Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by Subject "Antioxidants"

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    Antioxidant capacity of Pinotage wine as affected by viticultural and enological practices
    (Stellenbosch: University of Stellenbosch, 2006-12) De Beer, Dalene; Joubert, E.; Marais, Jeannine; Manley, M.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Food Science.
    The aim of the study was to provide the South African wine industry with guidelines for the production of Pinotage wines with optimal total antioxidant capacity (TAC), while retaining sensory quality. The contribution of individual phenolic compounds to the wine TAC is important in this regard. The wine TAC was measured with the 2,2 -azino-di(3-ethylbenzo-thiazoline-sulphonic acid radical cation) (ABTS +) scavenging assay. The contributions of individual phenolic compounds to the wine TAC were calculated from their content in the wines and the Trolox equivalent antioxidant capacity (TEAC) of pure phenolic standards. The effects of climate region, vine structure, enological techniques (pre-fermentation maceration, juice/skin mixing, addition of commercial tannins, extended maceration) and maturation (oak barrels, alternative oak products, oxygenation) on the phenolic composition, TAC and sensory quality of Pinotage wines were also investigated. The TEAC values of quercetin-3-galactoside, isorhamnetin and peonidin-3-glucoside were reported for the first time. TEAC values observed for most compounds were much lower than those reported previously, although TEAC values for gallic acid, caftaric acid, caffeic acid and kaempferol were consistent with some previous reports. Caftaric acid and malvidin-3-glucoside were the largest contributors to the wine TAC. The contents of monomeric phenolic compounds and procyanidin B1, however, only explained a small amount (between 11 and 24%) of the wine TAC, with the remaining TAC attributed to oligomeric and polymeric phenolic compounds and other unknown compounds. Some synergy between different monomeric phenolic compounds was also demonstrated. All the viticultural and enological factors investigated affected the phenolic composition of Pinotage wines, while the wine TAC was only affected by some treatments. Changes in wine TAC could not always be explained by changes in phenolic composition as the contribution of oligomeric, polymeric and unknown compounds could not be assessed, but could play a large role. Differences in wine colour were also difficult to explain due to the large number of factors involved and the dark wine colour, which made objective measurements difficult. The concentration of vitisin A, an orange-red pyranoanthocyanin, was increased consistently as a result of prefermentation maceration treatments and affected the wine colour of oxygenated wines. Increased wine TAC was observed when cultivating Pinotage grapes on bush vines and in cooler climatic regions, compared to cultivation on trellised vines in warmer climatic regions. All the climatic regions and vine structure treatments, however, resulted in wines with good sensory quality. In terms of enological techniques, pumping-over, as opposed to punching-down and rotor treatments, is not recommended as a juice/skin mixing technique, due to reduced wine TAC, colour and sensory quality. Pre-fermentation maceration, addition of commercial tannin preparations, and oak maturation using traditional and alternative treatments, resulted in improved sensory quality, but with no change in wine TAC. However, optimisation of the tannin addition protocol may result in increased wine TAC if additions are made after fermentation or higher dosages are used. Oxygenation of Pinotage wine needs further investigation to optimise the protocol, as improvements to the wine colour and fullness were observed for some treatments, but loss of sensory quality and TAC were observed in most cases.
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    Characterising the flavonoid profile of various citrus varieties and investigating the effect of processing on the flavonoid content
    (Stellenbosch : Stellenbosch University, 2016-03) Hunlun, Cindy; Sigge, G. O.; De Beer, D.; Van Wyk, J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Phenolic compounds in citrus fruit are specific for each species and variety and may be influenced by environmental conditions during the growing season and post-harvest practices. The exact chemical composition of citrus produced in South Africa is currently not known even though 2 million tonnes were produced in 2012. Various citrus varieties are produced for export, local fresh markets as well as processed into value-added products sold on the local market. In the current study South African citrus fruit (satsuma, clementine, navel and valencia) as well as products such as frozen concentrated orange juice (FCOJ), made-from-concentrate and not-from-concentrate orange juices produced from these varieties were characterised in terms of chemical and phenolic composition as well as total antioxidant capacity (TAC). Samples from two regions and three seasons were evaluated to determine the effect of variety as well as seasonal and regional differences. Citrus juice characteristics evaluated, included: °Brix, titratable acidity (TA), °Brix:acid ratio, pH as well as ascorbic acid (AA). Furthermore, the phenolic composition of the citrus fruit was quantified using high-performance liquid chromatography coupled with diode-array detection (HPLC-DAD). The TAC was determined using 2,2’-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay, Oxygen Radical Absorbance Capacity (ORAC) assay and Ferric Reducing Antioxidant and Ascorbic Acid (FRASC) assay. Eight phenolic compounds were quantified and included four flavanone-O-glycosides, two flavonols, a flavone and phenolic acid. The phenolic composition of different citrus varieties showed great variation between different seasons. Varietal difference was evident although there was some overlap between citrus varieties within the same season. Hesperidin and narirutin were the predominant flavanone-O-glycosides in sweet oranges, which included navel and valencia varieties, with vicenin-2 and ferulic acid-O-hexoside also present in high quantities. Regarding the FCOJ samples the results of the juice characteristics indicated that those from the WC sampling site were more mature compared to those of EC. Varietal differences were evident and variety proved to be the most significant factor that accounted for the variances in juice characteristics and phenolic composition. Seasonal differences were less evident. Variation that could be ascribed to regional differences was found for the individual phenolic composition. FCOJ from EC were characterised as having higher levels of the individual phenolics, total phenolic composition (TP) and TACDPPH and TACORAC. Of all the FCOJ varieties, navel was found to be the most mature, irrespective of season and region and was the variety with the highest TP. The predominant flavanone found in the MFC and NFC orange juices were hesperidin (HD) and narirutin (NART) followed by the flavone-C-glucoside vicenin-2 (VIC2) and a hydroxycinnamic acid namely ferulic acid-O-hexoside. Three other minor phenolic compounds where also quantified. The results indicated that NFC juices had higher levels of the individual phenolics as well as higher TACORAC. The results further showed that the phenolic composition of the MFC juices where dependent on the juice formulation, i.e. the quantity of orange juice added and not the treatment type (pasteurisation versus ultra-high temperature pasteurisation). Lastly, the results highlighted the lack of information pertaining to the processing, storage and shelf-life stability of the identified and evaluated phenolic compounds.