Abstract
Pomegranate fruit (Punica granatum L.) has gained attention for its rich polyphenols, which have health benefits. Despite these advantages, its hard external peel limits consumption. One solution is the commercialization of minimally processed, "ready-to-eat" arils. However, these arils are highly perishable, leading to significant postharvest losses. Traditional postharvest technologies like cold storage, modified atmosphere packaging (MAP), and chemical treatments have been employed to delay ripening and reduce moisture loss but have shown limited success in preventing browning and mould infestation during storage. Edible coatings offer a promising solution for extending the shelf-life of horticultural crops. These coatings form a semi-permeable barrier on the fruit surface, controlling moisture and gas exchange, thereby delaying ripening and extending shelf-life. This study aimed to develop a functional nanocomposite edible coating using alginate and cellulose nanofiber enriched with encapsulated organic pomegranate peel extracts. The goal was to offer a natural postharvest solution for reducing losses in minimally processed pomegranate arils.
The first experimental chapter explored the use of maltodextrin (MT), gum Arabic (GA), and waxy starch (WS) as wall materials for encapsulating bioactive components in pomegranate peel extracts, aiming to produce functional pomegranate peel extract powder (PPEP). Organic pomegranate peel extracts were obtained using 70% ethanol and encapsulated with 10% (1:10, w/v) of the selected wall materials before freeze-drying. GA-encapsulated PPEP exhibited higher flowability, density, water-holding capacity, encapsulation efficiency, ferric-reducing antioxidant power, and solubility than other powders. It also showed significant (p<0.05) antimicrobial activity. Conversely, MT-encapsulated PPEP yielded powders with higher total phenolic content and antimicrobial efficacy than other powders. Metabolites such as phenolic acids, flavonoids, and ellagitannins were identified in the powders. Notably, the levels of specific metabolites like ellagic acid, punicalin α, punicalin β, and punicalagin α were significantly (p<0.05) higher in MT-encapsulated powder. In conclusion, GA and MT are promising as wall materials for encapsulating organic pomegranate peel extracts, suggesting their potential utility in formulating natural food additives and antimicrobial agents.
The second experimental chapter focused on the development of alginate-based nanocomposite edible films reinforced with cellulose nanofibre (AL-CNF) and functionalized with encapsulated polyphenol-rich pomegranate peel extract powder (PPEP) using a blend of gum
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Arabic and maltodextrin, as informed by the findings from the previous chapter. The incorporation of cellulose nanofiber (CNF) into the alginate (AL) matrix significantly improved the film's barrier and mechanical properties (p<0.05). The addition of PPEP further enhanced film attributes such as thickness, water solubility, moisture content, total phenolic content, and antioxidant properties, albeit at the expense of tensile strength and elongation at break. The AL-CNF films exhibited a rough and irregular surface, which became smooth and homogeneous upon the addition of PPEP. Films with 0.5% PPEP (w/v) demonstrated superior total phenolic content, antioxidant properties, thickness and effective polyphenol release, making them suitable for active food packaging.
The third experimental chapter evaluated the effectiveness of alginate (AL) and cellulose-nanofiber (CNF) nanocomposite edible coating enriched with pomegranate peel extract powder (PPEP) at concentrations of 0.1, 0.3, and 0.5%. The study aimed to extend the shelf-life and maintain the quality of organic pomegranate arils stored at 5 °C and 95 ± 2% relative humidity (RH) over 15 days of storage. The AL-CNF coating significantly (p<0.05) reduced weight loss from 4.32% (in control) to 1% after 15 days of storage. This is noteworthy as high weight loss contributes to aril deterioration. The coating effectively acted as a semi-permeable barrier on the aril surface. In contrast, lacking this protective barrier, the control group exhibited the highest weight loss. The AL-CNF coatings also successfully maintained aril firmness and reduced the respiration rate, attributable to the strong barrier properties of CNF. Incorporating PPEP into the AL-CNF matrix increased total soluble solids (TSS) and improved phenolic content and antioxidant activity during the 15-day storage period. Microbial growth was significantly (p<0.05) reduced at the 0.5% PPEP concentration. This is likely due to the high content of phenolics and antioxidants in pomegranate peel, which act as free radical scavengers and antimicrobial agents.
The study underscores that alginate-cellulose nanofiber (AL-CNF) nanocomposite coatings, enriched with pomegranate peel extract powder (PPEP), offer a viable strategy for extending the shelf-life of minimally processed horticultural crops in cold storage conditions (5 °C and 95 ± 2% RH). Importantly, this research advocates valorizing agro-waste materials and utilizing them as feedstock for differentiated product development. It presents an innovative and environmentally sustainable alternative to conventional, potentially hazardous preservatives, contributing to human health and environmental safety. Furthermore, the study explores the potential of AL-CNF nanocomposite edible films enriched with PPEP as active
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food packaging materials. This research could be foundational for further advancements and practical applications, particularly within the South African pomegranate industry context.