Ozone, Heat Shock, and Microwave Differentially Promote Nutritional Quality and Antioxidant Capacity of Sweet Corn

TL;DR

This study compares ozone, heat shock, and microwave treatments to see which best preserves sweet corn's quality and antioxidants during storage.

Contribution

The study systematically compares three postharvest treatments for their effects on sweet corn quality and antioxidant capacity.

Findings

Ozone treatment best preserved sensory and appearance qualities of sweet corn.

Heat shock reduced weight loss during storage compared to the control group.

Microwave treatment maintained antioxidants but caused local scalding and later quality decline.

Abstract

In this study, the effects of ozone treatment (O3), heat shock treatment (HS), and microwave treatment (MW) on sensory quality, physicochemical properties, and oxidation levels of sweet corn were systematically investigated during storage. The results demonstrated that three treatments prolonged the postharvest quality of sweet corn to varying degrees. Specifically, the O3 group demonstrated the best sensory and appearance characteristics, with its sensory score being 1.18 and 1.38 folds higher than the HS group and MW group, respectively, and significant retardation of color deterioration. In addition, the O3 group effectively maintained the stability and hardness of the starch structure. The weight loss rate of the HS group decreased 0.78-fold compared to the CT group after storage. Moreover, both HS and MW treatments maintained the antioxidant properties of sweet corn, but MW had the limitations of local scalding damage and accelerated deterioration in later quality. The results of this study provide a scientific basis for the optimization and application of postharvest preservation techniques for sweet corn.

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Full text

1. Introduction

Sweet corn is rich in carbohydrates, vitamins, minerals, dietary fiber, antioxidants, and other nutrients [1]. It is also characterized by excellent sensory and functional traits, including tender and crisp texture, pleasant flavor, and easy digestibility, thus possessing high nutritional and economic value. However, sweet corn is marked by high water content and sugar content, as well as respiratory rate, and elevated water activity. These inherent properties make it prone to postharvest oxidation, causing significant losses in key nutritional quality indicators—texture, starch, and soluble sugar content. Its shelf life under ambient temperature is 2–3 days [2], which poses significant challenges to the preservation and market circulation of sweet corn. Thus, the adoption of effective measures to extend the postharvest storage period and enhance the commercial value of sweet corn is of great practical significance and necessity.

In recent years, modified atmosphere packaging (MAP) preservation, low-temperature preservation, and chemical preservation have been commonly employed for the postharvest preservation of sweet corn. However, MAP preservation and low-temperature preservation impose stringent requirements on transportation conditions and entail relatively high operational costs. Meanwhile, chemical preservation is associated with the risk of chemical residues, failing to meet consumers’ demands for healthy foods and clean-label products. Thus, low-cost, green, eco-friendly, and efficient preservation technologies that can rapidly inhibit spoilage factors, reduce chemical additive usage, and retain the “natural” properties of foods are more aligned with current societal development trends [3]. On this basis, three safe and highly applicable preservation methods—heat shock (HS), microwave (MW), and ozone (O_3_) treatments—were selected in this study to investigate their regulatory effects on the postharvest storage quality of sweet corn.

HS inhibits pathogenic bacteria via short-term heat stress, overcoming the limitation of traditional blanching that only inactivates enzymatic activity. By regulating the antioxidant system and the expression of defense-related proteins in produce through mild heat shock, it induces and enhances disease resistance in fruits and vegetables while maintaining their sensory quality. This achieves the dual effects of “stress resistance + quality stabilization”. Furthermore, no chemical residues are produced during treatment, which is consistent with the concept of green preservation [4,5]. HS is currently a cost-effective and widely utilized postharvest strategy [6]. Moreover, the activity of endogenous enzymes is inhibited by MW treatment via thermal effects. Bound phenolic compounds are effectively released and activated upon moderate treatment, with antioxidant activity enhanced simultaneously [7]. Significant impacts on the storage quality of fruits and vegetables are exerted by MW treatment, including delayed decay, maintained hardness, reduced browning, and a slowed weight loss rate. These effects are achieved by inhibiting the growth of pathogenic fungi, interfering with enzyme activity, and regulating physiological and biochemical metabolism [8]. With MW treatment, simultaneous internal and external heating is applied, accompanied by mild stimulation instead of ripening. Efficient action inside the kernels is achieved. Energy-saving and high-efficiency characteristics are exhibited, and industrial assembly lines can be easily adapted. Thus, the large-scale requirements of modern postharvest processing for fresh sweet corn are well satisfied. O_3_ is a strong oxidizing agent. Microorganisms can be inactivated and enzymes can be simultaneously deactivated by O_3_ treatment, thereby reducing water transpiration, maintaining firmness, and enhancing the antioxidant capacity of fruits and vegetables [9,10]. As a green non-thermal sterilization method used to replace traditional chemical preservatives, no residues or secondary pollution are produced, which meets the market demand for “safe and healthy” fresh sweet corn. O_3_ treatment yields safer products with extended shelf lives, making it a preservation technology with great development potential [11]. The selection of preservation methods is determined by inherent product characteristics, transportation conditions, and application costs. Significant variations in adaptability to preservation methods are observed among different products, requiring treatment strategies to be highly compatible with the properties of the target commodities.

On this basis, three green preservation methods (HS, MW, and O_3_) were employed in this work. The effects of fresh-keeping methods on the preservation of sweet corn were systematically evaluated. The assessment encompassed nutritional quality parameters, including sensory evaluation, hardness, and soluble sugar content, as well as antioxidant properties such as MDA content, total phenol content, DPPH radical scavenging activity, and ABTS radical scavenging activity. This study provides multiple technical pathways and theoretical references for the optimization of the postharvest quality of sweet corn.

2. Materials and Methods

2.1. Materials

The sweet corn ears (Zea mays saccharata cv. Crispy and Sweet 321) were harvested from Longyou County, Quzhou, Zhejiang Province, in early October 2024 and carefully transported to the lab. Sweet corn ears with uniform size and no pests or diseases were randomly selected for peeling and pre-freezing treatment. A total of 800 sweet corn ears were randomly divided into four groups. The optimal parameters for the three treatments were determined based on preliminary experiments and applied to sweet corn ears: HS treatment (50 °C warm water, 5 min), MW treatment (2450 MHz, 720 W, 40 s), and O_3_ treatment (90 mg/m^3^, 20 min). After treatment, samples were naturally cooled to room temperature, and the remaining ones without treatment were used as the control (CT). All samples were stored in black polyethylene bags at 20 °C in the dark, and sampling and statistics were conducted on days 0, 4, 8, and 12. All other chemicals used in this study were of analytical grade or above. The middle region of each of the processed cobs was then chosen as the measurement site, and the measurement procedure was repeated three times. All experiments included three biological replicates, each with three technical replicates.

2.2. Experimental Methods

2.2.1. Determination of the Quality of Sweet Corn

Apparent Characteristics and Sensory Evaluation

A total of 10 trained panelists participated in the Quantitative Descriptive Analysis (QDA) of stored sweet corn samples. All panelists had received standardized QDA training for the sensory evaluation of fresh sweet corn prior to the experiment. From each treatment group, 9 sweet corn ears were randomly selected, and only kernels obtained from the middle section of each ear were used for sensory assessment. Sensory scoring was performed independently by each panelist in accordance with the evaluation criteria presented in Table 1 [12].

Color Analysis

Color was measured using a CR-400 chromameter (INESA Scientific Instrument Co., Shanghai, China) colorimeter at a 2° viewing angle [13]. The following equation was used to calculate the total color difference:

$$[eqn]$$

where ∆L*, ∆a*, and ∆b* represent the differences in lightness, red green coordinate, and yellow blue coordinate, respectively.

Texture Profile Analysis

The hardness of sweet corn kernels was determined using a TA-XT2i texture analyzer (Stable Micro Systems, Surrey, UK), equipped with a P/50 probe with TPA mode. The parameters were set as follows: test speed was 2 mm/s and a trigger force was 5 g, with a penetration depth of 10 mm. Hardness was reported as the maximum peak force, with the unit of Newton [14].

Weight Loss

The weight loss percentage was calculated based on the mass of the sweet corn ear before storage (g_1_) and the mass after a period of storage (g_2_) [15].

$$[eqn]$$

Water Content Determination

For water quantification, 0.1 kg of minimally processed corn aliquots was randomly selected from each group for initial wet weight (g) measurement. The samples were subsequently enzyme-inactivated at 105 °C for 20 min, followed by drying at 80 °C until a constant mass was achieved. The desiccated mass (g) was recorded postdehydration. The water content (%) was calculated according to the standard gravimetric formula [16]:

$$[eqn]$$

where m_0_ represents the initial wet mass and m_1_ denotes the final dry mass.

Sugar–Acid Ratio

A PAL-BX/ACID F5 sugar–acid analyzer (ATAGO Co., Tokyo, Japan) was used to determine the changes in total soluble solid (TSS) and titratable acidity (TA) contents of sweet corn kernels during storage, so as to analyze the effects of different treatments on the quality of sweet corn ears [17].

Scanning Electron Microscopy (SEM)

A number of sweet corn kernels from different treatments were manually peeled off and fixed in 4% glutaraldehyde solution. After fixation, the fixative was discarded, and the samples were rinsed with an equal volume of PBS solution, followed by rapid freezing in liquid nitrogen. The corn kernels were longitudinally bisected under low-temperature conditions to prepare frozen-fractured cross-sections. Microstructural characterization was performed using an SU-1510 SEM-EDAX system (Hitachi High-Tech Corporation, Tokyo, Japan; EDAX module by AMETEK, Mahwah, NJ, USA) at an accelerating voltage of 10 kV, and cross-sectional micrographs were captured under a high-vacuum atmosphere [18].

Nutritional Components Testing

The amylose, amylopectin, total starch contents, α-amylase, and soluble sugar contents were tested according to the instructions of kits (Solarbio Technology Co., Beijing, China). The soluble protein was determined by the Coomassie Brilliant Blue staining method [19].

2.2.2. Oxidative Damage and Antioxidant Capacity

Carbonylated Protein Content, MDA, POV, and AV

Carbonylated protein content was determined following the instructions of the protein carbonyl assay kit (Solarbio Technology Co., Beijing, China). MDA content was determined according to the method of C. Xu et al. [20]. POV and AV of sweet corn samples were determined via the AOCS Cd 8–53 and Cd 3d-63 procedures, respectively, with partial modifications [21].

Total Phenol Content, DPPH, and ABTS

The total phenol content of sweet corn kernel samples from the four groups was determined by the Folin–Ciocalteu (FC) reagent method [22]. DPPH and ABTS radical scavenging activities were determined following the instructions of the corresponding assay kits (Solarbio Technology Co., Beijing, China).

Activities of Related Enzyme Systems

The activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were determined following the instructions of the corresponding assay kits (Suzhou Comin Biotechnology, Suzhou, China).

2.3. Statistical Analysis

Data analysis was performed using SPSS Statistics 27 software, and the results were presented as mean ± standard deviation (SD). Two-way analysis of variance (ANOVA) was conducted. Statistical significance was considered at the threshold of p < 0.05. Graphs were plotted using Origin 2024 software.

3. Results and Discussion

3.1. Determination of the Quality of Sweet Corn

3.1.1. Sensory Quality and Apparent Characteristics

In Figure 1, the CT group showed large-area shrunken grains on the 8th day of storage, with dull appearance and local black spots. The HS group performed stably throughout the storage period, with only small-scale shrunken grains at the top and lower parts of sweet corn ears, good appearance integrity, but overall whitish color; the MW group had local scald damage immediately after treatment, and white spots appeared around the damaged parts in the later storage period, but no large-area shrunken grains occurred. The O_3_ group had excellent overall apparent conditions without large-area shrunken grains or black and white spots. Similarly, the overall sensory score of sweet corn ears showed a gradual downward trend with the extension of storage, while the HS and O_3_ groups exhibited significant sensory quality advantages during storage (Table 2). On the 4th day, the sensory scores of all treatment groups were stable at around 8.4 points, while the CT group had dropped to 6.9 points, indicating that the three treatments (HS, MW, O_3_) effectively inhibited the quality deterioration of sweet corn ears. On day 8, the sensory score of sweet corn ears in the CT group reached the non-commercial level. In contrast, the scores of the treated groups remain at a satisfactory level and were significantly higher than the CT group, among which the sweet corn ears in the O_3_ treatment group still maintain an excellent sensory quality. At the end of storage, the sensory data of the O_3_ treatment group are 1.9-fold, 1.18-fold, and 1.38-fold higher than the CT, HS, and MW groups, respectively. Based on the above results, O_3_ treatment can more effectively maintain the shelf-life quality of sweet corn ears, endowing them with a sensory quality more acceptable to consumers.

3.1.2. Total Color Difference

Color is an important quality characteristic of food, which can serve as an intuitive indicator for assessing maturity and freshness [23]. As shown in Figure 2A, the total color difference (∆E) of postharvest sweet corn ears showed a gradual upward trend, which was mainly due to the combined effect of the enzymatic browning reaction and microbial activity [24]. All three treatments caused varying degrees of color difference changes in sweet corn ears. Among them, the HS and MW treatment groups exhibited significant changes in color difference immediately after treatment, which may be attributed to caramelization and Maillard reactions induced by heat treatment, thereby potentially contributing to sweet corn browning [25]. During the entire storage period, the O_3_ treatment group had the smallest variation range in the ∆E value (final value, 4.19), which was significantly lower than the CT group (final value 6.91). This indicated that O_3_ treatment can effectively delay the color deterioration process of sweet corn ears and is more conducive to maintaining their color quality.

3.1.3. Texture Properties

Fruit hardness mainly depends on the integrity of the cell wall structure [26]. During storage, the cell walls of fruits and vegetables are gradually damaged under the degradation of hydrolases, leading to decreased fruit hardness and an accelerated softening process. As shown in Figure 2B, the hardness of sweet corn kernels showed a trend of first increasing and then decreasing with the extension of storage time. The initial increase in hardness was mainly due to the accumulation of starch content in the kernels, while the subsequent decrease in hardness was caused by the deterioration of fruit quality under the storage environment in PE fresh-keeping bags. At 8~12 days, the decrease range of the hardness of sweet corn ears in all treatment groups was lower than that in the CT group, indicating that all three treatments could effectively maintain fruit hardness. Among them, the hardness of the HS and O_3_ groups increased 1.10-fold and 1.13-fold compared to the CT group, respectively, showing significant differences from the CT group. The good maintenance of the hardness of sweet corn kernels in the HS treatment group may be related to the potential inhibitory effect of HS treatment on the activity of fruit-softening enzymes. Previous studies have reported that mild heat shock can induce lignin accumulation and promote cell wall thickening in fruits and vegetables [27,28], which may also contribute to the hardness retention of sweet corn in this study. O_3_ treatment maintained the hardness of sweet corn ears and extended their shelf life by inhibiting the activity of fruit-softening enzymes and reducing the degradation of cell wall polysaccharides [29].

3.1.4. Weight Loss

Sweet corn undergoes varying degrees of water loss during postharvest storage, which leads to kernel shrinkage and reduced freshness, thereby significantly diminishing its commercial value [30]. According to Figure 2C, during the entire storage process, the weight loss rate of sweet corn ears showed a continuous upward trend with the progression of postharvest senescence. The water loss rate of the CT group was significantly higher than that of the treated groups, which was consistent with the observation of a large number of shrunken kernels in the CT group on the 8th day of storage. However, other treatment groups effectively delayed this water loss deterioration process. Previous studies have confirmed that HS, MW, and O_3_ treatments can reduce water loss by decreasing the respiratory intensity and transpiration of fruits and vegetables [31,32,33], which was highly consistent with the results in this experiment. At the end of storage, the weight loss rate of the CT group increased 1.28-fold that of the HS group, suggesting that postharvest senescence of sweet corn was effectively retarded by HS treatment.

3.1.5. Water Content Determination

As shown in Figure 2D, significant differences were observed between the HS and MW groups after treatment. This may be attributed to water absorption in sweet corn after HS treatment and water evaporation after MW treatment. The decrease in moisture content on day 4 was consistent with the increase in hardness. No significant difference was observed at the later stage of storage, which was due to the equilibrium of water exchange between sweet corn and the surrounding environment, thus stabilizing the moisture status and providing an environmental basis for the subsequent softening of sweet corn.

3.1.6. The Effect of the Sugar–Acid Ratio

The sugar–acid ratio is one of the core indicators determining the flavor quality of sweet corn, and its changes directly affect the palatability of sweet corn. In Figure 2E, the sugar–acid ratio of sweet corn ears showed a trend of first increasing and then decreasing as storage time was prolonged, which aligns with the general pattern of a gradual decrease in the sugar–acid ratio during the postharvest ripening of fruits and vegetables [34]. On day 0, the sugar–acid ratios of the HS group and MW group were significantly higher than the CT group (p < 0.05), and remained at a high level during the subsequent storage period. HS and MW treatments increase the reduction in sugar content in sweet corn kernels while delaying respiration and reducing nutrient substrate consumption [35]. Additionally, the sugar–acid ratio of sweet corn ears in the O_3_ treatment group is slightly lower than that of the HS and MW treatment groups but still higher than that of the CT group. This phenomenon may be attributed to the inactivation of enzyme activity, thus retarding the conversion of starch to carbohydrates. In summary, HS and MW treatments can significantly improve the flavor quality of sweet corn, while the flavor quality of the O_3_ treatment group was close to that of the CT group.

3.1.7. Microstructure Analysis

SEM was used to observe the microstructure of sweet corn starch granules. The SEM images on day 0 revealed that the sweet corn starch granules exhibited a regular spherical shape with a smooth and flat surface, and the granules were loosely and regularly arranged (Figure 3). On day 8, wrinkles appeared on the surface of starch granules in the CT group, the morphology transformed into polygonal, and the granules were in a dense accumulation state. At the end of storage, the starch granules in the HS treatment group still maintained a good overall morphology, but the edge clarity slightly decreased. The starch granules in the O_3_ treatment group still retained a spherical structure. However, the internal structures of starch granules in the CT and MW treatment groups were severely damaged, with significant shrinkage and fracture at the edges, showing an irregular volume density distribution. The above results indicated that HS and O_3_ treatments effectively maintain the normal microstructure of sweet corn kernels. Previous studies have pointed out that the complex hierarchical structure of starch granules determines kernel hardness, and the density of the granule packing structure is positively correlated with the hardness value [36]. The corresponding histogram of starch granule size distribution is presented in Figure 3, which quantitatively illustrates the distribution pattern of sweet corn starch granules under different treatments and storage periods. It can be clearly observed that the starch granule size shows an overall trend of increasing initially and then decreasing with prolonged storage, which is consistent with the change trend of sample hardness. Among all treatments, the starch granule size in the O_3_ treatment group was better maintained, suggesting that the uniformity and roundness of starch granules were well preserved during ozone treatment. This result is in agreement with the variation observed in total starch content [37]. In this work, the starch granules in the HS and O_3_ treatment groups had a denser structure, corresponding to higher kernel hardness. HS and O_3_ treatments can inhibit the softening deterioration of sweet corn during storage by maintaining the structural integrity of starch granules.

3.1.8. Total Starch Content and Amylose/Amylopectin Ratio

Starch is the core substance determining its stickiness and glutinousness. In Figure 4A, the total starch content of sweet corn in different treatment groups fluctuated to varying degrees, but generally showed a trend of “first increasing and then decreasing”, which was consistent with the change characteristics of the hardness of sweet corn kernels. There was no significant difference in total starch content between the HS and the CT group, suggesting that excessive starch degradation was one of the reasons for the occurrence of shrunken grains in the HS and CT groups. The MW group maintained the total starch content well in the early stage of storage, but the decrease rate of total starch content accelerated after 8 days. This phenomenon was closely related to the local deterioration caused by local scald damage of sweet corn ears induced by MW treatment. Starch hydrolysis would reduce cell turgor and thus promote fruit softening, while the O_3_ treatment group could significantly delay the change in total starch content (p < 0.05), which mutually corroborated the determination results of sweet corn kernel hardness [38]. The excellent texture of sweet corn is maintained by O_3_ treatment through retarding dynamic changes in total starch content, with its ripening and softening processes simultaneously delayed.

During storage, amylopectin converts to amylose and undergoes retrogradation in sweet corn kernels. The amylose/amylopectin ratio was a key indicator of sweet corn taste. Figure 4B shows that sweet corn starch was mainly amylopectin, and the amylose/amylopectin ratio generally shows an upward trend with the extension of storage time. This change was closely related to the changes in respiratory intensity and starch metabolism-related enzyme activities of sweet corn during storage. Among them, the change trend of the amylose/amylopectin ratio in the MW treatment group was the flattest. The heat treatment can effectively inhibit the activity of starch metabolism-related enzymes, thereby better maintaining the palatability of sweet corn.

3.1.9. α-Amylase Activity

Although α-amylase activity directly reflects the ability of starch to convert into carbohydrates, the increase in this enzyme activity is usually regarded as an important signal of postharvest senescence of fruits and vegetables. α-amylase activity of sweet corn ears in all groups showed an overall downward trend (Figure 4C). Among them, the CT group maintained a high α-amylase activity throughout the storage period, which would accelerate starch consumption and tissue collapse, leading to rapid quality deterioration of sweet corn. This conclusion mutually corroborated the change results of total starch content. Although the α-amylase activity of the HS group was significantly different from that of the CT group, the overall activity level was still high, with a wide fluctuation range (8.43~10.52 U/g). It is speculated that this may cause metabolic disorders inside sweet corn kernels and accelerate their aging process. In contrast, the MW and O_3_ treatment groups could effectively inhibit α-amylase activity, thereby delaying the conversion of starch to carbohydrates and ensuring the stability of starch content in sweet corn kernels.

3.1.10. Soluble Sugar Content

Soluble sugar content is one of the core quality indicators of sweet corn. Its content is affected by variety, harvest time, and postharvest treatment methods. At the same time, as a major osmoprotectant, it is directly related to the sweetness of sweet corn. As shown in Figure 4D, the soluble sugar content in all groups showed a trend of “first increasing and then decreasing” during storage, reaching a peak on the 4th day. The HS treatment group maintained a high soluble sugar content throughout the storage period. The reason may be that HS treatment can promote the conversion of starch and other polysaccharides into small-molecule soluble carbohydrates, or promote the conversion of some insoluble pectin into soluble pectin, thereby maintaining the sweetness of sweet corn kernels [39]. MW treatment inhibited the conversion of sugar to starch and promoted the accumulation of sugar in non-starch form, thereby slowing down the decrease rate of soluble sugar content [40]. In addition, O_3_ treatment inactivated enzymes related to sugar metabolism, thereby minimizing fluctuations in soluble sugar content, maintaining the stability of soluble sugar content, and ensuring the osmotic balance of cells.

3.1.11. Soluble Protein Content

Soluble protein is closely related to the texture characteristics of fresh-eating corn, such as hardness, breaking strength, breaking energy, and elasticity, and can affect product quality by regulating these characteristics. As shown in Figure 4E, the soluble protein content of sweet corn ears in all groups showed an overall downward trend with the extension of storage time. The HS and MW treatment groups exhibited a relatively small decrease in soluble protein content during days 0~8. On day 8 of storage, soluble protein contents were 1.39-fold and 1.26-fold that of the CT group, respectively, which indicated that the decrease in soluble protein content was significantly retarded by both HS and MW treatments. The internal mechanism may be as follows: heat treatment can up-regulate the expression of protein synthesis-related genes and induce the synthesis of new proteins, and this regulatory effect can last for a short period of time [41]. However, after 8 days of storage, the soluble protein content of the HS and MW groups decreases significantly. It is speculated that this may be due to the damage of the kernel cell membrane structure, leading to the slow recovery of the activity of protein-degrading enzymes that were inactivated in the early stage, which accelerated the degradation of soluble protein. The overall downward trend of soluble protein content in the O_3_ treatment group was relatively flat, indicating that this treatment could effectively maintain the stability of soluble protein content in sweet corn, thereby ensuring excellent texture characteristics such as hardness and elasticity of sweet corn.

3.2. Oxidative Damage and Antioxidant Capacity

3.2.1. Carbonylated Protein Content, MDA, POV, and AV

Carbonylated protein content is the most commonly used and critical biomarker for evaluating the degree of protein oxidative damage [42]. Figure 5A showed a significant upward trend overall with the extension of storage time. Specifically, the increase was relatively gentle in the HS and MW groups during 08 days of storage but significantly accelerated during days 912. The protein carbonyl content in the O_3_ group was lower than that in the CT and HS groups but higher than that in the MW group, which may be attributed to the mild oxidation of surface proteins induced by ozone. This trend was negatively correlated with the change in soluble protein content, as protein carbonylation modification not only changes protein solubility but also enhances sensitivity to proteases, thereby affecting protein physiological functions [43].

As a core indicator of cellular oxidative damage, MDA effectively characterizes the degree of lipid peroxidation in tissues and cell membranes [44]. As shown in Figure 5B, the MDA content in all groups increases progressively with prolonged storage time. Among these groups, the HS group induced the mildest cell membrane damage, with its MDA content lower than that of the other treatment groups. In contrast, the MDA value in the MW group was 1.22-fold that in the HS group, suggesting obvious initial membrane damage by microwave treatment, which is not the optimal choice for maintaining membrane system integrity. Although MW treatment delays microbial-mediated decay through sterilization, it is unable to inhibit senescence induced by membrane peroxidation [45].

POV reflects the oxidation degree of oils and fatty acids; its value directly corresponds to the accumulation of peroxide products. As shown in Figure 5C, POV increased with storage time, indicating progressive oil oxidation, which was basically consistent with the upward trend of MDA content. The HS treatment group exhibited a significant surge in POV on day 8, indicating that the lipid oxidation reaction was drastically accelerated at this stage. In contrast, the POV of the MW treatment group increased moderately during the early storage period, which is closely associated with the inactivation of enzyme activity and microorganisms induced by microwave treatment. However, the intensified lipid oxidation observed at the end of storage may be attributed to the reactivation of previously inactivated enzymes and the release of endogenous substances caused by damage to the cell membrane structure [46]. At day 0, the POV content in the ozone-treated group was 1.27-fold higher than that in the CT group. However, ozone can oxidize and scavenge free radicals as well as efficiently inhibit lipoxygenase activity, resulting in a significantly flattened upward trend of POV in the later stage and effectively reducing the rate of lipid oxidation [47].

AV measures oil hydrolysis and rancidity; exceeding the limit causes a characteristic rancid odor, and its change directly reflects the hydrolysis and rancidity process. As shown in Figure 5D, AV changes were roughly consistent with POV in all groups: oil hydrolysis accelerated, free fatty acids accumulated continuously, and AV increased over time. Notably, the MW group had an explosive AV increase to 1.24 mg/g on day 12, exceeding the limit and producing odor. This may be due to local tissue and membrane damage caused by MW treatment, accelerating oil hydrolysis.

3.2.2. Total Phenol Content, DPPH, and ABTS

Phenolic compounds protect key biomacromolecules from oxidative damage by directly scavenging free radicals and interrupting oxidative chain reactions through hydrogen atom donation [48]. As shown in Figure 6A, the total phenol content of sweet corn ears in the CT and MW groups decreased continuously with prolonged storage, indicating that the natural antioxidant defense capacity of sweet corn ears gradually weakened during storage. In contrast, both the HS and O_3_ treatment groups showed a dynamic change trend of “first increasing and then decreasing” in total phenol content, with peaks appearing on the 4th day of storage, reaching 4.26 μg/mL and 4.16 μg/mL, respectively. The increase or decrease trend of total phenol content usually corresponded to the opposite change in MDA content, reflecting the regulatory effect of phenolic compounds on the degree of lipid peroxidation in sweet corn ears. HS and O_3_ treatments induced a resistance response in sweet corn ears, significantly promoting the biosynthesis and accumulation of flavonoids and phenolics, and thus increasing total phenol content. However, with prolonged storage, the senescence process of sweet corn ears was irreversible, and total phenol content decreased accordingly [49]. For the O_3_ group, oxidative stress induced by ozone could activate the resistance mechanism of sweet corn ears, increasing total phenol content. Meanwhile, ozone treatment could inactivate enzymes related to phenolic compound degradation, thereby delaying the degradation process of phenolics [50].

DPPH and ABTS free radical scavenging rates are the key indicators for determining the antioxidant activity of sweet corn. As shown in Figure 6B,C, the free radical scavenging rate of the CT group decreased continuously with prolonged storage. The scavenging rates of the HS, MW, and O_3_ groups all increased to varying degrees on the 4th day of storage. The increased free radical scavenging rate in the HS and MW groups was attributed to high-molecular-weight complexes (e.g., polymeric phenolics, protein-polyphenol complexes) formed by heat treatment, which had strong antioxidant activity [51]. The O_3_ group enhanced the free radical scavenging rate by activating antioxidant enzyme activity and inhibiting pro-oxidant enzyme activity to reduce oxidative stress [9]. Notably, the enhanced antioxidant activity could significantly inhibit oxidative damage to sweet corn ears.

3.2.3. Activities of Related Enzyme Systems

Antioxidants in sweet corn can effectively reduce cell damage caused by free radicals, thereby delaying senescence. As shown in Table 3, the three treatments exerted significant differences in the antioxidant enzyme activities of sweet corn. Among them, O_3_ treatment most effectively activated and maintained the activities of SOD, POD, and CAT and delayed the decline in antioxidant enzyme activities at the later stage of storage, thereby enhancing the antioxidant capacity and reducing oxidative damage to sweet corn. This was closely related to its effect in delaying quality deterioration and extending the shelf life of sweet corn, which might be attributed to the up-regulation of related genes [9]. HS and MW treatments could induce a short-term increase in antioxidant enzyme activities but showed poor sustainability, which might be associated with the exposure of endogenous substances caused by cell structure damage during the late storage period [46].

4. Conclusions

The effects of three treatments (HS, MW, and O_3_) on the postharvest storage quality and antioxidant properties of sweet corn were systematically investigated. All three treatments improved the postharvest quality of sweet corn to different degrees. O_3_ treatment best maintains sensory quality, color, microstructure, and component stability, making it ideal for preserving sweet corn shelf-life quality. HS treatment effectively controls weight loss, protects cell membranes, and boosts antioxidant levels, thereby making it suitable for applications that demand high levels of freshness and antioxidant capacity. Although MW treatment had certain effects on improving shelf life in the early stage and some antioxidant indicators, it had problems such as local tissue damage and accelerated quality deterioration in the later stage. Thus, MW is not the optimal solution for postharvest preservation of sweet corn. The results provide theoretical and data support for postharvest corn preservation and promote the selection of green, sustainable sweet corn technologies.