Extending the storage life of Mexican lime through guar gum, jojoba oil, and oleic acid-based coatings

Abstract

Background

Chilling injury is a major factor reducing the postharvest quality and shelf life of fresh fruits. Edible coatings have proven to be a convenient approach for fruit preservation. This research evaluated guar gum (GG), jojoba oil (JO), oleic acid (OA), and their combination on the postharvest quality of Mexican lime (Citrus aurantifolia) stored at 8 ± 1 °C for 80 days.

Results

The results revealed that the edible coatings reduced weight loss and decay after 80 days of storage. GG/OA decreased weight loss by 32.51% and decay by 54.12% as compared to the control. GG/JO (57 N) and GG/OA (61 N) were the most effective treatments for maintaining firmness during storage. Compared with the control, all the treatments delayed the development of chilling injury. GG/JO most effectively reduced malondialdehyde (MDA) levels by 18.69%. GG/JO and GG/OA reduced electrolyte leakage (EL) by 13.08% and 15.44%, respectively. Additionally, GG/OA had the greatest effects on maintaining chlorophyll (0.36 mg100g^-1FW), total phenol (136 mgGA100g^-1FW), and flavonoid (2.30 mgQ100g^-1FW). Also, all coated fruits exhibited higher total soluble solids content (TSS) and titratable acidity (TA) compared with the control (4.77% and 4.30%).

Conclusions

The GG/OA and GG/JO composite coating were the most effective treatments, significantly improving the quality of Mexican lime under low-temperature conditions. These results highlight its potential as a practical, biodegradable, and consumer-safe postharvest treatment.

Background

Mexican lime (Citrus aurantifolia cv. Mexican lime) is a well-known citrus fruit belonging to the Rutaceae family, widely used in the food and beverage sector due to its distinctive flavor, acidity, and aroma [1]. The fruit is highly perishable and experiences 18–25% postharvest losses, primarily due to rapid moisture loss, decay, and sensitivity to low-temperature storage [2]. Cold storage is commonly employed to slow metabolic processes; however, temperatures below the species-specific threshold induce chilling injury (CI) symptoms such as peel discoloration, textural degradation, and increased susceptibility to microbial infection, which compromise fruit quality and market appeal [3, 4].

Edible films and coatings have emerged as safe, effective, sustainable, and high-potential alternatives to extend fruits’ shelf and storage life. These coatings act as semipermeable barriers that regulate gas exchange, reduce transpiration, delay oxidative damage, and maintain fruit firmness and biochemical integrity. Among the different coating materials, polysaccharide-based gums are widely recognized for their film-forming capacity, biodegradability, and ability to maintain postharvest quality [5–7]. Previous studies have shown the ability of edible coatings, including xanthan gum [8], coatings derived from wild sage seeds [9], and pectin [10], to improve the quality and storability of lime fruits.

Guar gum (GG) is a galactomannan derived from Cyamopsis tetragonolobus, exhibits favorable characteristics, including high molecular weight, film-forming ability, biocompatibility, and reported antimicrobial activity [11, 12]. Previous studies have demonstrated the effectiveness of GG-based coatings in extending the shelf life of mango [13, 14], Valencia orange [15], and sweet cherry [16].

Despite their specific physical and chemical characteristics, polysaccharide coatings alone typically exhibit high water-vapor permeability and suboptimal mechanical stability. Therefore, combining polysaccharides with hydrophobic compounds has gained attention as a strategy to enhance coating functionality. Oleic acid (OA), a monounsaturated fatty acid, is the main component of olive oil [17], enhances the water-barrier properties of hydrophilic films by reducing the water vapor permeability due to its lipid nature and liquid form at ambient temperature. It is highly miscible with polysaccharide matrices, such as chitosan [18] and Tara gum [19]. Additionally, incorporating an optimal amount of OA can improve the antimicrobial activity of chitosan coatings and influence fruit quality [18].

Jojoba oil (JO), extracted from the Simmondsia chinensis plant, is a liquid wax rich in tocopherols and long-chain esters, also exhibits strong oxidative stability and forms a protective hydrophobic layer that reduces moisture loss and microbial penetration [20, 21]. JO creates a protective layer on the fruit surface, which helps reduce moisture loss and prevents microbial penetration in stored fruits. JO has been shown to improve the quality of mango [22], guava [23] and peach [24] fruits by slowing water loss, improving the external skin layer, and regulating gas permeability and the transpiration rate.

For Mexican lime, previous studies have evaluated several coating materials yet there is limited evidence on the effectiveness of polysaccharide–lipid composite coatings in reducing CI and preserving biochemical quality during prolonged cold storage. Furthermore, no study to date has compared the combined effects of GG with JO or OA on CI suppression in lime under low-temperature conditions. Therefore, this study aimed to evaluate the effectiveness of these formulations in maintaining the physicochemical, biochemical, and visual quality of Mexican lime at cold storage. Particular attention was given to chilling injury development, lipid peroxidation, antioxidant retention, and overall storability to identify the most effective coating system for preserving lime quality under low-temperature conditions.

Materials and methods

Raw materials

Mexican lime (Citrus aurantifolia cv. Mexican lime) at physiological maturity (ºBrix = 10, color index = green light) was purchased from a commercial orchard in Jahrom, Fars Province, Iran (28° 29’ 39” N/53° 33’ 29” E). To evaluate all tested traits, fruits were transported to the postharvest physiology laboratory, University of Hormozgan, Bandar Abbas, Iran, within 4 h under shaded conditions and immediately sorted for uniformity in size, color, and maturity. Only fruits free from defects, mechanical injuries, or disease symptoms were selected for the experiment. Food-grade GG (Sigma Aldrich, UK), OA (Merck, Germany), JO (Samana Oils, Iran), and glycerol (Merck, Germany) were used to prepare the edible coatings.

Preparation of treatments

The optimal concentration of GG (2%) was determined through preliminary experiments. A 2% (w/v) GG solution was prepared by dissolving 20 g of GG powder in 1000 mL of distilled water under continuous stirring at 60 °C for 30 min to ensure complete hydration. Subsequently, 20 mL glycerol (plasticizer) and 10 mL Tween 80 (emulsifier) were added. The mixture was homogenized until fully incorporated and then cooled to room temperature [25]. To prepare the JO emulsion (1% v/v), 10 mL of JO was dispersed in 5 mL of Tween 80, and the volume was adjusted to 1000 mL with distilled water. The mixture was homogenized for 30 min to obtain a stable oil-in-water emulsion. OA coating (1% v/v) was prepared by emulsifying 10 mL of OA in 5 mL of Tween 80, then diluting with distilled water to 1000 mL and homogenizing for 30 min [18]. Composite coatings were formulated by mixing the prepared GG solution with JO or OA emulsion at a 1:1 ratio (v/v) under continuous stirring at room temperature to ensure homogeneity. Coatings were applied immediately after preparation to ensure freshness and optimal adhesion to the fruit surface.

Fruit sanitization and treatment application

A total of 1080 lime fruits were randomly divided into six treatments (three replications, with nine fruits per replication): control (distilled water), GG, JO, OA, GG/JO, and GG/OA. Before treatment, fruits were sanitized by dipping them in a 1% sodium hypochlorite solution for 1 min, rinsing with distilled water, and air-drying at room temperature. The fruits were subsequently immersed in the respective coating solutions for two minutes. After treatment, the fruits were air-dried at room temperature. Afterward, each treatment was individually packaged in one-kilogram perforated polyethylene containers in which 12 holes (two holes on each side of the container) of equal size were made to ensure air flow and stored at 8 ± 1 °C with a relative humidity of 90 ± 5%. The first evaluation was performed one day after treatment; subsequent analyses were performed at 20, 40, 60, and 80 days. To simulate marketing conditions, fruits from each treatment were held at room temperature for 24 h before quality assessments.

Weight loss

To measure weight loss, the initial weights of the samples (W₀) and their weights at 20, 40, 60, and 80 days (W_t) were recorded, and the WL (%) at each time point was determined via the following formula: WL (%) = [(W₀ − Wₜ)/W₀] × 100.

Firmness

The firmness (N) was evaluated via an Instron Universal Testing Machine (Model 53205, Turoni, Italy). Two points on each fruit were selected; an 8 mm-diameter probe was pressed into the fruit to a depth of 3 mm at a constant speed of 5 mm/s, and the required force (N) was recorded.

Decay index

The decay index (%) was quantified as the percentage of rotted lime fruits (with visible mycelial growth and fungal or microbial spoilage on the surface) in each treatment, calculated using the following formula: decay index (%) = (number of decayed fruits at the time of sampling/number of initial fruits) × 100.

Chilling injury

The occurrence of CI in lime fruit was assessed by counting the total number of fruits displaying CI symptoms, which included irregular brown lesions, yellow water-soaked spots, or even sunken areas on the fruit peel. Each fruit was scored according to the severity of chilling symptoms, and the CI was calculated using a standard scale [26].

Electrolyte leakage

Electrolyte leakage (EL) was assessed using 4 g of fresh lime peel, which was soaked in 40 mL of distilled water and shaken for 4 h at room temperature. The initial EL level (EL_i) was measured via a digital electrical conductivity meter (Weinheim, Germany). The samples were subsequently boiled at 100 °C for 30 min and cooled to 20 °C, after which the secondary EL (EL_s) was recorded. EL (%) was determined via the following formula: ([(EL_s - EL_i)/EL_s] × 100) [27].

Malondialdehyde content

Lipid peroxidation was evaluated by measuring the malondialdehyde (MDA) content following the method described by Pasquariello et al., [28]. In this procedure, 1 g of lime peel was blended with 2 mL of 50 mM phosphate buffer (pH 7.2) and then centrifuged (15 min at 14,000 × g and 4 °C). Subsequently, 1000 µL of a 20% trichloroacetic acid solution containing 0.5% thiobarbituric acid was added to 1000 µL of the supernatant. The mixture was heated at 95 °C for 30 min, then allowed to cool to room temperature and centrifuged again (5 min at 1000 × g and 4 °C). The absorbance of the reaction mixture was measured at 532 and 600 nm (microplate spectrophotometer, Epoch, Bio-Tek, USA) and was expressed as µmol 100 g^-1 FW.

Chlorophyll and carotenoid content

Total chlorophyll and carotenoid contents were determined according to the method described by Lichtenthaler [29]. Briefly, 0.5 g of fresh lime peel tissue was homogenized in a mortar with 5 mL of 80% (v/v) acetone. The extract volume was adjusted to 25 mL with 80% acetone and centrifuged at 4,800 × g for 20 min. The supernatant was used for the determination of photosynthetic pigments. Absorbance was measured at 663, 645, and 470 nm using a spectrophotometer. Total chlorophyll and carotenoid contents were expressed as mg 100 g⁻¹ fresh weight.

Ascorbic acid

The ascorbic acid content was measured with slight modifications according to the method described by O’Grady et al., [30]. One milliliter of fruit juice was mixed with 9 mL of metaphosphoric acid (1%), shaken for 1 min, and sonicated in cold water for 3 min. Then, 1 mL of the resulting mixture was blended with 4 mL of 2,6-dichlorophenolindophenol solution (0.0025%) and incubated in the dark for 10 min. The absorbance was measured at 515 nm (Epoch, Bio-Tek, USA). Ascorbic acid was expressed as mg ascorbic acid per 100 g of fresh weight (mgAsA100 g^-1 FW) on the basis of an ascorbic acid standard curve (0–500 mg L^-1).

Total phenol and flavonoid contents

The total phenol content (mgGA100g^-1FW) was assessed calorimetrically on the basis of a standard curve of gallic acid [31]. Lime juice methanolic extract (300 µL) and 10% diluted Folin–Ciocalteu reagent (1500 µL) were mixed and incubated for 5 min at room temperature. Then, 7% sodium carbonate (1200 µL) was added, and the mixture was placed on a shaker in the dark for 90 min at room temperature. The absorbances of the samples and the gallic acid standards were read at 750 nm (Epoch, Bio-Tek, USA).

The aluminum chloride colorimetric method and the standard curve of quercetin (0–500 mg/L) were used to assess the flavonoid content [32]. Lime juice (500 µL) was mixed with 85% methanol (1200 µL), 10% aluminum chloride (100 µL), 1 M potassium acetate (100 µL), and distilled water (2800 µL). The mixture was subsequently incubated in the dark at ambient temperature for 30 min. The absorbances of the samples and the quercetin standards were measured at 414 nm (Epoch, Bio-Tek, USA). The flavonoid content was expressed as mg quercetin per 100 g of fresh weight (mgQ100g^-1FW) on the basis of the quercetin standard curve (0–500 mg L^-1).

Antioxidant activity

The free radical scavenging DPPH (2,2-diphenyl-1-picrylhydrazyl) method was used to evaluate the antioxidant activity of lime fruit. Lime juice methanolic extract (100 µL) was mixed with 900 µL of DPPH (0.1 mM) and incubated for 30 min in the dark at ambient temperature. The absorbances (A) of the samples and the control were measured at 414 nm (Epoch, Bio-Tek, USA). The antioxidant activity (%) of the extracts was calculated via the following formula: ([(A Control − A Sample)/A Control] × 100) [33].

Total soluble solids (TSS) content, titratable acidity (TA), and pH

TSS, TA, and pH were measured to monitor changes in organic acids, flavor, and physiological responses during storage. TSS and TA represent total soluble solids and total organic acids, respectively, while pH reflects acidity strength and microbial stability. Together, they indicate the effectiveness of edible coatings in reducing metabolic activity and maintaining fruit quality. The TSS (%) was determined via a digital refractometer (DBR95, Taiwan), and the TA (%) was determined via titration of the diluted fruit juice (1/5 mL) to pH 8.2 with 0.1 N NaOH.

Phenylalanine ammonia-lyase, polyphenol oxidase, and peroxidase activity

Phenylalanine ammonia-lyase (PAL) activity was estimated according to the method described by Kováčik and Klejdus [34] with slight modifications. Fresh lime peel samples (0.3 g) were immediately frozen in liquid nitrogen and homogenized in 2 mL of cold sodium borate buffer (pH 8.8) containing beta-mercaptoethanol as a stabilizing agent. The homogenate was immediately centrifuged at 12,000 × g for 15 min at 4 °C to separate the supernatants containing the enzyme. The reaction mixture, consisting of 500 µL of buffer, 350 µL of homogenate, and 300 µL of 50 mM L-phenylalanine, was incubated at 40 °C for 60 min. The reaction was stopped by adding 50 µL of 5 N HCl, and the absorbance was measured spectrophotometrically at 290 nm (Epoch, Bio-Tek, USA).

Fresh lime peel samples (1 g) were immediately frozen in liquid nitrogen and homogenized in 5 mL of cold potassium phosphate buffer (50 mM, pH 7.0). The homogenate was immediately centrifuged at 12,000 × g for 15 min at 4 °C to separate the supernatants containing the enzymes for polyphenol oxidase (PPO) and peroxidase (POD) assessment. The reaction mixture used to assess PPO activity included 500 µL of buffer, 100 µL of supernatant, 200 µL of pyrogallol (0.02 M), and 2.7 mL of potassium phosphate buffer (50 mM, pH 7.0). The absorbance was measured at 420 nm (Epoch, Bio-Tek, USA), and the enzyme activity was expressed as UL^-1 [35]. A reaction mixture including the supernatant (300 µL), 100 µL guaiacol (4%), µL H₂O₂ (1%), and 2.77 mL potassium phosphate buffer (50 mM, pH 7) was used to assay POD activity [36]. The sample’s absorbance was read at 470 nm (Epoch, Bio-Tek, USA), and the enzyme activity was expressed as UL^-1.

Statistical analysis

A factorial experiment was conducted with a complete randomized design (CRD). The factors included six treatments (three replications and nine fruits per replication) and five storage times. Statistical analysis of the data was carried out via SAS software (version 9.1). Mean comparisons were performed via the least significant difference (LSD) test to determine significant differences among treatments at P < 0.05. Heatmap clustering analysis was conducted with R statistical software (R.3.3.2).

Results

Weight loss

The WL of the fruit was significantly affected by the treatments during the storage period (20–80 days) (Fig. 1A). After 20 days, the control treatment had the maximum WL (5.03%), while the GG/OA treatment had the minimum percentage (3.21%). On day 40, the OA treatment resulted in the highest WL (10.19%), whereas the GG/JO treatment resulted in the lowest percentage (6.87%). On day 60, the control treatment had the highest WL (9.99%), and the GG/OA coating had the lowest value (7.32%). Finally, the control presented the highest WL (14.12%), while the lowest was observed in the GG/OA treatment (9.53%).

Fig. 1.

Interaction effect of edible coatings and storage time on A weight loss and B firmness in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Decay

Our analysis revealed a significant effect of storage time on the probability of fruit decay in Mexican lime (P < 0.05). The estimated log odds of decay increased over time, indicating an increasing decay rate during storage. After 20 days, the estimated log odds ratio was − 1.82, corresponding to a mean decay probability of 13.9%. This probability reached 29.1%, and the increase was significant until 40 days. On day 60, the mean decay probability reached 44.3%. The estimated mean probability of decay ultimately reached 54.9%, which was not significantly different from that on day 60 (p = 0.24) (Table 1). Additionally, on the basis of the results of the GLIMMIX model with a logit link function, different coating treatments produced significant differences in controlling fruit decay (P < 0.05). The actual probability of decay in fruits treated with edible coatings was lower than that in the control. The combined treatment of guar gum + oleic acid resulted in the lowest decay rate, with a probability of 26.29%, which was not significantly different from that of the other coating treatments. Moreover, the control treatment resulted in the highest decay rate of 31.57% (Table 2).

Table 1.

Logit estimates and mean decay probabilities (± SE) of Mexican lime fruit during storage

Storage Time (day)	Estimate ± SE	Pr > |t|	Mean ± SE
20	-1.82 ± 0.24	< 0.0001	13.93 ± 0.03 c
40	-0.89 ± 0.18	< 0.0001	29.11 ± 0.04 b
60	-0.23 ± 0.16	0.16	44.28 ± 0.04 a
80	0.20 ± 0.16	0.24	54.88 ± 0.04 a

Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Table 2.

Logit estimates and mean decay probabilities (± SE) of Mexican lime fruit under different edible coating treatments during storage

Treatment	Estimate ± ER	Pr > |t|	Mean ± ER
Control	0.29 ± 0.21	0.160	57.31 ± 5.05 a
Guar gum	-0.82 ± 0.23	0.001	30.57 ± 4.95 b
Jojoba oil	-0.93 ± 0.23	0.000	28.26 ± 4.73 b
Oleic Acid	-1.03 ± 0.24	< 0.0001	26.29 ± 4.64 b
Guar Gum/ Jojoba oil	-0.82 ± 0.23	0.001	30.59 ± 4.94 b
Guar Gum/ Oleic Acid	-0.81 ± 0.23	0.001	30.78 ± 4.91 b

Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Firmness

The results revealed that treatment, storage time, and their combination had a significant effect on firmness (P < 0.05). A decreasing trend in firmness was observed for all the treatments throughout the storage time, while the highest values were recorded on the first day after treatment, and the lowest values were recorded on day 80. The control exhibited a notable decline in firmness from 78 N to 41 N over time. The GG/OA maintained the greatest firmness among the treatments, decreasing from 79 N on day 1 to 61 N on day 80. Additionally, the firmness of the plants in the other treatment groups was significantly greater than that of the control plants (Fig. 1B).

Chilling injury

As shown in Fig. 2A, no CI symptoms were visible during the first 20 days of storage. After this period, however, CI became progressively more apparent in all the treatment groups as the storage time increased. By day 40, the GG/JO treatment had the most severe CI, whereas the GG/OA treatment had the least severe CI, with a considerable difference of over 50% between the two. When storage was extended to 60 and 80 days, the untreated fruits (control) presented the greatest CI severity. In contrast, all the coated samples experienced notably less damage. Interestingly, the GG-treated fruits presented the lowest CI on day 60, whereas on day 80, the best results were observed in the GG/JO group. These findings suggest that applying edible coatings can be valuable in minimizing chilling-related damage during extended cold storage.

Fig. 2.

Interaction effect of edible coatings and storage time on A chilling injury, B simple effect of storage time and C treatments on electrolyte leakage, and D Malondialdehyde in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Electrolyte leakage

Analysis of variance indicated that only the main effects of storage time and treatment were statistically significant for EL, whereas their interaction effect was not significant (P < 0.05). As shown in Fig. 2B, EL increased significantly over the storage period, reaching a maximum of 99.87% on day 80. Moreover, all coating treatments significantly reduced the EL compared with that of the control. Among these treatments, the GG/JO and GG/OA treatments resulted in the greatest reductions, with EL values 15.44 and 13.07% lower than those of the control, respectively (Fig. 2C).

Malondialdehyde content

As shown in Fig. 2D, the MDA levels increased progressively during storage. The most significant increase in the MDA content was observed in the untreated fruits. Initially, no significant differences were detected among the treatments. After 20 days of storage, fruits treated with GG presented the smallest increase in the MDA content (40.05%), whereas OA-treated fruits presented the greatest increase (51.46%). By day 40, all the treatments except JO had lower MDA levels than did the control. After 60 days, the highest MDA concentration (2.35 µmol100g^-1 FW) was recorded in untreated fruit, whereas the GG and GG/OA combination had the lowest MDA. At the end of the storage period, the lowest and highest MDA contents were found in the GG/JO (2.23 µmol100g^-1 FW) and untreated fruits (2.75 µmol100g^-1 FW) respectively.

Chlorophyll and Carotenoid Contents

As shown in Fig. 3A, the total chlorophyll content gradually decreased as storage progressed. At the beginning of the storage period, there were no significant differences among the treatments. However, by day 20, fruits in the untreated fruit (0.58 mg100g^-1 FW), GG (0.55 mg100g^-1 FW), and JO (0.57 mg100g^-1 FW) treatments retained more chlorophyll than the other fruits did, although the differences were not statistically significant. By day 60, there were no significant differences among the treatments. The GG/OA- and GG/JO-treated fruits maintained higher chlorophyll contents than did the other treatment groups. On the other hand, the carotenoid content generally tended to increase throughout the storage period, suggesting that as the chlorophyll content decreased. No significant differences were detected among the treatments at the beginning of storage. On days 20, 40, and 60, the GG, OA, and combined treatment groups presented the lowest carotenoid levels. By the end of the storage period, fruits treated with GG (0.76 mg100g^-1 FW) or GG/OA (0.71 mg100g^-1 FW) presented lower carotenoid contents than those in the other treatments did (Fig. 3B).

Fig. 3.

Interaction effect of edible coatings and storage time on A total chlorophyll and B carotenoid content in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Ascorbic acid

As shown in Fig. 4, the ascorbic acid content in all the treatments decreased significantly over the 80-day storage period. Storage time, treatment, and their interaction had significant effects on ascorbic acid retention (P < 0.05). On day 1, the highest value was recorded in fruits treated with GG (50.25 mgAsA100g^-1 FW), whereas the lowest values were observed in the control and JO treatments (both 48.75 mgAsA100⁻¹ g FW). By day 20, a noticeable reduction was observed across all the treatments. The GG coating maintained the highest level (48.92 mgAsA100g^-1 FW), which was significantly greater than that of the control. The lowest value at this stage was detected in the GG/JO treatment (40.58 mgAsA100g^-1 FW). On day 40, the GG/OA treatment resulted in the highest amount of ascorbic acid (48.08 mg), followed by the GG treatment (46.42 mgAsA100g^-1 FW). By day 60, the GG/OA treatment resulted in the highest level of ascorbic acid (39.59 mg), whereas the control sharply decreased to 34.01 mg. On day 80, the lowest ascorbic acid content was found in the control (29.85 mgAsA100g^-1 FW). Fruits treated with GG/OA (35.10 mgAsA100g^-1 FW) or GG/JO (mgAsA100g^-1 FW) presented significantly greater levels, with no significant difference between the two combined treatments.

Fig. 4.

Interaction effect of edible coatings and storage time on ascorbic acid content in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

Total phenol and flavonoid contents

As shown in Fig. 5A, the TPC of all the treatments decreased significantly over the 80-day storage period (P < 0.05). Initially, there were no significant differences among the treatments. However, by day 20, the phenolic content of the GG-treated fruit (181.78 mgGA100g^-1 FW) substantially decreased, whereas the GG/OA-treated fruit presented the highest phenolic content (243.61 mgGA100g^-1FW). On day 40, the GG/OA treatment resulted in the highest TPC (256.33 mgGA100g^-1 FW), followed by the GG/JO (231.37 mgGA100g^-1FW) and GG (207.33 mgGA100g^-1 FW) treatments. On the other hand, the control and JO treatments had the lowest values (181.45 and 176.33 mgGA100g^-1FW, respectively). By day 60, GG/OA (170.67 mgGA100g^-1 FW) and GG/JO (165.45 mg) maintained significantly greater levels than did the control (120.94 mgGA100g^-1 FW). At the end of storage (day 80), the GG treatment had the lowest TPC (72.22 mgGA100g^-1 FW), whereas the GG/OA treatment had the highest TPC (133.56 mgGA100g^-1FW), followed by the GG/JO (108.95 mgGA100g^-1 FW) and GG (103.56 mgGA100g^-1 FW) treatments.

Fig. 5.

Interaction effect of edible coatings and storage time on A total phenol, B flavonoid, and C antioxidant activity in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

The data in Fig. 5B show that storage time, treatment, and their interaction significantly influenced the flavonoid content (P < 0.05). The changes in flavonoid levels closely matched those in TPC. On the first day, there were no significant differences among the treatments. By day 60, however, all the treatments except for GG/OA and GG/JO resulted in a significant decline in flavonoid content. After 80 days, the GG/OA treatment resulted in the maximum flavonoid content and the lowest reduction (1.55%) compared with the initial value.

Antioxidant activity

The results of the antioxidant activity measurements over the 80-day storage period indicated that storage time, treatment, and their interaction had significant effects on the antioxidant activity (P < 0.05). On the first day, no significant differences were observed among the treatments. On day 20, the antioxidant activity significantly decreased in all the treatments except for the OA treatment, which presented a 3.83% increase and presented the highest activity (73.23%), which was significantly greater than that in the other treatments. The antioxidant activity in the OA treatment remained the highest on day 40 (69.61%). The declining trend continued through day 60, when the OA and control treatments presented the highest (48.76%) and lowest (29.57%) percentages, respectively. At the end of the storage period, the control treatment had the lowest antioxidant activity (20.63%), while the OA treatment had the highest value (38.16%) (Fig. 5C).

Total Soluble solids content, titratable acidity, and pH

Storage time, treatment, and their interaction significantly influenced the TSS, TA, and pH values (P < 0.05). As shown in Fig. 6A, by day 20, the TSS content had decreased in all the treatments compared with that on the first day, except for the GG/OA ratio. By day 40, a significant increase in TSS was recorded in most treatments, and the OA (11.09%) and GG/JO (10.48%) treatments presented the highest TSS values. During the later stages of storage (60 and 80 days), all the treatments presented higher TSS levels than did the control, while the GG/OA-treated fruits presented significantly greater TSS contents than did those in the other treatments.

Fig. 6.

Interaction effect of edible coatings and storage time on A total soluble solids (TSS), B titratable acidity (TA), and C pH in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD’s test

As shown in Fig. 6B, TA decreased significantly over the storage period. Initially, there were no significant differences among the treatments. By day 20, TA levels had decreased in all the samples, but the JO treatment still had the highest value (7.8%), which was significantly greater than that of the control (6.1%). On day 40, the control had the highest TA (6.8%), while the OA treatment had the lowest (5.3%). After 60 days, the GG treatment resulted in the lowest TA (5.5%), whereas the GG/OA treatment resulted in the highest value (7%). Finally, the control had the lowest TA (4.3%), while the OA treatment had the highest value (5.5%). However, there were no significant differences among the treated fruits. As depicted in Fig. 6C, there were no significant differences in pH among the treatments at the start of the experiment. After 20 days of storage, the highest pH was recorded in the JO-treated fruit. At 40 and 60 days, the highest pH values were observed in the control and GG-treated fruits, with no significant differences between them. However, by the end of the storage period, these two treatments presented significantly lower pH values than did the other treatments.

Polyphenol oxidase, phenylalanine ammonia lyase, and peroxidase activity

Analysis of variance revealed that only the treatment effect on PPO and PAL activities was statistically significant (P < 0.05). In contrast, the main effects of time, treatment, and their interaction significantly influenced POD activity (P < 0.05). As shown in Fig. 7A, all treated fruits presented lower PPO activity than did the control, with the GG/OA treatment resulting in the lowest PPO activity. In contrast, all the treated fruits presented greater PAL activity than did the control fruits. The results revealed that, compared with the JO and GG/JO treatments, the GG treatment was more effective at increasing PAL activity, although this difference was not statistically significant (Fig. 7B).

Fig. 7.

The simple effect of treatments on A phenylalanine ammonialyase, and B polyphenol oxidase, and C the interaction effect of edible coatings and storage time on peroxidase in Mexican lime fruit stored at 8 °C plus 1 day at ambient temperature. Similar letters indicate a non-significant difference at 5% level of probability using LSD's test

The POD activity in all the treatments significantly decreased over the 80-day storage period. On day 1, there were no significant differences among the treatments. By day 20, all the treatments resulted in a significant decrease in POD activity. At this time, the GG/OA treatment had the highest POD activity (69.67 UL⁻¹). On day 40, the JO treatment resulted in the highest POD activity (65.32 UL^-1). The decrease in POD activity continued through day 60; at that time, the JO treatment resulted in the highest activity at 61.77 UL⁻¹, whereas the control treatment resulted in the lowest activity at 36.04 UL⁻¹. By the end of the storage period on day 80, the GG/OA treatment had the highest POD activity (46.71 UL⁻¹), which was not significantly different from that of the OA treatment (42.09 UL⁻¹) (Fig. 7C).

Visual acceptability

The overall visual acceptability of fruits decreased over the 80-day storage period (Fig. 8). During storage, the treatments that showed the highest levels of decay and mold symptoms had the lowest visual acceptability. On the other hand, the number of decayed fruits in the coating treatments that were used alone was lower compared to the combination treatments. At the end of day 80, the highest and lowest visual acceptability were observed in the GG/OA and control treatments, respectively.

Fig. 8.

Effect of different coatings: Guar gum (GG), Jojoba oil (JO), Oleic acid (OA), GG/OA, GG/JO, and control on overall visual acceptability of Mexican lime fruit cold stored at 8 ℃, for 80 days of storage

Correlation

As shown in Fig. 9, the statistical relationships between the quality traits of the Mexican lime fruit were illustrated through correlation analysis which revealed significant positive and negative relationships among the measured traits. Stress and senescence related parameters, including PPO, decay index, CI, WL and EL were strongly positively correlated with each other (r > 0.70). In contrast, fruit firmness, TSS, ascorbic acid, total phenolics, total flavonoids, and antioxidant activity exhibited strong positive correlations with one another. The strongest positive correlation was observed between total flavonoids and total phenolics (r = 0.97), indicating their close association during storage. Most quality related traits were negatively correlated with membrane damage indicators. Electrolyte leakage showed strong negative correlations with fruit firmness, antioxidant activity, total phenolics, and total flavonoids. Likewise, chilling injury was negatively correlated with fruit firmness, TSS, total flavonoids, and ascorbic acid. PPO activity also exhibited strong negative correlations with fruit firmness (r = − 0.83) and TSS (r = − 0.86).

Fig. 9.

Correlation between Mexican lime fruit quality properties. The color gradient ranging from red to blue represents correlation values between -1 and +1. Weight loss (WL), Firmness (F), decay index (DI), chilling injury (CI), electrolyte leakage (EL), Malon dialdehyde (MDA), Chlorophyll (Chl), Carotenoid (CAR), acid ascorbic (AsA), total phenol (TP), flavonoid (FLV), antioxidant activity (AA), total soluble solids content (TSS), Titratable acidity (TA), pH, peroxidase activity (POD), polyphenol oxidase (PPO) and Phenylalanine Ammonialyase (PAL)

Hierarchical cluster analysis grouped the variables into two major clusters. The first cluster consisted primarily of stress and senescence related parameters, including MDA, decay, CI, PPO, WL, carotenoids, TA and EL, whereas the second cluster comprised quality related attributes, including fruit firmness, TSS, ascorbic acid, total phenolics, total flavonoids, and antioxidant activity. This clustering further supports the antagonistic relationship between oxidative damage and fruit quality preservation during cold storage.

Discussion

During storage, fruits inevitably lose weight due to moisture transfer through transpiration and evaporation, as well as respiratory water losses. In the present study, the application of GG, OA, JO, and their combinations demonstrated varying efficiencies in reducing WL, with the GG/OA coating exhibiting the greatest effect. This reduction can be attributed to the formation of a compact gel-like matrix that limits water migration from the fruit surface and partially restricts O₂ and CO₂ exchange, thereby lowering respiration intensity and subsequent moisture loss [15]. Similar findings have been reported for GG-based coatings such as GG and GG/Eremurus in pomegranate [37], GG/Aloe vera in mango [13], GG/chitosan in mushroom [38], and GG/ginseng extract in sweet cherry [16], where the formation of a cohesive polymeric network markedly decreased WL.

OA, as a hydrophobic fatty acid, further enhances the barrier properties of coatings by forming a thin water-resistant layer that reduces transpiration-driven losses [18, 38]. Previous studies have also shown that incorporating OA into biopolymer coatings such as chitosan [18] or Arabic gum [27] improves film compactness and reduces pore formation, which strengthens the moisture barrier. The superior performance of the GG/OA coating in our study likely results from a synergistic interaction between the hydrophilic gum matrix and the hydrophobic OA domains, producing a more homogeneous and less permeable structure. Taken together, these mechanisms explain the pronounced reduction in WL observed in coated fruits [18]. Furthermore, the use of pea starch and GG blended with a lipid mixture (shellac and OA) in ‘Valencia’ oranges was the most effective at reducing the fruit respiration rate, ethylene production, and WL during storage [15]. Similarly, the application of JO-based coatings has demonstrated notable reductions in WL in fruits, such as peach [24] and pear [39], which is mainly attributed to the integration of liquid lipids into polysaccharide matrices, improving their hydrophobicity and reducing water diffusion [40].

Fruit decay is a major contributor to postharvest losses and is predominantly associated with fungal development that is favored by high moisture levels, abundant nutrients, and acidic pH conditions [41]. In recent years, considerable attention has been directed toward the development of eco-friendly biodegradable materials capable of reducing microbial activity on fruit surfaces [42, 43]. Consistent with previous studies, our results showed that GG and edible oil-based coatings effectively decreased decay incidence, which can be attributed to their ability to create semi-permeable barriers that restrict oxygen diffusion, reduce respiration, and delay the onset of senescence—conditions that are unfavorable for pathogen proliferation. Accordingly, the use of gum Arabic, JO, and moringa oil on pear [39] and pea starch and GG blended with a lipid mixture (shellac and OA) on ‘Valencia’ oranges [15] generally reduces the fruit decay rate. Plant-derived oils, particularly JO, have been shown to enhance microbial resistance in products such as peach [24], mango [22], and guava [23] by inhibiting ethylene action, lowering PPO-mediated browning reactions, and reducing oxidative processes that promote tissue breakdown. JO may further contribute to decay reduction by increasing phenolic content, which strengthens the fruit’s natural defense system against microbial attack [24]. In addition, documented antibacterial activity of JO against pathogens such as Escherichia coli, Klebsiella species, and Staphylococcus aureus provides additional support for its role as a natural preservative. Taken together, these mechanisms explain the lower decay percentage observed in JO- and GG-based coatings in the present study [40].

Firmness is closely associated with the structural stability of cell walls and the maintenance of cellular turgor. Softening during storage typically results from enzymatic depolymerization of pectic substances and hemicellulose, accompanied by moisture loss that reduces turgor pressure. Edible coatings can mitigate these processes by creating semi-permeable barriers that reduce water loss, slow down the activity of cell-wall–degrading enzymes such as polygalacturonase and pectin methyl esterase [44], suppress respiration rates, thereby delaying ripening [15]. Our results demonstrated a decrease in firmness in Mexican lime fruit throughout storage; however, the magnitude of softening was significantly lower in coated fruits, particularly those treated with GG- or JO-based formulations. The improved firmness retention is consistent with previous reports demonstrating that GG and GG/Eremurus in pomegranate [37], GG/Aloe vera in mango [13], GG/ginseng extract in sweet cherry [16], pea starch and GG/OA in ‘Valencia’ [15], and JO in peach [24] and pear fruits [39] maintained firmness. These findings collectively suggest that the coating matrices in our study likely reinforced cell-wall integrity by limiting moisture loss and modifying enzymatic softening processes, resulting in superior firmness preservation during storage.

CI typically disrupts cell membrane fluidity and increases membrane permeability. In our study, the combined application of GG, OA, and JO effectively reduced CI symptoms, which was consistent with the lower EL and MDA levels observed. GG and JO act as semipermeable coatings, whereas OA forms a thin, hydrophobic layer that limits oxidative stress during cold storage [18, 44, 45]. These coatings help preserve membrane integrity by reducing lipid degradation and stabilizing cellular structures [38, 45, 46]. GG also improves the antioxidant defense system by enhancing enzymes such as superoxide dismutase (SOD), POD, catalase (CAT), and ascorbate peroxidase (APX), thereby counteracting reactive oxygen species (ROS) accumulation typically induced under chilling stress [45]. Previous studies show that GG-based coatings reduce CI in fruits such as pear [46], Ponkan fruit [45], and mango [47] by lowering MDA accumulation. GG and GG/ginseng treatments have been shown to prevent increases in MDA by forming an oxygen barrier that limits lipid peroxidation and maintains membrane integrity [16]. Furthermore, GG, when used as a postharvest coating, can improve enzymatic antioxidant activities (SOD, CAT, POD, and APX), which help reduce oxidative damage [45, 46]. OA further supports membrane stability by reducing electrolyte leakage and delaying enzymatic browning, a common CI symptom, through its physical barrier properties and its ability to decrease oxidative stress [27]. JO, which contains natural antioxidants including tocopherols [48], also contributes to stress mitigation by modulating the activities of PPO, POD, CAT, and APX [24].

Mexican limes are generally harvested and marketed while still green, as this appearance is preferred by consumers [49]. However, chlorophyll degradation during cold storage leads to peel yellowing and reduces marketability. In our study, edible coatings based on GG, alone or combined with OA or JO, effectively conserved the green color during storage. The ability of these coatings to delay chlorophyll loss is likely related to their role in limiting respiration and ethylene production, both of which drive ripening and pigment changes. Similar effects have been reported in guava, where Arabic gum with OA significantly slowed chlorophyll degradation [27], and in tomatoes, where Arabic gum reduced gas exchange by forming a semipermeable barrier, thereby delaying ripening [50]. Coating-induced changes in internal gas composition, particularly increased CO₂, may also contribute to preserving green pigments. Other natural coatings, such as carnauba wax combined with galactomannan in guava, have also been shown to delay chlorophyll degradation by lowering respiration and carotenoid synthesis [51]. Similarly, xanthan gum reduced chlorophyll-degrading enzyme activity in mango, maintaining a greener appearance [7]. Overall, these findings support the use of gum-based coatings—especially when enriched with oils—as effective tools to retain peel greenness and delay ripening-related color changes.

Phenolic compounds contribute to antioxidant properties, flavor, and nutritional quality, yet they typically decline during storage because of oxidative reactions. In our study, GG alone did not prevent the reduction in total phenolics but effectively slowed the degradation of flavonoids and ascorbic acid. The addition of OA and JO further improved the preservation of all these compounds. Similar findings have been observed in lemons, where beeswax + naphthalene acetic acid increased antioxidant retention and hydroxyl radical scavenging capacity [52]. GG-based edible coatings enriched with plant extracts also maintained antioxidants, ascorbic acid, total phenolics, and flavonoids in lemon fruit [53]. These effects are generally associated with reduced oxygen permeability and enhanced antioxidant enzyme activity, which limit oxidative losses. Our results indicated that GG, OA, JO, and their combination decreased PPO activity while increasing PAL activity, indicating reduced phenolic oxidation and stimulated phenolic biosynthesis. Gum-based edible coatings commonly inhibit PPO and POD, whereas OA increases hydrophobicity and decreases respiration rates, further slowing oxidation. Similar reductions in oxidative degradation have been reported using chitosan/OA in strawberry [18], and gum arabic/chitosan in mango [54], and gum Arabic, OA, and cinnamon essential oils in guava [27]. JO is a stable wax ester with strong film-forming capacity, creating a semipermeable barrier that modulates gas exchange and reduces enzymatic oxidation. JO-based coating effectively preserved ascorbic acid in mango [22], peach [24], and guava [23] during cold storage, likely through reduced respiration and lower oxidation, accompanied by decreased PPO activity, increased in POD, CAT, and APX activities [24]. Overall, the combined application of GG, OA, and JO effectively mitigates oxidation-related losses of phenolics and ascorbic acid during storage.

Changes in TSS and TA are major indicators of fruit ripening and postharvest quality. TSS generally increases due to moisture loss and the hydrolysis of complex carbohydrates, whereas TA typically decreases as organic acids are consumed in respiration, leading to deterioration in flavor and overall quality [55]. Edible coatings can slow these changes by reducing respiration and metabolic activity, thereby delaying starch degradation and maintaining organic acid levels.

Our results showed that GG, OA, JO, and their combinations effectively slowed the increase in TSS and the decline in TA during storage. Similar inhibitory effects on TSS and TA changes have been reported for GG and CMC coatings on mango [14], GG-GSE and GG coatings on sweet cherry [16], chitosan/GG coatings on mushroom [38], and GG/Spiria platensis coatings on mango [13]. The preservation of TA is mainly attributed to reduced utilization of organic acids—such as citric and malic acid—which serve as primary substrates for respiration [14]. JO also contributed significantly to the maintenance of TA while limiting TSS elevation, consistent with earlier findings in peach [24], mango [22], guava [23], and pear fruits [39]. Its semipermeable film-forming properties reduce respiration and sugar catabolism, helping to stabilize soluble sugar and organic acid levels during ripening. Overall, GG/JO and GG/OA coatings effectively modulated metabolic processes, preserving both TSS and TA and thereby maintaining the postharvest quality of Mexican lime.

Conclusion

This study demonstrated that edible coatings based on GG enriched with JO and OA effectively maintained the postharvest quality of Mexican lime during cold storage. Among all treatments, the composite coating of GG/JO and GG/OA was the most effective, as it significantly reduced weight loss and decay incidence and better-preserved firmness, total phenol, flavonoid, ascorbic acid, TSS, and TA. The synergistic action of GG (as a structural matrix) and JO and OA (with hydrophobic and antimicrobial properties), contributed to improved maintenance of fruit physiology and reduced sensitivity to chilling injury. From a practical standpoint, GG/JO and GG/OA represents a promising, biodegradable, and consumer-safe coating that can be applied by growers, packers, and storage facilities to extend the shelf life and marketability of Mexican lime. Future research should explore optimization of the formulation concentrations, evaluation under commercial-scale conditions, and assessment of economic feasibility to support broader adoption in the citrus industry.

Authors’ contributions

Marzieh Shamshami: Conceptualization, Data curation, Formal analysis, Writing- Original draft preparation. Leila Jafari: Supervision, Project Administration, Writing - Review & Editing. Abdolmajid Mirzaalian Dastjerdi: Methodology, Project Administration, Writing-Reviewing and Editing.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.