Application of postharvest edible coatings to alleviate shrivel in plums and nectarines

The South African stone fruit industry faces several challenges when exporting fruit. The long handling chain that fruit is exposed to; from harvest through to consumption has detrimental effects on fruit quality. When exported, fruit is exposed to 4 to 6 weeks of cold storage during shipment, before a further 7 days of shelf-life. By the time it reaches its destination, many losses have already occurred, such as mass loss, shrivelling and softening. Shrivel is a huge problem within the industry. Various cultivars are prone to shrivelling as a result of moisture loss, and often must be repacked upon arrival, thus having a detrimental effect on the income generated by stakeholders in the industry. Edible coatings, a new strategy used to extend shelf-life and to improve food quality of whole fruits and fresh-cut fruits, have been applied to many products. There is no information on the use of these edible coatings to preserve the quality of South African plums and nectarines. The objective of this study is to investigate the potential edible coating application as a postharvest solution for nectarines and plums.

This research used the most susceptible cultivars – August Red for nectarines; African delight for plums. The various coatings were tested against a control (uncoated) and two commercial coatings used for stone fruit: one a locally-used coating and the other an internationally-used coating. Treatments were performed by dipping for 2 min. Fruits were packed into boxes with high density polyethylene (HDPE) shrivel sheets and stored at -0.5°C for 35 days to simulate shipping conditions. Thereafter, fruit were moved to 20°C, simulating shelf life conditions, for as long as possible. Fruit quality assessment took place at harvest and then at weekly intervals throughout the cold storage period and five-day intervals during shelf-life, pushing the fruit as far as possible until deemed unacceptable for commercial sale. Physio-chemical properties such as firmness, colour, total soluble solids and acidity, physiological responses such as mass loss, ethylene production, and physiological and pathological disorders were assessed. Another trial was conducted to investigate the potential of edible coating as a replacement for HDPE bags. In the commercial-scale trial, plums were coated with gum arabic and packed without HDPE bags during cold storage. At the end of the cold storage period (-0.5°C and 90% RH for six weeks), the physico-textural properties of coated fruit packed without HDPE bags resembled that of control plums packed with HDPE bags.

Alginate, chitosan and gum arabic performed best in reducing ripening and delaying physico-chemical changes in fruit throughout storage due to suppressed respiration and ethylene production. However, alginate and chitosan did not control weight loss and shrivel development, leading to postharvest losses. In addition to reducing changes related to ripening, gum arabic significantly (p<0.05) reduced weight loss, shrivel development and decay incidence throughout the storage period. The observed performance of the investigated edible coatings was similar for plums and nectarines.

Edible coatings had a significant effect on the postharvest quality of ‘African Delight™’ plums. Despite the challenge of moisture loss and shrivel incidence, gum arabic exhibited promise as a commercial postharvest edible coating for plums. Coating application proved viable for commercial packhouses, as pack lines were equipped with atomizers to apply postharvest solutions such as fungicides. Future studies should focus on optimizing coating moisture barrier properties and processing conditions in packhouses whilst considering commercial viability and the infrastructure available. Shrivel may be reduced if fruit are coated immediately after harvest. Furthermore, the prospect of pre-harvest edible coatings may be a potential solution to reducing postharvest moisture loss in the holding period before fruit are washed and then re-coated with a postharvest edible coating. In the commercial-scale trial, plums were coated with gum arabic and packed without HDPE bags during cold storage. However, weight loss and shrivel incidence were significantly higher in coated plums packed without HDPE bags compared to control plums packed with HDPE bags. Therefore, the potential of coating application to eliminate the need for HDPE bags is limited by coating moisture barrier properties. Future studies should focus on optimizing coating moisture barrier properties to eliminate the need for HDPE bags during export of plums and nectarines.

Although the commercial viability and technological readiness of GA 10% as a postharvest edible coating are limited in this study, we now know that applying polysaccharide-based (food grade) coatings such as gum arabic could reduce postharvest losses in stone fruit. In addition, coating application proved viable for commercial packhouses, as pack lines are equipped with atomizers to apply postharvest solutions such as fungicides. Future research towards using edible coating as a replacement for plastic should be encouraged. Future research can also focus on the combination of edible coating and paper covers or biodegradable liners in carton to further improve the efficacy of edible coating application. Furthermore, based on our experience, shrivel incidence may be reduced significantly if fruit are coated immediately after harvest, but this is logistically impossible; therefore, future research should also explore preharvest application of edible coatings on stone fruit – plums and nectarines. This may help reduce moisture loss and also help reinforce the membrane integrity of the fruit. In that case, there would be a need to investigate coatings specifically formulated for preharvest application and develop a regime protocol for their applications. There is an ongoing collaboration with reputable edible coating industry for this future research.