Annual mango fruit (Mangifera indica L.) production in Mexico is over 1.45 million tons, and one of every five tons is destined to the export market. Thus, Mexico is ranked as the fourth largest mango-producing country after Indonesia (Servicio de Información Agroalimentaria y Pesquera [SIAP], 2014). Postharvest management is important for increasing the export volume of fruit and vegetable products. Therefore, new technologies have been implemented that help preserve them; one of the alternatives that is being promoted is the use of biodegradable packaging, including edible coatings that present a viable alternative to prolong fruit shelf life and quality.
Edible coatings, made with polysaccharides from nonconventional sources, add value to fruit and vegetable products, since there is an important number of plant species with high contents of starch and pectin, which could serve as the raw material for making coatings. Species containing these materials include some fruits such as banana (Musa paradisiaca), which at physiological maturity has important amounts of these carbohydrates (Bello-Pérez, Agama-Acevedo, Sayago-Ayerdi, Moreno-Damían, & Figueroa, 2000).
Starch is the most commonly used raw material for making biodegradable films, mainly because it is a low-cost renewable polysaccharide, which is abundant and relatively easy to manage (Lourdin, Della-Valle, & Colonna, 1995). Pectin is another one of the complex carbohydrates found in fruits, and it is one of the principal components of the primary and medium cell wall in plant tissues (Arellanes et al., 2011). Pectin is a nontoxic biopolymer, biocompatible and biodegradable (Sriamornsak, Wattanakorn, Nunthanid, & Puttipipitkhachorn, 2008), which is why it has been employed as an edible coating.
Materials and methods
Fruits were harvested in the community 14 de Marzo in the municipality of Tepic, Nayarit, Mexico. The evaluated fruits were washed with distilled water and disinfected with 1 % sodium hypochlorite. They were later selected on the basis of size, shape, color and absence of mechanical or phytosanitary damage. The selected fruits were then treated and and stored at 10 ± 2 °C for 12 days; subsequently, they remained nine days at laboratory temperature (22 ± 2 °C) to simulate export conditions.
Preparation and application of starch, pectin and chitosan
A watery solution (1 %) of each polysaccharide (starch and pectin) was prepared. Additionally, 1 % chitosan, extracted from shrimp, was evaluated. The following treatments were generated: 1 % starch (T1), 1 % pectin (T2), 1 % chitosan (T3), starch-chitosan (T4), pectin-chitosan (T5) and control (T6).
All variables were measured every six days, starting from day 1 until day 21 of postharvest storage.
To determine this variable a digital scale (Snova ES-210®) and the following formula were used:
where WL is weight loss, fw is final weight, and iw is the weight of the fruits at the beginning of the experiment. The values were reported in percentage (%).
To measure fruit firmness, a penetrometer (Digital Fruits model GY-4) with a 0.8-mm diameter pressure head was used. The results are expressed in kilograms force per square centimeter (kgf∙cm-2).
Titratable acidity (TA)
TA was determined with NaOH at 0.1N and phenolphtalein at 0.5 % as indicator (Association of Official Analytical Chemists [AOAC], 2005). The values are reported as a percentage of malic acid present in the fruit. The formula used was:
where Ma is the % of malic acid, V the volume in mL of titratable NaOH, N the solution of NaOH (0.1 N), meq the weight in milliequivalents of malic acid (0.067 meq) and Y the volume in mL of sample.
Total soluble solids (TSS)
They were determined with a refractometer (Spectronics Instruments model 334610) by means of AOAC methodology (2005). The values were expressed in degrees Brix (°Brix).
The color was measured in the epidermis of the fruit and in two of its equatorial zones, using a Minolta CR-300 model colorimeter; the reading was related to the parameters L*, a*, b*, where L* is the luminosity reflected by the fruit, and the values go from 0 (black) until 100 (white); a* indicates the value from green (-) to red (+), and b* indicates the value of the color going from blue (-) to yellow (+), and these were converted to chromaticity (C) and hue angle (h°) parameters, which were calculated by applying the following equations (García-Tejeda, Zamudio-Flores, Bello-Pérez, Romero-Bastida, & Solorza-Feria, 2011).
C = (a*2 + b*2)½
h° = tan-1 (b*/ a*), when a* > 0 and b* ≥ 0
h° =180 + tan-1 (b*/a*), when a* < 0
The harvest index of mango fruits are based on the color of peel and pulp, ranging from yellow-green colors in peel, and from yellow to orange in pulp.
Experimental design and statistical analysis
A completely randomized experimental design with a 6x2 factorial arrangement was used. There were six treatments (T1 = starch, T2 = pectin, T3 = chitosan, T4 = starch/ chitosan, T5 = pectin/chitosan) and two temperature levels (10 ± 2 °C and 22 ± 2 °C). The experimental unit was a fruit with 5 replications. The statistical analysis of the results was carried out with an ANOVA test (Tukey, α = 0.01) by means of the SAS statistical package (SAS, 2000).
Results and discussion
There were no differences (P > 0.01) among the treatments (Table 1); nevertheless, the least weight loss occurred with the 1 % starch-chitosan mixture (3.51 %), followed by 1 % chitosan (3.60 %) and 1 % starch (3.67 %), with respect to the control (3.88 %). Regarding postharvest performance of ‘Ataulfo’ mango, it was observed that for day 6 the loss was 1.64 % to 3.46 %, and for day 12 it was 1.15 % to 2.2 % with respect to the control; weight loss, however, increased (7.60 %) from day 12 to day 18 (Table 2). This tendency was possibly due to the stress the fruits suffered at being transferred from 10 ± 2 °C to 22 ± 2 °C. Similar values were obtained by Valera, Materano, Maffei, Quintero, and Zambrano (2011), who reported that in ‘Bocado’ mango fruit covered with 2 % starch and stored at 15 °C for 16 days, weight loss was 3.5 % to 4 %. Almeida-Castro, Reis-Pimientel, Santos-Souza, Vieira-de Oliveira, y da Costa-Oliveira (2011) also recorded that in papaya coated with 2 % starch and stored at 8 °C for six days, weight loss was 1.33 %.
During storage of mango fruits at 10 ± 2 °C, differences (P > 0.01) in firmness (Table 1) were not observed; however, when they were transferred to 22 ± 2 °C, firmness diminished, with significant statistical differences being observed (P < 0.01) (Table 3). ‘Ataulfo’ mango fruits with 1 % starch had 3.34 kgf∙cm-2 firmness loss, followed by 1 % starch/chitosan (3.28 kgf∙cm-2), chitosan (3.14 kgf∙cm-2), pectin (3.26 kgf∙cm-2) and 1 % pectin/chitosan (2.90 kgf∙cm-2), with respect to the control fruits (2.26 kgf∙cm-2). This loss of firmness may be attributed to the fact that the non-water-soluble pectic fraction diminishes, implementing the soluble fraction, which caused the reduction in firmness. In addition to this, the carbohydrates, splitting from simpler units, reduce the fruit firmness (Pérez-Rivero, Bringas, Cruz, & Báez-Sañudo, 2003). Similar results were found by Zhu, Qiuming, Cao, and Jiang (2008) using chitosan at different concentrations in ‘Tainong’ mango. Valera et al. (2011) reported firmness of 6.5 kgf∙cm-2 with 2 % chitosan coatings and 5.5 kgf∙cm-2 with 2 % starch coatings in ‘Bocado’ mango fruits stored at 15 °C for 16 days.
Table 4 presents the changes in titratable acidity (TA). TA diminished while TSS (Table 5) increased during storage time, presenting differences (P < 0.01) during the 12 days of storage until day 18, where TA diminished (0.61 %) in all treatments, compared to the control fruits; for day 21 the fruits presented similar values (P > 0.01) in the treatments. TA decreased by 0.24 % in fruits coated with chitosan, starch/chitosan, pectin/chitosan, and control fruits, with respect to the fruits covered with starch (0.30 %) and with pectin (0.18 %). Zambrano, Maffei, Materano, Quintero, and Valera (2011), in assessing coatings of starch, methyl cellulose, and chitosan, obtained similar values of 0.38, 0.46, and 0.42 %, respectively. It is assumed that these changes in the reduction of content of organic acids are due to their being used in the respiration process or being turned into sugars (Alia-Tejacal, Colinas-León, Martínez-Damián, & Soto-Hernández, 2002). Another factor which could interfere in TA is the modified atmosphere generated by the coating, which by accumulating CO2 in the tissue results in higher fruit acidity (Zambrano et al., 2011).
Total soluble solids (TSS)
Table 5 shows the changes in total soluble solids (TSS) during the time of postharvest storage. Statistical difference (P < 0.01) in TSS was observed starting from day 12 of storage of coated mango fruits, with respect to the control. Zambrano et al. (2011) reported similar behavior in ‘Bocado’ mango fruits covered with starch, methyl cellulose, and chitosan. In the present study, the highest TSS values were obtained at 21 days of storage (15 to 17 °Brix).
Luminosity (L*) of the coated fruits showed no differences (P < 0.01) during postharvest storage compared to control fruits, so the coatings had no negative impact on the ‘Ataulfo‘ mango fruits (Table 6). Concerning the hue angle and chromaticity, the fruits stored for 12 days at 10 ± 2 °C had values between 102 and 108 degrees and chromaticity between 65 and 68, reasserting the green color of the mango fruit epicarp. It was observed that the fruits, upon being transferred to storage at 22 ± 2 °C (day 12 to day 21), on day 18 acquired a light yellow coloring, and up to day 21 they presented values of 87 degrees and chromaticity of 65, thus reaching its yellow coloring, typical of ‘Ataulfo’ mango (Tables 7 and 8). Color changes during the ripening process of most fruits are mainly the result of chlorophyll degradation and the synthesis of pigments such as carotenoid and anthocyanin (Brownleader et al., 1999). Similar values are reported using 2 % starch and 2 % chitosan coatings applied to ‘Bocado’ mango, obtaining L* = 61 and L* = 58, respectively; Pérez et al. (2003) recorded values of L* = 63.64 in ‘Tommy’ mango stored at 20 °C during a 12-day period.
‘Ataulfo’ mango fruits coated with starch from ‘Pear’ banana (Musa ABB) had greater firmness (3.34 kgf∙cm-2) and a high content of total soluble solids (16.96 °Brix) in relation to the control fruits (2.26 kgf∙cm-2 and 15.8 °Brix, respectively); the coating prolonged the postharvest period to 21 days.
The edible chitosan coating evaluated in the analyzed variables did not stand out when compared to the starch coating.
Edible coatings did not affect the cuticle color of ‘Ataulfo’ mango fruit and preserved its typical yellow color.