Doctoral Degrees (Food Science)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/537

Browse

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

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Comprehensive 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.
  • Thumbnail Image
    Item
    The 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.