Composite Coating Enriched with Lemon Peel Extract for Enhancing the Postharvest Quality of Cherry Tomatoes

Abstract

The present study investigated the efficacy of edible coatings formulated with gellan gum and lemon peel extract (LPE) in preserving the postharvest quality of cherry tomatoes (Solanum lycopersicum var. cerasiforme). Selected fruits exhibiting uniform ripeness and free from defects were sanitized and coated with solutions containing different HAG/LAG (high- and low-acyl gellan gum) ratios, incorporating 4.0% (w/v) LPE. Physicochemical and physiological parameters, including soluble solids content, weight loss, pH, titratable acidity, oxygen consumption, carbon dioxide and ethylene production, skin redness (a*/b* ratio), and decay incidence, were systematically assessed under storage conditions of 25 °C and 70% relative humidity. HAG-coated fruits showed the lowest weight loss (1.08%), higher soluble solids (7.11 °Brix), and greater firmness (3.11 N/mm²) compared to uncoated controls. Moreover, they exhibited reduced oxygen consumption (0.06 mg·kg⁻¹·h⁻¹), ethylene production (3.10 mg·kg⁻¹·h⁻¹), and decay rate (2%). Redness was better preserved, and decay rates were substantially (p < 0.05) reduced throughout the storage period. These findings highlight the potential of HAG-based edible coatings enriched with LPE as an innovative postharvest technology to extend shelf life, maintain quality attributes, and reduce postharvest losses in cherry tomatoes.

1. Introduction

Cherry tomatoes (Solanum lycopersicum var. cerasiforme) are recognized as an excellent source of phytochemicals, including lycopene, beta-carotene, polyphenols, and quercetin, all of which possess strong antioxidant properties and confer significant health benefits [1]. These bioactive compounds have been associated with a reduced risk of developing cancer, cardiovascular diseases, osteoporosis, and other health disorders.

Cherry tomato cultivation is of great importance in Colombia, with significant production concentrated in departments such as Cundinamarca, Antioquia, Boyacá, Nariño, and Valle del Cauca. Cherry tomatoes have gained popularity in recent years due to their sweet flavour and their use in salads and gourmet dishes [2]. Specifically, in the Bolívar department, production reached 62 tons, with a yield of 8.86 tons per hectare. Given the economic importance of tomato production and consumption, there is a growing need to implement strategies that preserve fruit quality during the postharvest period, with an emphasis on preventing fungal contamination. Previous studies have identified Aspergillus niger, Fusarium species, and Rhizopus stolonifer as some of the primary fungi responsible for the spoilage of cherry tomatoes [3].

As a climacteric fruit, cherry tomatoes exhibit a relatively short postharvest life due to their high respiration rate and rapid ripening and senescence, which, combined with their susceptibility to phytopathogenic fungal infections, can result in significant postharvest losses [4]. Various preservation technologies have been explored to extend the shelf life of cherry tomatoes, including low-temperature storage [5], modified atmosphere packaging [6], chemical preservatives, and edible coatings [7]. However, low-temperature storage is associated with substantial energy costs, limiting its widespread application. Similarly, while chemical fungicides remain a common strategy for managing fungal infections, their use raises serious concerns regarding environmental pollution, human health risks, and the emergence of resistant pathogenic strains, emphasizing the urgent need for safer and more sustainable alternatives [8].

Edible coatings have emerged as a promising, environmentally friendly solution to prolong fruit shelf life. They are simple to apply, cost-effective, and capable of forming a protective barrier that can slow down fruit respiration and ripening, reduce microbial growth, and minimize moisture loss [9,10,11]. Additionally, edible coatings can improve the visual quality of fresh produce. Typically, these coatings are formulated using biocompatible, biodegradable film-forming polymers such as natural polysaccharides and proteins [12].

Among the polysaccharides used for edible coating formulation, gellan gum stands out for its excellent film-forming properties [13]. Gellan gum is an extracellular anionic heteropolysaccharide produced by Sphingomonas elodea, and it is composed of repeating tetrasaccharide units (1,3-β-D-glucose, 1,4-β-D-glucuronic acid, 1,4-β-D-glucose, and 1,4-α-L-rhamnose) [14]. Native gellan is known as high-acyl gellan (HAG) because it presents both an acetate group (C6) and a glycerate group (C2) in its glucose residue. Low-acyl gellan (LAG) is obtained by the alkaline hydrolysis of native gellan gum at high temperatures, leading to a reduction in its acyl content [15]. The structural distinctions between HAG and LAG gums result in significant variability in their textural characteristics. HAG gum typically forms soft and elastic gels, while LAG gum yields firm and brittle gels. By combining both types in varying ratios, it is possible to obtain gels with a broad range of intermediate textural properties depending on the proportion of gellan used. LAG forms thermally stable gels that are resistant to acidic conditions and enzymatic degradation [16], making it particularly suitable for controlled release applications due to its porous three-dimensional network and biodegradability [17].

The incorporation of functional compounds into edible coatings is an increasingly attractive strategy to enhance the preservation of food matrices. For instance, González et al. [18] demonstrated that gellan gum-based liposomes loaded with Melissa officinalis extracts were effective in extending the shelf life of tomatoes, with HAG gum showing excellent performance in the gradual release of active compounds.

Lemon peel, a by-product of the lemon juice functional beverage industry, is often discarded despite being rich in valuable compounds such as phenolics, flavonoids, essential oils, fibre, and pectin [19]. The phenolic compounds from lemon peel can serve as natural food additives or as antioxidant and antimicrobial agents [20]. Furthermore, its pectin, cellulose, phenolics, and essential oils can be utilized in the development of active food packaging or edible coatings [21].

Based on this background, the present study aims to evaluate the effects of gellan gum-based edible coatings incorporating 4.0 wt.% lemon peel extract (LPE) on the quality attributes of cherry tomatoes during storage at 25 °C over a 10-day period. The quality parameters assessed include soluble solids content, weight loss, ethylene (C₂H₄) production, oxygen (O₂) consumption, titratable acidity, firmness, redness, and decay rate.

2. Materials and Methods

2.1. Fruit

The cherry tomatoes (Solanum lycopersicum var. cerasiforme) used in the experiments were purchased from a local market in Cartagena, Colombia. Tomatoes exhibiting physical damage, microbial contamination, or discoloration were discarded. Only healthy, intact fruits, free from mold infection and visually uniform in size and ripeness, were selected. The selected tomatoes were disinfected by immersion in a 2% (v/v) sodium hypochlorite solution for 2 min and then rinsed five times with water and air-dried at room temperature.

2.2. Preparation of Lemon Peel Powder Extraction Process

Initially, fresh lemons without physical damage were manually selected. The outer peel (flavedo) was then separated from the pulp. The peels were cut into small pieces (2 × 2 cm²) and dried at 50 °C for 18 h until the moisture content reached 10% [22]. Once dried, the peels were ground using a mill to obtain a fine powder. To ensure uniform particle size, the lemon peel powder was sieved through a 60-mesh screen and stored in a desiccator until further use. The lemon peel powder was then dissolved in water at a 1:10 ratio (w/v) [22]. The resulting solution underwent ultrasound-assisted extraction for 60 min at 30 °C using a Labscient Model KSL5120-5 ultrasonic processor (Frequency 40 kHz, ultrasonic power 120 w, Stuttgart, Germany) attached with a sonotrode Model Ezodo with the following precision: ±1.5 dB (94 dB ref @ 1 KHz). After extraction, the LPE solution was centrifuged at 4000 rpm for 10 min. The supernatant was subsequently filtered using vacuum filtration. The obtained filtrate was concentrated using a rotary evaporator at 50 °C and 60 rpm for 6 h. Finally, the LPE was weighed and stored in dark bottles for further analysis.

2.3. Edible Coating Manufacture

The dispersions were prepared separately in deionized water under continuous stirring at 80 °C for 10 min at a concentration of 0.5% w/w, using a simple design of mixtures of HA and LA gellan and mixtures of 25HA/75LA; 50HA/50LA; and 75HA/25LA. Subsequently, 1.5% (w/v) glycerol was added, and the solution was stirred at 600 rpm and 80 °C for an additional 10 min to ensure proper incorporation. The coating solution was then allowed to cool to 30 °C. Once cooled, 4.0% (w/v) LPE was added to the solution and homogenized using an ultrasonic homogenizer for 10 min to achieve a uniform dispersion. Tomato samples (20 fruits for each analysis) were immersed in the coating solution for 10 s under aseptic conditions to minimize microbial contamination and then dried at room temperature in a laminar flow cabinet for 1 h to allow for proper film formation. Tomatoes immersed only in distilled water served as controls and were designated as uncoated fruits (UFs). Finally, both coated and uncoated tomatoes were stored at 25 °C with a relative humidity of 70%.

2.4. Weight Loss and Soluble Solids

For the assessment of weight loss, entire tomatoes were weighed, with approximately 20 fruits per measurement. The weights were recorded at days 0, 2, 4, 6, 8, and 10. Weight loss was calculated using Equation (1):

$$weight loss={\displaystyle \frac{w_0-w_t}{w_0}}\times 100$$

(1)

where w₀ is the initial weight of the fruit, and w_t represents the weight at each sampling time.

The fruits were macerated using a mortar, and the soluble solids (SSs) content was determined by placing four drops of the macerate onto the prism of a refractometer (Extech Model 2132, Extech Instruments, Nashua, NH, USA). The results were expressed in °Brix.

2.5. Titratable Acidity

Tomato fruits were ground in a blender for homogenization. A 10 mg portion of each homogenized sample was mixed with distilled water, and titratable acidity (TA) was determined by titration with 0.1 N NaOH following the AOAC 942.15 method [23]. Sampling was performed on days 0, 2, 4, 6, 8, and 10. The results were expressed as the percentage (%) of citric acid equivalent per 100 g of sample using the following equation:

$$TA={\displaystyle \frac{V_{NaOH}\times {meq}_{citric acid}\times N_{NaOH}}{W}}\times 100$$

(2)

where V_NaOH represents the volume of NaOH used in the titration, meq_{citric acid} is the equivalent value for citric acid (0.0064), N_NaOH stands for the normality of NaOH, and W is the weight of the homogenized sample.

2.6. Consumption of O₂ and Production of Ethylene (C₂H₄)

The evaluated fruits were stored in an environmental chamber equipped with a Felix F-950 gas analyser (F-950, Felix Instruments, QA Supplies LLC, Norfolk, UK) at 25 °C and 70% relative humidity (RH) to simulate the environmental conditions of tropical regions. Subsequently, ethylene (C₂H₄) production, as well as O₂ consumption, were measured on days 0, 2, 4, 6, 8, and 10.

2.7. Firmness

Firmness was evaluated using a texture analyzer (Shimadzu EZ Test, Kyoto, Japan). During the analysis, the probe penetrated the fruit to a depth of 5 mm, with a penetration distance set at 10 mm and a speed of 50 mm min⁻¹. Six fruits were randomly selected from each treatment group for measurement. For each fruit, five points were randomly chosen along the cross-section for testing. Measurements were conducted on days 0, 2, 4, 6, 8, and 10.

2.8. Redness

The colour of the tomato skin (20 fruits) was assessed using a CR-20 colorimeter (Konica Minolta, Tokyo, Japan) by taking five measurements at equidistant points on each fruit. The measurements were taken on days 0, 2, 4, 6, 8, and 10. The device was calibrated with black and white reference standards. The CIE Lab colour space parameters L*, a*, and b* were recorded, and the impact of the treatments on colour was evaluated based on the redness values. Redness was determined using the following equation:

$$Redness={\displaystyle \frac{{\mathrm{a}}^{\mathrm{*}}}{{\mathrm{b}}^{\mathrm{*}}}}$$

(3)

where a* is the degree of red-green coordinate, and b* is the yellow-blue coordinate

2.9. Decay Rate

The number of decayed fruits was recorded on days 0, 2, 4, 6, 8, and 10. The decay rate was determined using the equation proposed by Chen et al. [24] as follows:

$$Decay rate \left(\%\right)={\displaystyle \frac{N_t}{N_0}}\times 100$$

(4)

where N₀ represents the initial number of cherry tomatoes (20 fruits), and Nₜ corresponds to the number of rotten fruits at each respective time point (days 0, 2, 4, 6, 8, and 10).

2.10. Statistical Analysis

All analyses were conducted in triplicate, and the results were expressed as the mean ± standard deviation. Analysis of variance (ANOVA-one way) was performed to evaluate the significance of the model, with a p-value of less than 0.05 considered statistically significant.

3. Results and Discussion

3.1. Physicochemical Quality

Weight loss in fruits is mainly driven by nutrient depletion resulting from respiration and moisture loss through transpiration [24]. Figure 1a illustrates the trend in weight loss in cherry tomatoes throughout the storage period, revealing a consistent decline in weight across all treatment groups. None of the treatments led to weight losses surpassing 5%, as can be seen in Figure 1a. A longer storage duration was associated with a gradual increase in weight loss in all samples. Notably, uncoated fruits exhibited a significantly greater reduction in weight compared to those treated with edible coatings [24].

By the end of the 10-day storage period, the control group (uncoated fruits) showed the highest weight loss (p < 0.05), approximately 4.53%, likely due to their direct exposure to ambient conditions, which enhances internal water evaporation. In contrast, the application of edible coatings appears to serve as a physical barrier, reducing oxygen permeability and consequently lowering the respiration rate and associated metabolic activity [24]. Among the treatments, fruits coated exclusively with HAG exhibited the lowest weight loss, around 1.08% at the conclusion of the storage period. The ability of edible coatings to reduce weight loss is largely attributed to their semi-permeable characteristics, which restrict water vapor and gas exchange while also sealing microcracks in the fruit’s surface, effectively covering stomatal openings [25,26]. This barrier effect contributes to the mitigation of postharvest physiological processes, such as transpiration and respiration, which are responsible for moisture loss [27].

As shown in Figure 1b, the soluble solids content increased progressively up to the fourth day of storage. Both coated and uncoated fruits exhibited an initial rise in total soluble solids, followed by a subsequent decline. However, this decline is quite significant (p < 0.05) in the control fruits. The early increase can be attributed to metabolic processes that convert carbohydrates into sugars and other soluble compounds [28], as well as water loss due to dehydration. The later decrease in total soluble solids is likely associated with the consumption of nutrients during respiration [29].

At the end of the storage period, the lowest soluble solids content was observed in the control fruits (5.58 °Brix), whereas coated fruits maintained higher values ranging from 6.65 to 7.11 °Brix. Overall, coated tomatoes were more effective in retaining soluble solids throughout storage compared to uncoated ones. Considering that the majority of soluble solids are composed of sugars and organic acids [30] and given the lower content in uncoated fruits, it can be inferred that the application of edible coatings may slow down the ripening process of cherry tomatoes [29] by delaying the metabolic consumption of sugars and acids. Conversely, Álvarez et al. [31] reported higher soluble solids levels in uncoated cherry tomatoes compared to those coated with an exopolysaccharide and a lactic acid bacterium. This discrepancy may be attributed to the presence of lemon peel extract used as an active agent in the coatings evaluated in the present study. The levels of soluble solids and total titratable acidity are important indicators of ripeness and flavour in cherry tomatoes, making it relevant to also evaluate changes in acidity during storage. As shown in Figure 1c, total titratable acidity decreased significantly (p < 0.05) over the storage period, which can be attributed to the utilization of organic acids as substrates in the respiration process [32]. The primary organic acids typically found in cherry tomatoes are citric, malic, and oxalic acids [33].

At the end of the storage period, fruits coated exclusively with HAG exhibited the highest acidity levels (0.52%), followed by those coated with 75HAG/25LAG and 50HAG/50LAG, which showed acidity values of 0.50% and 0.48%, respectively. In contrast, the control fruits (uncoated) displayed the lowest final acidity level at 0.38%. These differences are likely related to the progressive decrease in organic acid concentration, which occurs as a consequence of fruit maturation and moisture loss [34].

In general, coated fruits retained higher titratable acidity than uncoated ones throughout the storage period, likely due to a slower consumption of organic acids [35]. The HAG-based coating likely acts as an effective barrier to oxygen penetration, thereby delaying the fruit’s respiration rate during storage and reducing organic acid degradation. Coatings based on HAG significantly reduce weight loss and the breakdown of organic acids in cherry tomatoes while also preserving soluble solids, making them a promising strategy for extending the shelf life of this fruit.

3.2. Oxygen Consumption and Ethylene Production

The quality of tomato fruit is influenced by multiple factors, with respiration rate being one of the most critical, as tomato ripening follows a climacteric pattern characterized by a marked increase in CO₂ production. Figure 2a shows the oxygen consumption of the coated and uncoated fruits. The application of coatings based on gellan gum and LPE significantly (p < 0.05) reduced O₂ consumption, with control fruits exhibiting the highest (0.15 mg·kg⁻¹·h⁻¹) oxygen uptake compared to coated samples. Among the coated fruits, those treated with HAG showed the lowest oxygen consumption (0.06 mg·kg⁻¹·h⁻¹), followed by those coated with a 75HAG/25LAG blend, which consumed 0.7 mg·kg⁻¹·h⁻¹. This reduction may be attributed to the ability of the polymer matrix to act as a barrier to oxygen diffusion, consistent with previous findings on polysaccharide coatings [36]. HAG undergoes gelation due to the formation of multiple hydrogen bonds between gellan helices, resulting in a more interwoven structure that leads to the formation of a more compact gel [13]. This, in turn, may create a more effective gas barrier, which explains the lower oxygen consumption values observed in fruits coated with HAG. The influence of edible coatings on fruit respiration is primarily linked to their capacity to limit oxygen permeability, which in turn reduces the availability of O₂ required for respiration [29]. These findings suggest that the gellan gum-based coatings may have altered the internal atmosphere of the cherry tomatoes, contributing to the preservation of important quality attributes, such as reduced weight loss and the slower degradation of organic acids and soluble solids during storage—all of which are respiration-dependent parameters [29].

In terms of ethylene (C₂H₄) production, Figure 2b illustrates a significant reduction (p < 0.05) in ethylene emission in coated fruits compared to the control group. Although no significant differences (p > 0.05) were observed during the initial days of storage, differences became apparent after day 4. At the end of the storage period, control fruits exhibited the highest ethylene production (6.20 mg·kg⁻¹·h⁻¹), while fruits coated with HAG showed the lowest values (3.10 mg·kg⁻¹·h⁻¹), followed closely by those treated with the 75HAG/25LAG coating (3.41 mg·kg⁻¹·h⁻¹). The reduction in ethylene production in fruits coated with HAG can once again be attributed to the formation of a more compact structure by HAG, as previously mentioned. These results indicate that the application of HAG-based and LPE-based coatings effectively reduced both oxygen consumption and ethylene production in whole cherry tomatoes. Since tomato ripening is regulated by ethylene, which drives various physical, biochemical, and physiological changes, the suppression of ethylene biosynthesis and respiratory activity provides a valuable strategy to delay ripening and extend postharvest shelf life [37].

3.3. Firmness Analysis

Firmness is a key quality attribute of tomatoes, and it is largely determined by the composition of polysaccharides and the structural integrity of the cell wall in the pericarp [38,39]. Nonetheless, both the harvest stage and storage duration significantly influence firmness loss [40]. Firmness is also a major criterion used by consumers to assess the ripeness and freshness of cherry tomatoes.

In this study, a decline in firmness was observed over storage time across all samples, including both coated and uncoated fruits, aligning with previous findings [41]. Figure 3 shows a marked and statistically significant (p < 0.05) decrease in firmness in the control tomatoes starting from the sixth day of storage. Similar trends were reported by Alvarez et al. [31], who found differences in firmness between uncoated cherry tomatoes and those coated with exopolysaccharides and lactic acid bacteria after six days of storage. At the end of the storage period, the lowest firmness was recorded in control fruits (2.30 N/mm²), whereas the highest firmness was maintained in tomatoes coated with HAG (3.11 N/mm²), demonstrating the beneficial effect of the coating on firmness retention. This effect is likely due to reduced water loss provided by the coating, as reported by Wu et al. [42].

Changes in firmness serve as an important indicator of ripening progression and overall fruit acceptability. During the respiration process, pectin degradation alters the cell wall structure and, combined with the breakdown of cellular architecture and intracellular components, results in fruit softening [43,44]. Therefore, applying a coating reduces the respiration rate, thereby delaying ripening and helping to preserve firmness during storage. These findings suggest that the combination of HAG and lemon peel extract effectively reduces firmness loss in cherry tomatoes during storage.

3.4. Redness

The redness (a*/b* ratio) of the cherry tomatoes during storage is illustrated in Figure 4. Initial colour measurements indicated that the cherry tomatoes exhibited a light red stage (0.78–0.73), which is considered an acceptable colour for consumption and commercial purposes. At the beginning of storage, a non-significant increase (p > 0.05) in redness values was observed in the control tomatoes compared to the coated ones. However, after six days of storage, this difference became significant in the control samples, which reached a value of 1.23, while the coated tomatoes exhibited redness values ranging between 1.16 and 0.86, corresponding to a transitional stage between light-red and red (0.95–1.21) [45].

The observed maintenance of redness in coated samples may be attributed to the incorporation of LPE into the coating matrix. These results are consistent with those reported by Salas-Méndez et al. [46], who found that tomatoes coated with nanolaminate films containing F. cernua extracts maintained a light red colour throughout a 15-day storage period, in contrast to other treatments. Therefore, the application of an edible coating formulated with gellan gum and lemon peel extract contributed to preserving the redness stage in cherry tomatoes.

Additionally, the increase in the a*/b* ratio of the samples suggests that the fruits were not fully ripened, as they had not reached their maximum ripening point. The lack of significant differences in colour between control and coated fruits at the onset of storage could also be attributed to similarly non-significant changes in natural pigment concentration during the storage period [47].

3.5. Decay Rate Analysis

Figure 5 presents the decay rate of the analyzed fruits, showing that during the early stage of storage, cherry tomatoes exhibited no visible decay. This phenomenon can be attributed to the integrity of the cell wall structure in freshly harvested fruits, which remains closely linked to pectin content [48]. As storage time advanced, particularly after the sixth day, a gradual increase in decay was observed in the coated samples. Nevertheless, coated fruits exhibited a significantly lower decay rate (p < 0.05) compared to the control group, where a 2% decay rate was already evident by day 4. Moreover, the effectiveness of the coatings persisted beyond the sixth day of storage. By the end of the storage period, the lowest decay percentages were recorded in fruits coated with HAG and 75HAG/25LAG formulations, both showing only 2% decay. This outcome may be attributed to the sustained release of the active compound (lemon peel extract) from the HAG matrix, providing prolonged protection to the cherry tomatoes. It is also important to note that significant differences were observed among the coated fruits after the 8th day of storage, suggesting that both HAG and LAG may also influence the decay rate. This effect could be attributed to the different gelling mechanisms of gellan gum and potential variations in their permeability values. The application of edible coatings delays fruit senescence by minimizing direct exposure to the external environment. Similar results were reported by Chen et al. [24], who noted that while both control and konjac glucomannan/curdlan-coated cherry tomatoes exhibited decay starting on the sixth day, the coated fruits maintained significantly better preservation compared to the controls.

4. Conclusions

The application of edible coatings based on gellan gum and LPE proved to be an effective strategy for preserving the postharvest quality of cherry tomatoes during 10 days of storage. Among the different formulations tested, the coating composed exclusively of high-acyl gellan gum (HAG) consistently demonstrated the best performance across various quality parameters. Specifically, HAG-coated fruits exhibited the lowest weight loss (1.08%) compared to 4.53% in uncoated control fruits, and this is likely due to the coating’s ability to reduce moisture evaporation and gas exchange. In terms of soluble solids, coated fruits maintained higher values (6.65–7.11 °Brix) than the control (5.58 °Brix), suggesting a delay in ripening. Similarly, titratable acidity was better retained, with HAG-coated tomatoes reaching 0.52%, while uncoated fruits decreased to 0.38%. The coatings also significantly reduced the respiratory activity of the fruits. HAG-treated samples exhibited the lowest oxygen consumption (0.06 mg·kg⁻¹·h⁻¹) and ethylene production (3.10 mg·kg⁻¹·h⁻¹) compared to 0.15 mg·kg⁻¹·h⁻¹ and 6.20 mg·kg⁻¹·h⁻¹, respectively, in the control group. These results highlight the effectiveness of the coatings in delaying physiological processes associated with ripening and senescence. In terms of texture, HAG-coated fruits maintained greater firmness (3.11 N/mm²) compared to the control (2.30 N/mm²), and they also exhibited significantly lower decay rates (2% by day 10) compared to the uncoated fruits, which began to deteriorate as early as day 4. These findings suggest that HAG-based edible coatings enriched with lemon peel extract can serve as a promising postharvest technology to extend shelf life and maintain the quality attributes of cherry tomatoes, reducing postharvest losses and enhancing their marketability.