Effects of 1-Methylcyclopropene, Gibberellic Acid, and Aloe vera Coating on Lime Storage Life and Fruit Quality

Abstract

Limes (Citrus aurantifolia Swingle) have a short postharvest life. Therefore, effective treatments increasing storage life to lengthen the time that produce remains fresh is desirable. This study evaluated the effects of postharvest treatments of 1-methylcyclopropene (1-MCP), gibberellic acid (GA₃), and Aloe vera on lime storage life and fruit quality. The optimum individual treatments of 50% Aloe vera coating, 300 mL·L⁻¹ GA₃, and 750 nL·L⁻¹ 1-MCP were identified as most effective in extending lime postharvest life. Under ambient conditions (26.4 ± 1.0°C, 64.4 ± 7.4% RH), 1-MCP fumigation was the best treatment, increasing lime postharvest life to 28.2 days. The 1-MCP-treated fruit had a higher juice content than the control fruit. Also, soluble solid contents and titratable acidity were not significantly different, while ascorbic acid was lower than the control. Under cold room conditions (10.0 ± 0.3°C, 68.9 ± 12.4% RH), 1-MCP fumigation followed by Aloe vera coating was the best treatment, increasing lime storage life by 38.7 days. The longer storage time did reduce juice content in treated limes compared to untreated fruit. However, the treatment did not affect the soluble solid contents, titratable acidity, or ascorbic acid level. In conclusion, for optimum postharvest life in limes stored under ambient conditions, fumigation with 1-MCP is best. While for limes stored in cold rooms, a cotreatment of 1-MCP fumigation followed by Aloe vera coating maximizes postharvest life.

Introduction

Lime (Citrus aurantifolia Swingle.) is an economically valuable acidic citrus fruit cultivated in nearly all tropical and subtropical regions, including Brazil, Argentina, Mexico, India, Pakistan, and the Middle East. It accounts for 5% of global citrus production (Donkersley et al., 2018). Each component of the fruit is a valuable commodity, with uses as an essential ingredient for cuisines, beverages, refreshing drinks, processing, and rind-extracted oils in cosmetics and perfume industries. Limes are also medically beneficial, as a rich source of vitamin C, increasing resistance to conditions, such as scurvy, respiratory disorders, and cardiovascular diseases (Kalatippi and Hota, 2020). Hence, the already large volumes of fresh lime produced are increasing, driven by World Health Organization recommendations and high demand in the global marketplace (Donkersley et al., 2018).

In 2021, Thailand’s lime production reached 161,000 tons, with the value of limes sold at $158 million, mainly for domestic consumption (Office of Agricultural and Economics, 2021). Most consumers in Thailand prefer green limes, with price strongly dependent on availability and appearance. During postharvest management, mature-green limes have a much longer postharvest life than those picked when yellowish-green or yellow; the latter must be senesced soon after harvest. Therefore, limes are generally harvested before ripening, while the peel color is still green. However, the harvested fruit undergo chlorophyll degradation, resulting in yellowing, a major determinant in shelf- and storage life (Kaewsuksaeng et al., 2015). A short postharvest life due to fruit deterioration is a significant problem for fresh commercial limes (Kaewsuksaeng et al., 2015; Win et al., 2006). In practice, maintaining limes at 9–10°C or in the refrigerator is a common approach to increase storage life and maintain quality for 3–4 weeks. However, further increases to postharvest life to extend peak periods of fresh lime fruit during storage are still required.

Most limes are purchased within domestic retail markets and consumed fresh (Paull, 1999). Thus it is important to preserve limes under both ambient conditions during sale and within a cold room for extended storage. To achieve this, it is imperative to develop treatments that extend postharvest and storage life while maintaining the fruit quality to ensure a year-round supply of fresh limes for domestic processing and export markets (Champa and Gamage, 2020). In this regard, postharvest treatments such as plant growth regulators (Win et al., 2006) and coatings (Bosquez-Molina et al., 2004) are potential methods to increase lime postharvest life. If lime postharvest life can be increased, it will beneficially extend the market window of commercial limes during the peak period.

Limes are a genus of Citrus species and, based on respiration rate and ethylene production, are classified as a non-climacteric fruit (Palou et al., 2015; Paul et al., 2012). Therefore, citrus does not undergo peak respiration and ethylene production after harvest. However, endogenous ethylene and exogenously applied ethylene can impact postharvest life and fruit quality of many Citrus spp. (Porat et al., 1999). 1-Methylcyclopropene (1-MCP), an effective inhibitor of ethylene action, is well-known and widely used for many fresh commodities, including limes (Watkins, 2015; Win et al., 2006). The application of 1-MCP inhibits ethylene action and suppresses ethylene production, delaying chlorophyll degradation in lime peel and consequently increasing postharvest life (Win et al., 2006). Preharvest application of gibberellic acid (GA₃), an ethylene antagonist, is a regular practice to maintain a green peel and improve Citrus fruit quality on the tree (Garmendia et al., 2019). Also, GA₃ is applied after harvest to delay peel degreening of navel oranges and California grapefruit (Ismail, 1997), ‘Oroblanco’ citrus (Porat et al., 2001), and ‘Fallglo’ tangerines (Ritenour et al., 2005). For limes, currently only one report evaluates the effects of post-harvest GA₃ application on shelf life and quality for three Mexican lime varieties, namely Colimex, Colimón, and Lise (Zea-Hernández et al., 2016). However, the effects of pre-storage GA₃ treatment on the postharvest life of Thai acid limes is unreported.

Aloe vera gel is an edible polysaccharide with significant nutritional benefits, such as essential vitamins, antioxidants, and beneficial phytochemicals, and it is also a natural, tasteless, colorless, and odorless product (Misir et al., 2014). These properties enable this coating to potentially reduce plastic packaging, while meeting food safety and quality requirements (Hasan et al., 2021; Misir et al., 2014). Aloe vera coating applications are reported to increase postharvest life and maintain fruit quality in many fruit species, such as grapes, papaya, oranges, nectarines, cherries, apples (Misir et al., 2014), and guava (Rehman et al., 2020), along with lime (Buapuean, 2016; Buapuean and Lerslerwong, 2014). In the case of limes, Aloe vera coating was found to increase postharvest life by almost a week.

Therefore, this study investigated the effects of 1-MCP, GA₃, and Aloe vera coating on lime postharvest life. As most lime fruit are consumed in their tropical production countries, room temperature storage is the regular practice of retailers in these regions. Thus, this study will be performed at both ambient and low temperatures. Improving lime postharvest life and extending storage times will greatly benefit commercial acid lime production.

Materials and Methods

Plant materials

Extra class fruit, 4.3–5.0 g and diameter 4.5–5.5 cm, limes (Citrus aurantifolia Swingle ‘Pan’) were purchased from the wholesale market in Songkhla Province. The fresh limes came from Ratchaburi Province, a large plantation in Thailand. Fresh limes at the fully green mature stage were harvested, graded based on size, packed into the 25 kg net bag, roughly 400 fruit, and immediately transported from the farmer’s field by truck/minitruck to the wholesale markets within 15 h. Then, limes were purchased and transported to the laboratory within 1 h. Limes were again sorted for uniformity, diameter > 49 mm, and green color based on the THALANDAISE AGRICULTURAL STANDARD: LIME (National Bureau of Agricultural Commodity and Food Standards, 2017). For fruit surface sterilization and pesticide residue removal, the selected fruit were soaked in a 100 mg·L⁻¹ sodium bicarbonate solution for 15 min, washed twice with tap water, then air-dried at room temperature for 30 min before experimental treatments.

Preparation of and treatment with the Aloe vera coating

Harvested green mature leaves of Aloe vera (A. barbadensis Mill.) were received from the field at the Faculty of Natural Resources, Prince of Songkla University. The Aloe vera gel matrix under the outer bark of the leaf was separated from the outer cortex of the leaves and then processed in a blender. The mixture was filtered through a gauze cloth to remove fiber yielding a 100% Aloe vera liquid extract, then pasteurized at 70°C for 45 min. The mixture was immediately cooled to room temperature and the extracted gel diluted with sterile distilled water to prepare Aloe vera solutions at 10%, 25%, 50%, and 75% before use. One hundred fruit were coated with each treatment for 30 min and allowed to air-dry at room temperature.

Preparation of and treatment with 1-MCP and GA₃

One hundred limes each were fumigated with 0, 250, 500, 750, and 1000 nL·L⁻¹ 1-MCP for 18 h. 1-MCP was obtained from EthylBlock^® powder (0.14% active ingredient) (Rohm and Hass Company, Philadelphia, PA, USA) with gaseous 1-MCP generated by adding water to the powder in a 3-mL vial in an airtight plastic chamber (50 cm × 50 cm × 50 cm). Water was poured into a vial of EthylBlock^® powder (ratio 1:16) to produce the final concentration of 1-MCP gas in each chamber. The control limes were placed in identical chambers without 1-MCP. For the GA₃ treatments, one hundred fruit each were soaked in solutions of GA₃ at concentrations of 0, 100, 200, and 300 mL·L⁻¹ for 30 min and allowed to air-dry at room temperature. The 1 L GA₃ solution of each concentration was obtained from 0.5% w/v commercial GA₃ liquid (0.5 L SL).

Fruit packing before storage in the experiment of optimized treatment concentration of 1-MCP, GA₃, and Aloe vera coating

After Aloe vera coating, 1-MCP, and GA₃ treatments, the fruit were packed in a foam tray wrapped with an 11-micron food packaging film (M Wrap^®; MMP Corporation Ltd., Bangkok, Thailand) and placed at room temperature under ambient conditions.

The experiment of optimized treatment concentration of 1-MCP, GA₃, and Aloe vera coating

Three separate experiments determined the suitable 1-MCP, GA₃, and Aloe vera coating concentrations. The optimum identified concentration of each treatment was then used in the subsequent storage experiments at ambient and low temperatures. All experiments were performed using completely randomized designs, and each treatment comprised ten fruit replicates.

Treatment with 1-MCP and GA₃ combined with Aloe vera coating and storage conditions

Two hundred fruit were used in each storage experiment. The experiment comprised of six treatments: control (untreated), 1-MCP, GA₃, Aloe vera coating, 1-MCP + Aloe vera coating, and GA₃ + Aloe vera coating. Concentrations of 1-MCP, GA₃, and Aloe vera coating were determined according to the preliminary experiments. For the combination treatment of growth regulators and coating, fruit were treated with 1-MCP and GA₃ before coating. After treatment, the fruit were packed in a 25 μm low-density polyethylene (LDPE) MAP bag made of active breathable 12 × 18-inch packaging (Fresh & Fresh^®; Thantawan Industry Public Company Ltd., Thailand). Finally, the packed limes were placed under ambient and low temperatures.

Data records

1.  Evaluation of the postharvest life and senescence score

Fruit peel color was measured on two sides of each fruit using a Colorimeter (Model CR-400; Minolta Co., Ltd., Osaka, Japan) with the standard CIE illuminant at the hue angle, L*, a*, and b*. For the ambient and low-temperature experiments, fruit from each treatment were measured daily and weekly, respectively. Six senescence scores, ranging from 0–5, were defined by the hue angle value and visible yellowing on the peel: green fruit receives a score of 0 (hue angle ≥ 121.00 and green color), followed by score 1 (hue angle 119.01–120.99 and 10% yellow), score 2 (hue angle 116.01–118.99 and 25% yellow), score 3 (hue angle 113.01–115.99 and 50% yellow), score 4 (hue angle 111.01–112.99 and 75% yellow), and score 5 (hue angle < 110.99 and > 95% yellow). A senescence score of 3 was used to indicate the end of postharvest life. The time from treatment to the end of postharvest life was recorded as the number of days.

2.  Assessment of internal quality

The juice content was measured using a juice machine, with each treatment’s fruit juice sieved through a cheesecloth. Lime juice content was calculated from the whole fruit and final juice weight. The soluble solid concentration was measured by a hand-held refractometer (Brix RHB-32ATC; Jiangsu Victor Instrument Meter Co., Ltd., Shanghai, China) calibrated with distilled water. The titratable acidity was determined in triplicate titrations of 5 mL juice with 1 N NaOH and 0.1% phenolphthalein as an indicator, and the results are expressed as grams of milliequivalent weight citric acid. The ascorbic acid contents were determined by titration with 2,6-dichlorophenolindophenol (AOAC, 2000) and are expressed in μmol·L⁻¹.

Ten fruit or replications were evaluated for postharvest life, with all biological fruit quality parameters measured upon fruit reaching a senescence score of 3 over 10 replications; 3 were sampled per replication.

Statistical analysis

Data statistical analyses were performed using R version 2.14.0. Analysis of variance was performed using the F test, and least significant differences (LSD) were calculated at the 5% probability for comparing means between treatments.

Results

1.  Effects of Aloe vera coating concentration on postharvest life and fruit quality

The effects of Aloe vera coating on postharvest life and fruit quality are presented in Table 1. Coating with Aloe vera gel increased lime fruit postharvest life by 3–11 days at ambient temperature, 34.5 ± 1.8°C, 60.5 ± 5% RH. The 50% Aloe vera coating increased lime postharvest life the most. The coating treatments effectively delayed decreases in the hue angle value (Fig. 1a and 3a). For the control fruit, the senescence index reached score 3 within 5 days, while fruit coated with 50% Aloe vera lasted for 15 days. In terms of fruit quality, the limes coated with the 50% Aloe vera solution had a lower juice content compared to the control. Also, soluble solid concentration and titratable acidity were unaffected by Aloe vera coating treatments.

Table 1

Effects of postharvest treatments with Aloe vera (AV) coating on postharvest life and internal quality of ‘Pan’ lime under ambient conditions (34.5 ± 1.8°C, 60.5 ± 5% RH).

Fig. 1

Effects of postharvest treatment with Aloe vera (AV) coating (a), 1-MCP (b), and GA₃ (c) on the hue angle of stored ‘Pan’ limes under ambient conditions. Values are the means of ten replicates (3 fruits per replication) ± SE (n = 30). The asterisks above bars represent a significant difference (P < 0.05 and P < 0.01).

2.  Effects of the concentration of 1-MCP on postharvest life and fruit quality

1-MCP fumigation enhanced the lime postharvest life by 5–10 days at an ambient temperature, 26.2 ± 1.4°C, 82.7 ± 9.5% RH (Table 2). The lime postharvest life was greatest with 750 nL·L⁻¹ 1-MCP fumigation treatment, greatly delaying the hue angle value decrease. The hue angle value yielding a senescence score 3 of the control fruit occurred within 10 days, while fruit fumigated with 750 nL·L⁻¹ 1-MCP lasted for 20 days (Fig. 1b and 3b). Regarding fruit quality, the fruit fumigated with 250–750 nL·L⁻¹ 1-MCP exhibited no significant differences in juice content, soluble solids concentration, or titratable acidity.

Table 2

Effects of postharvest treatments with 1-MCP fumigation on postharvest life and internal quality of ‘Pan’ lime under ambient condition (26.2 ± 1.4°C, 82.7 ± 9.5% RH).

3.  Effects of the concentration of GA₃ on postharvest life and fruit quality

At an ambient temperature (27.4 ± 1.1°C, 81.6 ± 7.3% RH) postharvest GA₃ application did not increase lime postharvest life, and differences between groups were not statistically significant (Table 3). However, the 300 mL·L⁻¹ GA₃ treatments tended to delay decreases in hue angle. The hue angle of the control fruit decreased within 12 days, while the hue angle of the soaked fruit decreased by 17 days (Fig. 1c and 3c). However, fruit treated with 300 mL·L⁻¹ GA₃ had a relatively low soluble solid concentration, while GA₃ treatments did not affect juice content or titratable acid.

Table 3

Effects of postharvest treatments with GA₃ soaking on postharvest life and internal quality of ‘Pan’ lime under ambient condition (27.4 ± 1.1°C, 81.6 ± 7.3% RH).

4.  Effects of 1-MCP, GA₃, and Aloe vera coating on storage life and fruit quality at ambient storage temperature

Applying 1-MCP, GA₃, and the Aloe vera coating to limes increased postharvest life by 3–11 days at ambient temperature, 26.4 ± 1.0°C, 64.4 ± 7.4% RH (Table 4). The 1-MCP fumigation with and without the combined Aloe vera coating treatment was performed, significantly enhancing postharvest life by 7 and 11 days, respectively. The postharvest life spans of fruit fumigated with 1-MCP alone and combined with Aloe vera coating were 29 and 25 days, respectively, compared to the control of 18 days. These results demonstrate that 1-MCP with and without Aloe vera coating under ambient conditions effectively delayed the decrease in the hue angle, but treatment with 1-MCP alone was more effective than the 1-MCP and Aloe vera coating cotreatment (Figs. 2a and 3d). Also, GA₃ cotreatment with the Aloe vera coating increased postharvest life and delayed hue angle decrease by 3 days. Regarding fruit quality, for all treatments, juice contents were higher than that of untreated fruit, while soluble solid concentration and the titratable acidity were unaffected. For ascorbic acid content, the 1-MCP and Aloe vera coating cotreatment showed a yield comparable to the control.

Table 4

Effects of postharvest treatments with 1-MCP fumigation, GA₃ soaking and Aloe vera (AV) coating on the storage life and internal quality of ‘Pan’ lime under ambient conditions (26.4 ± 1.0°C, 64.4 ± 7.4% RH).

Fig. 2

Effects of postharvest treatment with Aloe vera (AV) coating, GA₃, and 1-MCP on the hue angle of stored ‘Pan’ limes under ambient conditions (26.4 ± 1.0°C, 64.4 ± 7.4% RH) (a) and in cold room conditions (10.0 ± 0.3°C, 68.9 ± 12.4% RH) (b). Values are the means of ten replicates (3 fruits per replication) ± SE (n = 30). The asterisks above bars represent a significant difference (P < 0.05 and P < 0.01).

Fig. 3

The appearance of ‘Pan’ lime fruit treated with Aloe vera (AV) coating (a), 1-MCP (b), and GA₃ (c) and that of the lime fruit under ambient conditions (26.4 ± 1.0°C, 64.4 ± 7.4% RH) (d) and in cold room conditions (10.0 ± 0.3°C, 68.9 ± 12.4% RH) (e), when compared to the control lime at senescence score 3.

5.  Effects of 1-MCP, GA₃, and Aloe vera coating on storage life and fruit quality at low temperature

The application of 1-MCP, GA₃, and Aloe vera coating on limes stored at low temperature showed increases in postharvest life ranging from 2–39 days at cold-room conditions of 10.0 ± 0.3°C, 68.9 ± 12.4% RH (Table 5). The cotreatment incorporating the most effective concentration of 1-MCP fumigation combined with the 50% Aloe vera coating increased the postharvest life significantly by 39 days. The total storage life of fruit with the cotreatment of 1-MCP fumigation with the 50% Aloe vera coating was 62 days compared to the control at 23 days. The cotreatment of 1-MCP and Aloe vera coating effectively delayed the decrease in hue angle (Figs. 2b and 3e). Regarding fruit quality, treatment of limes both for Aloe vera coating alone and cotreatment of 1-MCP with Aloe vera resulted in lower juice content. Aloe vera coating treatment yielded the lowest soluble solid concentration and titratable acidity. Postharvest treatments of 1-MCP, GA₃, and Aloe vera coating did not affect the ascorbic acid content.

Table 5

Effects of postharvest treatments with 1-MCP fumigation, GA₃ soaking and Aloe vera (AV) coating on the storage life and internal quality of ‘Pan’ lime under cold room conditions (10.0 ± 0.3°C, 68.9 ± 12.4% RH).

Discussion

Limes have a short postharvest life, and commercial production requires improved treatments to increase postharvest life to cover periods of storage. We performed a preliminary comparison of 1-MCP, GA₃, and Aloe vera coating applied to limes to determine optimal individual conditions to improve lime storage times for both ambient and low temperatures.

For the Aloe vera coating experiment, the results (Table 1) confirmed previous work (Buapuean, 2016; Buapuean and Lerslerwong, 2014) indicating that 10–100% Aloe vera coating increased postharvest life. The optimal concentration for the coating treatment was identified as a 50% concentration of Aloe vera. This coating did not affect fruit quality, soluble solid concentration, or titratable acid. Alongside the film-like properties of the Aloe vera gel, acting as both a gas exchange barrier and preventing weight loss in many fruits, the nutritional components of Aloe vera gel include anthraquinones, vitamins, enzymes, saccharides, carbohydrates, non- and essential amino acid, inorganic compounds, among others (Misir et al., 2014). Additional components identified include gibberellin and salicylic acid, which are reported to extend Citrus spp. postharvest life. Thus, these components may also act to extend lime postharvest life.

For the 1-MCP treatment, the most effective concentration for increasing lime postharvest life was 750 nL·L⁻¹ after 18 h of fumigation (Table 2). However, this result does not reflect the findings of Win et al. (2006), who reported that concentrations of 750 and 1000 nL·L⁻¹ were detrimental to their limes, increasing the senescence index faster than the control, coupled with an increase in ethylene production. The unfavorable results reported for treatment with 1-MCP at a high concentration included increased tissue stress in high-temperature storage and low relative humidity (24–31°C and 73–81% RH). However, our results demonstrated that 750 and 1000 nL·L⁻¹ 1-MCP fumigation effectively delayed the decrease in hue angle without affecting internal fruit quality when storing lime fruit at 26.2 ± 1.4°C and 82.7 ± 9.5% RH. The different results of both studies may be due to the shorter 18 h fumigation treatment in our study than their 24 h protocol. This indicates that, in addition to 1-MCP concentration and the storage conditions of temperature and relative humidity, the effective application of 1-MCP also may depend upon fumigation period. Nevertheless, 1-MCP treatment clearly has tremendous potential to improve lime postharvest life, with an effectiveness depending upon applied concentration, storage conditions, and treatment duration.

For the GA₃ experiments, the treatments yielded limited effectiveness on improving lime postharvest life (Table 3). However, a concentration of 300 mL·L⁻¹ GA₃ did increase postharvest life by four days compared to the control. As a postharvest treatment, the dip treatment of 100 mL·L⁻¹ GA₃ delayed the yellowing of ‘Oroblanco’ citrus (Porat et al., 2001). The response of citrus fruit to GA₃ is influenced by different species and cultivars. However, GA₃ is very persistent in orange peel, with a half-life of 80 days (Ashebre, 2015). In this study, the results indicated that much higher GA₃ concentrations may be required to increase lime postharvest life, which is likely to negatively affect lime quality for safe consumption. This detrimental aspect of GA₃ will thus limit the use of high GA₃ concentrations on postharvest limes in a commercial context.

For the storage of limes at an ambient temperature, 1-MCP fumigation was more effective than the treatments of GA₃, Aloe vera, and other combined treatments (Table 4). However, at low temperatures, the effectiveness of 1-MCP in increasing lime postharvest life did not differ from other treatments (Table 5). The results confirmed that low temperatures are the primary factor determining lime postharvest life, as with other perishable tropical fruit. However, the cotreatment of 1-MCP and Aloe vera coating showed increased effectiveness by extending the low temperature storage period to two months. These results clearly demonstrate that the combination of Aloe vera coating improves the effectiveness of 1-MCP in delaying lime senescence, increasing the storage life of lime. These findings highlight that the choice to apply 1-MCP with or without Aloe vera coating depends on the storage temperature. Usually, the fixed cost of constructing a cold room, supporting facilities, and providing energy for fruit storage in Thailand is relatively high (Ketsa, 1994). Thus, other efficient treatments are desirable for commercial operation. Additionally, our experiment identified that the Aloe vera coating method used is inconvenient for practical commercial applications. Therefore, improvements to the application method for the cotreatment of 1-MCP and Aloe vera coating to increase the postharvest life of stored lime requires additional investigation.

For the ambient temperature storage experiments, GA₃ effected no extension of postharvest life. In contrast to the preliminary experiments determining optimum GA₃ concentration, in which the GA₃ treated lime had a longer postharvest life than the control. This may be due to relative humidity differences between storage conditions, which was higher in the preliminary experiment (81.6 ± 7.3%RH) than in the storage experiments at ambient (64.4 ± 7.4%RH) and low (68.9 ± 12.4%RH) temperatures. This suggests that GA₃ application to limes may succeed for high relative humidity (> 80%). In addition, the use of 1-MCP fumigation and Aloe vera coating increased lime postharvest life greater than that of GA₃ treatment. It is not entirely surprising that 1-MCP yields better enhancements in lime postharvest life than GA₃, as 1-MCP directly inhibits ethylene formation while GA₃ competes for the ethylene formed (Porat et al., 2001).

Depending on its use, juice content is an essential indicator of lime quality. In general, lime juice content gradually decreases over the life of the fruit. In this study, the 1-MCP and cotreatment of 1-MCP and Aloe vera coating clearly yielded greater juice content than that of the control fruit (Table 4). However, limes coated with Aloe vera yielded a relatively low juice content. In contrast, after two months in cold storage, the lime cv. Egypt Banzahir coated with gum arabic yielded the highest juice content. Maintaining juice content directly correlates with low weight loss (El-Eryan and Tarabih, 2020). The critical desired function of the Aloe vera coating is to reduce weight loss due to a reduction in fruit respiration (Misir et al., 2014). However, in this study, the Aloe vera coating, 1-MCP, and GA₃ had no significantly different effect on decreasing weight loss throughout the experiment (data not shown). Both Aloe vera and gum arabic are natural coatings of water-soluble polysaccharides. Gum arabic is obtained from the gum exudates of the Acacia senegal tree and used as a natural film preservative for its water solubility, film-forming, antioxidant activity, and emulsification properties (Nieto, 2009). Aloe vera gel has poorer film-forming properties compared to gum arabic. In addition, a more effective treatment on guava using gum arabic in combination with Aloe vera increased the storage time while diminishing losses of fruit quality and flavonoid contents (Anjum et al., 2020). This result supports Aloe vera coating as a suitable additive treatment or cotreatment with a primary procedure to increase lime postharvest life with consideration of storage conditions.

The postharvest treatment of 1-MCP, GA₃, and Aloe vera coating alone and combined did not affect the soluble solid concentration, titratable acidity, or the ascorbic acid content of limes stored at ambient temperature. The effects of 1-MCP and GA₃ on soluble solid concentration and lime titratable acid were as previously reported for ‘Oroblanco’ citrus (Porat et al., 2001) and ‘Kinnow’ mandarin (Baswal et al., 2020). The magnitude of these results may be explained by Win et al. (2006), who identified that differences in lime soluble solid concentration and titratable acidity depend upon 1-MCP concentration, with excessive treatments showing a negative response. El-Otmani and Coggins Jr (1991) reported that GA₃ slowed rind color change in ‘Clementine’ mandarins and ‘Washington’ navel oranges, both on the tree and post-harvest, but no GA₃ effects were noted for weight loss, soluble solid concentration, and titratable acidity. These results noted that 1-MCP and GA₃ only affected the peel coloration, without impacting soluble solid concentration, titratable acidity, and yielded similar weight losses in all treatments (data not shown).

However, the Aloe vera coating applied alone did not maintain the soluble solid concentration and the titratable acidity of lime stored at low temperatures, indicating that the Aloe vera coating did not effectively maintain the internal quality of limes stored for long periods, especially in a cold room. Therefore, for limes stored at low temperatures, the combined treatment of 1-MCP with Aloe vera coating is optimal for the reasons discussed previously.

Regarding the ascorbic acid content, the most effective treatment in maintaining lime ascorbic acid content stored at ambient temperature was the cotreatment of 1-MCP with Aloe vera coating. In comparison, the ascorbic acid content of the remaining treatments appears lower than the control. Although limes coated with Aloe vera alone showed the lowest ascorbic acid content, it is noteworthy that combined application with 1-MCP resulted in the highest content. The cotreatment with 1-MCP may directly inhibit ethylene action, thus delaying lime senescence, improving the efficiency of Aloe vera in maintaining ascorbic acid content. This result was also confirmed in the storage experiment in the cold room as described above.

In conclusion, postharvest treatments of Aloe vera coating, 1-MCP with and without Aloe vera coating, and GA₃ with Aloe vera coating can increase lime postharvest life. The recommended treatment for ambient temperature lime fruit storage is 1-MCP fumigation alone. For storing limes at a low temperature, the optimum treatment is the combined application of 1-MCP and an Aloe vera coating, extending postharvest life to up to two months. The cotreatment of 1-MCP and Aloe vera coating maintained lime internal quality, except for juice content, over storage at low temperatures.

Acknowledgements

This research received grants from the Postharvest Technology Innovation Center (PHTIC), Ministry of Higher Education, Science, Research and Innovation, and Thailand Toray Science Foundation (TTSF). Additionally, we would like to thank the Center of Excellence in Agricultural and Natural Resources Biotechnology, Faculty of Natural Resources, and Graduate School, Prince of Songkla University, for supporting the graduate student grant.

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