The contribution of phenolics to the bitter taste of honeybush (Cyclopia genistoides) herbal tea

Alexander, Lara (2018-12)

Thesis (PhDFoodSc)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die bitter smaak van sommige produksielotte van Cyclopia genistoides kruitetee beperk nie alleen die heuningbosbedryf se doelwit om konstante gehalte te verseker nie, maar het ook ‘n negatiewe impak op verbruikersaankope. Vorige studies het probeer om hierdie verskynsel te verstaan deur assosiasies tussen bitter smaak intensiteit en die konsentrasie van ‘n aantal fenoliese verbindings te bepaal. Ten spyte van 'n aantal betekenisvolle korrelasies tussen spesifieke verbindings en bitter intensiteit, was afdoende bewys van die oorsaak van bitterheid nie moontlik nie. Die huidige ondersoek was dus daarop gemik om beslissende bewys te lewer van die bydrae van fenoliese verbindings tot die bitterheid van C. genistoides. Om hierdie doel te bereik het die eerste fase van die studie behels dat 'n warm water ekstrak van groen C. genistoides plantmateriaal (infusie bitter intensiteit van ~45 op ‘n 100-punt skaal) d.m.v. kolom-chromatografie in drie fraksies geskei is, onderskeidelik ryk aan bensofenone, xantone en flavanone. Die bitter smaak van die onderskeie fraksies is bepaal deur beskrywende sensoriese analise (BSA), asook diskriminasietoetse. Die individuele fenoliese verbindings in elke fraksie is d.m.v. hoë-druk vloeistofchromatografie gekwantifiseer. Die bensofenoon-ryke fraksie was nie bitter nie (< 5), die flavanoon-ryke fraksie was effens bitter (~13) en die xantoon-ryke fraksie was duidelik bitter (~31). Verdere ondersoek van die bitter xantoon-ryke fraksie het vergelyking van die hoof heuningbos xantoon verbindings en regio-isomere, mangiferien en isomangiferien, deur middel van BSA ingesluit. Hierdie vergelyking het getoon dat isomangiferien (~15) effens bitter is en die bitter smaak van mangiferien (~30) onderdruk het (~22). Die tweede fase van die projek het gefokus op die moontlike vermoë van die bensofenoon- en flavanoon-ryke fraksies, sowel as hul belangrikste individuele fenoliese verbindings, om die intensiteit van bitter smaak te moduleer. BSA is ook hiervoor aangewend. Die resultate het aangetoon dat die modulerende effek dosis-afhanklik is. Dit is bevestig dat 3-β-D-glukopiranosiel4-β-D-glukopiranosieloksiriflofenoon (IDG) and naringenien-O-heksose-O-deoksiheksosied B (NHDB) bitter smaak moduleer weens hul vermoë om onderskeidelik die intensiteit van bitter smaak te onderdruk en te versterk. Daarbenewens is dit ook bevestig dat ‘n mengsel van NHDB en sy isomeer, NHDA (gevorm gedurende gesimuleerde fermentasie van NHDB), geen modulatoriese effek op bitter intensiteit het nie. Hierdie resultaat regverdig verdere ondersoek. Vir die derde en finale fase van die studie is ‘n groot datastel gebruik om ‘n robuuste statistiese voorspellingsmodel vir die intensiteit van bitter smaak op grond van fenoliese samestelling te ontwikkel. Monsters van fermenteerde en ongefermenteerde plantmaterial van verskeie genotipes van C. genistoides en C. longifolia, tans deel van die Landbounavorsingsraad se heuningbos plantverbeteringsprogram, is ontleed. Beide spesies bevat hoë xantoon- en bensofenoonvlakke en kan bitter infusies lewer. Die data het ook ondersoek na die effek van fermentasie op bitter smaak en fenoliese saamestelling moontlik gemaak. Die finale onafhanklike gevalideerde stapsgewyse lineêre regressiemodel kon die infusie se bitter smaak voorspel (R2 = 0.859) deur slegs van vyf fenoliese verbindings (IDG, hesperidien, 3-β-D-glukopiranosielmaklurien, mangiferien and isomangiferien) en inhoud van oplosbare vastestowwe, wat in beide C. genistoides en C. longifolia voorkom, gebruik te maak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105145
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