Doctoral Degrees (Food Science)
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Browsing Doctoral Degrees (Food Science) by browse.metadata.advisor "Joubert, E."
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- ItemAntioxidant 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.
- ItemDevelopment and validation of prediction models and rapid sensory methodologies to understand intrinsic bitterness of Cyclopia genistoides(Stellenbosch : Stellenbosch University, 2018-03) Moelich, Erika Ilette; Joubert, E.; Muller, M.; Næs, T.; Stellenbosch University. Faculty of AgriScience. Dept. of Food Science.ENGLISH ABSTRACT: Cyclopia species, endemic to South Africa, is used for the production of honeybush tea. This herbal tea has grown from a product consumed only locally to one currently marketed worldwide. Cyclopia species is high in polyphenols, well-known for its health related properties, but these compounds could contribute to bitterness, which may elicit negative consumer response. One of the species, C. genistoides, is high in polyphenols but also associated with bitterness, contrary to the characteristic sweet taste and honey-like flavour associated with honeybush. The polyphenolic content of four Cyclopia species were quantified with high-performance liquid chromatography diode-array detection while descriptive sensory analysis (DSA) was used to determine the taste intensities of these samples. The link between phenolic content and bitterness was investigated. Based in the phenolic content of C. genistoides and C. longifolia, partial least squares (PLS) regression analysis identified four compounds, mangiferin, isomangiferin, iriflophenone-3-C-glucoside-4-O-glucoside and iriflophenone-3-C-glucoside, as candidate predictors of bitterness. This model will find application as screening tool in cultivar development research programs. Production of honeybush lags behind demand, forcing the industry to use blends of Cyclopia species, including C. genistoides, to supply in the increased demand. The distinct differences in the sensory profiles associated with different Cyclopia species require that special care is given to blending to ensure a consistent, high quality product. DSA was used to evaluate the effect of blending of C. genistoides with other Cyclopia species on bitterness. Blending of C. genistoides with C. subternata, C. intermedia or C. maculata in a ratio of 2:3 were effective in reducing bitterness to below perceptible levels. The sensory profile of C. genistoides- C. subternata blends were further quantified using DSA and was described as “fynbos floral”, “apricot”, “woody”, “fruity sweet” and “fynbos sweet” aroma and a sweet taste. Based on these results, inclusion of C. genistoides at 40% when blending different Cyclopia species, is recommended. Blending at this standardised ratio will result in a well-rounded product with bitterness below perceptible levels. The herbal tea industry expressed the need for time- and cost-effective methods for sensory screening of infusions to improve quality and product consistency. The validity of three rapid profiling methods {sorting, projective mapping and polarised sensory positioning (PSP)} for the sensory characterisation of honeybush infusions were investigated using a trained panel. The efficacy of partial (aroma or palate attributes) or global (all attributes) evaluation was compared within each rapid method. Product configurations similar to that of DSA demonstrated the validity of all three methods for broad sensory profiling of Cyclopia species. Sorting on palate attributes resulted in additional differentiation between samples with only subtle differences. Sorting demonstrated to be the most effective method for the broad sensory profiling of honeybush infusions and could find application in the honeybush industry as screening tool. PSP on the other hand could find application in quality control programs where poles that represent specific quality attributes, should be included. Implementation of valid, scientific methods, such as sorting and PSP, will aid the honeybush industry in their effort to supply a product with consistent quality and high consumer appeal.
- ItemExposure to polyphenol-enriched rooibos (Aspalathus linearis) and honeybush (Cyclopia spp.) extracts : implications of metabolism for the oxidative status in rat liver(Stellenbosch : Stellenbosch University, 2012-12) Van der Merwe, J. Debora; Gelderblom, W. C. A.; Joubert, E.; Manley, M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.ENGLISH ABSTRACT: Potential beneficial and/or adverse modulatory effects of polyphenol-enriched extracts of rooibos (Aspalathus linearis) and honeybush (Cyclopia spp.) on the antioxidant homeostasis in the liver were investigated. Phase II metabolism of aspalathin and mangiferin, the major polyphenols of rooibos and honeybush respectively, was assessed for potential glucuronidation and sulphation. Glucuronidation resulted in a loss of antioxidant activity for aspalathin and mangiferin in post-column HPLC-DAD-DPPH• and HPLC-DAD-ABTS•+ assays and also a decreased activity of iron chelating properties of mangiferin in the FRAP assay. Two independent studies for 28 and 90 days with polyphenol-enriched extracts (PEEs) of unfermented rooibos [Aspalathus linearis (PER)] and honeybush [Cyclopia. subternata (PECsub) and C. genistoides (PECgen)] in male Fischer rats were conducted to assess possible beneficial and/or adverse biological effects. PECgen was only included in the 28 day study. PEEs were characterised by in vitro antioxidant assays and HPLC analysis. The importance of detailed chemical characterization of rooibos and honeybush when investigating biological effects in vivo is clear as distinctive biological effects and major differences in compositions were evident. Biological parameters included were serum chemical parameters, activities of selected antioxidant enzymes, levels of glutathione and the modulation of expression of oxidative stress and antioxidant defense related genes in the liver. Sub-chronic (90 days) exposure of rats to PER and PECsub both adversely affected iron absorption significantly (P<0.05) and significantly (P<0.05) and markedly lowered levels of reduced glutathione (GSH) in the liver. The high levels of polyphenol intake were implicated and interaction with glutathione was postulated to occur via catechol o-quinone conjugations with GSH. This was also implicated in the significantly (P<0.05) increased activity of glutathione reductase (GR) following 28 days. These findings suggest that PEEs from rooibos and honeybush have the potential to alter the glutathione homeostasis, which could contribute to oxidative status in the liver. PECsub resulted in alterations in the liver biliary system which was manifested as significantly (P<0.05) increased serum total bilirubin (Tbili) and alkaline phosphatase (ALP), depending on the age of the rats, level of total polyphenols and duration of exposure. The expression of a number of oxidative stress and antioxidant defense related genes were differentially altered by the PEEs of rooibos and honeybush in rat liver and further indicated potential oxidative stress. Modulatory effects of PEEs on expression of 84 of these genes in rat liver were assessed with a quantitative real-time reverse transcription polymerase chain reaction (RT-PCR) array and provided additional insights into the possible adverse and protective effects of rooibos and honeybush. Further investigation on total polyphenol dose levels and time of exposure in the application of PEEs of rooibos and honeybush as dietary supplements and functional foods is recommended and will also be of value in anticipated regulatory requirements for future substantiation of safety and efficacy.
- ItemThe 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.
- ItemXanthones 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.