The effect of the composite of Aloe vera gel and peppermint essential oil as natural coatings on the postharvest characteristics of kiwifruits

Abstract

For climacteric fruits like kiwifruit, postharvest deterioration presents a serious problem with storage and marketability. In this study, the effectiveness of an edible coating made of peppermint essential oil (Es) and Aloe vera gel (ALVG) in maintaining the quality and prolonging the shelf life of “Hayward” kiwifruit kept in cold storage for three months was examined. Treatments were applied in various concentrations (25% and 50% ALVG with 0, 500, and 1000 ppm Es), and quality parameters were evaluated at monthly intervals. The coating significantly reduced weight loss (by 41%) and fruit spoilage (by 65%), and microbial content particularly at ALVG _50%-Es_{1000 ppm}. Additionally, it enhanced physicochemical characteristics such vitamin C retention, titratable acidity (TA), pH, and total soluble solids (TSS). Moreover, it improved antioxidant-related metrics, including phenolic and flavonoid concentrations, catalase activity, and total protein levels, while diminishing oxidative stress indicators such as malondialdehyde (MDA) and reactive oxygen species (ROS). The findings indicate that an edible coating infused with ALVG and peppermint essential oil provides an effective, natural method to improve postharvest quality and prolong storage life in kiwifruit.

Introduction

Kiwifruit (Actinidia deliciosa), a dioecious species native to China and belonging to the Actinidiaceae family, holds a prominent position in global fruit production. The yearly global kiwifruit production, grown on about 286,100 hectares, is over 4.5 million tons, according to the Food and Agriculture Organization (FAO). Over recent decades, the cultivation and commercialization of diverse kiwi varieties and hybrids have expanded considerably¹. However, this rapid growth has been accompanied by an increase in fungal infections during storage and transport, leading to significant postharvest losses that compromise fruit quality and economic value². Despite advances in storage technologies, effective, eco-friendly strategies to reduce spoilage and extend shelf life remain a critical need in the industry.

Kiwifruits are climacteric, rich in vitamins C and K, and highly perishable postharvest due to their sensitivity to microbial decay and physiological deterioration³. The high susceptibility to spoilage not only hampers domestic consumption but also poses substantial barriers to export markets, emphasizing the necessity for innovative preservation methods that are safe, sustainable, and environmentally friendly. Current approaches, including chemical preservatives, raise concerns regarding health and environmental impacts, underscoring the demand for natural alternatives.

Recent research highlights the potential of plant-derived substances and edible coatings as protective agents to enhance postharvest fruit quality. Due to its inherent antibacterial, antioxidant, and moisture-retention qualities, ALVG and essential oils have attracted significant interest^4–6. ALVG, containing water, vitamins, glucomannans, sterols, and amino acids, has demonstrated efficacy in reducing microbial load, controlling respiration and reducing oxidative browning in a variety of fruits, including limes and grapes^7,8. The researchers showed that essential oil of pepper mint (Mentha piperita L.) had good antioxidant and antimicrobial activity. The major component of the oil is menthol. The essential oil effects could be attributed to their monoterpene compounds. The essential oil of this plant is one of the natural sources of antioxidants which could be suggested to replace the synthetic source⁹. Shahdadi et al., reported Aloe vera gel edible coating infused with rosemary essential oil had not a significant effect on taste, smell, or texture of the strawberry fruits¹⁰.

Despite encouraging results, there remains a notable lack of comprehensive understanding regarding how combined natural agents, such as ALVG and essential oils, can work together to enhance kiwifruit preservation. Most existing research has examined these treatments individually, leaving their combined effects on microbial stability, physiological traits, and sensory qualities during storage largely unexplored. Additionally, optimizing application techniques, dosages, and formulations specifically tailored for kiwifruit could significantly improve postharvest preservation strategies. In order to close this gap, this study will assess how well an edible, biodegradable coating enhanced with ALVG and peppermint essential oil extends the shelf life of kiwifruit. The innovative aspect of this research lies in integrating two natural, safe compounds into a single coating system to leverage their combined antimicrobial and antioxidant properties. Furthermore, the study investigates how variations in coating composition influence quality parameters, microbial control, and sensory acceptance, offering valuable insights for commercial application.

The use of natural plant-based coatings for postharvest preservation has shown promising outcomes across different fruits. Essential oils, particularly peppermint oil, have demonstrated antimicrobial activity against fungal pathogens and the ability to influence enzymatic processes related to browning and decay¹¹. However, employing these agents separately often presents limitations, such as sensory changes at higher concentrations or limited efficacy against a broad spectrum of pathogens.

Recent advances suggest that combining natural antimicrobials with edible coatings offers a more effective and sustainable approach to postharvest preservation¹². Nonetheless, few studies have specifically examined the combined use of ALVG and essential oils on kiwifruit. There is a significant need to identify optimal formulations that maximize preservation benefits while preserving sensory quality and consumer acceptance.

Biodegradable edible coatings infused with bioactive substances have shown considerable potential as sustainable alternatives to traditional preservation methods. In this research, a semi-permeable barrier was created on the fruit’s surface using a coating composed of ALVG and peppermint essential oil. So far, no study has been conducted on the use of ALVG with peppermint essential oil coating on postharvest characteristics of kiwifruit. Therefore, the aim of this study was to evaluate how well this eco-friendly coating prolongs kiwifruit shelf life and reduces postharvest deterioration under refrigerated conditions.

Materials and methods

Preparation of essential oil (Es)

Aerial parts of the peppermint plant were dried at an ambient temperature (25–38 °C) and in the shade. Then were grounded and passed through a sieve (mesh 40). The Es was extracted by steam distillation over a Clevenger system (Aria Exir, Iran) for 4 h. The obtained Es was dehydrated using sodium sulfate and stored at 4 °C according to previous research¹³.

Preparation of Aloe vera Gel (ALVG)

Aloe vera leaves were collected from University of Jiroft Research Farm. The leaves were washed and their jagged edges were cut with a knife. The top layer of the leaf was removed lengthwise and the gel was carefully separated from the leaf. The gel parts were blended thoroughly and put through a clean metal sieve (mesh 20) to form a homogeneous solution, and the extract was finally pasteurized at 65 °C for 15 min¹⁴.

Preparation of treatments

ALVG was prepared at concentrations of 25% and 50% according to the method described by Shahdadi et al.^10,15. ‘Hayward’ kiwifruits at the pre-commercial maturity stage sourced from a local market. Fruits were free of physical damage, pests, and diseases, and they were all the same size, shape, and color. After washing, the fruits were air-dried completely and then immersed for 5 min in ALVG solutions at 0% (control), 25%, and 50% concentrations, each combined with peppermint essential oil at 0, 500, and 1000 ppm concentrations (To prepare a mixture of ALVG and different concentrations of peppermint essential oil, the solution was homogenised in an Ultraturrax IKA T-18 system for 5 min at 10000 rpm). Following dipping, the fruits were allowed to air dry at ambient temperature (27 °C), after which they were put into lidded high-density polyethylene (HDPE) containers, appropriately labeled, and kept at 4 °C with a relative humidity of 85%. Quality assessments and related analyses were conducted at 30-day intervals over a three-month storage period. In this study 186 fruits were selected for non-destructive tests (i.e., Percentage of fruit spoilage and Weight Loss Percentage) and destructive tests (i.e., pH, titratable acidity, total soluble solids, Vitamin C, phenol, flavonoids, ROS, MDA, proline, protein, catalase, and Sensory evaluation) and microbiological determinations (i.e., Total Microbial and Mold and Yeast Count). All experiments were performed in three replicates and each experimental unit consisted of three fruits.

Percentage of weight loss

A digital balance with an accuracy of 0.01 g was used to measure the weight of the fruit. The weight loss percentage was then computed using Eq. (1)¹⁶.

1

Where W1 and W2 are the primary and secondary weights in grams, respectively, and WL is the weight loss percentage.

Fruit spoilage

The spoilage rate of the fruits was assessed by monitoring visible signs such as mold development, discoloration, and softening of texture. The proportion of spoiled fruits compared to the total quantity of fruits assessed was computed using these indicators¹⁷.

pH, and titratable acidity (TA)

Using a digital pH meter (Sartorius, Professional Meter PP-50, Germany), the pH was measured. In order to evaluate the titratable acidity (TA), 10 mL of fruit juice were diluted with 10 mL of distilled water, and titrated with sodium hydroxide (0.1 N) until the pH reached 8.2. After recording the amount of NaOH used, Eq. (2) was used to determine the titratable acidity as a percentage of citric acid equivalent¹⁸.

2

Where TA equal citric acid percentage, V equal the volume of NaOH consumption and the coefficient 0.064 is typically used to express the acidity in terms of citric acid.

Total soluble solids (TSS)

A digital refractometer (model PDR-108-1, manufactured in Taiwan) was used to determine the fruit juice’s total soluble solids (TSS) level.

Flavor index (TSS/TA)

The ratio was considered a flavor index or fruit maturity index¹⁹.

Vitamin C content

The iodometric titration method was used to measure the amount of vitamin C¹⁹. This approach involved adding 2.5 mL of starch solution as an indicator after diluting 10 mL of fruit juice with 20 mL of distilled water. Using 0.01 N iodine-potassium iodide (I₂/KI), the solution was titrated until the endpoint was indicated by a gray color. The volume of iodine used was recorded and used in a standard calculation to determine the vitamin C concentration, reported as mg of ascorbic acid per mL of juice, using Eq. (3).

3

Where C value indicates that 0.88 mg of ascorbic acid (vitamin C) are equal to one mL of the 0.01 N iodine solution, V1: volume of consumed solution and V2: volume of fruit juice.

Phenol contents

In order to measure the total phenolic content, 0.5 g of fruit juice was combined with 2 mL of 95% ethanol and left in the dark for 24 h. Following 10 min of centrifugation at 5000 rpm, 250 µL of the extract was mixed with sodium carbonate, ethanol, and Folin-Ciocalteu reagent. After 15 min of incubation at 40 °C, the mixture’s absorbance at 725 nm was measured. The standard curve was created using Gallic acid²⁰.

Flavonoid content

The colorimetric technique with aluminum chloride was used to measure the total flavonoids. The methanolic extract mixed with 10% aluminum chloride, 1 M potassium acetate, and distilled water, and it was then allowed to sit at room temperature in the dark for 30 min. At 415 nm, absorbance was then measured. Results were reported in mg per g of fresh weight, and a standard curve was created using Quercetin²¹.

Reactive oxygen species (ROS)

The ROS concentration was measured by centrifuging 0.1 g of fresh tissue at 10,000 rpm for 20 min at 4 °C after it had been homogenized in phosphate buffer (pH 7.4). Next, 900 µL of acidic xylenol orange reagent was mixed with 100 µL of the supernatant. At 560 nm, the absorbance was determined with a spectrophotometer. ROS levels were represented as micromoles per gram of fresh weight and were computed using a standard curve based on various hydrogen peroxide concentrations²².

Catalase (CAT) activity

The assay solution was made up of 0.05 mL of enzyme solution and 2.25 mL of hydrogen peroxide made with 0.1 M sodium phosphate buffer (pH 7.4). After three minutes at 25 °C, the increase in absorbance at 240 nm was noted²³.

Malondialdehyde (MDA)

In order to measure the MDA levels, a 0.1 g sample of fresh tissue was homogenized in phosphate buffer (pH 7.4) and centrifuged for 20 min at 10,000 rpm and 4 °C. After mixing the supernatant solution with a trichloroacetic acid-thiobarbituric acid (TCA-TBA) solution, it was heated for 20 min at 94 °C. Following cooling, the absorbance at 532 and 600 nm was measured using a spectrophotometer²².

Proline

Proline levels were measured by centrifuging 0.05 g of fruit juice with 2 mL of 70% ethanol for 10 min at 8000 rpm. After reacting with ninhydrin, the resultant extract was heated for 20 min to 97 °C. The mixture was centrifuged for one minute at 2500 rpm after cooling in an ice bath. The absorbance was then measured with a spectrophotometer at 520 nm. A standard curve was used to measure the amount of proline²⁴.

Protein content

The protein content was ascertained by centrifuging fresh tissue after it had been homogenized in phosphate buffer. After mixing the resultant extract with the Biuret reagent, the absorbance at 595 nm was determined. Protein concentration was determined using a standard curve utilizing bovine albumin and was reported in mg per g of fresh weight²⁵.

Total microbial and mold and yeast count

To evaluate the total microbial count and total molds and yeast counts, were used PCA (Plate Count Agar) and PDA (Potato Dextrose Agar) culture media, respectively. First, the kiwifruit were crushed in a sterile porcelain mortar, and 10 g were transferred to 100 ml of physiological serum dilution solution. Then 10², 10³, and 10⁴ dilutions were prepared from it. From each of the dilutions made in the amount of 0.1 ml, it was transferred as a surface culture on PCA medium for 24–48 h at 37 °C (total microbial counts) and PDA medium for 3 to 5 days at 25 °C (total molds and yeast counts) under aerobic conditions, then resulting colonies were counted and reported as CFU g^{− 1} sample²⁶. Two plates containing PCA and PDA media without any sample were used as negative controls.

Data analysis

A factorial (5 × 4: Composites of ALVG-Es× storage time) experiment based on a completely randomized design (CRD) was used in this investigation. SAS software version 9.4 was used for data analysis. Duncan’s multiple range test will be used at a significance level of P ≤ 0.05 to ascertain differences between treatment means. Pearson’s correlation coefficients between the examined attributes and principal component analysis (PCA) were computed using OriginPro (2024) software.

Results

Weight loss

The results show that weight loss significantly (p ≤ 0.05) increased consistently for all treatments during the storage period. Notably, the treated fruits with ALVG _50%-Es_{1000 ppm} showed much less weight loss on day 90 compared to the untreated fruits. This suggests reduced metabolic activity and enhanced preservation. In particular, the weight loss was 41% lower with the ALVG_50%-Es_{1000 ppm} treatment (Fig. 1).

Fig. 1.

The effect of edible coatings (ALVG -Es) on kiwifruit weight loss during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Fruit spoilage

Over time, the rate of fruit spoilage increased, especially in untreated samples. At the end of the storage period, treatment with ALVG _50%-Es_{1000 ppm} showed the lowest rate of fruit spoilage. In general, the coated treatments had the lowest spoilage percentage compared to the control. Using ALVG _50%-Es _{1000 ppm}, fruit spoilage decreased by 65% by day 90 of storage (Fig. 2).

Fig. 2.

The effect of edible coatings (ALVG -Es) on kiwifruit spoilage during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

pH

The pH values showed a slight decrease during storage, indicating a shift toward more acidic conditions; however, an overall increasing trend was observed. Treatment with ALVG _25%-Es_{1000 ppm} maintained the pH at a relatively stable level, indicating delayed spoilage and improved preservation of the fruit’s internal quality. In contrast, using ALVG _50%-Es_{1000 ppm} the pH increased by 2% on day 90 (Fig. 3a).

Fig. 3.

The effect of edible coatings (ALVG -Es) on kiwifruit pH (a), TSS (b), TA(c) and (TSS/TA) during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

TSS and TA

Storage time led to an increase in TSS (Fig. 3b and c). Specifically, TSS levels rose by 120%, 104%, and 97% on days 30, 60, and 90, respectively, relative to day 1. Treated samples, particularly during days 60 and 90, maintained significantly higher TSS levels than untreated samples (Fig. 3b). The effect of Aloe vera gel (ALVG) and peppermint essential oil (Es) concentrations did not have a significant (p ≤ 0.05) effect on the TA. Only the effect of storage time on this factor was significant and with increasing storage time, the TA decreased. By days 60 and 90, TA decreased by 22%, 28%, and 61% (Fig. 3c).

TSS/TA ratio

Storage time significantly increased the flavor index or TSS/TA levels. The TSS/TA ratio rose by 2.9-, 3.2-, and 5.5-fold on days 30, 60, and 90, respectively, relative to day 1. Treated samples maintained a higher TSS/TA level than untreated samples during storage time. Elevated ratios in treated samples indicate a more favorable balance of sweetness and acidity, enhancing taste quality. In particular, ALVG_25%-Es_{1000 ppm} increased the TSS/TA ratio by 18% on day 90 (Fig. 3d).

Vitamin C

The amount of vitamin C decreased as storage time increased. Vitamin C on days 30, 60, and 90 decreased by 2.9, 3.2, and 5.5-fold, respectively. Treatments significantly slowed the degradation of vitamin C, indicating improved retention of antioxidants. On day 60 of storage time, using the ALVG 25%-Es500 ppm, ALVG _25%-Es_{1000 ppm}, ALVG_50%-Es_{500 ppm}, and ALVG _50%-Es_{1000 ppm} treatments increased the phenolic content by 66, 10, 50, and 64%,. On day 90 of storage time, using the ALVG _25%-Es_{500 ppm} treatment, the phenolic content increased by 40% (Fig. 4).

Fig. 4.

The effect of edible coatings (ALVG-Es) on kiwifruit vitamin C during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Phenols and flavonoids

In coated samples, both phenol and flavonoid contents increased during storage time. On the thirty-first day of storage, using the ALVG_25%-Es_{500 ppm}, ALVG_25%-Es_{1000 ppm}, ALVG_50%-Es_{500 ppm}, and ALVG_50%-Es_{1000 ppm} treatments increased the phenolic content by 56, 66, 95, and 97%, respectively. On day 60 of storage time, using the ALVG_25%-Es_{500 ppm}, ALVG_25%-Es_{1000 ppm}, ALVG_50%-Es_{500 ppm}, and ALVG_50%-Es_{1000 ppm} treatments significantly (p ≤ 0.05) increased the phenolic content by 77, 40, 63, and 52%, respectively. On day 90 of storage time, using the ALVG_25%-Es_{500 ppm} treatment, the phenolic content increased by 40% (Fig. 5a). On day 90, flavonoids decreased by 44% in untreated samples. Using the ALVG_25%-Es_{500 ppm}, ALVG_25%-Es_{1000 ppm}, ALVG_50%-Es_{500 ppm}, and ALVG_50%-Es_{1000 ppm} treatments, increased the flavonoid content by 1.2, 1.6, 1.8, and 3.2 folds on day 30, by 1.2, 1.8. 2.1, and 2.6 folds on day 60, and by 1.5, 2.1, 1.8, and 1.8 folds on day 90 (Fig. 5b).

Fig. 5.

The effect of edible coatings (ALVG-Es) on phenol (a) and flavonoids (b) contents of kiwifruits during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Reactive oxygen species (ROS) and malondialdehyde (MDA)

In control group, ROS levels increased during storage time. ROS levels were considerably lower in treated groups, indicating that oxidative damage had been mitigated. Using the ALVG_50%-Es_{1000 ppm} treatments decreased the ROS content by 18, 15, and 17% on days 30, 60, and 90, compared to the untreated group (Fig. 6a). MDA, a measure of oxidative damage and lipid peroxidation, significantly (p ≤ 0.05) increased during the kiwifruit’s storage period, especially on day 60. Using ALVG_25%-Es_{1000 ppm} on days 60 and 90 of storage time decreased MDA levels by 39 and 53% in contrast to the untreated samples, respectively (Fig. 6b). These findings imply that ALVG-based coatings successfully lower oxidative stress and lipid peroxidation in kiwifruit, particularly when they contain significant concentrations of peppermint oil. Because peppermint oil has antioxidant qualities and ALVG has a protective barrier, lower MDA levels assist maintain fruit freshness and membrane integrity during storage.

Fig. 6.

The effect of edible coatings (ALVG-Es) on kiwifruit ROS (a) and MDA (b) during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Catalase (CAT) activity

CAT activity decreased over the storage period. Treated samples maintained higher CAT activity levels, indicating enhanced enzymatic defense mechanisms against reactive oxygen species. Using ALVG_25%-Es_{1000 ppm}, on day 90 of storage time, CAT activity increased by 20% (Fig. 7).

Fig. 7.

The effect of edible coatings (ALVG-Es) on kiwifruit Catalase (CAT) activity during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Proline

Up until day 90 of storage, the proline content of kiwifruit exhibited an increasing trend. Using ALVG_50%-Es_{1000 ppm}, on day 90 of storage time, proline content significantly (p ≤ 0.05) decreased by 19% (Fig. 8). The reduction or stabilization of proline levels in treatments containing ALVG -Es indicates the effectiveness of these coatings in alleviating oxidative and chilling stress. This suggests that the fruit experienced less stress, maintained better physiological status, and had its quality better preserved during storage time.

Fig. 8.

The effect of edible coatings (ALVG-Es) on kiwifruit proline content during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Protein

According to our results the amount of protein significantly (p ≤ 0.05) increased in uncoated fruits over the course of storage. Protein content increased by 24, 13, and 45% in 30, 60, and 90 days of storage time. Using ALVG50%-Es1000 ppm, on day 30 of storage time, increased protein content by 54% compared to untreated fruits, while no significant difference was observed between the protein content in coated fruits with other treatments and uncoated fruits. At the other storage times, no significant difference was observed between the protein content in fruits coated with the treatments and uncoated fruits too (Fig. 9).

Fig. 9.

The effect of edible coatings (ALVG-Es) on kiwifruit protein content during storage. The standard deviations are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Microbial content

Total microbial and total mold and yeast counts are shown in Fig. 10. With increasing concentration of ALVG and essential oil, total microbial and total mold and yeast counts significantly (p ≤ 0.05) decreased in 90 days of storage time. The highest amount of total and total mold and yeast counts was observed in the control and the lowest amount in the ALVG_50%-ES_1000ppm treatment. On the 90th day of storage time, using the ALVG_25%-Es_{500 ppm}, ALVG_25%-Es_{1000 ppm}, ALVG_50%-Es_{500 ppm}, and ALVG_50%-Es_{1000 ppm} treatments decreased total microbial by 1.67, 1.73, 2.99, and 4.1 fold, and total mold and yeast counts by 1.84, 1.84, 2.76, and 3.67 fold, respectively.

Fig. 10.

The effect of edible coatings (ALVG-Es) on microbial content (a: total microbial count and b: total mold and yeast) of kiwifruits on the 90th day of storage time. The standard errors are shown by error bars. According to Duncan’s multiple range test, there is no significant (p > 0.05) difference between the same letters.

Visual symptoms

A summary of the effect of edible coatings (ALVG-Es) on the external and internal quality of kiwifruit on the 90th day of storage is shown in Fig. 11.

Fig. 11.

The summary of the effect of edible coatings (ALVG-Es) on external and internal quality of kiwifruits on the 90th day of storage time. ALVG: Aloe vera gel; Es, essential oil of peppermint.

Correlation analysis

Figure 12 shows the correlations between the correlations between the 14 evaluated traits using the Pearson correlation. At p ≤ 0.05, Pearson’s correlation coefficient is proportionate to the severity of the squares’ size and color. Positive correlations are shown by purple squares, whereas negative correlations are shown by red squares. The results showed a positive correlation between weight loss (WL) and fruit spoilage (FS), TSS/TA, and proline. FS, TSS, TSS/TA, and proline. pH and TSS, and TSS/TA. TSS and TSS/TA, total phenol compounds (TPC), proline, and protein. TA Catalase (CAT) activity. TPC and TFC (total flavonoid compounds) and protein. TFC and protein. ROS and CAT. MDA and proline. Protein and proline. However, there was a negative correlation found between TA and WL, FS, pH, TSS, and TSS/TA. Vit C, WL, FS, pH, TSS, and TSS/TA. ROS and TSS, TPC and TFC. CAT and TSS, TPC and TFC. Proline and TA, Vit C, and CAT. Protein and Vit C, ROS and CAT.

Fig. 12.

Pearson correlation analysis (p ≤ 0.05) of storage time and edible coatings treatments with variable trait relationship physiological and biochemical parameters of Kiwi fruits. WL (weight loss), FS (fruit spoilage), TSS (total soluble sugar); TA (titratable acidity); TSS/TA (flavor index); Vit C (vitamin C); TPC (total phenol compounds); TFC (total flavonoids compounds); ROS (reactive oxygen species); MDA (Malondialdehyde), CAT (Catalase).

Principal component analysis (PCA): PCA reveals treatment- and storage-dependent variation in physiological and biochemical traits of kiwifruit. PCA illustrated the interrelationships among 14 physiological and biochemical traits across four storage periods (S1–S4) and five coating treatments (T1–T5). The first two principal components (PCs) explained 68.9% of the total variance, with PC1 accounting for 49.2% and PC2 for 19.7% (Fig. 13). PC1 represented a quality–stress axis, showing strong positive loadings for oxidative and stress-related parameters (CAT and proline) and quality (TSS, TSS/TA and FS) while exhibiting negative loadings for quality-associated traits (vitamin C, and TA). PC2 primarily captured variation associated with storage duration, distinguishing early (S1–S2) from late (S3–S4) storage stages, where later months were characterized by increased ROS and MDA accumulation. Treatment projections on the biplot revealed distinct clustering patterns. The control (T1) and low-concentration coating (T2) groups were positioned on the positive side of PC1, aligning with higher stress indicators and spoilage. In contrast, treatments containing higher concentrations of Aloe vera gel and essential oil (T4: ALVG_50%−ES_500ppm, and T5: ALVG_50%−ES_1000ppm) clustered on the negative PC1 side, associated with higher vitamin C, phenolic, and flavonoid contents, suggesting effective maintenance of fruit quality and antioxidant status. Overall, the PCA demonstrated that the combined coating of ALVG_50%−ES_1000ppm (T5) was the most effective treatment, maintaining fruit quality, reducing oxidative damage, and preserving biochemical integrity throughout storage.

Fig. 13.

Principal component analysis (PCA) storage time (S1: first month, S2: second month, S3: third month, and S4: fourth month), and edible coatings treatments (T1: control, T2:ALVG25%-ES500ppm, T3: ALVG25%-ES1000ppm, T4: ALVG50%-ES500ppm and T5: ALVG50%-ES1000ppm; ALVG: Aloe vera gel and ES: essential oil of peppermint) with variable trait relationship physiological and biochemical parameters of kiwifruit. Positive or negative correlations between various variables are indicated by lines emanating from the center of the PCA biplot of the treatment-variable connection. WL (weight loss), FS (fruit spoilage), TSS; TA; TSS/TA; Vit C (vitamin C); TPC (total phenol content); TFC (total flavonoids content); ROS; MDA, CAT.

Discussion

The current study assessed how well different doses of peppermint essential oil and Aloe vera gel (ALVG-Es) preserved fruit quality over an extended period of storage. The results indicate that ALVG-Es treatments significantly improved storage time outcomes across multiple physiological and biochemical parameters.

Weight loss and fruit spoilage

Fruit weight loss is a prevalent and important postharvest problem that is mostly caused by physiological processes including respiration and transpiration. These processes cause produce to lose moisture, which eventually lowers its quality and shortens its shelf life. Both weight loss and spoiling were found to gradually rise during storage time across all treatments in the current investigation (p ≤ 0.05), with the control group exhibiting the most severe degradation. Notably, treatment with ALVG_50%-Es_{1000 ppm} significantly mitigated these losses, reducing weight loss by 41% and spoilage by 65% on the 90th day of storage. These findings suggest lower metabolic activity and improved fruit preservation under this treatment.

This outcome supports previous research indicating that ALVG-based coatings reduce water vapor permeability, thereby slowing down moisture evaporation and respiratory rates²⁷. Given the critical role of moisture in maintaining fruit quality, its loss is a primary contributor to postharvest spoilage²⁸. However, in certain contexts, controlled dehydration has been shown to enhance flavor and overall product quality²⁹.

ALVG coatings were found to be useful in preventing weight loss in fruits like strawberries, which is in line with previous research³⁰. This action is mostly caused by a semi-permeable coating that forms on the fruit’s surface, sealing small surface wounds and lowering evaporation to stop water loss^31,32. When essential oils are incorporated into the gel, their hydrophobic properties further enhance the coating’s ability to prevent dehydration¹⁸. For example, by creating a barrier that prevents water evaporation, the application of 500 µL L⁻¹ of Mentha piperita L. essential oil considerably decreased weight loss in pomegranate arils³³. Chitosan and clove essential oil have been used to reduce water loss in pomegranate arils with similar results³⁴.

Uncoated fruits displayed the highest decay rates after 90 days, mainly due to increased microbial activity, which typically accelerates spoilage and reduces fruit quality³⁵. Essential oils possess antimicrobial properties that disrupt microbial cell membranes and interfere with key cellular functions, thereby helping to delay decay in perishable fruits like raspberries³⁶. Research has demonstrated that films made from guar gum, either by themselves or combined with essential oils, exhibit potent antimicrobial effects against harmful microorganisms^37,38. Moreover, coatings with higher antioxidant capacity not only protect tissues against physiological stress but also enhance resistance to microbial infection, further contributing to reduced spoilage³⁹.

pH, TSS, TA, and TSS/TA ratio

During storage time, pH levels slightly decreased overall, indicating increasing acidity; however, a general increasing trend was observed, but treatment with ALVG_25%-Es_{1000 ppm} helped maintain pH stability. All samples showed a consistent decline in titratable acidity (p ≤ 0.05), with treated fruits showing a 61% decrease by 90 days. The TSS/TA ratio increased during storage, and higher ratios in treated fruits reflected improved taste quality through a better sweetness–acidity balance.

Maintaining fruit quality during storage requires maintaining the pH at an ideal level. The pH level plays a role in how flavors are perceived, as acidity can alter the way sweetness and sugar are experienced; in some instances, increased acidity may lessen the direct perception of sugar but enhance the overall sense of sweetness, contributing to a balanced flavor profile⁴⁰. As fruits ripen, a slight rise in acidity typically occurs, which could explain the variations in pH between coated and uncoated fruits observed during the storage time⁴¹.

The conversion of starch to sugars during fruit maturation raises the amounts of total soluble solids (TSS), a process that is sped up by increased ethylene production⁴². Fruits coated with edible films show slower respiration rates and altered internal atmospheres characterized by reduced oxygen and ethylene and increased carbon dioxide, which helps limit the rise in TSS compared to uncoated fruits, especially toward the end of storage time^16,43. Throughout development, ripening, and postharvest stages, changes in plant tissue composition significantly influence fruit quality. The balance between soluble solids and acidity is crucial for determining harvest time and processing suitability, particularly for climacteric fruits that continue ripening after harvest. Fruit flavor depends on sugars, organic acids, phenolics, and aromatic compounds, all of which can diminish due to ripening, poor storage, or enzymatic activity. Despite providing essential nutrients and antioxidants, fruits and vegetables often suffer postharvest losses caused by physical damage, inadequate storage, or cold sensitivity⁴⁴. Fruits usually have a higher total soluble solids content during ripening and storage due to a decrease in moisture content and an increase in free sugar accumulation⁴⁵. In a study, also observed a similar increase in TSS in arils treated with flaxseed gum and lemongrass essential oil. Enhancing the guar gum coating with Mentha piperita L. essential oil and increasing the amounts of guar gum and essential oil help to maintain TSS values by creating a modified atmosphere, reducing the respiration rate, and possibly affecting the fruit’s metabolic processes through the action of the essential oil³³.

The main cause of TA in fruits is the existence of organic acids. As these acids are metabolized during respiration, a gradual decline in TA is typically observed throughout storage^45,46. This reduction occurs because organic acids are utilized as energy substrates during fruit ripening and senescence, supporting metabolic processes through the tricarboxylic acid cycle⁴⁷.

Vitamin C, Phenolic, and flavonoids content

Vitamin C content declined over time in all groups (p ≤ 0.05), with the control showing the greatest decrease; however, treatments notably slowed this degradation, enhancing antioxidant retention. Specifically, ALVG_25%-Es_{500 ppm} increased vitamin C by 54% by 90 days. Similarly, phenol and flavonoid levels decreased during storage time but remained higher in treated samples, contributing to improved antioxidant activity and extended storage life. On day 90, ALVG_25%-Es_{500 and 1000 ppm} boosted phenolic and flavonoid contents by 40% and 52%, respectively, relative to day 1. Vitamin C and phenolic compounds (TPC) were likely positively correlated with coated treatments, suggesting a protective role of edible coatings in preserving antioxidant compounds.

Vitamin C is an important measure of a fruit’s nutritional value because it acts as a powerful antioxidant⁴⁸. The primary cause of ascorbic acid reduction is oxidation, as this vitamin is susceptible to oxidative reactions in the presence of oxygen⁴⁹. Non-enzymatic antioxidants like phenolic and flavonoid compounds are essential for strengthening fruits’ antioxidant defenses against oxidative stress during ripening⁵⁰. By scavenging free radicals and stopping hydrogen peroxide from transforming into additional harmful radicals, these bioactive compounds help maintain the quality of fruit⁵¹. It is well known that essential oils extracted from medinical plants increase the activity of the enzyme phenylalanine ammonia-lyase (PAL), which in turn promotes the synthesis of phenolic compounds. Additionally, essential oils themselves possess antioxidant properties mainly due to their phenolic and flavonoid content⁵¹. Studies have demonstrated that applying ALVG on pineapples and oranges slows down the loss of ascorbic acid and inhibits its oxidation by limiting oxygen penetration into the fruit. However, it is notable that using high concentrations of the gel significantly decreased the ascorbic acid content, probably because elevated internal CO₂ levels within the fruit promote the breakdown of ascorbic acid⁴⁴.

Antioxidant activity and oxidative stress markers (ROS, CAT, MDA)

During storage time, ROS levels rose across all samples during storage time, but treated groups showed significantly lower ROS accumulation, demonstrating the protective effect of treatments against oxidative damage (p ≤ 0.05). While catalase (CAT) activity generally declined over time, treated samples maintained higher CAT levels, reflecting enhanced enzymatic defense. ALVG_25%-Es_{1000 ppm} increased CAT activity by 20% on day 90. Particularly in the control, MDA levels and proline content rose, suggesting increased oxidative stress. However, treatments significantly reduced MDA accumulation and moderated proline increases. Specifically, ALVG_25%-Es_{1000 ppm} reduced MDA levels and proline by 53% and 18%, respectively, by day 90.

Reactive oxygen free radicals can be eliminated during fruit aging by using vitamin C, POD, and CAT to effectively stop membrane lipid peroxidation. Fruit storability may be impacted by the concentrations of antioxidant chemicals and the activity of antioxidant enzymes⁵². According to recent studies, ALVG coatings play a significant part in strengthening fruits’ antioxidant defense systems during storage time. ALVG application, for instance, dramatically raised the activity of important antioxidant enzymes, such as catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX), while simultaneously lowering levels of reactive oxygen species (ROS), such as hydrogen peroxide and malondialdehyde (MDA), according to a study on guava fruit⁵³. ALVG coatings also helped maintain increased levels of CAT and peroxidase (POD) activity after a 20-day storage period at room temperature, according to a study on persimmon fruit. By day 20, coated fruits showed CAT activity approximately 1.25 times and POD activity 1.43 times greater than uncoated samples⁵⁴.

In a study, the effect of psyllium coating on apricot fruit enzymes during storage was investigated. The results showed that treatments regulated related enzymes activities such as catalase, peroxidase, superoxide dismutase, and phenylalanine-ammonia lyase⁵⁵.

Proline (Osmolyte compound)

Additionally, proline, an essential natural osmolyte and antioxidant is essential in stabilizing enzymatic activity and protecting cellular components from oxidative damage. Beyond its role in osmotic regulation, proline acts as a protective agent under oxidative stress by supporting enzymes such as CAT and POD^54,56. Studies have shown that treatment with methyl jasmonate and gamma-aminobutyric acid increases the activity of enzymes involved in proline synthesis in peach and Japanese quince fruit, facilitating proline accumulation. Finally, it increased proline content during storage⁵⁷. This effect is also observed in oxalic acid-treated honeydew melons under cold storage conditions, that proline content increases with prolonged exposure to low temperatures⁵⁸.

Protein content

Storage generally leads to protein degradation; however, treated samples showed better protein preservation. ALVG_25%-Es_{1000 ppm} led to a 22% increase in protein content by day 60. This may be due to ALVG’s protective effects on structural proteins and enzymes, minimizing oxidative breakdown⁵⁹. However, a decline in protein content was observed in the fourth month, which appears to be related to increased oxidative stress during storage that activates endogenous proteolytic enzymes, leading to protein degradation; proteomic studies on kiwifruit have shown that postharvest metabolic pathways associated with sugar and amino acid catabolism become more active, correlating with protein changes in the fruit⁶⁰.

Margi et al. developed an active edible coating on freshly cut pears by combining oxalic and citric acid in carboxymethyl cellulose and sodium alginate and showed that this treatment significantly reduced protein degradation and preserved fruit protein during storage⁶¹. According to the results of the study, it was observed that combining Aloe vera gel with peppermint essential oil and using them as a coating had a greater anti-spolage effect, antioxidant compounds, and flavor index than Aloe vera gel alone.

Microbial content (Total microbial and total mold and yeast counts)

Various studies have demonstrated the antimicrobial effects of ALVG, and peppermint essential oils. The antibacterial activity of ALVG has been demonstrated against both ram-positive and Gram-negative bacteria. The antibacterial compounds of the whole leaf of Aloe vera include anthraquinones and saponins. While the polysaccharides in the gel exert their antibacterial activity directly by stimulating phagocytic leukocytes to kill bacteria⁶². In a study, the main components of peppermint were carvone and limonene. Carvone is an oxygenated monoterpene and limonene is a monoterpene hydrocarbon. Most of the antimicrobial activity in these oils has been attributed to the oxygenated monoterpenes⁶³.

Conclusion

This study demonstrated how kiwifruit quality may be successfully maintained over a three-month storage period by incorporating ALVG and peppermint essential oil into a biodegradable edible covering. The treatment reduced spoilage and oxidative damage, while enhancing nutritional content and maintaining the fruit’s physicochemical properties during refrigerated storage time. Among the tested formulations, the ALVG_50%-Es_{1000 ppm} coating was most effective. This natural, environmentally friendly method offers a viable substitute for artificial preservatives and has great promise for the commercial postharvest handling of climacteric fruits, such as kiwifruit. To expand their application, future studies should look at how these organic, biodegradable coatings affect a greater range of climacteric and non-climacteric fruits. Incorporating advanced technologies such as nano-encapsulation of active ingredients may further improve the coatings’ effectiveness and durability. Although promising, some limitations exist. The production and application costs of these biodegradable coatings, particularly those with essential oils and complex formulations, may be higher than traditional methods, affecting commercial viability. Scaling up from lab to industry presents challenges, and differences among fruit types necessitate tailored formulations, requiring further research for widespread, cost-effective application.

Author contributions

A.S. was responsible for conceptualizing and designing the experiments, laying the foundation for the research. A.K. handled the practical execution of the experiments, ensuring they were carried out according to the planned design. S.A. conducted the data analysis, interpreting the results and deriving key insights. The drafting and proofreading of the main manuscript were performed by S.A. and F.S., who collaborated to ensure clarity and accuracy. All authors reviewed the final manuscript and approved it for publication, indicating their agreement with the content and findings presented in the study.

Data availability

The data used in this study is openly available, and the data used are available upon request from the corresponding authors.

Declarations

Competing interests

The authors declare no competing interests.

Compliance with ethical standards

This article does not contain any studies involving animals or human participants as research subjects.

Consent for publication

All authors approved this manuscript before submission.