Abstract
Fruits and vegetables are associated with physiological and biochemical variations that lead to
undesired disorders, decay, and subsequent loss of market value along the supply chain. Postharvest
pathogens such as Penicillium expansum, Penicillium italicum, Alternaria alternata, Botrytis
cinerea, Rhizopus stolonifer, etc., are the key reasons for the quality loss of fruits and vegetables.
To maintain the quality of fruits and vegetables, the horticulture industry relies on the application
of postharvest conventional synthetic antifungal agents to control these postharvest pathogens.
Nevertheless, these synthetic fungicides have been reported to have negative effects on human
health and the environment at large; thus, consumers have requested harmless and natural options
to maintain the quality of horticultural crops. Medicinal plants such as Bidens pilosa, Lippia
javanica, Syzygium cordatum, and Ximenia caffra have been reported in indigenous knowledge
systems (IKS) for food preservation; therefore, their biological potential can be applied in
maintaining the postharvest quality of fruits and vegetables. These biological activities can also be
used to improve the functional characteristics of edible films and coatings. Therefore, composite
coatings can be developed as a substitute for conventional synthetic antifungals to maintain the
postharvest quality of fruits and vegetables.
The application of nanocomposites is another field of study that can be exploited by the South
African horticulture industry to maintain the postharvest quality of horticultural crops. Although
green synthesis of nanoparticles is trending, limited studies have been done in South Africa on the
application of green synthesized nanoparticles in edible coating matrix in maintaining postharvest
quality of horticultural crops, especially as a green alternative to replacing the banned and harmful
conventional synthetic fungicides in the South African fruit industry. Due to their use in IKS for
food preservation and traditional medicine, B. pilosa, L. javanica, S. cordatum, and X. caffra can
be used as bio-factories in the green synthesis of nanoparticles. Different metal oxides can be
synthesized using the ‘green chemistry’ approach, and among the nanoparticles, zinc oxide has been
reported to be eco-friendly. Thus, green synthesized zinc oxide nanoparticles have the potential to
be fused into an edible coating matrix to form a nanocomposite for postharvest protection and
preservation of fruits by the South African fruit industry.
This study could open new possibilities for the application of bio-inspired nano/composites
edible coatings to preserve the quality of fruits and vegetables during storage or export. Specifically,
this study aimed to develop green composite edible nano/composite coatings and characterize them
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for practical applications as alternatives to conventional synthetic compounds in maintaining the
quality of selected horticultural crops. To achieve this, the following specific objectives were
executed: a) Characterization of medicinal plant extracts and the green synthesis of zinc oxide
nanoparticles; b) Formulation of composite films and characterize their properties relevant to
postharvest protection and preservation of horticultural crops; c) Application of composite and
nanocomposite edible coatings on selected horticultural crops and establish their mode of action.
In Theme I, the research overview and literature studies were analyzed. Paper 1 focused on the
potential of therapeutic herbal extracts as a substitute for synthetic antifungal agents in postharvest
protection and preservation of fruits and vegetables. The paper reviewed the need to explore
medicinal plants used in indigenous knowledge systems (IKS) that are known to have high
antimicrobial and/or antioxidant capacity and their potential to maintain the quality of horticultural
crops. Paper 2 documented the application of bio-inspired nanoparticles as a substitute for banned
postharvest fungicides in maintaining the quality of fruits and vegetables. The low-priced, ecofriendly,
and green manufactured nanoparticles fused in an edible coating matrix have a huge ability
to be applied in maintaining the quality of fruits and vegetables. For this purpose, it is important to
broaden our scope of research on the application of bio-inspired nanoparticles fused in edible
coating/s in maintaining the quality of fruits and vegetables.
In Theme II, the characterization of medicinal plants (B. pilosa, L. javanica, S. cordatum, and
X. caffra), green synthesized zinc oxide nanoparticles, and composite films was examined. In Paper
3, 52 metabolites among the medicinal plants were identified using liquid chromatography-mass
spectrometry (LC-MS). These included gluconic acid, a known antioxidant commonly used in the
food, beverage, and pharmaceutical industries, and aconitic acid, which has been reported to be an
inhibitor of microorganisms that cause fermentation. Other polyphenolic compounds included
quercetin and ferulic acid, a good inhibitor of reactive oxidation stress on fruits and a good synergist
actor with other antioxidants, apigenin 7-O-glucoside, a good constituent of antioxidants that have
been used for functional food preservation, luteolin a known polyphenolic plant flavonoid that has
antioxidant properties that are applied in food to enhance its shelf life, kaempferol acetyl dihexose,
and di-O-methyl ellagic acid, that is highly recommended for its antioxidant and antimicrobial
properties capable of reducing lipid oxidation and protect the functional properties of fruits and
vegetables cell wall. The occurrence and concentration of these polyphenolic compounds from the
different medicinal plant extracts varied. For instance, procyanidin B1 was more abundant in S.
cordatum, while ferulic acid was more dominant in B. pilosa. This indicates that these medicinal
plant extracts possess different beneficial properties for food and pharmacological properties.
Drawing on the rich knowledge of phytochemistry, diverse natural metabolites, and their varied
antioxidant activities indicate that these plants may offer significant benefits for functional foods
and hold promising food preservation potentials.
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The study in Paper 4 (Theme II) focused on synthesizing and characterizing zinc oxide
nanoparticles (ZnO-NPs) using leaf extracts of B. pilosa, L. javanica, S. cordatum, and X. caffra as
bio-factories. The XRD recorded in the 2 Ɵ range of 20º to 90º varied for the ZnO-NPs obtained
from the different bio-factories; however, they were all indexed and confirmed the presence of pure
crystalline shapes of ZnO-NPs. UV-vis spectroscopy analysis results revealed that the major
absorption peaks for the ZnO-NPs generated from the different bio-factories were 375 nm,
indicating characteristics of ZNO-NPs. In all the ZnO-NPs, FTIR analysis showed a sharp and
intense band at 550 cm-1, indicating the existence of zinc and oxygen bonding vibrations. SEM and
TEM results showed the presence of nearly spherical ZnO-NPs varied in size with aggregation.
These spectra results indicate that the synthesized nanoparticles possess strong antioxidant
properties that can reduce reactive oxygen species, which causes physiological disorders in stored
fruit. Hence, they provide ample opportunities for postharvest preservation of horticultural crops.
In Paper 5 (Theme II), composite films were formulated using chitosan-based films enriched
with medicinal plants, and their antifungal property against Penicillium expansum and Botrytis
cinerea was studied. The results showed that incorporating the different medicinal plant extracts
improved the antifungal of the chitosan-based films against P. expansum and B. cinerea. The study
also showed that enriching the chitosan-based films with the selected medicinal plant extracts
improved the physical and mechanical properties of the films. However, variations in other
properties such as thickness, density, swelling degree, water solubility, total phenolic content,
radical scavenging activity, and ferric-reducing antioxidant capacity of the composite films were
also observed. This indicates that the composite films can be further optimized for further use in
the industry as active packaging material.
Theme III focused on the postharvest application of edible composite coatings and bio-inspired
zinc oxide nanocomposite coatings in the postharvest preservation of selected horticultural crops.
In Paper 6, the study evaluated the efficacy of chitosan edible coating enriched with different
medicinal plant extracts in maintaining the quality of purple passion fruit (Passiflora edulis var.
Ester) during storage. Coating the fruit with chitosan alone maintained the fruit quality
characteristics more than the control, but composite chitosan-coated fruit improved the fruit
marketability. The best coating was chitosan fused with B. pilosa extract, as it was more effective
in alleviating shrivel incidence and better maintained the quality of passion fruit during storage,
showing potential for commercial applications when optimized.
In Paper 7, the study evaluated the efficacy of chitosan fused with B. pilosa extract in
maintaining the quality and shelf life of raspberry fruit. Results showed that chitosan enriched with
1% Bidens pilosa was the most effective in preserving the fruit's quality during storage. The study
found that coated fruit had a delayed peak in respiration rate and ethylene production due to the
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extension of shelf life by 3 d, while uncoated fruit reached senescence faster. There were significant
differences in weight loss, decay incidence, juice content, and fruit texture. Based on the principal
component analysis, chitosan enriched with 1% B. pilosa was recommended as a viable option for
postharvest preservation of raspberry cultivars.
In Paper 8 (Theme III), the research aims to evaluate the effectiveness of using gum Arabic
(GA) enriched with green synthesized zinc oxide nanoparticles (ZnO-NPs) to reduce postharvest
physiological disorders and maintain the quality of mandarin (cv. Kinnow) during export and
marketing. The study utilized B. pilosa zinc oxide nanoparticles from Paper 4, which were
incorporated into GA (2% w/v) at concentrations of 0%, 0.25%, 0.5%, and 1% to form composite
coatings: GA, GA+ZnO-NP 0.25%, GA+ZnO-NP 0.5%, and GA+ZnO-NP 1%, respectively.
Results showed that GA+ZnO-NP exhibited potential as an optional postharvest treatment for
reducing postharvest physiological disorders and maintaining the quality of Kinnow mandarin.
GA+ZnO-NP 0.5% distinctly reduced weight loss, electrolyte leakage, and chilling injury rate at
the end of the storage, while GA+ZnO-NP 1% significantly lessened the rind pitting rate. The gum
Arabic coating fused with zinc oxide nanoparticles at strengths between 0.5% and 1% can be
proposed as a possible option for maintaining the marketability of ‘Kinnow’ mandarin fruit. The
findings suggest that incorporating green synthesized zinc oxide nanoparticles in a gum Arabic
matrix offers a novel and eco-friendly approach for the South African fruit industry to replace the
use of harmful synthetic fungicides in postharvest preservation of horticultural crops. By utilizing
this approach, the fruit industry can ensure both the nutritional and health benefits of the crops, as
well as the safety of the environment, without compromising the quality and longevity of the
produce.
Overall, this study has successfully linked indigenous knowledge systems with modern
technology in reducing postharvest losses of horticultural crops without using hazardous synthetic
fungicides. The research has demonstrated the effectiveness of eco-friendly methods prioritizing
the quality and safety of horticultural crops and the environment while ensuring economic stability
for farmers and exporters. The study explored the potential of using edible films enriched with
medicinal plant extracts as a source of polyphenols and functional constituents to develop a new
active food packaging material that prolongs the shelf life of perishable crops. Furthermore, the
study established scientific guidelines for reducing postharvest losses of horticultural crops by
developing nano/composite coatings using medicinal plants used in indigenous knowledge systems
for food preservation. Additionally, the study presented scientific evidence for the novel application
of nanotechnology in postharvest protection and preservation, which can serve as a foundation for
further optimization and adoption by the South African horticulture industry.