Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by browse.metadata.advisor "De Beer, D."

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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.
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    The physicochemical properties and stability of aspalathin in micro- and nanoencapsulated green rooibos extract formulations
    (Stellenbosch : Stellenbosch University, 2019-04) Human, Chantelle; De Beer, D.; Joubert, E.; Sigge, G. O.; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Green rooibos extracts with high aspalathin content have potential as nutraceutical food ingredients based on their properties relating to the prevention of metabolic syndrome. However, delivery of green rooibos extracts in convenient beverage products is a challenge due to poor stability of aspalathin in the presence of moisture. Thus, the development of alternative ingredients and convenience products is required. Microencapsulation of a green rooibos extract (GRE) with maltodextrin as control carrier and inulin and chitosan as low kilojoule functional alternatives was achieved by spray-drying. Spray-dried GRE and powders containing maltodextrin or inulin had similar yields (>76%), moisture content (<3.5%) and aspalathin retention (>95%), whereas microencapsulation with chitosan resulted in lower yields (<66%), higher moisture content (>3.4%) and lower aspalathin retention (<83%). Accelerated stability tests (40 °C/75% relative humidity (RH)) revealed similar aspalathin degradation rates (based on fractional conversion model) for GRE, inulin and maltodextrin formulations, but significantly higher degradation rates for chitosan formulations. Given the low incompatibility between GRE and inulin, inulin-microencapsulated GRE (1:1 ratio; IN50) was selected as the most suitable green rooibos nutraceutical beverage ingredient. IN50 was added to iced tea powder formulations, which contained various food grade ingredients (sucrose, xylitol, citric acid and ascorbic acid). Shelf-life trials (30 °C and 40 °C/65% RH for 5–12 months) in different packaging materials (semi-permeable vs impermeable) revealed more aspalathin degradation (based on first order reaction rates), more discolouration and clumping after the addition of crystalline ingredients. These changes were more pronounced at 40 °C and for powders stored in the semi-permeable packaging. The formulation containing IN50, xylitol and citric acid, which showed the most drastic physical and chemical changes during storage, was subjected to descriptive sensory analysis, which confirmed significant changes also in its sensory profile. Nanoencapsulation of an aspalathin-rich fraction (GRAF) prepared from green rooibos was also investigated. Combinations of natural (chitosan and lecithin) and synthetic [poly(lactide-co-glycolide) and Eudragit S100® (ES100)] polymers with suitable conventional methods and electrospraying were investigated. Overall, ES100 electrosprayed particles had the best combination of properties, i.e. encapsulation efficiency (EE, 55.4%), loading capacity (LC, 11.1%), release rate at pH 7.4 (1.67 h-1) and size (190 nm). Further optimisation of the ES100-GRAF loaded nanoparticles was achieved using a central composite design. Responses included yields between 78.2–78.3%, EE between 73.9–76.4% and LC between 9.9–12.9%. Pure aspalathin was subsequently encapsulated using the optimal conditions, resulting in a similar yield, EE, LC, particle size and particle morphology to that of GRAF loaded nanoparticles. The stability of the aspalathin and GRAF loaded nanoparticles was investigated at fixed pH-time combinations. Nanoencapsulation offered a more stable environment for aspalathin. Overall, pure aspalathin was less stable than when in GRAF. Even though intestinal permeability could theoretically be improved with nanoencapsulation, the parallel artificial membrane permeability assay and Caco-2 cell model indicated that pure aspalathin and aspalathin nanoparticles both have equally low permeability. These methods offered an alternative for the production of GRE convenience products and ingredients, whilst providing insight on the effects of encapsulation and ingredients of powder formulations.
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    Xanthones and benzophenones from Cyclopia genistoides (honeybush) : chemical characterisation and assessment of thermal stability
    (Stellenbosch : Stellenbosch University, 2016-03) Beelders, Theresa; Joubert, E.; De Beer, D.; Sigge, G. O.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Numerous health-promoting benefits may be derived from the consumption of honeybush tea, a herbal infusion prepared from the leaves and fine stems of the endemic Cape fynbos genus, Cyclopia. These health-promoting benefits are attributed to its phenolic constituents and therefore insight into the nature, quantities and biological activities of individual compounds are required. Information regarding the thermal stability of these compounds is also crucial, as the plant material is subjected to a high-temperature chemical oxidation process (“fermentation”) to develop the sought-after characteristic sensory attributes of the herbal tea product. In this study, the phenolic composition of Cyclopia genistoides, a commercially important species, was comprehensively characterised by high-performace liquid chromatography (HPLC) coupled with diode-array and mass spectrometric detection. A species-specific HPLC method was developed for the qualitative analysis of aqueous extracts prepared from “unfermented” and “fermented” C. genistoides plant material and was subsequently validated for the quantification of 18 phenolic compounds in these types of extracts. The major phenolic constituents included the C-glucosyl xanthone mangiferin (1) and its regio-isomer isomangiferin (2), and the benzophenone 3-β-D-glucopyranosyliriflophenone (3). The presence of novel benzophenone and xanthone derivatives in C. genistoides was demonstrated for the first time, including an iriflophenone-di-O,C-hexoside derivative, present in large quantities. This compound was isolated and unambiguously identified by nuclear magnetic resonance spectroscopy as 3-β-D-glucopyranosyl-4-β-D-glucopyranosyloxyiriflophenone (4) – a novel benzophenone unique to Cyclopia. 3-β-D-Glucopyranosylmaclurin (5), present in small quantities, was also isolated. The isolated benzophenones (4 and 5) exhibited mammalian α-glucosidase inhibitory activity, while 4 and 3 were also marginally effective in increasing glucose uptake in vitro. Compound 4 was ineffective as antioxidant in the DPPH assay, but the most effective in the ORAC assay, compared to the other compounds tested (1, 2, 3, 5). Degradation of compounds 1-4 in C. genistoides plant material under simulated fermentation conditions (80 °C/24 h and 90 °C/16 h) followed first-order degradation kinetics and their thermal stability decreased in the order 4 > 2 > 3 > 1. An increase in the degree of glucosylation significantly increased the thermal stability of the benzophenones, whereas glucosylation at C-4 of the dibenzo-γ-pyrone structure, as opposed to C-2, increased the stability of the tetrahydroxyxanthones in the plant material matrix. This was also confirmed for individual compounds (1-5) in aqueous model solutions (pH 5). Inclusion of 5 in the model systems provided additional insight into structure-stability relationships. Increased B-ring hydroxylation significantly increased the first-order degradation rate constants of the benzophenones. Oxidative coupling of the polyhydroxybenzophenone 5 with the formation of its corresponding xanthones (1 and 2) led to substantial increases in the thermal stability of 1 and 2 compared to that of 5. Increased temperatures increased the degradation rates of all compounds in both the plant material matrix and model solutions. The thermal stability of 1, tested at pH 3-7, was found to be pH-dependent, with increased degradation rates observed at higher pH. Thermally-induced reactions principally included isomerisation, dimerisation and cleavage of O-linked sugar moieties; conversion of all benzophenones to the xanthones occurred to varying degrees. Of special interest was the rapid and predominant conversion of 5 to 1 and 2.