green tone is indicated by a*, and the yellow-blue tone is represented by b*.

Three parallel browning rates were separately calculated for each treatment. Browning was defined as the presence of brown spots or patches on the surface, and the browning rate was determined by dividing the browning mass by the total mass, as shown in Equation (2):

$$\mathrm{Browning} \mathrm{rate}/\%=\frac{\mathrm{Browning} \mathrm{mass}}{\mathrm{Total} \mathrm{mass}}$$

(2)

Three parallel decay rates were separately calculated for each treatment, and the average value was taken. Decayed fruit was defined as having water, mold, cracking, or dents on the surface, and the decay rate was determined by dividing the decayed mass by the total mass, as shown in Equation (3):

$$\mathrm{Decay} \mathrm{rate}/\%=\frac{\mathrm{Rot} \mathrm{mass}}{\mathrm{Total} \mathrm{mass}}$$

(3)

The entire cluster of “Crystal” grapes was lifted out of the polyethylene bag, gently shaken, and the mass that falls off into the bag was counted. This mass was then divided by the total mass of each bag to obtain the threshing rate, as calculated in Equation (4):

$$\mathrm{Shedding} \mathrm{rate}/\%=\frac{\mathrm{Shedding} \mathrm{mass}}{\mathrm{Total} \mathrm{mass}}$$

(4)

The water content was measured using the weighing method, which involves subtracting the constant weight after drying from the fresh weight, and then dividing it by the fresh weight of the fruit before drying.

2.4. Changes in Physiological and Nutritional Quality during Storage

Respiratory Intensity, Soluble Solids Content (SSC), Soluble Proteins, Free Amino Acids, Titratable Acids, Reducing Sugars

Fruit respiration intensity was determined using the method described by Zhang et al. [19] with some modifications. A total of 1.0 kg of “Crystal” grapes were sealed in a 2.5 L jar at 25 °C. After 2 h, 1.0 mL of gas was aspirated and the increase in CO₂ concentration in the sealed jar was monitored using a portable O₂/CO₂ analyzer (PBI-Dansensor Co., Denmark, UK). The respiration intensity was calculated as the amount of CO₂ increased per hour per kilogram of fresh fruit mass, expressed as mg·kg⁻¹·h⁻¹.

The SSC of each fruit group was measured using the handheld digital refractometer (PAL-1, Tokyo, Japan), with 15 replicates per treatment.

According to the method described by Bradford [21], the soluble protein content in grape pulp was determined with a slight modification. A 5.0 ± 0.05 g sample was ground with 3.0 mL of 200.00 mM sodium phosphate buffer (pH 7.0), followed by centrifugation at 10,000 rpm for 30 min at 4 °C using a high-speed refrigerated centrifuge (Pingke Scientific Instrument Co., Ltd., Changshao, China). The resulting liquid was collected for the determination of soluble protein and reducing sugar levels. To prepare soluble proteins, 0.1 mL of the supernatant was combined with 0.9 mL of distilled water and 5.0 mL of Coomassie brilliant blue G-250 solution (Macklin Inc., Shanghai, China). After a duration of 5 min, the absorption at a wavelength of 595 nm was measured using a spectrophotometer (Shimadzu, Tokyo, Japan). The amount of soluble protein was expressed as mg·g⁻¹ of fresh weight.

The method described by Yan et al. [22] was utilized with a slight modification for the determination of free amino acid content. A grape fruit sample weighing 1.0 g was mixed with 3.0 mL of ultrapure water by vortexing. The mixture was then incubated at 100 °C for 30 min and subjected to centrifugation at 10,000 rpm for 10 min. The resulting supernatant was collected. Then, 0.1 mL of the sample supernatant was combined with 3.0 mL of 2% ninhydrin reagent, 1.0 mL of acetic acid buffer (pH 5.4), and 0.3% ascorbic acid. The mixture was then heated at 100 °C for 15 min and rapidly cooled using ice water. Subsequently, 2.0 mL of 95% ethanol was added. Measurements were taken at a wavelength of 570 nm using a spectrophotometer. The overall amount of free amino acids was calculated using leucine as the reference curve. The content of free amino acid was expressed as mg·100 g⁻¹ fresh weight.

The method described by Zhang et al. [23] was employed to quantify the TA. A total of 5.0 mL of grape juice was mixed with 50.0 mL of distilled water, followed by the addition of 2 drops of 1% phenolphthalein. The mixture was then titrated using 0.1 mM sodium hydroxide (pH 8.3). The acidity was expressed as the equivalent amount of malic acid.

The reducing sugars content was determined using the dinitrosalicylic acid method described by Miller et al. [24], with slight modifications. To determine the reducing sugars content, 0.2 mL of the liquid sample was mixed with 1.5 mL of 3,5-dinitrosalicylic acid and 1.8 mL of deionized water. The mixture was heated at 100 °C for 5 min, then cooled and diluted with 25.0 mL of purified water. The concentration of reducing sugars was measured using spectrophotometer at a wavelength of 540 nm. The content of reducing sugars was expressed as mg·g⁻¹ fresh weight.

2.5. Changes in Browning-Related Enzyme Activity and Composition during Storage

PPO, POD, LOX, SOD, AsA Content, Polyphenols Content, MDA

The determination of PPO activity was determined according to the method described by GALEAZZI et al. [25]. Firstly, 5.0 g of “Crystal” grapes were placed in a mortar and mixed with 20.0 mL of 0.05 mol/L phosphate buffer (pH 7.8) and 0.5 g of Polyvinylpyrrolidone (PVP). The mixture was then ground into a pulp at a temperature of 4 °C. The resulting mixture was filtered by centrifuging the filtrate at a speed of 19,000 r/min for 20 min, resulting in a crude enzyme extract. Next, 3.9 mL of phosphoric acid buffer (0.05 mol/L, pH 7.8) was measured out, followed by the addition of 1.0 mL of 0.1 mol/L 4-methylcatechol and 3.0 mL of the enzyme extract. The mixture was then incubated at 37 °C for 10 min, quickly placed in an ice bath, and 2.0 mL of 20% trichloroacetic acid was immediately added to stop the reaction. The absorbance at 420 nm was then determined. PPO activity was calculated as U·min⁻¹ per gram of fresh weight.

The determination of POD activity was conducted following the method established by Wan et al. [26]. The fruit sample (1.0 g) was weighed and mixed with 10.0 mL of 50 mmol·L⁻¹ phosphate buffered saline (PBS) at pH 7.0, containing 50.0 g·L⁻¹ PVP, 5.0 mmol·L⁻¹ dithiothreitol, and 1.0 mmol·L⁻¹ polyethylene glycol. The mixture was then centrifuged at 12,000 rpm for 20 min at 4 °C, and the supernatant was collected for an enzyme activity assay. The reaction mixture consisted of 0.8 mL of the extraction solution, 1.1 mL of guaiacol solution (0.025 mol·L⁻¹), and 0.1 mL of H₂O₂ (0.5 mol·L⁻¹). The absorbance at 470 nm was measured, and POD activity was calculated in U·min⁻¹ per gram of fresh weight.

LOX enzyme activity was determined according to the method described by Liu et al. [27]. Grape samples weighing 1.0 g were mixed with 10.0 mL of PBS (pH 7.0) at a concentration of 0.05 mol·L⁻¹, containing 1% Triton X-100 and 4% PVP. The mixture was then centrifuged at 12,000 rpm for 15 min at 4 °C. The reaction mixture was composed of 0.2 mL of the extracted enzyme solution, 2.75 mL of acetic acid-sodium acetate solution (0.1 mol·L⁻¹), and 0.05 mL of sodium linoleate solution (0.1 mol·L⁻¹). The mixture was incubated at 30 °C for 10 min, and the absorbance at 234 nm was measured. LOX activity was calculated as U·min⁻¹ per gram of fresh weight.

The determination of SOD activity was determined following the method described by Ezzat et al. [28]. Frozen fruit tissue weighing 3.0 g was homogenized in 5.0 mL of sodium phosphate buffer at a concentration of 50.0 mmol·L⁻¹ with a pH of 7.0. The homogenate was then centrifuged at 10,000 rpm for 20 min at 4 °C, and the resulting supernatant was used for enzyme activity determination. The reaction mixture consisted of EDTA at a concentration of 3.0 μmol·L⁻¹, sodium phosphate buffer at 50.0 mmol·L⁻¹ with a pH of 7.8, tetrazolium nitroblue (NBT) at 1.0 μmol·L⁻¹, methionine at 14.0 mmol·L⁻¹, riboflavin at 60.0 μmol·L⁻¹, and 0.1 mL of crude enzyme extract. The absorbance of the formed blue formazan was measured spectrophotometrically at 560 nm. SOD activity was calculated in U·min⁻¹ per gram of fresh weight.

The determination of AsA was determined using the 2,6-dichloroindinophenol method as described by Luo et al. [29]. First, 3.0 g of grape sample was mixed with 15.0 mL of 0.05 mol·L^–1 oxalic acid-EDTA. The homogenate was then centrifuged at 10,000 rpm for 30 min and the supernatant was collected. Then, 2.0 mL of the sample supernatant was combined with 3.0 mL of oxalic acid-EDTA, 0.5 mL of metaphosphoric acid-acetic acid, and 1.0 mL of 5% H₂SO₄. The mixture was left to settle at 80 °C for 10 min, cooled to room temperature, and then absorbance was read at 760 nm. The results were expressed as mg·100 g⁻¹ fresh weight.

The polyphenol content was determined following the method described by Kobori et al. [30]. First, 10.0 mg of a crude extract of sample polyphenols was accurately weighed and diluted with 2.0 mL of methanol to create a working solution. Next, 100.0 μL of the sample working solution was added to a tube, followed by the sequential addition of 3.0 mL of distilled water, 250.0 μL of Folin-Ciocalteu reagent, 750.0 μL of 20% Na₂CO₃, and 900.0 μL of distilled water, creating a final volume of 5.0 mL. The mixture was then shaken using a vortex oscillator and incubated at 40 °C for 30 min in the dark. After incubation, the absorbance value was measured at 760 nm using a spectrophotometer. The results were expressed as mg·g⁻¹ fresh fruit.

MDA content was determined using an improved version of the thiobarbituric acid method [31]. Initially, 2.0 g of pulp was ground together with 3.0 mL of 0.05 M sodium phosphate buffer (pH 7.8) in an ice bath to obtain a homogenate. This homogenate was centrifuged at 12,000 rpm for 15 min at 4 °C. Subsequently, 2.0 mL of 5.0 g/L thiobarbituric acid solution was added to 2.0 mL of the supernatant, and the mixture was soaked at 95 °C for 15 min, cooled, and centrifuged again for an additional 15 min. The optical density (OD) values at 450 nm, 532 nm, and 600 nm were determined, and the MDA content in μmol·g⁻¹ fresh weight was calculated using the following formula:

$$\mathrm{C}=6.452 \times ({\mathrm{OD}}_{532} - {\mathrm{OD}}_{600}) - 0.559 \times {\mathrm{OD}}_{450}$$

(5)

2.6. Changes in Cell Wall Composition during Storage

Protopectin, Soluble Pectin Content, PG Activity

The protopectin and soluble pectin content was determined following the method of Zhao et al. [32]. To begin, 1.0 g of grape pulp was crushed in 25.0 mL of 95% ethanol. The mixture was then heated at 100 °C for 30 min, and then cooled to room temperature. Afterward, the mixture was filtered, and the filter residue was washed twice with 75% ethanol and left to dry at room temperature. For the determination of soluble pectin content, the alcohol-insoluble solid was suspended in 20.0 mL of sterile water and incubated at 50 °C for 30 min. It was then centrifuged at 8000 rpm for 15 min to obtain the soluble pectin. As for the protopectin content, the ethanol-insoluble solid was suspended in a 0.5 mol·L⁻¹ sulfuric acid solution and subjected to a water bath at 100 °C for 1 h to hydrolyze the protopectin. Finally, the content of soluble pectin was determined using carbazole colorimetry with galacturonic acid as the standard [33]. The results were expressed as mass scores.

The quantification of PG activity was carried out following the method of Wang et al. [34]. Initially, 2.0 g of grape pulp were weighed and homogenized in 8.0 mL of pre-cooled 95% ethanol. The resulting mixture was transferred to a 10.0 mL centrifuge tube and allowed to stand at 4 °C for a duration of 10 min. Next, 3.0 mL of 2.0 mol·L⁻¹ sulfuric acid was added to the tube, followed by centrifugation (4 °C, 13,500 rpm) for 10 min. The resulting solution was then dissolved in 5.0 mL of pre-chilled 50.0 mmol·L⁻¹ (pH = 5.5) sodium acetate buffer and centrifuged once again for 10 min. The mixture consisted of 0.5 mL of 1% polygalacturonan acid (PGA), 1.0 mL of 0.1 mol·L⁻¹ sodium acetate buffer (pH = 4.6), 0.5 mL of distilled water, and 1.0 mL of enzyme extract. The assay mixture was incubated at 37 °C for a duration of 1 h, after which the reaction was terminated by adding 1.0 mL of 3,5-dinitrosalicylic acid (DNS). The absorbance at 540 nm was then measured, using D-galacturonic acid as the reference standard. Finally, PG activity was calculated based on the fresh weight of the sample, expressed as U·min⁻¹.

2.7. Changes in Texture Characteristics during Storage

TPA, Tests the Springiness, Hardness, Gumminess, Chewiness, Adhesiveness, Resilience, Cohesiveness Brittleness of the Fruit

For each treatment, 15 high-quality grape fruits were randomly selected. The fruits were placed horizontally on the texture analyzer, with the fruit stems positioned towards the left side of the texture analyzer. The P/36R probe was utilized to measure the TPA of the fruits. The following test parameters were applied: the downward speed prior to measurement was set at 60.0 mm·min⁻¹, the upward speed after measurement was also 60.0 mm·min⁻¹, the compression set was set to 3.0 mm, the probe returned to its original position for 5.0 s after the initial compression, and then compressed again. The trigger force employed was 5.0 g, followed by an additional 5.0 mm compression set.

2.8. Changes in Aroma Quality during Storage

According to Xu et al. [35], the volatiles produced by “Crystal” grapes were assessed using electronic noses (AIRSENSE Analytics Co., Schwerin, Germany). Each treatment group consisting of 30.0 g of grapes was placed in a closed container with a capacity of 100.0 mL for a duration of 30 min. The headspace volatile sampling was conducted utilizing an electronic nose. Prior to sampling, the clean channel was flushed with zero gas to regenerate the gas sensor. The operating parameters of the electronic nose were as follows: a sampling interval of 1 s, a rinsing time of 220 s, a presampling time of 5 s, a measurement time of 60 s, and an injection flow rate of 240.0 mL·min⁻¹. For data analysis, stable response values observed between 56 and 59 s were selected.

2.9. Data Analysis

All experiments and analyses were conducted using three biological replicates and using Origin 2021 for graphing and correlation analysis. One-way ANOVA was performed using SPSS 19.0 software, and data differences were analyzed for significance using Duncan’s new complex range method. PCA was performed on the electronic nose data using WinMuster-GDA 1.2 Software to analyze, classify, and reduce the dimensionality of the data set. This analysis aimed to examine the aroma of samples at different sampling points and establish the relationship between aroma and different processing methods.

3. Results

3.1. Changes in Appearance Quality such as Firmness, Decay Rate, Browning Rate, Color Difference, and Shedding Rate

Figure 2A presents the overall appearance and quality changes of “Crystal” grapes stored for 0~40 days after fumigation with different concentrations of 1-MCP. In comparison to the fruits on the day of harvest, the control group’s peel exhibited slight yellowing and fruit browning on the 10th day of storage. However, the other three treatment groups did not show any noticeable change in peel color. Additionally, the control group experienced significant fruit shedding during the middle and late stages of storage. In contrast, the M10 and M50 treatments were able to maintain the good color and appearance quality of “Crystal” grapes after harvest. The control group decayed rapidly after 20 d, while the M50 treatment delayed the rot of the grapes by 20 days compared to the control group. From 30 to 40 days, the decay rate in the M50 treatment group was lower, only 10.85~16.15%, while the decay rate in the control group was greater than 20% (Figure 2B).

On the 10th day of storage, the peel of the control group exhibited noticeable yellowing and slight brown spots, resulting in a browning rate of 16.2%. By the 30th day of storage, the M50 treatment group had a browning rate of 14.3%. This indicates that the M50 treatment delayed the browning of “Crystal” grapes by 20 d (Figure 2C). Similarly, the M10 treatment delayed the occurrence of browning by 10 days. At the end of storage, the control group had severe browning and a fruit shedding rate of 34.5% (Figure 2D). Both the M10 and M50 treatments were able to inhibit the deterioration of peel color to some extent (Figure 2G).

According to Figure 2E, the control group experienced a faster softening rate of “Crystal” grapes during storage. During the 30~40 d storage period, the fruit firmness of the M10 and M50 treatment groups was approximately 1.1 times and 1.0 times that of the control group, respectively. This indicates that the M10 and M50 treatments effectively slowed down the decrease in fruit firmness during storage.

3.2. Changes in Respiration Intensity, SSC, Reducing Sugar Content, Titratable Acid Content, Soluble Protein Content, and Free Amino Acid Content

The “Crystal” grapes remain living organisms after being harvested, and they rely on their own stored organic matter and water for energy through respiration. This leads to an accelerated consumption of nutrients in the “Crystal” grapes. Our study discovered that the respiratory intensity of “Crystal” grapes increased after harvest. The control group exhibited the highest increase, from 7.454 mg·kg⁻¹·h⁻¹ to 26.290 mg·kg⁻¹·h⁻¹, which was significantly greater than the increase observed in the group treated with 1-MCP. Overall, compared to the control group, the application of 1-MCP treatment at all three concentrations effectively suppressed the rise in fruit respiration rate (Figure 3A).

In the M10 and M50 treatment groups, the levels of soluble protein were significantly reduced compared to the control group (Figure 3B). This study found that the soluble protein content of each group steadily grew during the early phases of storage, which could be explained by the fact that as fruit matures, protein synthesis increases. According to Figure 3C, the levels of free amino acids in the treated fruits showed a gradual increase. During the middle and late phases of storage, there was a moderate increase in the concentration of free amino acids in the control group, which was significantly lower compared to the 1-MCP treatment. This suggests that 1-MCP treatment promotes the accumulation of free amino acids in “Crystal” grapes after harvest.

Additionally, the M50 treatment consistently maintained higher SSC and reducing sugar content in the fruits, closely followed by the M10 treatment group. Both treatments effectively slowed down the decrease in SSC and reducing sugar content in “Crystal” grapes, as depicted in Figure 3D,E. It is important to note that there was no significant difference in the titratable acid levels between fruits treated with 1-MCP and the control group, indicating that the application of 1-MCP did not significantly affect the titratable acid content in “Crystal” grape berries, as shown in Figure 3F.

The findings suggest that the application of 1-MCP has the potential to delay the decrease in SSC and reducing sugars in “Crystal” grapes by suppressing fruit respiration intensity, thereby slowing down respiratory metabolism and nutrient consumption. Furthermore, it has been observed that 1-MCP treatment can facilitate the accumulation of free amino acids in fruits while inhibiting the synthesis of soluble proteins.

3.3. LOX, POD, PPO, SOD Activity and Changes in Polyphenols, AsA, and MDA Content

Throughout the entire storage period, LOX activity in “Crystal” grape berries exhibited an increasing trend, with the control group and M2 treatment showing the highest increases, from 2.546 U·min⁻¹ on the storage day to 257.63 U·min⁻¹ and 255.44 U·min⁻¹, respectively. In contrast to the control group, the M10 and M50 treatments significantly suppressed the increase in LOX activity, particularly during the later stages of storage (Figure 4A). These findings indicate that the M10 and M50 treatments can effectively inhibit membrane lipid peroxidation during “Crystal” grape storage and maintain membrane integrity.

Figure 4B demonstrates that the activity of POD in the fruit exhibited a similar ascending trend. Notably, the M10 and M50 treatments mitigated the increase in POD activity to some extent. After 40 d of storage, fruit treated with M50 exhibited a PPO activity of 1.274 U·min⁻¹, which was lower than the PPO activity of the control group on the 20th day of storage (1.413 U·min⁻¹). The delay in the increase of PPO activity in the M50 treatment group aligns with the browning rate of “Crystal” grapes, suggesting that M50 treatment effectively inhibits PPO activity and reduces peel browning during storage (Figure 4C). Figure 4D demonstrates that, apart from the continuous rise in SOD activity observed in the M50 treatment group, the SOD activity of fruit in other treatment groups displayed a pattern of initial growth followed by decline. Later in storage, SOD activity gradually decreased in the control group and the M2 treatment group, while the M10 and M50 treatments maintained high SOD activity in the fruit. This helped detoxify various free radicals and minimize oxidative damage, ultimately preventing the deterioration of grape fruit quality.

Generally, browning can be divided into two types: enzymatic browning and non-enzymatic browning. In postharvest non-enzymatic browning of fruits and vegetables, there are two main reaction types: AsA browning and polyphenol auto-oxidative browning. AsA and polyphenols are important antioxidants that are commonly found in plant tissues and contribute to enhancing the overall antioxidant activity of plants [36]. Figure 4E shows that the polyphenol levels in the different treatment groups of “Crystal” grapes initially increased and then decreased during the storage period. The control group and the M2 treatment group exhibited a rise in polyphenol content, reaching a maximum of 1.085 mg·g⁻¹ and 0.932 mg·g⁻¹, respectively. The content of AsA gradually decreased during storage, but the M10 and M50 treatments slowed down the decline of AsA content. These treatments also delayed the oxidation of cellular AsA, resulting in higher AsA content in grape fruits. This suggests that higher concentration treatments positively influenced the maintenance of fruit AsA content (Figure 4F).

As shown in Figure 4G, the MDA content gradually increased with storage time, indicating a deepening of lipid peroxidation in the fruit membranes. However, the fruits treated with M10 and M50 maintained a low MDA content of 7.077 μmol·g⁻¹ and 5.998 μmol·g⁻¹, respectively. On the other hand, the control group and M2 treatments experienced a sharp increase in MDA content to 37.052 μmol·g⁻¹ and 32.622 μmol·g⁻¹ after 30~40 d of storage. The M10 and M50 treatments effectively inhibited the rise of MDA content during the storage of “Crystal” grapes. This slows down the fruit’s oxidative deterioration, guaranteeing ideal storage quality.

3.4. Changes in PG Activity, Protopectin, and Soluble Pectin Content of Fruit Cell Wall

In this study, fumigation with 1-MCP had an inhibitory effect on the increase of PG activity in fruits. During the 30 d before storage, PG activity increased rapidly in the control group and M2 treatment, by 6.6 times and 6.2 times, respectively, compared to the day of harvest. In contrast, the M10 and M50 treatment groups showed relatively smaller increases in PG activity, at 4.2 times and 3.7 times, respectively, and a delay in the peak PG activity (Figure 5A). These results indicate that the M10 and M50 treatments significantly inhibit the enhancement of PG activity during the storage of “Crystal” grapes and delayed the postharvest softening of the fruits.

As depicted in Figure 5B,C, this study demonstrates that the application of 1-MCP treatment could inhibit the decrease in propectin mass fraction and the accumulation of soluble pectin to a certain extent. The M50 treatment exhibits the most inhibitory effect on the degradation of protopectin, leading to a significant delay in postharvest softening of “Crystal” grapes. In the later stages of storage, protopectin degradation occurs rapidly, accompanied by a rapid increase in the content of soluble pectin. This may be related to the enhanced activity of PG in the fruit and the accelerated decomposition of protopectin into soluble pectin.

The correlation analysis conducted between various physiological indicators and fruit quality of “Crystal” grape revealed that significant positive correlations between the browning rate of “Crystal” grapes and PPO, LOX, POD, and MDA (p = 0.96, p = 0.91, p = 0.87, p = 0.95). Furthermore, firmness exhibited positive correlations with PG, soluble pectin, and browning-related enzymes, while showing a significant negative correlation with propectin. Respiration intensity and MDA content demonstrated a significant positive correlation with rot rate, browning rate, and shedding rate. This indicates that higher respiration levels lead to more severe membrane lipid peroxidative damage, resulting in increased fruit rot, shedding, and browning. The acceleration of fruit respiration also contributes to the aging process and significantly affects the visual appearance quality of “Crystal” grapes. Conversely, there was no significant correlation found between polyphenol and reducing sugar content and the various indicators (Figure 6).

3.5. Effects of 1-MCP Fumigation at Different Concentrations on the Change of Texture Characteristics of “Crystal” Grapes during Storage

The TPA method was employed to investigate the texture alterations in fruit during postharvest storage. The findings revealed that the M10 and M50 treatments effectively postponed the decrease in fruit hardness, chewiness, springiness, cohesiveness, and adhesiveness of “Crystal” grapes. It has a positive effect on maintaining the postharvest storage texture characteristics of “Crystal” grapes and effectively preserved the original form of the “Crystal” grapes. Notably, the M50 treatment exhibited the most favorable impact (Figure 7A).

3.6. Effects of 1-MCP Fumigation at Different Concentrations on Aroma Quality Changes during Storage of “Crystal” Grapes

To more comprehensively analyze the differences in aroma substances during the postharvest quality deterioration of “Crystal” grapes, PCA analysis was conducted on the electronic nasal data of “Crystal” grape samples treated with different concentrations of 1-MCP and stored for varying periods. The first principal component accounted for 61.3% of the contribution rate, while the second principal component accounted for 35.6% (Figure 8A). Excluding the volatile constituents of the fruits at the time of harvest, the primary principal component showcased a contribution rate of 78.1% from the 10 sensors, while the secondary principal component exhibited a contribution rate of 16.4% and the cumulative contribution rate of 94.5% (Figure 8B).

Figure 8C illustrates a clear distinction between the smell of “Crystal” grapes on the day of harvest, and the resulting odor after storage and the odor characteristics differ significantly. However, there is a noticeable clustering observed among some samples treated with different concentrations of 1-MCP, indicating a lower level of differentiation. PCA analysis effectively distinguishes the 1-MCP treatment group, which exhibits significant differences in volatile components in “Crystal” grapes. However, it has limitations in differentiating treatments with similar volatile components overall. Therefore, in addition to volatile odors on the day of harvest, the distribution of odor substances in grape fruits during different storage periods was analyzed (Figure 8D). The application of 1-MCP treatment facilitates the release of W2W, W1S, W5S, W2S, and W1W within the fruit (Figure 7D,

Table 1. Sensors of response characteristics in electronic nose PEN3.

No.	Sensor	Sensitive Compound
1	W1C	Aromatic compounds
2	W5S	Nitrogen oxides
3	W3C	Ammonia, aromatic compounds
4	W6S	Hydrogen
5	W5C	Short-chain alkane aromatic compounds
6	W1S	Methyl aromatics
7	W1W	Sulfides and terpenes
8	W2S	Aromatic compounds of alcohols and aldehydes and ketones
9	W2W	Organic sulfides and aromatic compounds
10	W3S	Long-chain alkanes

).

4. Discussion

4.1. Effects of Different Concentrations of 1-MCP Treatment on Appearance, Physiological Nutrition, and Aroma Quality of “Crystal” Grapes during Storage

The results of this research demonstrate that compared with the control group, the M10 and M50 treatment groups significantly inhibited the increase in browning rate and shedding rate of the grape fruit and significantly delayed the decrease in fruit hardness. The changes in water content and hardness of grapes are basically the same. Lv et al. [37] confirmed that 1-MCP can inhibit fruit color loss or yellowing, and this study also provides evidence for the inhibitory effect of 1-MCP treatment on fruit color transition, which is directly related to changes in browning rate, thereby keeping the fruit with a good appearance quality. The evaluation of fruit aroma relies on several important factors, such as the content of SSC, TA, and reducing sugar. After harvest, table grapes still demonstrate a strong respiratory intensity, and the aroma of grape fruits can be impacted by the consumption of a large amount of sugar substances through respiration [38]. Moreover, vigorous respiration leads to the consumption of nutrients, release of respiratory heat and water, creation of high temperature and humidity environments, and acceleration of the deterioration of harvested results’ quality. Throughout this study, both the control group and the 1-MCP treatment group exhibited a consistent decline in SSC and TA as the storage time increased. Additionally, the levels of reducing sugars initially increased during the early stages of storage, but gradually decreased due to fruit maturity; the results of reducing sugar changes aligns with the results of a previous study on the antioxidant activity of guava conducted by Jim et al. [39]. The main reason for this lies in the ability of 1-MCP treatment to maintain a low respiration intensity, thereby reducing the consumption of carbohydrates and preserving the aroma quality of the fruit. These findings are consistent with the results of the study on the treatment of lychee fruits with oxalic acid by Ali et al. [40]. Strauss et al. [41] unearthed that the volatile aroma compounds existing in grapes predominantly encompass alcohols, esters, terpenes, etc. In parallel, according to this study, the overall flavor of grape fruits that were fumigated with 1-MCP was significantly better. 1-MCP treatment promotes the release of aroma substances in the fruit. The response of an electronic nose to these volatile gaseous components can be utilized to evaluate the excellence of table grapes.

4.2. Relationship between Postharvest Grape Fruit Browning and Browning-Related Enzyme Activity and Antioxidant Content

The effect of 1-MCP on slowing down the aging process is attributed to its ability to better maintain enzymatic and non-enzymatic antioxidants, which scavenge ROS [42]. Browning in table-grape fruit is predominantly caused by enzymatic browning, wherein phenolic substances are oxidized to quinones by PPO and POD enzymes, resulting in the formation of brown pigments. Peel browning is a main form of postharvest grape fruit quality deterioration, thus reducing the activity of PPO and POD enzymes is crucial in preventing fruit browning [43]. We observed that the activities of POD and PPO enzymes in the 1-MCP treated group were significantly lower compared to the control group, effectively delaying the oxidation of fruit phenols into quinones to form brown substances, consequently reducing the incidence of fruit browning. A study conducted by Javed et al. [44] also found that the activities of POD and PPO enzymes in grape fruit subjected to vanillin maceration after 6 d of storage were 40.73% and 21.42% lower than the control group, respectively, leading to a decrease in browning, which supports the findings of our study. SOD possesses strong antioxidant properties and is capable of removing harmful substances produced during physiological metabolic processes in postharvest fruit. It is also associated with plant stress responses related to aging, as well as the direct and indirect clearance of ROS [45]. SOD activity was initially inhibited during early storage stages after 1-MCP treatment of “Crystal” grapes. However, SOD activity in the fruits continued to enhance antioxidant properties and remove ROS in the later stages. LOX can catalyze membrane lipid peroxidation, resulting in the loss of cell membrane integrity, and providing a reaction site for polyphenol oxidase and phenolic substrates. This ultimately causes irreversible damage to tissues and metabolic disorders, such as browning. LOX is considered a key enzyme in regulating the maturation and aging of fruits [46]. The role of LOX in catalyzing membrane lipid peroxidation and causing fruit browning has also been confirmed in studies on lychee [27] and longan [47]. In our experiment, we found that 1-MCP treatment inhibited the increase in LOX activity during storage and delayed membrane lipid peroxidation of cell membranes in “Crystal” grape berries, consequently reducing browning of the peel tissue and this study also confirms this conclusion.

The fruit produces numerous secondary metabolites during postharvest ripening. Phenolic substances are a prevalent class of secondary metabolites in plants. The accumulation of phenolic compounds is associated with disease resistance during plant–pathogen interactions, effectively inhibiting the infection of pathogens [48]. Ascorbic acid serves as an essential antioxidant in living organisms, playing important roles in antioxidant and antistress functions, cell expansion and division, as well as photoprotection [42]. Elevated levels of Ascorbic acid in 1-MCP-treated kiwifruits significantly alleviates oxidative stress and inhibits membrane degradation, thereby retarding fruit aging [49]. This experiment confirmed that 1-MCP treatment maintains a high content of polyphenols and ascorbic acid, enhancing the antioxidant capacity of “Crystal” grapes. These results are consistent with previous findings on the effects of salicylic acid, which increased the content of ascorbic acid, polyphenols, and other antioxidants in sweet cherries, thereby enhancing the antioxidant capacity of the fruit [50]. Furthermore, the production of the peroxidation product MDA exacerbates membrane lipid peroxidation, leading to damage to the cell structure and disturbances in metabolic processes, further accelerating fruit aging [51]. Studies have shown that both 1-MCP and MT inhibit membrane lipid peroxidation by suppressing MDA content and LOX activity, either individually or in combination [52]. Additionally, 1-MCP treatment suppresses the expression of MDA content and LOX activity in pear fruits, thereby reducing the accumulation of reactive oxygen species (ROS) and effectively minimizing oxidative damage to the membrane system and browning of southern pears [53]. This finding is consistent with previous research and demonstrates that 1-MCP treatment inhibits the accumulation of MDA and LOX activity in grape fruit, consequently reducing postharvest browning in grape fruit.

4.3. Relationship between Postharvest Grape Fruit Softening and Changes in Cell Wall-Related Substances and Texture Characteristics

Possible causes of fruit softening include hydrolysis of polysaccharides and alterations in polymer bonds, which lead to increased cell separation and softening of the cell wall. Our study discovered a negative correlation between fruit hardness and the activity of the fruit PG enzyme, whereby higher softening enzyme activity was associated with lower fruit firmness and increased softening. Additionally, a decrease in softening enzyme activity may be responsible for the reduction in fruit softening due to 1-MCP treatment, as the activities of several softening enzymes were significantly negatively related to fruit hardness [54]. Moreover, the results of the correlation analysis revealed a significant positive correlation (p = 0.94) between fruit firmness and protopectin content, while a noteworthy negative correlation was observed with soluble pectin (p = −0.87).

During the storage period, alterations in fruit texture frequently coincide with a decline in edible quality and aroma, as well as the softening of internal tissues and reduced tolerance to extrusion, thereby diminishing the storage capacity of grapes. This study of the substantial change of fruit treated with 1-MCP showed that, compared with the control fruit, 1-MCP treatment reduced the elasticity and firmness loss of “Crystal” grape fruit. This reduction may be attributed to a decrease in the activity of softening enzymes in fruit treated with 1-MCP, leading to greater intercellular tissue cohesion, chewing, and adhesion. It is well-known that fruit softening enzymes are involved in the reduction of intercellular adhesion and tissue rigidity during fruit ripening [55], and an increase in fruit softening degree is associated with an increase in softening enzyme activity. A similar conclusion was reached in the study of 1-MCP-treated nectarines by Ullah et al. [56], which found that untreated nectarine pulp tissues exhibited decreased firmness and intercellular adhesion, while nectarines treated with 1-MCP showed increased chewiness, potentially due to decreased softening activity in the fruit. The substantial impact of 1-MCP treatment on fruit softening can be attributed to its active involvement in reducing ethylene levels within fruits, thereby impeding fruit softening and delaying ripening. This inhibition of ethylene, induced by 1-MCP, results in the postponement of fruit softening throughout storage [57]. Furthermore, the impact of 1-MCP treatment on fruit ripening has been reported in various other fruits, including plum [58] and kiwifruit [59]. Finally, a model of 1-MCP fumigation’s effect on inhibiting “Crystal” grape postharvest quality deterioration was developed in this study (Figure 9).

Currently, the Karst areas are susceptible to anthropogenic harm, leading to an escalating issue of rocky desertification, which consequently leads to a gradual decline in vegetation coverage and exacerbated soil desertification. Conducting further research on the physicochemical properties of “Crystal” grapes in the Karst mountains can serve the dual purpose of introducing appropriate flora for the restoration and remediation of rocky desertification in the Karst mountains of southwestern China. Additionally, it produces edible table grapes for enhanced economic and ecological benefits.

5. Conclusions

The results showed that higher concentrations of 1-MCP fumigant significantly suppressed the activity of PPO, POD, and LOX browning related enzymes, improved SOD activity, and eliminated ROS in fruit tissues. Additionally, it hindered the decline of AsA and polyphenol antioxidant content and the accumulation of MDA content, reduced the membranes lipid peroxidation damage of fruit, and maintained the integrity of cell membranes. It also postponed the peak PG activity of the fruits and delayed the degradation of protopectin into soluble pectin, which helped to delay fruit softening and sugar consumption, thus inhibiting the increase of fruit browning rate and decay rate. In general, there is minimal difference in maintaining the storage quality of “Crystal” grapes between the 10.0 and 50.0 μL/L 1-MCP treatments. Considering cost, the industry recommends treating “Crystal” grapes with 10.0 μL/L 1-MCP. This study investigates the postharvest storage techniques employed for “Crystal” grapes, elucidates the physical and chemical properties of “Crystal” grapes during postharvest, extends the storage period of grapes, offers technical assistance for the expansion of “Crystal” grape cultivation in the Karst mountains, stimulates local economic and ecological progress, and effectively manages desertification in the Karst mountainous regions.