Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by browse.metadata.advisor "Fawole, Amos Olaniyi"

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    Impacts of preharvest and postharvest handling and processing on bioactive compounds and functional properties of pomegranate fruit fractions and by-products
    (Stellenbosch : Stellenbosch University, 2016-03) Mphahlele, Rebogile Ramaesele; Opara, Umezuruike Linus; Fawole, Amos Olaniyi; Stander, Marietjie; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Pomegranate fruit (Punica granatum L. Punicaceae) is highly valued owing to its high concentration of bioactive compounds found in the arils and peel. In fact, evidence from literature indicates that pomegranate fruit consumption has been associated with reduced risk of life threatening non-communicable diseases such as cancers and cardiovascular disorders. Although substantial amount of research has been reported on the effects of preharvest factors on phytochemical and functional properties of pomegranate, including cultivar and micro-climatic differences, little is known about the effects of postharvest and processing techniques on individual phenolic concentrations of fruit fractions such as arils and peel. The aim of this study was therefore to examine the impacts of preharvest and postharvest handling factors and processing methods on bioactive components and functional properties of pomegranate fruit and by-products. Drying characteristics and a thin-layer drying model for pomegranate peel over a wide temperature range were included in this study given the importance of drying as a commonly applied processing method in the processing of high-moisture products such as fruit. The results showed that concentrations of total phenolic and total tannin as well as radical scavenging activity (RSA) by DPPH assay declined as fruit maturity advanced, while ferric reducing antioxidant power (FRAP), total anthocyanin, total flavonoid and vitamin C concentration increased significantly (P<0.01). Principal component analysis (PCA) demonstrated that fruit grown in areas with lower altitude were associated with higher bioactive compounds at the full ripe stage. The study also showed significant (P<0.05) interaction effect between fruit maturity and altitude of the growing location on the phenolic compounds concentration. Studies on the effect of different extraction methods on phenolic compounds and antioxidant properties of pomegranate juice did not show significant influence (P>0.05) on fructose and total soluble solid concentration of pomegranate juice. Juice obtained from arils plus seed had the lowest citric acid concentration (18.96 g/L) and high juice colour saturation (2.69). Juice obtained by pressing fruit cut in half along the longitudinal axis (halved fruit) had significantly higher total phenolics, total tannins, radical scavenging activity and ferric reducing antioxidant power, which highlights the impact of extraction method on the quality of pomegranate juice. The influence of packaging and long term cold storage of whole pomegranates on phenolic compounds and antioxidant properties of fruit fractions and by-products thereof was also investigated. The result showed that total phenolics in pomegranate juice and peel decreased significantly (P<0.05) with prolonged storage duration regardless of package type. Catechin increased by 65.43% under modified atmosphere package (MAP) while rutin increased by 139.39% in individual shrink wrap package after 4 months of cold storage. Rutin was the predominant flavonoid in peel (3446.24 mg/kg dry matter), and its concentration decreased by 65% in fruit peel stored in MAP at the end of the storage (4 months). The study showed that punicic acid constituted 68.09% of total fatty acids in the seed oil and the concentration did not change significantly after 4 months under MAP and individual shrink wrap packaging, respectively. Fruit peel of whole pomegranates stored in individual shrink wrap package showed poor inhibitory activity against Gram negative bacteria (Klebsiella pneumonia), with minimum inhibitory concentration (MIC) of 1.56 mg/mL while seed oil showed better activity against diphenolase with inhibitory concentration (IC50) of 0.49 μg/mL after 4 months of storage. The effects of drying on the phenolic concentration, antioxidant, antibacterial and anti-tyrosinase properties were also studied. Freeze dried peel extracts had the highest total phenolic, tannin and flavonoid concentration compared to oven dried peel at the temperatures studied (40°C, 50°C and 60°C). Pomegranate peel extracts dried at 50°C showed the highest inhibitory activity with MIC value of 0.10 mg/mL against Gram positive bacteria (Staphylococcus aureus and Bacillus subtili) and monophenolase (22.95 mg/mL). Drying behaviour of pomegranate peels showed that drying time decreased as the oven drying temperature increased. The effective moisture diffusivity of pomegranate peel ranged from 4.05 x 10-10 to 8.10 x 10-10 m2/s over the temperature range investigated, with mean activation energy (Ea) of 22.25 kJ/mol. Empirical models were successfully applied to describe drying kinetics of pomegranate peel and these models could be used as analytical tools for future drying performance assessment.