Chitosan coating forming solution was prepared by dissolving 1% (w/v) of chitosan in 1% (v/v) acetic acid solution. 1% (w/v) of Tween 20, 0.5% (w/v) glycerol and 0.5% (w/v) of glucose were added before the solution was homogenized by using an ultraturrax (Polytron, Switzerland) at 11000 rpm for 4 min.

Starch coating forming solution was prepared according to Santacruz et al. (2015). A 0.5% (w/v) cassava starch suspension in water was heated from room temperature up to 90 °C, under stirring, where it was kept for 5 min. 1% of Tween®-20, 0.5% (w/v) glycerol, and 2 mmol L^-1 of SA were added before cooling. Once the solution was cooled to room temperature, glucose (0.5%, w/v), 0.15% (w/v) cinnamaldehyde (>95%) and 0.15% (w/v) thymol (98.5%) were added. Finally, the coating forming solution was homogenized by using an ultraturrax (Polytron, Switzerland) at 11000 rpm for 4 min.

Weight loss was calculated by the following equation.

Where: WL: Percentage of weight loss, W0: Fruit weight at time zero, Wt: Fruit weight at any storage time.

Titratable acidity was determined by titration with 0.01 M NaOH solution according to the AOAC method (1990), the results of three measurements were reported as a percentage of citric acid. Total soluble solids were determined according to the AOAC method (1990). Three fruits were disintegrated using a household blender and the obtained juice was filtered with textile and analyzed with a refractometer (Atago, Japan). The results of three measurements were reported as °Brix.

Puncture tests were performed in triplicate at the central part of three fruits according to Castro et al. (2014). A Shimadzu texturometer (Model EZ-LX, Japan) together with a stainless-steel probe of 3 mm diameter and 8 cm length were utilized. The probe was inserted 15 mm into the fruit at a speed of 10 mm s^-1 and the maximum penetration force was recorded.

All measurements were performed in triplicate. ANOVA and Tukey test with a significance level of 5% were run using InfoStat statistics software (Infostat version 2014, Argentina).

Mangoes coated with cassava starch containing salicylic acid (SSA) showed the highest weight loss, whereas fruits coated with starch-cinnamaldehyde-thymol (SCT) showed the lowest weight loss throughout the whole storage time (Table 2). SCT mangoes showed a weight loss that varied between 4.4 and 15.7%, which was statistically different than SSA samples. After the first week of storage, no difference in weight loss was found among uncoated mangoes and mangoes coated with either chitosan or SCT. The hydrophilic nature of starch may contribute to losing water in a higher amount in mangoes coated with SSA compared to uncoated samples. The use of cinnamaldehyde-thymol together with starch (SCT) led to lower weight loss compared to SSA. The hydrophobic properties of cinnamaldehyde-thymol (Man et al., 2019) may be the reason for such behavior. The incorporation of EOs into the coating-forming solution can confer water-resistance properties to coatings because this oil has a hydrophobic nature (Sánchez-González et al., 2011).

Table 2

Titratable acidity showed a decrease along the four weeks of storage. Biochemical changes, e.g., ascorbic acid content on mangoes during ripening may lead to a reduction of titratable acidity (Pandarinathan and Sivakumar, 2010). There was no difference in titratable acidity among samples up to the second week of storage (Table 3). However, on the third week of storage, mangoes coated with chitosan, SSA, and SCT samples, ripened more slowly as indicated by higher acidity than uncoated samples. On the fourth week of storage, only mangoes coated with SSA showed lower acidity than the uncoated sample. A decline in acidity demonstrates advancement of maturation (Maftoonazad et al., 2008), thus the coated fruits contributed to delaying the fruit maturation/ripening. Higher acidity in coated fruits may be the result of the formation of carboxylic acid by dark fixation of CO₂ (Maftoonazad et al., 2008).

Table 3

The four treatments showed an increase in total soluble solids along the storage time. Total soluble solids had no differences among samples up to the first week (Table 2), however, an increase for the four samples during the second week of storage was noticed. At the end of storage, the uncoated sample reached the highest value of total soluble solids (approximately 14 °Brix), which was statistically different than the three coated mangoes (total soluble solids between 9 and 11 °Brix). No statistical difference was found among total soluble solids for coated mangoes. The low value of mangoes coated with SSA suggests that SA may reduce the rate of ripening. This ripening reduction may be probably through inhibition of ethylene biosynthesis (Yin et al., 2013). In fact, Lo'ay (2017) found that an exogenous supply of SA delays the ripening of grapes.

Results of puncture tests showed that the maximum force of penetration decreased during the storage time for all samples. Mangoes coated with SSA and chitosan showed statistically similar forces of penetration which were also higher than SCT and uncoated samples (Table 3). SSA and chitosan coatings led to mangoes with low total soluble solids and low penetration forces which may be the result of a low respiration rate (Cissé et al., 2015) and an inhibition of ethylene production by the presence of SA (Hayat et al., 2007). Chitosan coating caused substantial delays to some processes involved in ripening most notably weight loss, total soluble solids, and texture of the fruit. These effects may be linked to the reduced rates of ethylene production and respiration that might be the result of lower internal oxygen levels (Jitareerat et al., 2007).