Effects of chitosan coating on postharvest quality and storage life of ‘Nam Dok Mai’ mango fruit

Abstract

‘Nam Dok Mai’ mango (Mangifera indicaL. cv Nam Dok Mai) is an important exported fruit of Thailand. However, postharvest problems are the main problem which cause the reduction of fruit qualities during transportation and storage. Qualities of ‘Nam Dok Mai’ mango fruit were investigated after coating with high molecular weight chitosan (HM-CTS: 360 kDa), medium molecular weight chitosan (MM-CTS: 270 kDa) and low molecular weight chitosan (LM-CTS: 40 kDa). Our results demonstrated that HM-CTS had the best chitosan coating film properties by representing the thickest of coating film on mango peel (3.47±0.50 µm) and it completely covered mango surface. Stomata were not observed after coating hence reducing gas exchanges. Fruit coated with MM-CTS showed thinner film (1.29±0.45 µm) than HM-CTS while LM-CTS showed the least thickness of chitosan film among treatments. Stomata of LM-CTS fruit were detected on mango peel as same as uncoated fruit (control). Fruit coated with HM-CTS could delay respiration rate, ethylene production, weight loss, fruit softening and changes of physicochemical (TA and TSS) when compared with other treatments. Moreover, chitosan coated fruit exhibited high antioxidant contents by presenting high contents of ascorbic acid, DPPH inhibition, free flavonoids, high activities of catalase and ascorbate peroxidase and low content of H2O2. Furthermore, fruit coated with HM-CTS did not show any disease symptom until the end of storage life. Therefore, HM-CTS was the optimum treatment for using as fruit coating in ‘Nam Dok Mai’ mango after harvest. Combination of HM-CTS and spermidine (SPD) (0.1, 1 and 10 ppm) were applied on ‘Nam Dok Mai’ mango to evaluate the effect of SPD addition. HM-CTS alone and HM-CTS combined with 0.1 ppm SPD showed thicker chitosan film than HM-CTS combined with 1 and 10 ppm SPD (3.05±0.06 µm and 3.07±0.06 µm, respectively). Fruit coated with HM-CTS combined with 0.1 ppm SPD fully covered stomata on mango peel. Delayed respiration rate and ethylene production were obtained in fruit coated with HM-CTS combined with SPD. Fruit coated with HM-CTS combined with 0.1 ppm SPD also maintained fresh weight, fruit firmness, peel color and controlled disease development. Fruit softening usually occurs during ripening stage caused by increased cell wall degrading enzyme activities (polygalacturonase (PG) and pectin methyl esterase (PME)) and soluble pectin content. An addition of SPD resulted in lower soluble pectin content and PG and PME activities than control treatment (HM-CTS alone), especially fruit coated with HM-CTS combined with 0.1 ppm SPD showed the lowest values. In addition, endogenous SPD was induced in fruit coated with HM-CTS combined with 0.1 ppm SPD. Therefore, HM-CTS combined with 0.1 ppm SPD was the optimum coating for improving qualities of ‘Nam Dok Mai’ mango after harvest.