Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by Subject "Antioxidant activity"

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    Value addition of pomegranate seed oil: Effect of seed pretreatment methods on yield, quality attributes and functional properties
    (Stellenbosch : Stellenbosch University, 2021-04) Kaseke, Tafadzwa; Opara, Umezuruike Linus; Fawole, Olaniyi Amos; Sigge, G. O.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Efficient and cost-effective processing protocols which enhance oil recovery and quality are required by processors in the seed oil industries. Currently, there is no established pomegranate seed oil processing procedure in South Africa, which could be one of the reasons hindering the development of pomegranate seed oil industry, despite the readily available fruit raw material. Furthermore, cold pressing, which is the current and most preferred pomegranate seed oil extraction technique by oil processors and consumers is associated with low recovery of oil. Physical or chemical pretreatment of seeds has been demonstrated to improve oil recovery and quality in other fruit seeds and field crops; however, the application of these pretreatments to enhance pomegranate seed oil extraction efficiency and quality is limited. Therefore, the overall aim of this study was to establish a suitable pomegranate seed pretreatment method for high oil yield, quality, and functional properties. In Theme B, the effect of seed pretreatment on three common pomegranate cultivars in South Africa was examined. To evaluate if blanching as a pretreatment technique for oil extraction adds value to pomegranate seed oil (PSO), seeds of ‘Wonderful’ pomegranate fruit were blanched at 80, 90, and 100 °C for 3 and 5 min. Blanching pomegranate seeds at 90± 2 °C for 3 to 5 min significantly improved oil yield, stigmasterol, punicic acid, total phenolic content (TPC) and 2.2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging capacity. Given the significance of cultivar on seed pretreatment efficacy and oil quality, blanching (95± 2 °C for 3), microwave heating (261 W for 102 s) and enzyme pretreatment (1.7 %, 40 °C, pH=4.5 and 5 h) were investigated on the seeds of three different pomegranate cultivars (‘Wonderful’, ‘Herskawitz’, and ‘Acco’). Blanching and microwave heating of ‘Wonderful’ and ‘Acco’ seeds improved the oil yield and colour, whilst they enhanced the antioxidant capacity of oil extracted from ‘Herskawitz’ seeds. However, oil extracted from ‘Acco’ after seed enzyme pretreatment exhibited higher oil yield, total carotenoids content (TCC) and, DPPH radical scavenging capacity relative to ‘Wonderful’ and ‘Herskawitz’. The results showed that the quality of PSO from pretreated seeds is cultivar dependent. The study reported in Theme C investigated the influence of blanching (95± 2 °C/3 min) and microwave heating (261 W for 102 s) pomegranate seeds on the quality of oil extracted by cold pressing, the most preferred seed oil extraction technique by oil processors and consumers but with low oil yield. Both blanching and microwave pretreatment of seeds prior to pressing enhanced oil yield, TCC, TPC, DPPH and 2.2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity. Although the levels of oil oxidation indices were significantly higher in microwaved than blanched seeds, they remained within the limits of Codex Alimentarius Commission (CODEX STAN 19-1981) standard on cold pressed vegetable oils. The oil palmitic acid, oleic acid, and linoleic acid significantly increased after microwave heating of seeds, whilst punicic acid decreased, which could be attributed to increased heat penetration and oxidation of the fatty acid. Conversely, the fatty acid composition of PSO was not significantly altered by seed blanching, indicating that the nutritional quality of the oil was not affected. Blanching of seeds is, therefore, a valuable step that could be incorporated into the PSO production process. The studies in Theme D, focused on blanched seed storability, oxidative stability, shelf-life and functionality of PSO from blanched seed and blended oils. Blanching pomegranate seed did not cause significant deterioration of the oil quality after seed storage. More so, storing pomegranate seed at 25 and 35 °C for 6 months did not result in a considerable reduction in oil quality, with respect to peroxide value, anisidine value and total oxidation value. In paper 9, the lipid oxidation kinetics, thermodynamics parameters, and shelf-life were estimated, given the improved extractability of bioactive compounds and enhanced antioxidant capacity of PSO from pretreated seeds. The Arrhenius model and activated complex theory were applied to calculate the activation energy (Ea), enthalpy (ΔH‡), entropy (ΔS‡) and Gibbs free energy (ΔG‡), which ranged from 6.2 to 8.59 kJ mol⁻¹, 3.69 to 6.38 kJ mol⁻¹, -146.62 to -155.06 J K⁻¹ mol⁻¹ and 48.13 to 64.74 kJ mol⁻¹, respectively. These thermodynamic parameters showed that the lipid oxidation reactions in all PSO extracts were non-spontaneous, endothermic, and endergonic. Moreover, the developed Arrhenius models established that blanching seeds may increase the PSO shelf-life at 25 °C from 21 to 24 days. In contrast, microwave heating may not change the shelf-life. The higher initial level of peroxide value and increased polyunsaturated fatty acids could explain the insignificant effect of seed microwaving heating on the oil shelf-life. The study provided valuable insights useful in the design of PSO packaging, the establishment of storage conditions and application of novel technologies to preserve the storage life. In paper 10 (Theme D) the functionality and oxidative stability of PSO from unblanched and blanched seeds (95± 2 °C/ 3 min) were investigated by blending with semi-refined sunflower oil (SO) considering the detrimental health effects of synthetic antioxidants. The study showed that blended oils (85: 15 w/w) had better oxidative stability, a lower rate of antiradical activity depletion and concentration of volatile oxidation compounds than SO, although this did not significantly (p > 0.05) vary between the blended oils. Despite this, the formulation of PSO blends is a novel and desirable development for the food industry, which is currently interested in specialty oils and functional foods formulations to improve human nutrition and health. Overall, this study has established that the improvement of PSO quality through seed pretreatment is a function of cultivar. This could be ascribed to the genetic variation of the pomegranate cultivars investigated. The study has also demonstrated that seed pretreatment is essential to improve the performance of safe and green but low oil yielding technologies such as cold pressing. Blanching of pomegranate seed may enhance the oxidative stability and shelf- life of PSO. Furthermore, the study provides scientifically based information that can be used to develop strategies to improve the storability of PSO from pretreated seeds and retain the health promoting properties.