Abstract
Edible coatings are considered an innovative and environmentally friendly alternative to synthetic chemicals and the extensive use of plastics in postharvest quality preservation of fresh horticultural produce. However, the application of this technology remains limited for pomegranate fruit due to the development of physiological disorders like chilling injury and shrivelling following prolonged cold storage. In contrast, exogenous phytohormone treatments improve resistance to chilling injury and other physiological disorders. Furthermore, exogenous phytohormone treatments delay the ripening and senescence of fresh produce. Thus, the combination of edible coatings and exogenous phytohormone treatments emerges as a promising and innovative strategy to enhance the quality management of pomegranate fruit during storage. Consequently, this study aimed to formulate and optimize phytohormone-enriched edible coatings as natural postharvest approaches to improve the postharvest quality preservation of pomegranate fruit and its minimally processed products. Moreover, this study aimed to investigate the mode of action of the phytohormones-enriched composite coatings in regulating the postharvest behaviour of pomegranate fruit and minimally processed products.
The review of the literature further demonstrated the crucial role of exogenous phytohormone treatments in preserving the postharvest quality of fresh horticultural produce during storage. Moreover, the biological properties of plant hormones were highlighted, which demonstrates their suitability for food preservation. The review further showed a need to optimize concentrations and formulations of exogenous phytohormone treatments for different fruit types.
The research was structured into different of experimental sections using randomized complete block design (RCBD). Section 2 focused on the formulation and optimization of composite coatings, using chitosan (CH) as a base edible coating, with the incorporation of 24-epibrassinolide (EBR) and melatonin (MT) for whole pomegranate fruit and minimally processed arils, respectively. The optimal coating concentrations were achieved through preliminary experiments that evaluated critical quality parameters, including color retention, firmness, total soluble solids (TSS), decay incidence, and respiration rate under storage conditions.
Subsequent studies (Section 3) were designed to elucidate the mechanisms of action of the optimized CH+10 μM EBR and CH+0.8 mM MT coatings. Advanced biochemical analyses were employed to monitor changes in key metabolites and enzyme activities. Antioxidant capacity was
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assessed using multiple assays (DPPH, ABTS+, and FRAP), while lipid peroxidation, membrane integrity, and electrolyte leakage were quantified to evaluate chilling tolerance. Metabolomics profiling were performed to identify and quantify bioactive metabolites, including flavonoids and phenolic acids, which contribute to stress resistance.
Section 4 explored the application of Response Surface Methodology (RSM) to optimize a methylcellulose-based coating system for ‘ready-to-eat’ arils. This section focused on enhancing functional properties by incorporating salicylic acid, canola oil, and Tween-80, aiming to reduce mass loss, maintain sensory attributes, and boost antioxidant properties during extended storage.
The optimized CH+EBR coating significantly enhanced the postharvest quality of whole pomegranate fruit by delaying chilling injury symptoms up to 12 weeks of cold storage, compared to control fruit, which showed visible chilling injury within 8 weeks. This effect was mediated by a higher unsaturated/saturated fatty acid ratio, reduced malondialdehyde (MDA) levels, and a significant upregulation of antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Enhanced membrane stability was linked to the accumulation of metabolites with high antioxidant potential, such as ellagic acid rhamnoside, epicatechin, and punicalagin derivatives. These biochemical markers were associated with reduced oxidative stress and improved membrane integrity.
Similarly, the CH+MT coating demonstrated efficacy in extending the shelf life of minimally processed aril-sacs by suppressing surface browning, reducing weight loss, and maintaining higher levels of phenolic compounds and antioxidant activity. The increased PAL/PPO enzyme activity ratio and elevated antioxidant enzyme activities (CAT, POD) contributed to mitigating oxidative browning and preserving the overall quality of the minimally processed aril-sacs.
Optimization studies using RSM demonstrated that the incorporation of canola oil and salicylic acid into methylcellulose-based coatings significantly enhanced the antioxidant capacity of ‘ready-to-eat’ pomegranate arils, reducing mass loss and preserving total phenolic content. The quadratic interactions between coating components were shown to have a synergistic effect on maintaining the quality of fresh-cut produce, as confirmed by high model accuracy and validation.
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This PhD research provides significant contributions to the field of postharvest horticulture by offering a novel approach to extending the shelf life and maintaining the quality of pomegranate fruit through phytohormone-enriched edible coatings. The findings underscore the potential of combining chitosan-based coatings with phytohormones like EBR and MT to enhance chilling tolerance and mitigate physiological disorders. The study advances our understanding of the biochemical mechanisms underpinning the efficacy of these treatments, providing a scientific basis for their broader application in the horticultural industry. This research not only addresses critical challenges in pomegranate postharvest management but also opens avenues for developing sustainable alternatives to synthetic preservatives, aligning with current trends in green technology and consumer demand for natural food preservation solutions.