Authors M. Bill, D. Sivakumar, L. Korsten, A.K. Thompson,
The avocado fruit plays an important role in human nutrition due to its nutritional properties such as oleic, palmitic, linoleic, palmitoleic acids, trace amount of stearic acid, vitamin A, B, C, E, K, and high fibre content. The common postharvest disease anthracnose (Colletotrichum gloeosporioides Penz.) affects the fruit quality, marketability, and shelf life of avocados during marketing. Both field spraying and postharvest treatments are necessary to achieve high quality fruit. Copper sprays are commonly used in the orchard to control post-harvest diseases. Limited control of the anthracnose disease can be achieved with an application of preharvest copper oxychloride. The latter application leaves undesirable patches on the fruit surface, and it is a time-consuming process to remove them manually in the packhouse prior to packing. At the packhouse, after cooling the fruit is commonly treated with a synthetic non-systemic fungicide prochloraz as a first defense mechanism in the packing line to control anthracnose and it is a common commercial packhouse treatment adopted in South Africa, New Zealand, and Australia. Postharvest loses due to anthracnose can increase up to 80% if the fruits are not treated with prochloraz at postharvest stage. However, there is a need for safer methods to control postharvest decay development due to an increase in consumer concern regarding food safety and demand for organically produced fruit. The importing countries have enforced stringent regulations regarding the maximum residue limits (MRL) in the skin of the fruit and the MRL for South African avocado is 2 mg kg?1. It is also important to note that countries like Netherlands and France which are biggest importers of the fruit are more stringent with MRLs below 0.5 mg kg?1. In addition to this, development of fungicide resistant strains, and growing global pressure on the fruit industry to lower the associated environmental pollution footprint have necessitated the need to search for natural novel products to replace the prochloraz fungicide application at postharvest stage. Commercially, Avoshine® canuba wax coating is used for avocados. Green-skinned cultivars may develop surface discoloration if the proper wax formulation and application methods are not employed. It is essential that the applied wax coating must not leave any deleterious residues or affect the natural glossiness of the fruit, the eating quality or alter the characteristic fruit flavour. The EU does not allow morpholine in wax emulsions. There is some resistance to waxing of fruits including avocados in the EU due consumer pressure. Application of methyl cellulose or gelatin-starch coatings to avocado fruits have shown beneficial effects especially delaying the ripening behavior. However, it is necessary to investigate the effect of edible coatings on the incidence of decay. Biocoat or Natralife a mixture of beeswax and olive oil was shown to increase the shelf live with effective control of decay incidence. Application of essential oils or their volatile compounds at postharvest stage has been shown to control postharvest diseases in different fruits. Antifungal activity of thyme oil is well documented and proven to inhibit the fungal growth of C. gloeosporioides in vitro or in vivo in avocado cultivars Hass and Fuerte and Hass also showed that the thyme oil application in vapour phase in modified atmosphere packaging enhanced activities of defense enzymes (PAL, chitinase, 1,3-?-glucanase, peroxidase), antioxidant enzymes (catalase and superoxide dismutase) as well as high total phenols. Biodegradable polymers are often referred to as edible coatings and are mainly used to improve food appearance and to preserve fruit quality. Therefore, the incorporation of thyme oil into edible-coatings could be an effective method to control its high volatility thus minimizing losses and improving its effectiveness than when applied directly on the surface of the fruit. Some of the most used edible coatings are chitosan, Aloe vera gel and Gum Arabic to improve fruit quality and to suppress decay during postharvest storage. Chitosan, a copolymer consisting of ?-(14)-2-acetamido-d-glucose and ?-(14)-2-amino-d-glucose units which is derived from chitin has excellent film-forming properties, nontoxic, has antimicrobial activity and is biodegradable. Application of chitosan was observed to be effective in controlling postharvest diseases in strawberries, litchi, sweet cherries, and papaya, by activating defense-related enzymes such as phenylalanine ammonia-lyase and production of total phenols. Preventative chitosan coatings containing tea tree oil were found to be effective in reducing the incidence of Penicillium italicum (blue mold rot) in citrus fruits. Due to its emulsifying properties Gum Arabic is a potential coating component and incorporating either lemon grass or cinnamon oil into Gum Arabic was reported to control Colletotrichum musae in bananas and C. gloeosporioides in papayas. A. vera gel obtained from the leaf pulp of Aloe plants showed antimicrobial properties and has also been identified as a novel coating agent. Growth of yeasts and molds in grapes were inhibited following the application of A. vera gel during cold storage at 1 “C . From the literature and the authors’ experience no work has been reported on the incorporation of thyme oil into edible coatings to control postharvest decay and maintenance of fruit quality in avocados. This study is comprised of a threefold objective. Firstly, to investigate the effect of three different edible coatings incorporated with thyme oil on control of radial mycelial growth in vitro, secondly decay inhibition in artificially inoculated fruits (in vivo) (curative and preventative) and finally to determine the induction of defense related enzymes chitinase, 1, 3-?-glucanase, PAL, POD, antifungal compound phenol and antioxidant enzymes (catalase and superoxide dismutase.)
