Development of edible composite coatings with antifungal activity on citrus fruits

Abstract

In the citrus industry, very important economical losses are primarily caused by postharvest green and blue molds, caused by the pathogens Penicillium digitatum (Pers.:Fr.) Sacc. and Penicillium italicum Wehmer. For many years, chemical fungicides have been widely applied to control these diseases. However, consumer concerns about prolonged and extensive use of chemical fungicides to control citrus postharvest decay lead researchers to look for alternative nonpolluting methods that do not deposit harmful residues in/on fruit and contaminate the environment. The use of edible films and coatings is an environmentally-friendly alternative method to increase the shelf-life of many food products including fruits and vegetables. However, very little research has been focused on the development of edible composite coatings with the addition of antifungal compounds as a new method to control major fungal postharvest diseases of fresh citrus fruit. The general objective of the present doctoral thesis was to develop new edible composite coatings with the addition of antifungal food additives for the control of posharvest green and blue molds on commercially important citrus cultivars. Firstly, new hydroxypropyl methylcellulose (HPMC)-lipid edible composite films containing food additives or generally recognized as safe (GRAS) compounds with antifungal properties were developed and selected according to their capability of forming stable emulsions. Films from stable emulsions were evaluated for their in vitro activity against P. digitatum and P. italicum and their mechanical and barrier properties (Chapter 1). Then, selected emulsions were used in in vivo tests to coat commercially important citrus species and cultivars and determine their curative (fruit coated after fungal inoculation) and preventive (fruit coated before fungal inoculation) antifungal activity against green and blue molds (Chapter 2). Next, the effect of the application of selected antifungal coatings on the development of penicillium molds and the physico-chemical and sensory quality of ‘Valencia’ oranges, and ‘Ortanique’ and ‘Clemenules’ mandarins during long-term cold storage was assessed (Chapters 3, 4, and 5). Finally, the effectiveness of chitosan edible coatings to control green and blue molds of ‘Valencia’ oranges and ‘Oronules’ mandarins was determined (Chapter 6). Summary Only around 5% of about 470 emulsion formulations prepared by incorporating food preservatives (mostly mineral salts, salts of organic acids, salts of parabens) to HPMC-lipid were stable. Edible films and coatings containing parabens, potassium sorbate (PS), sodium benzoate (SB), and some mixtures were the most effective to inhibit the in vitro growth of P. digitatum and P. italicum, and control in vivo the development of green and blue molds on previously inoculated ‘Valencia’ oranges, and ‘Ortanique’ and ‘Clemenules’ mandarins (curative action). Important differences in antifungal activity were observed between in vitro and in vivo tests. In general, the curative activity of the coatings after incubation at 20 ºC for 7 d was higher on oranges than on mandarins. The activity was fungistatic rather than fungicidal. The tested coatings did not provide any preventive activity against both molds. The lipid composition and the properties of added food preservatives greatly influenced the barrier and mechanical properties of selected films. During cold storage at 5 ºC for up to 2 months, the PS+SP- and SBbased coatings were the most effective to inhibit both molds on ‘Valencia’ oranges and ‘Ortanique’ mandarins, being the antifungal activity higher on oranges than on mandarins. Although the coatings did not reduce weight loss of ‘Valencia’ oranges, they did not adversely affect the fruit physico-chemical quality. The coatings significantly reduced weight loss and maintained the firmness of coated ‘Ortanique’ and ‘Clemenules’ mandarins. Sensory quality was evaluated as acceptable in all cases. Since selected HPMC-lipid edible coatings effectively reduced mold development and preserved fruit quality during cold storage, they showed promise as nonpolluting commercial alternatives to conventional fungicide-amended citrus waxes. Further research should focus on the modification of some physical characteristics of the coatings to improve their gloss and visual aspect, and their combination with other alternative control methods to find synergistic or complementary activities in an integrated disease management approach. In general, the effectiveness of chitosan-based coatings was highly dependent on inoculum density, fruit cultivar, and storage conditions, but it was not consistently high enough for commercial decay control.