Doctoral Degrees (Food Science)
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Browsing Doctoral Degrees (Food Science) by browse.metadata.advisor "Joubert, Elizabeth"
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- ItemComprehensive characterisation of the phenolic composition of Cyclopia pubescens (honeybush)(Stellenbosch : Stellenbosch University, 2019-12) Walters, Nico Albertus; De Beer, Dalene; Joubert, Elizabeth; Williams, Paul James; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.ENGLISH ABSTRACT: Cyclopia pubescens Eckl. & Zeyh, endemic to South Africa, is under threat of extinction. This threat can be negated through commercial cultivation if utilised as honeybush tea. Lack of knowledge of the phenolic composition of this Cyclopia species is a hurdle in the development of value-added products such as nutraceuticals, providing the incentive for a comprehensive investigation of the phenolic profile of C. pubescens. A reversed-phase (RP) high-performance liquid chromatography-diode-array detection (HPLC-DAD) method using a biphenyl column was developed and validated. Eight phenolic compounds, representing major compounds (3-β-D-glucopyranosyl-4-O-β-D-glucopyranosyliriflophenone, 3-β-D- glucopyranosylmaclurin, 3-β-D-glucopyranosyliriflophenone, mangiferin, isomangiferin, vicenin-2, narirutin and hesperidin), were identified and a further six compounds tentatively identified by comparison of their retention times, UV–Vis spectra and high-resolution mass spectrometric (HR-MS) characteristics with those of authentic reference standards and literature, respectively. The predominant accumulation of the compounds in the leaves or stems of C. pubescens was determined for a small number of available genotypes (n = 17), affording some indication of variation between plants in the field genebank of the Agricultural Research Council. For further elucidation of the phenolic profile, the focus fell on minor phenolic compounds. Increased separation power was required and therefore an analytical comprehensive two-dimensional (2D) separation method utilising normal-phase (NP) high- performance countercurrent chromatography-DAD (HPCCC) as the first dimension and RP ultra(U)HPLC-DAD as the second dimension. High loading capacity and selectivity of HPCCC and high selectivity and resolving power of UHPLC resulted in ca 81% orthogonality. A total of 32 compounds were (tentatively) identified by comparing their UV– Vis and HR-MS characteristics to those of reference standards and literature. The HPCCC method was subsequently upscaled to semi-preparative mode and eight phenolic compounds were isolated in adequate quantities for unambiguous structural elucidation using 2D nuclear magnetic resonance, gas chromatography-MS analysis after acid- catalysed hydrolysis of the glycoside, derivatisation and determination of optical activity. (2R)-5-O-[α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranosyl]eriodictyol, (2S)-5-O-[α-L- rhamnopyranosyl-(1→2)-β-D-glucopyranosyl]eriodictyol and (2S)-5-O-[α-L-rhamnopyranosyl- (1→2)-β-D-glucopyranosyl]-5,7,3ʹ,4ʹ-tetrahydroxyflavan have not been previously reported in literature. The five other compounds include the two flavanones, (2S)-5-O-[α-L- rhamnopyranosyl-(1→2)-β-D-glucopyranosyl]naringenin and R-neo-eriocitrin, the two phenolic acids, 3-O-α-L-arabinopyranosyl-3,4-dihydroxybenzoic acid and 4-O-β-D-glucopyranosyl-cis-4-hydroxycinnamic acid, and an anisole, 4-(4ʹ-O-β-D-glucopyranosyl-3ʹ- methoxyphenyl)-2-butanone. Natural deep eutectic solvents (NADES) (n = 7) were evaluated to improve extraction efficiency and obtain a more representative phenolic profile of C. pubescens. Hot water and 40% EtOH, commonly used for extraction of Cyclopia plant material, represented the benchmark solvents. Stereoisomers were selectively extracted by the different NADES solvents. Application of an on-line HPLC-ABTS assay showed that these stereoisomers also differed in Trolox equivalent antioxidant capacity. Glutamic acid based NADES (n = 2) provided superior extraction efficiency, but low eutectic stability made these solvents unsuitable for high-throughput extraction; therefore, the most suitable NADES for extracting the phenolic compounds of C. pubescens was choline chloride:proline (CcP) (1:3, molar ratio). The optimal extraction temperature for NADES, tested over the range 40 to 90 °C, was 70 °C instead of 93 °C for water when using a 20 min extraction time. The best extraction solvent for phenolic compounds remains 40% EtOH, but NADES can be used to selectively extract phenolic compounds.
- ItemThe contribution of phenolics to the bitter taste of honeybush (Cyclopia genistoides) herbal tea(Stellenbosch : Stellenbosch University, 2018-12) Alexander, Lara; Joubert, Elizabeth; De Beer, Dalene; Muller, Magdalena; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.ENGLISH ABSTRACT: The occurrence of bitter taste in some production batches of Cyclopia genistoides herbal tea not only challenges efforts of the honeybush industry to achieve consistent product quality, but also adversely affects consumer purchase intent. Previous studies have attempted to understand this phenomenon by determining associations between the bitter intensity of honeybush infusions and their individual phenolic concentrations. Despite some significant correlations between specific compounds and bitter intensity, the data did not give conclusive evidence of the cause of bitterness. The current investigation thus aimed to provide decisive proof of the role of phenolic compounds in the bitterness of C. genistoides herbal tea. To achieve this, the first phase of the study utilised a hot water extract of unfermented C. genistoides plant material (yielding an infusion with a bitter intensity of ~45 on a 100-point scale), separated by column chromatography into three fractions rich in benzophenones, xanthones and flavanones, respectively. The bitter taste of the fractions was determined by descriptive sensory analysis (DSA) and discrimination tests, and their individual phenolic content was quantified by high-performance liquid chromatography. The benzophenone-rich fraction was not bitter (< 5), the flavanone-rich fraction was somewhat bitter (~13) and the xanthone-rich fraction was considered distinctly bitter (~31). Further investigation of the bitter xanthone-rich fraction included a focussed DSA comparison of the major xanthones and regio-isomers, mangiferin and isomangiferin. This comparison revealed that isomangiferin was only somewhat bitter (~15) and modulated the distinct bitter taste of mangiferin (~30) by suppressing it (~22). The second phase of the study focussed on possible bitter taste modulation by the benzophenone- and flavanone-rich fractions, as well as their major individual phenolic compounds using DSA. The results indicated that modulation is dose-dependent, and identified 3-β-D-glucopyranosyl-4-β-Dglucopyranosyloxyiriflophenone (IDG) and naringenin-O-hexose-O-deoxyhexoside B (NHDB) as novel bitter modulators for their respective bitter suppressing and enhancing activities. In addition, a mixture of NHDB and its isomer, NHDA, formed upon heating of NHDB (to simulate the effect of fermentation), did not have any modulatory effect on bitter intensity and should be investigated further. For the third and final phase of the study, a large data set was utilised to produce a robust statistical model for the prediction of bitter intensity of infusions from their individual phenolic concentrations. Fermented and unfermented samples of several genotypes of C. genistoides and C. longifolia in the Agricultural Research Council’s honeybush plant breeding programme were analysed. Both species contain high xanthone and benzophenone levels and have been found to produce bitter infusions. The data also allowed the investigation of the effects of fermentation on bitter intensity and individual phenolic concentrations of the infusions. The final independent validated stepwise linear regression model was able to predict bitter taste of the infusion (R2 = 0.859) using the concentration of only five phenolic compounds (IDG, hesperidin, 3-β-D-glucopyranosylmaclurin, mangiferin and isomangiferin) and soluble solids content, common to both C. genistoides and C. longifolia.
- ItemDevelopment of a gastroretentive anti-diabetic nutraceutical incorporating polyphenol-enriched fractions of Cyclopia genistoides(Stellenbosch : Stellenbosch University, 2020-04) Miller, Neil; Joubert, Elizabeth; Manley, Marena; Malherbe, Christiaan Johannes; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.ENGLISH ABSTRACT: Extracts of honeybush (Cyclopia genistoides) containing glycosylated xanthones, mangiferin (1) and isomangiferin (2), and benzophenones, 3-β-D-glucopyranosyliriflophenone (3) and 3-β-D-glucopyranosyl-4- O-β-D-glucopyranosyliriflophenone (4) inhibit α-glucosidase (AG), a key digestive enzyme and treatment target for postprandial hyperglycaemia associated with type 2 diabetes. Ultrafiltered green C. genistoides extract served as the starting material for the development of an optimised production protocol for xanthone- and benzophenone-enriched fractions (XEFs and BEFs) by macroporous adsorbent resin chromatography. Inter-batch variation in the phenolic content of the raw material manifested as variation in the composition and degree of enrichment of target compounds in XEFs and BEFs. The in vitro AG inhibitory effects of C. genistoides phenolics, extract, XEF and BEF, combined with the commercial AG inhibitor (AGI), acarbose, were investigated using the combination index. The single-compound AGIs demonstrated potency in the descending order: acarbose (IC50 = 44.3 µM) > 1 (102.2 µM) > 2 (119.8 µM) > 3 (237.5 µM) > 4 (299.4 µM). Potency of the extract and fractions was strongly linked to their xanthone content. XEFs (xanthone content = 22.3–48.1 g/100 g) were produced using ten different batches of plant material and tested at a fixed concentration (160 µg/mL), achieving 63 to 72% enzyme inhibition. BEFs (benzophenone content = 11.4– 21.7 g/100 g) achieved enzyme inhibition of 26 to 34%. There was a weak linear correlation (R² < 0.43) between the target compound content of the fractions and their AG inhibition potency. Synergistic AG inhibition at > 50% effect levels was observed for all combinations of acarbose with fractions (XEFs, BEFs) or target compounds (1–4). Combinations of acarbose with 1 and 2 gave the highest theoretical in vitro acarbose dose reduction (> six-fold) across all effect levels. XEFs showed greater theoretical acarbose dose reduction (≈ four-fold at 50% inhibition) than BEFs, demonstrating the potential of XEFs as a supplement to acarbose. In a subsequent in vivo oral sucrose tolerance test in normal and diabetic Wistar rats, XEF (single orally administered dose of 300 mg/kg body weight) did not result in significantly lowered postprandial blood glucose or in an improved effect in combination with acarbose (5 mg/kg body weight). The suitability of C. genistoides phenolics as non-toxic active pharmaceutical ingredients (APIs) was confirmed in a liver cell model, which indicated no cytotoxicity following acute or chronic exposure. Ex vivo intestinal transport studies using porcine jejenum showed that the target compounds (1–4) are poorly absorbed, confirming their suitability as APIs aimed at an intestinal target, and re-emphasising the low risk of systemic toxicity. XEF and BEF were subsequently incorporated (alone and combined) in a non-effervescent gastroretentive tablet formulation containing low-density styrene-divinylbenzene co-polymer as floating agent. The tablets floated in an in vitro medium (0.1 N HCl) for at least 8 h and released APIs through a diffusion-based process, described by the Weibull model (R² > 0.99). API degradation during storage under adverse conditions (12 weeks at 40 °C) followed first order reaction kinetics with the order of compound stability: 4 > 1 > 2 > 3.
- ItemDevelopment of a quality grading system for the honeybush (Cyclopia spp.) tea industry(Stellenbosch : Stellenbosch University, 2020-12) Du Preez, Brigitte Von Pressentin; Joubert, Elizabeth; Muller, Magdalena; Moelich, Erika Ilette; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.ENGLISH ABSTRACT: Honeybush tea, produced from Cyclopia species endemic to South Africa, has attained an international footprint within the global herbal tea sector. As demand is exceeding supply to a primarily export market, all production batches should meet optimum quality standards. The lack of standardised sensory quality criteria and assessment methods within the commercial sector has resulted in tea of variable sensory quality reaching the market. The trade of inconsistent and inferior quality products will be detrimental to the reputation of honeybush tea and consumer acceptance, and ultimately the honeybush industry. The need for a scientifically founded quality grading system to evaluate, differentiate and communicate the sensory quality of honeybush tea was addressed through four quality control elements, i.e. a sensory lexicon and wheel, sensory quality standard, quality scoring method, and rapid quality classification methods. The previously developed honeybush aroma lexicon and wheel were revised, based on a newly established comprehensive sensory and physicochemical dataset. Data of samples of the main commercial Cyclopia species (C. intermedia, C. subternata and C. genistoides), processed on laboratory- and commercial-scale, were incorporated to represent the sensory space in terms of different qualities. Universal chemical-based reference standards were developed and validated to replace food-based reference standards in the aroma lexicon to facilitate standardised assessment of honeybush sensory quality. The established sensory quality standard was founded on the comprehensive dataset and input from industry. Sensory quality parameters for the tea infusions and dried plant material were identified, and parameter specifications for ‘high’, ‘moderate’, ‘low’ and ‘poor’ quality classes were defined through expert focus groups. A user-friendly quality scoring method that incorporates a scorecard and colour reference card, was developed and validated for the assessment and classification of production batches based on obtained parameter and total score values and citation frequencies of specific attributes. The validity of reference-based rapid methods, polarised sensory positioning (PSP) and polarised projective mapping (PPM), were investigated for their discrimination ability as time-efficient classification tools to distinguish between infusions of large samples sets of variable sensory quality within commercial and research context. The efficacy of the use of physical (p) poles (tea infusions) and novel theoretical (t) poles (descriptions), representative of the four sensory quality classes, as references, were compared within each method, using a trained panel. Product configurations similar to that of a classical sensory profiling method, descriptive sensory analysis, demonstrated the validity of the method variations for broad quality classification based on key sensory quality parameters. PPM-p indicated the highest discrimination ability between the quality classes. Recommended amendments to theoretical pole descriptions would improve feasibility for commercial application. The quality scoring method and PPM-t were tested by a panel of industry representatives, and the need for industry assessor training in sensory quality parameters was emphasised. Implementation of the proposed integrated quality grading system will equip honeybush industry role-players in delivering a final product of consistent sensory quality within the honeybush value chain.
- ItemMulti-stakeholder perspectives on food labels and the future of healthy eating(Stellenbosch : Stellenbosch University, 2022-12) Todd, Melvi; Joubert, Elizabeth; Guetterman, Timothy; Sigge, Gunnar; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.ENGLISH ABSTRACT: South Africa, like many other countries throughout the world, is grappling with the increasing burden of non-communicable diseases, caused in part by an unhealthy diet. This study investigated how changes to the presentation of nutritional information on food labels can potentially help South Africans judge the healthiness of food, thereby potentially guiding them towards healthier choices and beneficial health outcomes. As a point of departure for the study, the value-adding potential of legislating health claims on food labels was considered. Draft legislation to allow health claims have been pending since 2014, and the legislation also does not address any claims pertaining to the health potential of our indigenous food plants, despite notable research spending in this area. Interviews (n = 49) with stakeholders with varied expertise revealed strong evidence for economic value creation potential from health claims, however, this was skewed towards businesses. The findings were less clear about other forms of value and the potential beneficiaries. Developing metrics that people understand and that relate to all forms of value will be required to ensure that health claims on food labels, should they be permitted, do not only benefit a select group of stakeholders. Delving deeper into the question of whether legislating health claims on food labels would be useful for consumers revealed more challenges than opportunities. Persistent impediments to use of the information and unresolved questions about the substantiation and enforcement of health claims were of particular concern. At the same time, no solution appeared apparent for reaching consumers in the informal market with nutritional information. While legislating health claims is not recommended at present, numerous front-of-pack (FOP) label proposals were made by the interview participants. In the final stage of the study, consumers (n = 1 261) were asked to compare FOP labels and a health claim relative to a no-label control, applied to a fictitious cereal product, a commonly consumed processed food in South Africa. Both health warnings and health star rating showed promise in terms of assisting consumers in identifying less healthful products, but effect sizes were largest for the warning label. This outcome requires validation with more product categories, as well as testing in real-world settings. The study culminated with the presentation of a three-phased nutritional labelling strategy for South Africa: ‘Make it clearer’, ‘make it simpler’, ‘make it smarter’ - intended to illustrate the need to take action now whilst simultaneously planning for the future. At a policy level, while South Africa plans for the implementation of FOP labels, we should be actively planning for the labelling of the future – one which will be more technologically enabled. The findings of this study may provide policy-makers and decision-makers, such as the Department of Health, with valuable information about the presentation of nutritional information on food labelling (i.e. selecting FOP labels for further real- world testing), as well as setting the course for the future.