Browsing by Author "Alexander, Lara"

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    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.
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    Effects of steam treatment and storage on green honeybush quality
    (Stellenbosch : Stellenbosch University, 2015-12) Alexander, Lara; Joubert, Elizabeth; Muller, Magdalena; De Beer, Dalene; Stellenbosch University, Faculty of Agrisciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Application of steam treatment to green Cyclopia maculata and C. longifolia to modulate their aroma and flavour profile, without detrimental effects on colour and individual phenolic content, was investigated. Steam treatment (96 °C, atmospheric pressure) of different time periods was applied to the shredded, fresh plant material before drying (STBD; 0, 30, 60, 90 and 120 s) or the herbal tea product after drying (STAD; 0, 1, 2, 3, and 4 min). Steam treatment of 60 s or longer resulted in a significant (p < 0.05) decrease in vegetative (‘green grass’ and ‘hay/dried grass’) and cereal (‘oats/porridge/grains’) aroma and flavour intensities, with an increase in some fruity (‘tropical fruit’ and ‘guava’) and ‘fruity-sweet' aroma intensities. These changes manifested to a greater degree for plant material subjected to STBD, while they were less prominent for C. longifolia compared to C. maculata. Additionally, 19 aroma compounds were instrumentally identified in the volatile fraction of C. maculata infusions prepared from STBD plant material. Of these, seven correlated well with green honeybush vegetative and cereal aroma attributes associated with unsteamed samples. Steaming thus resulted in the rapid loss of some of these highly volatile ‘green’-associated aroma compounds. Steam treatment of more than 60 s, regardless of STBD or STAD, was not detrimental to green colour, and individual phenolic content was not greatly affected. The 30 s STBD treatment of C. maculata, however, led to the rapid loss of green colour and oxidation of phenolic compounds, without considerable aroma improvement. This highlights the need for process control during green honeybush production. Storage stability of steamed (60 s, STBD) and unsteamed green C. maculata was investigated under low (0 °C; moisture impermeable packaging) and normal (25 °C at 60% relative humidity (RH); semi-moisture-permeable sachets) temperature storage conditions (LTS and NTS, respectively) over 6 months and high temperature storage (HTS) conditions (40 °C at 75% RH; semimoisture- permeable sachets) for 1 month. HTS conditions, after 1 month, seemed to emulate changes occurring over 6 months at NTS conditions, with little change detected at LTS conditions. Despite noticeable green colour loss over the respective storage periods at NTS and HTS conditions, individual phenolic compounds were not severely affected. NTS and HTS led to the progressive development of sought-after fermented honeybush sensory attributes, especially in unsteamed samples. These include prominent fruity (‘stewed fruit’, ‘apricot jam’ and ‘marmalade’) aromas and flavours, ‘general sweet’ and ‘fruity-sweet’ aromas and a sweeter taste. The collated descriptive sensory data were used to generate a preliminary sensory wheel for green honeybush aroma, and another for flavour, taste and mouthfeel. The sensory profile of green honeybush was finally described as a dominant vegetative aroma and flavour, prominent sweetassociated and slightly fruity aroma, with sweet and notably bitter tastes and an astringent mouthfeel. By combining the sensory data, it was clear that the major effects of storage outweighed those of steam treatments, suggesting that storage of 3 to 6 months may improve sensory quality, although colour may be slightly compromised. Immediate sensory manipulation and thus improvement, however, may be achieved to a lesser degree in the short term by STBD.