Edible Composite Coating of Chitosan and Curdlan Maintains Fruit Quality of Postharvest Cherry Tomatoes

Abstract

Postharvest cherry tomatoes are prone to senescence, decay and nutrient loss during the storage period owing to microbial invasion and their own metabolism. In this work, postharvest cherry tomatoes were treated with a composite coating of 1% chitosan and 1% curdlan, and the characteristics of postharvest cherry tomatoes during storage were investigated. Compared to control samples, after 21 d of storage under ambient conditions, the cherry tomatoes treated with the chitosan and curdlan coatings showed less rottenness, less weight loss, a lower respiration rate, reduced ethylene production, lower malonaldehyde (MDA) content and reduced membrane permeability. After the samples were treated with the composite coating, the activities of free radical scavenging enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were maintained at higher levels; the activities of related disease-resistant enzymes such as chitinase (CHI) and glucanase (GLU) were also maintained at higher levels. The soluble solids, titratable acidity, firmness, vitamin C content, lycopene content and antioxidant activities of postharvest cherry tomatoes treated with the composite coating exhibited higher levels as well. The composite coating of chitosan and curdlan might be a potentially promising method for preserving postharvest cherry tomatoes and other fruits.

1. Introduction

Cherry tomato is popular with consumers owing to its rich nutrients, sweet and sour taste, and wonderful flavor. It has relatively high economic value and is one of “four fruits” preferentially promoted by FAO. The other three are apple, grape and banana [1,2]. With the promotion of rural industrial revitalization, the planting area of cherry tomatoes in China has continued to expand in recent years. Cherry tomato is juicy and its peel is thin. During postharvest storage, it is prone to senescence, decay and nutrient loss due to respiratory action, mechanical damage, the infection of pathogenic microorganisms, etc. Such deterioration leads to a decrease in the commercial value of fruits, causing great economic losses for farmers and fresh logistics enterprises [3]. Therefore, effectively preserving postharvest cherry tomatoes, delaying their aging process and maintaining their commodity value are of great significance. At present, some remedies such as edible coating, modified atmosphere packaging and irradiation were used to prolong the shelf life of postharvest cherry tomatoes [4,5,6].

Cherry tomato is a typical respiration climacteric fruit, and there are obvious respiratory and ethylene peaks during postharvest storage [7]. As a living organism, postharvest tomato could produce free radicals such as O²⁻. These free radicals could oxidize the lipid of the cell membrane, damage the cell membrane and promote the senescence of cherry tomato [8]. Senescent fruit is susceptible to microbial infestation, leading to decay. Fruits will use their own relevant protective enzyme systems to minimize the damage caused by free radicals to the cell membrane. SOD, CAT and POD are important free radical scavenging enzymes in fruit tissues, which could remove excessive free radicals from postharvest fruit. Thus, postharvest fruit might slow down senescence. CHI and GLU are enzymes that are related to disease resistance in postharvest fruit. The higher the activity is, the stronger the ability to resist the invasion of pathogenic fungi is [9].

Edible coating preservation technology is to coat the surface of fruits and vegetables with a thin layer of substances. The components of coating materials include polysaccharides, proteins and other macromolecules. Sometimes, certain small molecules of molding agents are added. Traditionally, coating treatment could properly adjust the permeability of CO₂, oxygen and water vapor on the surface of fruits and vegetables, decrease respiration, reduce water evaporation and improve the appearance and quality of fruits and vegetables [10,11]. Thus, the shelf life of postharvest fruits and vegetables was extended accordingly [12]. Meanwhile, coating treatment might also be used as the carrier of antioxidants and preservative antibacterial components, thus effectively inhibiting postharvest pathogen microbial invasion. In addition, it also has a certain protective significance for reducing the mechanical damage to the surface of postharvest fruits and vegetables. Coating preservation technology has been applied in many postharvest fruits, such as apple, grape, citrus, etc. [13,14].

Polysaccharides composed of hexose or amino-hexose are one of the main components of fungal pathogen cell walls in postharvest fruits and vegetables. Chitin and (1-3)-β-D glucan are important polysaccharides of postharvest fungi cell walls. Chitin could trigger immune responses in plants. (1-3)-β-D glucan is the antigen component of fungi cell walls and widely exists in various fungi, accounting for more than 50% of the cell wall polysaccharides in postharvest fungal pathogens [15]. The chemical name of chitosan is poly (1,4)-2-amino- β-D-glucose. Chitosan is a biological polysaccharide macromolecule obtained from chitin through a certain degree of deacetylation. In the food industry, chitosan is mainly used as a preservative for postharvest fruits and vegetables [16]. Previous studies have confirmed that chitosan has the ability to resist pathogens and kill some invading pathogenic microorganisms. Additionally, chitosan can trigger the immune response of plants, so that the plants might take corresponding defensive measures. For example, plants could synthesize chitinase to degrade the cell wall of invading pathogens [17]. Curdlan is a polysaccharide macromolecule linked by β-1,3-D-glucoside bonds of glucose structural units. Each curdlan molecule is composed of 300~500 glucose residues [18]. Curdlan is safe, non-toxic and has many unique physical and chemical properties, so it has a wide range of application prospects in biomedicine, food processing and other fields [19]. In China, curdlan is allowed to be applied in the food industry as a new food additive, and its national standard has been issued [20].

Both chitosan and curdlan conform to food safety, so they were used to prepare a composite coating to preserve postharvest cherry tomatoes in this experiment. The effect of the composite coating on the preservation of cherry tomatoes was investigated. The postharvest physiological index, biochemical index and nutrient composition were determined. The aim of this research was to provide a novel method for the preservation of postharvest cherry tomatoes. Meanwhile, this research might provide a theoretical basis and technical support for the efficient and economical preservation of postharvest fruits and vegetables, based on coatings with preservatives that conform to food safety regulations.

2. Materials and Methods

2.1. Materials

Cherry tomatoes (Solanum lycopersicum var.saopola) were purchased from Shanxi Tiansen DU’S Tomato Technology Co., Ltd., located in Fancun Town, Taigu District, Jinzhong City, Shanxi Province. Those selected as experimental materials were of uniform size, had reached maturity and had no deformation, mechanical damage, pests or diseases. Water-soluble chitosan (a molecular weight of approximately 200 kDa, deacetyl degree ≥ 85%, viscosity ≤ 100 cps and solubility ≥ 99%) was purchased from AK Biotech Ltd. (Jinan, China). Curdlan (implementation standard GB28304-2012 [20]) was purchased from Shandong Cuiyuan Yikang Biotechnology Co., Ltd. (Binzhou, China). Other reagents were provided by Alfa Aesar Company (Tianjin, China), which were all of analytical grade.

2.2. Preparation of the Coating Solution and Cherry Tomato Treatment

A total of 10 g of water-soluble chitosan and 10 g of curdlan were separately dissolved in 1000 mL distilled water and stirred at 45 °C for 1 h with a constant temperature magnetic agitator (85-2A, Jintan Jincheng Guosheng Experimental Instrument Factory, Jintan City, China). Thus, a 1% chitosan coating solution and a 1% curdlan coating solution were obtained, respectively. For the preparation of the composite coating solution, 10 g of water-soluble chitosan and 10 g of curdlan were separately dissolved in 500 mL of distilled water and stirred at 45 °C for 1 h with a magnetic agitator. Afterward, the two solutions were mixed evenly. The mixture was continued to be stirred for another 15 min. Then, the composite coating solution of 1% chitosan and curdlan was obtained.

The damaged or diseased cherry tomatoes were removed, and the fruits with uniform size and shape were selected. They were randomly divided into 4 groups with 3 kg in each group. Cherry tomatoes were washed clean with tap water and treated as follows. The first group was soaked in distilled water as the blank control group, the second group was soaked in the 1.0% chitosan solution, the third group was soaked in the 1.0% curdlan solution and the fourth group was soaked in the 1.0% composite solution of chitosan and curdlan. Each group of cherry tomatoes was soaked for 3 min. The soaked cherry tomatoes were taken out and then put on the filter paper prepared before. The water on the surface of the cherry tomatoes was dried using an electric fan so that the coating layer was formed on the cherry tomato surface [21]. The 4 treatment groups were placed separately in plastic crisper containers made of PE material, and stored at 85% relative humidity under ambient temperature for 21 days. Cherry tomatoes were randomly selected from each group every 3 days, and related indexes were measured.

2.3. Determination of Indexes Related to Cherry Tomato Preservation

2.3.1. Determination of Appearance of Rottenness

The appearance of rottenness of the postharvest cherry tomato was directly observed, and photos were taken as records.

2.3.2. Determination of Weight Loss, Firmness, Soluble Solids and Titratable Acidity

Weight loss was measured using gravimetry. Cherry tomatoes were weighed every three days during storage, and the weight loss was calculated as follows: weight loss (%) = [(m₀ − m₁)/m₀] × 100, where m₀ is the initial weight before storage and m₁ is the weight measured during storage.

Firmness was determined using a fruit sclerometer (GY-3, Chengdu Bsida Instrument Co., Ltd., Chengdu, China). The equator locations of cherry tomato were selected and directly determined. The unit of firmness was kg/cm².

Soluble solids were determined using a refractometer (WYT-II, Qingyang Optical Instrument Co., Ltd., Chengdu, China). Three tomatoes were crushed to extract juice for determination and converted to standard value by 20 °C. The unit was Brix.

Acidity was determined by acid–base titration and data were expressed as the percentage of [H⁺].

2.3.3. Determination of Respiration and Ethylene

Respiration and ethylene were determined according to the method of Cao, Jiang and Zhao [22]. A total of 500 g of cherry tomatoes was placed into a container with a volume of 5 L and sealed for 2 h at room temperature. Afterward, the concentrations of CO₂ and ethylene in the container were measured using a portable gas analyzer (F-940 portable ethylene/oxygen/carbon dioxide analyzer, Beijing Sunshine Yishida Technology Co., Ltd., Beijing, China). The respiration unit was expressed using mg CO₂/Kg/h/FW and the respiration unit was mL/Kg/h/FW. FW represents the fresh weight of cherry tomatoes.

2.3.4. Determination of Enzymatic Activities

SOD activity was determined according to the previous method [23]. First, 2.0 g of samples was homogenized with 2 mL of 50 mmol/L pH 7.8 phosphoric acid buffer (pH 7.8 phosphoric acid buffer is composed of 50 mL 0.2 mol/L KH₂PO₄ + 45.2 mL 0.2 mol/L NaOH) in an ice bath and centrifuged at 12,000× g for 15 min at 4 °C with a centrifuge (Neofuge 23R, Hong Kong Health Force Development Co., Ltd., Hong Kong, China). The supernatant was collected as an enzyme extract of SOD. The reaction mixture (3 mL) containing 0.1 mL of enzyme extracts, 50 mmol/L sodium phosphate buffer (pH 7.8), 13 mmol/L methionine, 75 μmol/L nitrobluetetrazolium (NBT), 10 ηM EDTA and 20 ηM riboflavin was illuminated using a fluorescent lamp (60 mol m⁻² s⁻¹) for 20 min. The absorbance at 560 nm was recorded using a UV spectrophotometer (752 N, Shanghai Yidian Analytical Instrument Co., Ltd., Shanghai, China). An aliquot of an identical solution was kept in the dark and served as the blank control. One unit of SOD activity was defined as the amount of enzyme that catalyzed a 50% decrease in the SOD-inhibitable NBT reduction.

POD activity was analyzed according to the previous method [24]. The enzyme of POD was prepared as the SOD enzyme was extracted. The assay mixture contained 1.5 mL of enzyme extract, 2 mL of 50 mmol/L sodium phosphate buffer (pH 7.8), 0.6 mL of 0.04 M guaiacol and 0.1 mL of 15% H₂O₂. POD activity was measured using a UV spectrophotometer by an increase in absorbance at 470 nm. One unit of POD activity was defined as a 0.01 increase in absorbance at 470 nm per min.

CAT activity was assayed according to the method of Zeng, Tan and Liu [25]. First, 2 g of samples was homogenized with 15 mL of 50 mmol/L pH 7.0 phosphoric acid buffer (pH 7.0 phosphoric acid buffer is composed of 50 mL 0.2mol/L KH₂PO₄ + 29.6 mL 0.2 mol/L NaOH) containing 1% polyvinyl-polypyrrolidone (PVPP) and centrifuged at 12,000× g for 15 min at 4 °C. The supernatant was collected as a crude extract of CAT. The assay mixture contained 2 mL of 50 mmol/L phosphoric acid buffer (pH 7.0) and 1 mL of distilled water. The mixture was preheated at 40 °C for 10 min, and 0.6 mL crude enzyme was added. After that, 1 mL of 30% H₂O₂ was applied to start the reaction. The absorbance was measured at 240 nm per min. One unit of CAT activity was defined as a 0.1 decrease in absorbance at A₂₄₀ per min.

CHI activity was determined according to the previously described method with some modifications [9]. Cherry tomato was frozen for about 30 s using liquid nitrogen, and then ground into a powder with a grinder. Then, 3 g of cherry tomato powder was added to 4 mL of acetone precooling. After blending, the mixture was centrifuged at 12,000× g for 15 min at 4 °C. The reaction system was successively added with 0.5 mL 50 mmol·L⁻¹ acetic acid–sodium acetate buffer (pH 5.2), 0.5 mL 10 g·L⁻¹ colloidal chitin suspension and 0.5 mL enzyme solution. The control sample was boiled for 5 min. The mixture was incubated at 37 °C for 1 h. Afterward, 0.1 mL of 30 g·L⁻¹ desalinized snail enzyme was added to the mixture. The mixture was continued to be incubated for 1 h, and then 0.2 mL of 0.6 mol·L⁻¹ potassium borate solution was added to the mixture. The mixture was boiled for 3 min, and then cooled rapidly. The absorbance value was measured at 585 nm. One unit of CHI activity (U) was defined as the amount of enzyme necessary to form 1 × 10⁻⁹ mol N-acetylglucosamine per second by decomposing chitin polysaccharides.

GLU activity was determined according to the method [26]. The cherry tomato powder was prepared as CHI. 2.0 g cherry tomato powder was added into 2.0 mL pre-cooled extract (containing 1 mmol·L⁻¹ EDTA-Na₂, 5 mmol·L⁻¹ β-mercaptoethanol and 1 g·L⁻¹ L-ascorbic acid). The mixture was centrifuged for 15 min at 12,000× g at 4 °C, and the supernatant was collected. Then, 100 μL of laminae polysaccharide (4 g·L⁻¹) was added to two test tubes. Next, 100 μL of supernatant was added to one test tube, and 100 μL of boiled supernatant was added to the other test tube. The mixtures in two test tubes were incubated at 37 °C for 40 min. Afterward, 1.8 mL distilled water and 1.5 mL 3,5-dinitrosalicylic acid (DNS) were added, and finally diluted to 25 mL with distilled water. The absorbance values at 540 nm were measured and denoted as OD_S (absorption value of sample tube reaction solution) and OD_C (absorption value of blank tube reaction solution). One unit of GLU activity (U) was defined as the amount of enzyme necessary to form 1 × 10⁻⁹ mol glucose per second by decomposing laminae polysaccharides.

2.3.5. Determination of MDA Content and Membrane Permeability

MDA content was measured according to the previous method [25]. Cherry tomato tissue (0.5 g) was homogenized with 10 mL of 10% trichloroacetic acid containing 0.5% (w/v) thiobarbituric acid. The mixture was then heated at 100 °C for 10 min. After the rapid cooling of the sample to room temperature and centrifugation at 4000× g for 15 min at 25 °C, the absorbance of the supernatant was measured at 450, 532 and 600 nm. The MDA content was calculated using the following formula: 6.45 × (D532 − D600) − 0.56 × D450. The unit of MDA content was mmol/g.

Membrane permeability was determined according to the method of Chen et al. using electrolyte leakage by conductivity (P₁/P₀) measurements [27]. The whole cherry tomatoes were cut into small discs (1 cm in diameter, 20 discs from five fruits) and washed three times with deionized water. After blotting dry with filter papers, the discs were placed into flasks with 40 mL of deionized water. The flasks were gently shaken on a rotary shaker at 25 °C for 10 min and the electrical conductivity (P₁) of the solution was measured using a conductivity meter (DDX-11AT, Shanghai Yizheng Scientific Instrument Co., Ltd., Shanghai, China). Flasks with the solution were then heated to 100 °C for 10 min, cooled quickly and P₀ was measured. The membrane permeability was calculated as follows: P (%) = P₁/P₀ × 100%.

2.3.6. Determination of Vitamin C and Lycopene

The vitamin C content was measured by 2,6-dichloroindophenol titration [5]. Briefly, 2 g of cherry tomato tissue was homogenized in 10 mL of 2% oxalic acid solution, and then centrifuged at 8000× g for 15 min at 4 °C. Afterwards, 2 mL of supernatant was titrated to a permanent pink color using 0.1% of 2,6-dichloroindophenol titration. The vitamin C concentration was calculated according to the titration volume of 2,6-dichloroindophenol, and expressed as mg·Kg⁻¹ fresh weight.

Lycopene was determined using spectrophotometry [6]. Lycopene was extracted from fresh tomato samples after homogenization of the whole fruit. The homogenized sample (2 g) was weighed into a 100 mL glass vial wrapped with aluminum foil to exclude light, and 50 mL of a mixture of hexane/acetone/ethanol (2:1:1) was added to extract lycopene. Samples were kept in constant agitation for 90 min. The solution was left to be separated into a distinct polar layer (35 mL) and a non-polar layer (25 mL) containing lycopene. The content of lycopene was obtained by comparing the absorbance of the lycopene hexane solution with a calibration curve obtained using different concentrations of standard lycopene, and measurements were performed at 473 nm. The unit of lycopene was mg/Kg.

2.3.7. Determination of Antioxidant Activities

The antioxidant activities of postharvest were evaluated in terms of DPPH radical scavenging capacity and reducing power. Tissues (10 g) were homogenized in 30 mL of 150 mmol/L sodium phosphate buffer (pH 7.8) and then centrifuged at 5000× g for 15 min at 4 °C using a centrifuge. The clear supernatant was collected.

DPPH radical scavenging capacity was assayed according to a previously described method [28]. Briefly, 3 mL of the supernatant was added to 3 mL of DPPH (120 μmol/L) in methanol. The reaction mixture was incubated for 1 h at 30 °C in the dark. Afterward, the absorbance at 517 nm was determined using a spectrophotometer. The scavenging rate of DPPH radicals was calculated as scavenging rate (%) = [1 − (A₁ − A_s)/A₀] × 100, where A₀ is the absorbance of the control solution (3 mL of phosphate buffered saline in 3 mL of DPPH solution), A₁ is the absorbance of the supernatant in DPPH solution, and A_s, which is used for error correction arising from the unequal color of the sample solutions, is the absorbance of the sample extract solution without DPPH.

The reducing power of the fruit samples was determined using the method of Jayaprakasha, Singh and Sakariah [29]. Then, 0.2 mL of the supernatant was mixed with 2.5 mL of phosphate buffer (0.2 M, pH 6.6) and 2.5 mL of 1% potassium ferricyanide in 10 mL test tubes. The mixtures were incubated for 20 min at 50 °C. After incubation, 1 mL of 10% trichloroacetic acid was added to the mixtures, followed by centrifugation at 2000× g for 10 min. The upper layer (2.5 mL) was mixed with 2.5 mL of distilled water and 1 mL of 0.1% ferric chloride. The mixture was mixed well and reacted for 10 min at room temperature. The absorbance of the mixture was measured at 700 nm. The reducing power was expressed using the absorbance value of 700 nm.

2.4. Statistical Analysis

Experimental data were analyzed by DPS9.01 statistical software (Refine Information Tech. Co., Ltd., Hangzhou, China). Experimental data were the means ± SD of three replicates of determinations for each sample. Tukey’s method was used for multiple comparative analysis, and p < 0.05 was considered to indicate statistical significance.

3. Results

3.1. Rottenness

As shown in Figure 1, the rottenness of postharvest cherry tomatoes occurred gradually with the extension of storage days. After 12 d, the cherry tomatoes in the control group began to rot. The samples treated with the composite coating showed no rottenness. On day 21, serious rottenness occurred in the control sample, and there was a relatively large area of rottenness in samples treated with either chitosan coating or curdlan coating. The cherry tomatoes treated with the composite coating still showed no rottenness on day 21. In general, treated samples with chitosan or curdlan coating showed some positive effects compared to control samples. The preservation effect of cherry tomatoes treated with the composite treatment was the best among all groups during storage time.

3.2. Weight Loss, Firmness, Soluble Solids and Titratable Acidity

The weight loss of postharvest cherry tomatoes increased gradually with the extension of time during storage (Figure 2a). The weight loss of the control group increased the fastest, followed by that of curdlan treatment. The weight loss of samples treated with chitosan increased more slowly, and samples treated with the composite coating showed the lowest increase in weight loss during the whole storage time. On day 21, the weight loss of samples treated with the composite coating was 10.4%, which was 27.2%, 33.8% and 44.2% lower than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). These results indicated that the composite coating treatment could effectively slow down the water loss of postharvest cherry tomatoes and contribute to the preservation of cherry tomatoes. Figure 2b exhibited the firmness changes in postharvest cherry tomatoes. The firmness of cherry tomatoes treated with the composite coating decreased the slowest, while the other three groups declined faster. On day 21, the firmness of samples coated with chitosan and curdlan was 0.77 kg/cm², which was 51.8% higher than that of the control group (p < 0.05)

Soluble solids of postharvest cherry tomatoes decreased during storage time (Figure 2c). The control group showed the fastest decline, while the composite-coated sample showed the slowest decline. The amount of soluble solids in samples treated with the chitosan or curdlan coating was higher than that of the control sample and lower than that of composite-treated samples. On day 21, the soluble solids of cherry tomato treated with the composite coating was 5.4 Brix, which was 5.74% higher than that of the control group (p < 0.05). The titratable acidity of postharvest cherry tomatoes decreased during the whole storage period (Figure 2d). However, the titratable acidity of samples treated with the composite coating was consistently the highest among all treatments. The titratable acidity of chitosan or curdlan-coated samples also maintained a high level, while the control group showed the fastest decrease in titratable acidity. After 21 d, the acidity of the composite coating samples was 33.67% higher than that of the control group (p < 0.05).

3.3. Respiration Rate and Ethylene

The respiration rate of postharvest cherry tomatoes first increased and then decreased with the extension of time during storage (Figure 3a). The peak value of the respiration rate in control samples appeared on day 6, while the peak value of treated samples appeared on day 9. Moreover, among all samples, the respiration rate of treated samples with composite coating showed the lowest peak value, 71.4 mg CO₂·Kg⁻¹ h⁻¹ FW, which was 6.0%, 10.0% and 23.7% lower than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). Between days 9 and 21, the respiratory intensity of cherry tomatoes in each treatment showed a downward trend, and the composite-coated samples consistently showed the lowest respiratory intensity during storage. Figure 3b illustrates the variation trend of ethylene production in postharvest cherry tomatoes. The ethylene production showed a similar trend as respiration intensity changed. It first increased and then decreased. The ethylene production of cherry tomatoes treated with the composite film was the lowest during the whole storage period. Its peak value was 3 days later and the peak intensity was 19.0% lower than that of control samples. Compared with the curdlan treatment, the samples coated with chitosan also showed low ethylene production.

3.4. SOD, POD, CAT, GLU and CHI Activities

As shown in Figure 4a, the SOD activity of postharvest cherry tomatoes first increased and then decreased during storage time, reaching a peak value on day 9. Among all groups, the samples treated with the composite coating maintained the highest SOD activity during the whole storage period. Its peak SOD activity was 728.3 U·min⁻¹·g⁻¹, which was 11.8%, 26.2% and 34.9% higher than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). Figure 4b shows the changes in POD activity. Similar to SOD, the POD activities of postharvest cherry tomatoes also first increased first and then decreased. The samples treated with the composite coating also maintained higher enzyme activity. On day 12, the peak value of 8.28 U·min⁻¹·g⁻¹ appeared. The peak value of samples treated with the composite coating was 5.3% higher than that of the control group (p < 0.05). As can be seen from Figure 4c, CAT enzyme activity also first increased and then decreased. The CAT activity of cherry tomatoes treated with the composite coating was the highest during the whole storage period. The peak value appeared on day 12 and was 13.2% higher than that of the control group (p < 0.05).

The activity of CHI first increased and then decreased during the storage period (Figure 4d). During the whole storage period, the CHI activity of cherry tomatoes treated with the composite coating was the highest. Its peak value was 5.1 U·mol·s⁻¹·g⁻¹, appearing on day 12. It was 5.4%, 13.1% and 31.3% higher than that of the chitosan coating, the curdlan coating and the control sample, respectively (p < 0.05). Figure 4e shows the change in GLU enzyme activity. Similar to CHI, it also first increased and then decreased. The composite-coated samples always maintained the highest enzyme activity, while the control samples maintained the lowest enzyme activity during the whole storage period.

3.5. MDA Content and Membrane Permeability

Figure 5a indicates that the MDA contents of postharvest cherry tomatoes showed an increasing trend during storage time. The MDA contents of the control group increased the fastest. The samples treated with the composite coating maintained relatively low MDA content, followed by samples treated with chitosan or curdlan. On day 21, the MDA content of samples treated with the composite coating was 0.22 mmol·g⁻¹, which was 6.7%, 14.2% and 19.8% lower than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). Membrane permeability represents the ion permeability of the cell membrane after the senescence of postharvest fruits and vegetables. The higher the degree of fruit and vegetable senescence is, the greater the membrane permeability is. According to Figure 5b, the membrane permeability of postharvest cherry tomatoes increased during storage. The membrane permeability of samples treated with the composite coating was the lowest during the whole storage period, and the control group showed the highest membrane permeability. On day 21, the membrane permeability of samples treated with the composite coating was 14.5% lower than that of the control group (p < 0.05).

3.6. Vitamin C and Lycopene

Vitamin C content in postharvest cherry tomatoes showed a downward trend during the storage time (Figure 6a). Samples treated with the composite coating showed the slowest decline, followed by chitosan-treated samples and curdlan-treated samples. The control sample exhibited the lowest vitamin C content during the whole storage period. On day 21, the vitamin C content of samples treated with the composite coating was 9.0%, 15.3% and 23.1% higher than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). The lycopene contents of postharvest tomatoes generally presented a trend of first increasing and then decreasing (Figure 6b). The samples treated with the composite coating maintained the highest lycopene content, while the lycopene content of the control group showed the lowest lycopene content during the whole storage period.

3.7. Antioxidant Capacities in Terms of DPPH Radical Scavenging Capacity and Reducing Power

As depicted in Figure 7a, the DPPH radical scavenging capacity of postharvest cherry tomatoes first increased slightly and then tended to decline. During the whole storage period, the samples treated with the composite coating maintained the highest DPPH scavenging capacity, followed by samples treated with the chitosan coating and samples treated with the curdlan coating, while the control group exhibited the lowest DPPH scavenging capacity. On day 21, the DPPH radical scavenging capacity of samples treated with the composite coating was 90.7%, which was 3.0%, 4.9% and 6.1% higher than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05). As described in Figure 7b, the reducing power of postharvest cherry tomatoes decreased during the storage time. The three groups of samples treated with the composite coating, chitosan coating or curdlan coating maintained a higher reducing power, while the control group showed a lower reducing power. On day 21, the reducing power of samples treated with the composite coating was 7.0%, 11.6% and 45.9% higher than that of the chitosan coating, curdlan coating and control group samples, respectively (p < 0.05).

4. Discussion

After the composite coating treatment, the SOD, CAT and POD activities of postharvest cherry tomatoes maintained high levels during the whole storage period compared with the control group (Figure 4a–c). Thus, in postharvest cherry tomatoes, excessive free radicals might be effectively removed and the senescence caused by free radicals might be reduced to a certain extent. Jahani et al. also found that treating cherry tomatoes with a chitosan nano-biopolymer could increase the activities of CAT and POD [30]. Among all groups, the MDA content and membrane permeability of cherry tomatoes treated with the composite coating were relatively low (Figure 5a,b). This suggested that the senescence degree of cherry tomatoes treated with the composite coating was the lowest, and also confirmed that free radical scavenging enzymes played a positive role [31]. The CHI and GLU activities of postharvest cherry tomatoes treated with the composite coating maintained the highest activity during the whole storage period (Figure 4d,e). This result indicated that the composite coating enhanced the disease resistance of cherry tomatoes, reduced the invasion of postharvest pathogens to some extent and slowed down the occurrence of decay (Figure 1) [26].

Soluble solids and organic acids are two of the substrate components of respiration, and their decline also slowed down with the lower respiration intensity of postharvest cherry tomatoes (Figure 2c,d) [32]. Composite coating treatment maintained less weight loss and a high firmness of postharvest cherry tomatoes (Figure 2a,b). The samples treated with the composite coating exhibited better quality. Lycopene and vitamin C are important antioxidant nutrients of cherry tomatoes [30]. Samples treated with the composite coating slowed down the senescence of fruit and retained more lycopene and vitamin C (Figure 6a,b). This result was similar to Razali et al., who treated cherry tomatoes with mucilage from dragon fruit [5]. As for the antioxidant activities of postharvest cherry tomatoes, Vc content and reducing power, lycopene content and DPPH radical scavenging capacity displayed relatively high correlations (Figure 8). In summary, we inferred that composite coating of chitosan and curdlan might slow down the senescence process of postharvest cherry tomatoes to a certain extent, thus maintaining relatively good fruit quality (Figure 9). With regard to the mechanism of preservation effect, does the composite coating directly stimulate the defense response of postharvest cherry tomatoes, or interfere with the invasion of postharvest pathogens? Or do both effects exist? This mechanism needs to be further studied and confirmed in the future [33,34,35]. At present, our research group is continuing to carry out relevant research.

5. Conclusions

The composite coating of chitosan and curdlan could effectively maintain the quality of postharvest cherry tomatoes. Treated samples displayed higher activities of free radical scavenging enzymes and related disease-resistant enzymes. The nutrients Vc and lycopene were also more preserved. Therefore, the edible composite coating of chitosan and curdlan might be a potentially promising method for preserving relevant fruits and vegetables.