Postharvest Life Extension of Fresh-Cut Mango ( Mangifera indica cv . FaLun ) Using Chitosan and Carboxymethyl Chitosan Coating

Postharvest life extension of fresh-cut mango (Mangifera indica cv. Fa-Lun) using chitosan and carboxymethyl chitosan (CMCH) coating was studied. Fresh-cut mango was treated with chitosan and carboxymethyl chitosan solution of 0.5-1.5% w/v, after that fresh-cut mango was placed on foam tray, over-wrapped with PVC film and then stored at 6 °C. Weight loss, texture analysis, soluble solid content, color and sensory quality were evaluated. The shelf life of non-coated fresh-cut mango was only 2 days while that of fresh-cut mango coated with chitosan and carboxymethyl chitosan was 4 and 6 days, respectively. Effect of chitosan concentration on quality of fresh-cut mango was significantly different but of carboxmethyl chitosan concentration was not. In this study, Coating with carboxymethyl chitosan could extend shelf life of fresh-cut mango by delayed flesh browning which correlated to the sensory score.


Introduction
Seafood industry in Thailand has brought a considerable amount of income.However, the scraps after seafood processing, mainly the shells of crustaceans and squids are priceless.Many researchers try to use these scraps as raw materials for the production of chitin polymer.Chitin is one of the most abundant natural polymers next to cellulose, found particularly in the shell of crustaceans such as shrimps, crabs and insects and also cell wall of fungi (Kittur et al., 2002;Abdou et al., 2008;Elieh-Ali-Komi & Hamblin, 2016;Philibert et al., 2017).Chitosan is produced by chemical deacetylation of chitin.It is a biopolymer with low toxicity and had been used in various applications.In biomedical and pharmaceutical applications, chitosan film is used as drug-releasing-control agent (Shu & Zhu, 2002;Islam et al., 2017).In waste water management, chitosan can adsorb toxin and humic acid that can be got rid out of water later.Chitosan is also used as a supplementary food (Yinsong et al., 2007;de Abreu and Campana-Filho, 2009;Gómez-Mascaraque et al., 2016).In agricultural application, chitosan coating have been widely used on fruit and vegetable mainly with the purpose of providing it with a modified atmosphere, reducing gas transfer, retarding moisture and flavor loss, delaying color changes, and improving the general appearance of the produce through storage time.Pen and Jiang (2003) studied the storage life of fresh-cut Chinese water chestnut using chitosan coating and found that the treatment could inhibit the activities of polyphenol oxidase, peroxidase and phenylalanine ammonia lyase (PAL).They also found that chitosan coating delayed the decreasing of anthocyanin and inhibited polyphenol oxidase activity.Chien et al. (2007) studied the effect of chitosan coating on quality and storage life of fresh-cut mango and found that the treatment could preserve quality and prolong the storage life of the product.Vargas et al. (2009) studied the effect of chitosan coating in combination with vacuum condition on the storage of fresh-cut carrot and found that the treatment could reduce water permeability and color change.However, there are also problems in using chitosan.The solubility of chitosan is restricted to only acid solution (de Abreu and Campana-Filho, 2009;Ryu et al., 2015;Wang et al., 2015;Rui et al., 2017) and it represents a serious drawback to many of its potential applications, especially the odor of acid on the product which is not accepted by the consumers.Therefore, the chemical modifications of chitosan structure are normally designed to increase the polymer solubility in a wide pH range by the carboxymethylation of chitosan through the direct alkylation (El-Sherbiny, 2009).de Abreu and Campana-Filho (2009) studied the synthetic pathway of carboxymethyl chitosan and the influences of various variables (the proportion of water/isopropanol, chloroacetic acid content, temperature and reaction duration) on the substitution degree of carboxymethyl chitosan.In the synthesis of carboxymethyl chitosan or sodium carboxymethyl chitosan, it started with chloroacetic acid as a reactant (etherifying agent) in alkaline condition (pre-alkalized chitosan) with alcohol solvent.The carboxymethylation reaction of chitosan occurred at the site of amino group and hydroxyl group of the polymer.Due to the protonation and highly alkaline properties of amino group in chitosan, these made it soluble in only acid aqueous solution (pH < 6.5) which restricted its applications.Making chitosan derivatives could solve this solubility problem (Miao et al., 2008;An et al., 2009;de Abreu and Campana-Filho et al., 2009).At present, there are various applications in using carboxymethyl chitosan as edible coating because of its solubility property.Zhou et al. (2008) coated carboxymethyl chitosan in pear and found the reducing of respiration and weight loss, but the efficacy was still less than when coated with shellac.The carboxymethyl chitosan had a good effect on decreasing malondialdehyde content and delaying spoilage of peach fruits suggesting their possible use as preservative and antioxidant coating (Elbarbarya & Mostafa, 2014).Wu and Yang (2015) suggested that brassinolide and carboxymethyl chitosan coating has the potential to maintain the quality parameters of asparagus spears and extend its storage life.Benhabiles et al. (2013) concluded that the effect of a chitosan coating seems to be comparable to an N,O-carboxymethyl chitosan coating in improving postharvest preservation of tomato fruit.There has little report about using carboxymethyl chitosan for coating fresh-cut product.This research aimed to study the effect of carboxymethyl chitosan (CMCH) as coating agent for fresh-cut mango and compare the efficacy of carboxymethyl chitosan to chitosan at various concentrations on the quality and storage life of fresh-cut mango.

Carboxymethyl Chitosan (CMCH) Synthesis
Carboxymethyl chitosan was synthesized according to Tantala et al. (2012aTantala et al. ( , 2012b)).The chitosan powder was suspended in a solution (NaOH: IPA: distilled-water) for 1 h.The appropriate amount of monochloroacetic acid was added into the solution for 30 min.The mixture was continuously reacted at 50 °C for 4 h.The solid phase was separated from the liquid phase and then suspended in 95% methanol.The reaction was stopped by adding glacial acetic acid and the pH was adjusted to neutrality.After that, the solid was washed in 70% ethanol to desalt and filtered and then rinsed once with 95% ethanol.The carboxymethyl chitosan powder was dried in a hot air oven at 50 °C overnight.

Minimally Processed Mangoes
Mango fruits were washed, peeled and cut into strip pieces.Fresh-cut mangoes were coated with 0.5, 1.0, 1.5% w/v chitosan and 0.5, 1.0, 1.5% w/v carboxymethyl chitosan compared with control treatment (non-coated).The samples were placed in the foam tray and wrapping with PVC film.Finally, all samples were stored at 6±2 °C.Five pieces per treatment were used to study each quality parameter every two days.

Weight Loss
Weight loss of all samples was individually monitored every two day with an analytical balance.In a pectin

Firmness
Table 1 shows the firmness of chitosan and carboxymethyl chitosan-coated fresh-cut mango.Flesh firmness decreased with storage period in every treatment with no statistical difference (p < 0.05) between treatments, in accordance with weight loss percentage in Figure 1.The degradation of the cell wall constituents and polysaccharides by the action of pectin esterase and polygalacturonase on the solubilization of pectin substrates were causes of fruit softens (Yashoda et al., 2007;Razzaq et al., 2013).The firmness of fresh-cut mango significantly reduced throughout storage life, but this decrease was more pronounced for chitosan and non-coat sample from the fourth day, meaning that the CMCH was able to maintain flesh texture.According to Chiumarelli et al. (2011) in fresh-cut mangoes coated with sodium alginate or cassava starch, higher firmness preservation in coated samples along the postharvest time.Note.A, B, C : Different letters in the same row indicate significant differences among formulation (p < 0.05); a, b, c : Different letters in the same column indicate significant differences among formulation (p < 0.05).

Soluble Solid Contents (SSC)
The effect of chitosan and carboxymethyl chitosan coating on soluble solid contents of fresh-cut mango is shown in Table 2.The SSC of fresh-cut mango was about 7.4% (day 0), and it slightly increase until the end of storage (day 8).Soluble solid contents increased with storage period.The result was in accordance with Srinivasa et al. (2002) who also reported the increasing in soluble solid contents in mango with storage period.Note.A, B, C : Different letters in the same row indicate significant differences among formulation (p < 0.05); a, b, c : Different letters in the same column indicate significant differences among formulation (p < 0.05).

Color Changes
The effect of chitosan and carboxymethyl chitosan coating on color measurement of fresh-cut mango is shown in Figure 2. Light value (L*) tended to decrease in all treatments, especially in non-coating group.Coating with carboxymethyl chitosan gave the best result in delaying the decrease of L* value, followed by coating with chitosan as shown in Figure 2a.The different concentrations of chitosan gave significantly different results in L* value while this difference was not found in carboxymethyl chitosan treatments.The effect of chitosan and carboxymethyl chitosan coatings on color measurement on green to red color (a* value) of fresh-cut mango is  carboxymethyl chitosan gave the best physical qualities (color).The carboxymethyl chitosan could delay browning reaction better than non-coating and coating with chitosan.

Figure
Figure 2. C

Table 1 .
Firmness of chitosan and carboxymethyl chitosan-coated fresh-cut mango

Table 2 .
SSC of chitosan and carboxymethyl chitosan-coated fresh-cut mango