Effectiveness of Chitosan against spoilage moulds in harvested mangoes and its preservative effect on Ngowe mangoes (Mangifera Indica L. var. Ngowe

Abstract

Trade in mangoes contributes significantly to the Kenyan economy and the mango is an important fruit for its nutritional and sensory qualities. There are several mango varieties grown in Kenya. Ngowe mangoes are the dominant variety and they constitute 70% of the mangoes grown in the coast region. They are popular in local and export markets because of their excellent fruit qualities that include the attractive fruit colour, fibre-free flesh and good taste. Mangoes are climacteric fruits and as such postharvest losses associated with their ripening are high. Currently, the method used to reduce spoilage of mangoes is primarily by application of fungicides. Inappropriate use of fungicides in Kenya has already been reported. This project investigated the possibility of an alternative preservation technique using chitosan. Chitosan (1-4, 2 - amino -2 - deoxy ~-D glucan) is natural and non toxic, and there is potential for its production in Kenya. It is a polycationic complex polymer which is soluble in dilute acetic acid and can be made into gels, films, powder, fibres, flakes, among other products and has antimicrobial properties. The biopolymer was investigated for its effectiveness in inhibiting three moulds that cause postharvest spoilage in mangoes and in extending the shelf-life of the Ngowe mangoes. The inhibition effects of both irradiated and non-irradiated chitosan were studied on the germination of the isolated moulds spores and mycelia growths were determined using chitosan solutions of different concentrations ranging between 50 and 3000 ppm for spore germination assays, and 500 and 4000 ppm for mycelia growth assays. The effect of pH on inhibition efficacy of chitosan against mycelia growth in solid media was determined using three pH levels representing the low, medium and high acid foods. The preservative effect of chitosan was also tested. Mangoes were coated by dipping them in 1% chitosan solutions and their shelf life was determined by changes in skin colour, pulp texture as a measure of softening through ripening and spoilage activities and the observation of overall visual quality. Results showed that inhibition of spore germination was significant in chitosan treatments equal to or greater than 100 ppm for both irradiated and native chitosans within the concentration ranges tested. The effectiveness of inhibition as measured by percent reduction on spores germination increased from 1.5%, 0.5%, and 1.5% at 100 ppm to 46%, 38%, and 39% at 3000 ppm irradiated chitosan, for Aspergillus spp, Penicillium spp and Rhizopus spp, respectively, while for native chitosan at the same concentrations, the increase in inhibition was from 1.5%, 0.5% and 2% to 46%, 39.5% and 39% for Aspergillus Penicillium and Rhizopus moulds, respectively. However, the difference in inhibition of germination by irradiated and native chitosans was not significant (p:S0.05). Mycelia growth inhibition increased from 2.5% and 5.9% at 500 ppm to 67.2% and 63.5% at 4000 ppm irradiated chitosan for Aspergillus and Penicillium moulds respectively while for native chitosan, the increase was from 1.2% and 2.7% to 64.1% and 57.7% for Aspergillus spp and Penicillium spp, respectively. The efficacy of inhibition of mycelia was significantly different (p:S0.05) and so was the difference between the irradiated and native chitosans. Increasing pH resulted to a significant decrease in mycelia growth inhibition with growth being 13.43 mm at pH 4 and 18.17 mm at pH 6 for native chitosan. Coating mangoes with chitosan significantly prolonged their shelflife. Colour scores on day 12 were 6, 5.6 and 3.6 for control, those coated with Irradiated and Native chitosans respectively. The pulp firmness was 2.5, 3.5 and 4 lbs for the control, those coated with irradiated chitosan and Native chitosan, respectively. The overall visual quality was 6, 4.6 and 2.1 on day 20 for the control, those coated with native and irradiated chitosan, respectively. The study showed that chitosan has potential for application in preservation of mangoes and is a possible technology for export mangoes. Irradiation of chitosan in addition to decreasing its viscosity and making it easy to handle also showed improved antimicrobial and preservative effects. The cost benefit analysis of irradiating chitosan against the improved properties should be further investigated to justify the extra processing.