Introduction

It is estimated that postharvest losses in fruits and vegetables are 62%, which are generated mainly during activities related to transport, storage, packaging, and the marketing process, which is why the challenge arises to ensure that the product reaches the consumer with quality similar to that at the time of harvest (^{IICA, 2018}). This makes it necessary to apply sustainable, economical, environmentally friendly, non-destructive, loss-reducing techniques based on food safety criteria.

Among the alternative technologies is the study of edible coatings to extend the shelf life of a fruit against the use of plastic materials. An edible coating (EC) is the product of the formation of three-dimensional networks derived from the denaturation of edible substances, such as proteins, polysaccharides, or lipids, and is defined as a thin layer of edible material formed directly on the surface of the food by immersion or spraying (^{Otoni et al., 2017}).

Among the functions of ECs are: prolonging the shelf life by controlling the transfer of gases, avoiding the loss of firmness and moisture, regulating ripening, reducing metabolic processes during storage, allowing the addition of other compounds; in addition, they can improve the appearance and quality of the coated product to make it more striking due to its brightness or color; bio-based ECs are gaining importance due to their biodegradability, sustainability, and respect for the environment (^{Mora-Palma et al., 2021}).

Research has been carried out applying EC to fruits: ^{Kumar et al. (2021}) managed to extend the shelf life of the green bell pepper since the quality of storage at 4 °C, the phenolic content, firmness, and antioxidant activity were maintained, this due to the application of an EC based on pomegranate peel extract and chitosan. ^{Mthembu et al. (2021}) studied the effect of an EC based on moringa leaf extracts and carboxymethylcellulose on tomato fruits, they reported decreases in the changes of texture, total soluble solids, and reduced spoilage compared to untreated fruit, so shelf life was extended by delaying ripening.

Coatings based on basil seed gum and oregano essential oil in apricots improved their quality, odor, and acceptability, while the essential oil decreased water vapor permeability and showed an increase in moisture in the coating (^{Hashemi et al., 2017}). Most of the matrices that make up ECs are intended to be from renewable sources or byproducts of agribusiness, in order to make them biodegradable and reduce the use of non-biodegradable plastic packaging.

One of the polymers used is agar, which is extracted from red algae, considered as Gras, it forms good quality, colorless, flexible, and low moisture content films, with excellent oxygen barrier properties (^{Mostafavi et al., 2020}). The mango peel represents 20% of the weight of the fruit, is considered an agro-industrial waste and has a high potential to be used to obtain molecules with antioxidant activity due to the presence of bioactive compounds such as polyphenols, flavonoids, and carotenoids.

The mango peel of the Ataulfo variety contains 68.13 mg of polyphenols per gram of dry weight (^{Lizárraga and Hernández., 2018}) and can be used to incorporate it through powder, oils, or extracts into ECs. Capsicum chinense var. habanero is one of the most important crops in Mexico, it is a product with a high socioeconomic impact that is characterized by the presence of various secondary metabolites of great value for the pharmaceutical and food industries, such as capsaicinoids, polyphenols, vitamins, and carotenoids, which give it added value (^{Oney-Montalvo et al., 2020}).

Nevertheless, it is a perishable fruit, has a shelf life of two weeks with storage at 22 °C, while at 7 °C in combination with modified atmospheres using perforated polyethylene bags, it can be kept for up to 20 days followed by 5 days at 22 °C (^{Pérez-Ambrocio et al., 2018}). Therefore, the objective is to evaluate the effect of an edible coating based on agar and mango peel extract on the quality and shelf life of Capsicum chinense Jacq. var. habanero.

Materials and methods

Results and discussion

The results of the characterization of the coatings are shown in Table 1; it was observed that the EC1 with the lowest percentage of mango peel extract (2%) presented greater thickness compared to the one containing 3% of the extract, there is no significant difference between them with a p≤ 0.5. Agar is a hydrocolloid that binds water within the film, by increasing the extract content and having less water content, it allows an easier exit of water during drying and consequently, a slight decrease in thickness.

Most ECs are hydrophilic in nature and it has been found that the thicker the film, the greater the resistance to the transfer of water and gases through them, since the thickness determines the distance that the permeate (water vapor and gases) must travel to diffuse from one side of the film to the other (^{Park and Chinnan, 1995}). It is mentioned that this parameter must be considered so that the edible coating in general forms a fine and thin layer on the food (^{Al-Hassan and Norziah, 2012}).

Significant differences (p≤ 0.05) were found in opacity, where it was higher in EC1 compared to EC2; this property is desirable since light catalyzes the oxidation and degradation processes of nutritional compounds, such as vitamin C content (^{Solano-Doblado et al., 2018}), in this case, mango peel extract gave the opacity by acting as a barrier to light.

Regarding the percentage of moisture, no significant differences were observed between the coatings, so the content of mango extract did not have an effect on this parameter but it did have an effect on the solubility, this has been an important factor that determines the integrity of the coatings in an aqueous medium, and on the biodegradability, when used as a means for food protection, this parameter involves the penetration of water molecules into the polymer matrix, followed by the breakdown of Van de Waals forces (^{Archundia et al., 2016}).

ECs had a moderate solubility, with EC1 having the lowest percentage, which is desirable during storage as it limits the exudate of fresh products, so the use of low solubility films is important in foods that require protection against moisture (^{Montalvo et al., 2012}).

Both coatings presented high values in the percentage of free radical inhibition, but they were not statistically significant between them. This antioxidant capacity was provided by mango peel extract, which provides antioxidant substances due to the presence of polyphenols (^{Lizárraga and Hernández, 2018}).

According to the previous results, the EC1 formed by 2% agar, 2% glycerol, and 2% mango peel extract was used due to solubility, opacity, and percentage of free radical inhibition to be used in fresh habanero chili as a coating.

Regarding the biodegradation of EC1, in the first three days, its weight decreased drastically (56%) and at the end of the 30 days, there was a total weight loss of 60%, which is related to the loss of water-soluble materials or low molecular weight compounds (glycerol) and microbial activity, especially due to enzymatic degradation.

The microorganisms, represented by CFU g^-1 soil, were higher by 278.57% after 30 days of weight loss; likewise, the nitrogen content of the soil also increased (0.324 ±0.011 at the beginning and 0.46 ±0.009 at the end of 30 days), which may indicate the presence of nitrogen-fixing bacteria, therefore, the biodegradation process does generate significant changes in the soil.

It has been mentioned that this process depends on several factors such as temperature, moisture, the type of soil used, the microbial load present, among others (^{Emadian et al., 2016}). In habanero chilies, the decrease in moisture content is one of the main causes of spoilage leading to loss of weight, appearance, softening and nutritional value and is partly due to the limited water-holding capacity of the fruits due to their hollow nature, but differs between species, varieties, and maturation stages (^{Calvo et al., 2018}).

During refrigerated storage, there was an increase in moisture content, from 60.89% ±1 on day zero to 63.7% ±0.5 and 64.43% ±0.487 in uncoated and coated chilies, respectively, at the end of the fifth week. As it is a non-climacteric fruit, the low storage temperature, 3 °C, and the relative humidity conditions (> 95%) resulted in a decrease in the transpiration of the fruit and less permeability of the membranes, which caused a positive effect in the delay of its senescence and in the loss of moisture; this was reflected in the shelf life of the chilies, 35 days for the coated ones and 25 to 30 days for those in the control batch. ^{Chenlo et al. (2005}) reported an increase in moisture content in bell peppers and showed that storage with an external film affects the kinetics of moisture loss.

The acidity content of habanero chilies (Figure 1), was significantly different between W/C (coated) and U/C (uncoated) chilies during storage, where W/C chilies showed an increase in acidity content, this behavior is associated with the edible coating that acts as a modified atmosphere that slows down the metabolic process in the fruits, which results in an accumulation of organic acids that, together with temperature, delays the degradation of this acid, this causes the chilies to remain green and acidic (^{Figueroa et al., 2013}). On the other hand, in control chilies, this variable tends to remain constant throughout storage, with a decreasing dynamic due to an increase in the volatilization of organic acids as there is no coating.

Figure 1 Acidity content (% citric acid) in chilies with and without edible coating. 

Vitamin C showed a tendency to decrease throughout storage and although the values found were slightly higher in the coated fruits, statistically there is no significant difference between them and the control batch (p≥ 0.05) during weeks 0, 2, 3 and 4; significant differences were observed at weeks 1 and 5, where the uncoated fruits of the latter showed a more pronounced reduction (Figure 2).

Figure 2 Vitamin C content (% ascorbic acid) in chilies with and without edible coating. 

W/C chilies managed to retain 86.11% of the vitamin C at the end of storage, this decrease in the loss of its content can be attributed to the low permeability to O₂ induced by the coatings, which delays the oxidation reactions that degrade this nutrient. EC had a positive effect on the preservation of vitamin C, since ^{Espinosa-Torres et al. (2010}) reported losses of up to 40% concentration in manzano chilies coated with pliofilm and stored at 5 °C, so it is advisable to consume it freshly harvested.

Polyphenols are one of the main metabolites found in habanero chilies, which contribute to the antioxidant effect of this type of fruit. The results obtained revealed that the polyphenol content in the control fruits and in the coated fruits showed a tendency to increase until the second week, and then decrease and increase again in the 4^th and 5^th week for W/C chilies (Figure 3).

Figure 3 Polyphenol content in chilies with and without edible coating. 

This increase in the content of polyphenols in chilies is related to the activity of the phenylalanine ammonium lyase (PAL) enzyme, as this is increased by the stress of the treatments (temperature control, coating + temperature), since it causes deterioration and generates the decomposition of the cell structure with the presence of a higher content of polyphenols (^{Oms-Oliu et al., 2008}), the values obtained are within the range reported by ^{Oney-Montalvo et al. (2018}), for green chilies grown in red, brown and black soils (64.9, 49.8 and 53.3 mg GAE 100 g^-1, respectively).

The loss of firmness caused by changes in the structure of the cell wall and by the decrease in moisture as a result of the decrease in cell turgor is one of the parameters that has the greatest impact on the quality of the habanero chili. According to the results obtained, the coating had a positive effect on the texture of the chilies, since the W/C fruits showed a significantly higher texture in terms of firmness with respect to the U/C fruits, the maintenance of this characteristic is due to the fact that the coating acts as a modified microatmosphere by creating a partial barrier to water and decreasing the concentration of oxygen, the enzymatic activity causing cell wall degradation is reduced, allowing the retention of firmness in chilies (Figure 4).

Figure 4 Texture in chilies with and without edible coating. 

Similar results were found by ^{Ordoñez-Bolaños et al. (2014}) during the conservation of pepper (Capsicum annuum L.), who demonstrated that the application of an edible coating based on modified cassava starch and thyme essential oil presented significant changes in the barrier properties of the samples, delay in the loss of firmness, and decrease in deterioration compared to uncoated fruits.

The capsaicinoids responsible for the spiciness in chili fruits showed significant variation between those with the coating and the control batch, where the former were the ones with a higher content of this variable throughout refrigerated storage.

This increase in W/C fruits is due to an increase in the rate of synthesis and a decrease in degradation levels due to a decrease in the activity of the isoenzymes involved in oxidation, due to the oxygen-poor atmosphere created by the coating by also reducing the activity of peroxidases, which are involved in the degradation of capsaicin and dihydrocapsaicin (^{Barbero et al., 2016}), other authors mention that capsaicin tends to increase under conditions of greater stress, in the case of habanero chilies due to the coating plus temperature (^{Pedraza-Chaverri et al., 2017}) (Figure 5).

Figure 5 Capsaicin content in chilies with and without edible coating. 

The values found for this metabolite are higher than those reported by ^{Oney-Montalvo et al. (2018}) for habanero chilies, of 247.5 to 537.6 mg 100 g^-1, this variation in capsaicin content is attributed to botanical variety, plant maturation, geographic origin of plants, soil type, and extraction and analytical methods.

Since the total antioxidant capacity of a fruit is the sum of its antioxidant capacity in the hydrophilic phase and lipophilic phase (^{Repo and Encina., 2008}), the percentage of free radical inhibition was determined by the ABTS radical assay, this solution presents a decrease in absorbance when it is reduced by the hydrogen or electron donor species (antioxidant substances contained in the habanero chili).

The percentage of free radical inhibition for W/C chilies was significantly higher compared to U/C chilies, with values in a range of 96 to 93% inhibition, these percentages are higher than those reported by ^{Oney-Montalvo et al. (2018}) for habanero chilies, whose inhibition values range from 88 to 91% depending on the color of the soil in which they were sown (Figure 6).

Figure 6 Percentage of free radical inhibition in chilies with and without edible coating. 

The antioxidant activity of a food is the expression of the different polyphenolic components, which use different mechanisms of action to neutralize reactive oxygen species (^{Zapata et al., 2013}); in this case, a moderately strong correlation was observed between the content of polyphenols and the percentage of inhibition of free radicals (r= -0.5798), this indicates that the inhibition of these radicals depends mainly on polyphenolic compounds.

Conclusions

According to the results found in this work, the edible coating of 2% agar, 2% glycerol, and 2% mango peel extract presented good characteristics in solubility, antioxidant activity and opacity; it is a biodegradable coating friendly to the environment and easy to prepare, its application in Capsicum chinense var. habanero helped to maintain quality characteristics with a shelf life of up to 35 days at 3 °C compared to uncoated fruits, which reached only 30 days.

The coated chilies managed to maintain acidity, firmness, polyphenol, and capsaicin content, reached high values of free radical inhibition, so this coating is considered as an alternative method to extend the shelf life of the habanero chili, without altering its quality characteristics.