Effectiveness of submicron chitosan dispersions in controlling anthracnose and maintaining quality of dragon fruit
Introduction
Dragon fruit (Hylocereus polyrhizus (Weber) Britton & Rose; family Cactaceae) has gained global attention due to its prominent purple red colour and high antioxidant content. The major constraints in dragon fruit production are attributed to biotic and abiotic factors, for example anthracnose disease caused by the fungus Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. which can be devastating at the farm level and during storage. In addition, dragon fruit has a short storage life due to high respiration, weight loss and accelerated ripening, and shrivelling of fruit is evident from eight days following harvest (Arevalo-Galarza and Ortiuz-Hernandez, 2004). Chemical composition of the fruit during ripening changes dramatically and depends on texture, flavour, titratable acidity and ascorbic acid content.
Currently, synthetic fungicides such as benomyl, carbendazim, propineb and difenoconazole are commonly used to control dragon fruit diseases. However, rising public awareness about the toxicological effects of fungicides to human health, their environmental impact and the build up of fungal pathogen resistance towards fungicides, have necessitated the development of non-toxic biofungicides (Ali et al., 2010).
Chitosan is a polycationic compound derived from deacetylation of chitin. The antifungal properties of chitosan are well documented, both in vitro and in vivo (El Ghaouth et al., 1992). It has the potential to extend the storage life of fresh fruit and vegetables (El Ghaouth et al., 1992), however, chitosan used at concentrations of more than 1.5% has shown some negative effects on the quality of papaya fruit during storage (Ali et al., 2011).
Chitosan in the form of a submicron dispersion, where the average size of the droplets is 1000 nm, has been used in the present study. Due to their smaller size and higher kinetic stability, such droplets have been successfully used in pharmaceutical and agrochemical industries. Such dispersions can encapsulate functional ingredients within their droplets, and can facilitate reduction in chemical degradation (McClements and Decker, 2000). This paper is the first report on the use of SCD as a coating material for the control of postharvest anthracnose and maintenance of quality of dragon fruit during storage.
Section snippets
Materials
Low molecular weight chitosan from crab shell (50 kDa; 75–85% deacetylated) was purchased from Sigma–Aldrich, USA. Brij 56 (Merck KGaA, Darmstadt, Germany) and Span 20 (Sigma–Aldrich, USA) were used as emulsifiers. Potato dextrose agar (PDA) was obtained from Merck chemicals (Merck KGaA, Darmstadt, Germany).
Isolation and culture conditions for C. gloeosporioides
Small pieces of the skin of dragon fruit infected with C. gloeosporioides showing symptomatic lesions were placed in Petri plates containing PDA and incubated at ambient temperature (28 ± 2 °C).
Dynamic light scattering (DLS) of submicron chitosan dispersions
Table 1 shows the results of SCD diameters obtained from DLS. All the particle diameters are within the range (±25 d nm) of required size.
Antifungal effects of conventional chitosan and submicron chitosan dispersions
There were significant differences (P < 0.05) in disease incidence (DI) among the treatments during 28 days of storage at 10 ± 2 °C and 80 ± 5% RH. In control fruit, DI reached a maximum of 99.9% (Table 2), whereas in fruit treated with 2.0% CC, DI was reduced by 80%. SCD with 600 nm droplets showed the most promising results in terms of reduction in DI compared with
Conclusions
Results of the present study indicate that chitosan at 1.0% SCD with a droplet size of 600 nm reduced the onset of anthracnose in dragon fruit and that the quality of the fruit treated with SCD was comparable with CC. The minor differences observed in the decrease in quality of the fruit treated with SCD when compared with CC could have been due to the low viscosity of SCD that may have reduced the filmogenic properties of the solutions. Control of disease using SCD can add market value to fresh
Acknowledgements
The authors thank the Ministry of Agriculture (MOA), Malaysia for financing this study under the project grant 05-02-12-SF1003.
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