Browsing by Author "Sishi, Manzimase"

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    Evaluation of rooibos waste plant material for the development of a high-value herbal tea product
    (Stellenbosch : Stellenbosch University, 2018-03) Sishi, Manzimase; Joubert, E.; De Beer, D.; Muller, M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) is a South African fynbos plant species that is predominantly harvested for consumption as herbal tea. The demand for this herbal tea has grown significantly along with the industry over the years. However, declining production yields and export volumes, partially as a result of drought, are a threat to the industry. Rooibos processing generates a noteworthy volume of waste plant material in the form of fine dust and coarse stems. Therefore, the purpose of this study was to determine the sensory profiles of fermented rooibos waste plant material, separately and as blends, to gauge the feasibility of possibly reutilising rooibos waste for the production of rooibos products of acceptable quality for the herbal tea market. Firstly, three commercial enzymes (Rapidase, Validase and Filtrase) were tested for their effectivity to increase the soluble solids content of rooibos dust extract. Enzyme-assisted extraction (EE) of rooibos dust resulted in a minor increase in the extract yield. Rapidase at the highest dose of 10% (1000x dosage recommended by supplier) resulted in the largest increase in extract yield (8.4%). EE of rooibos dust is therefore impractical. Hot water extraction (HWE) conditions were therefore optimised using response surface methodology. Preliminary “one-factor-at-a-time” experiments demonstrated that extraction time, extraction temperature and plant material-to-water ratio had significant effects (P ≤ 0.05) on the extract yield. A central composite design was used to optimise these three variables, followed by identification of the optimal extraction conditions using desirability profiling, but taking cost-efficiency and practicality into consideration. Satisfactory predictive ability for the extract yield (R2adj = 0.988) was verified confirming suitability of the prediction model. Extract yields varied between 16.4% and 27.9% when the practically optimal extraction conditions (94 °C, 20 min and 1:20 plant material-to-water ratio (m.v-1)) were applied to different batches (n=20) of rooibos dust. Secondly, sensory attributes (aroma, flavour, taste and mouthfeel) associated with diluted dust extracts (at “cup-of-tea” strength) and stem infusions individually, as well as diluted dust extract and stem infusion combinations (50/50 and 75/25 ratios), were characterised using descriptive sensory analysis. Diluted dust extracts, as well as diluted dust extract and stem infusion combinations, produced infusions of similar sensory quality as normal rooibos infusions. In contrast, stem infusions produced weak infusions, indicating that the use of stem plant material alone would result in rooibos infusions with decreased quality. Additionally, unusual “planky/pencil shavings”, “raisin” and “almond” aroma attributes were perceived in the stem infusions. The “planky/pencil shavings” aroma note was perceived as non-typical and undesirable. This attribute was carried through into all dust extract and stem infusion combinations. A reduction of the stem plant material content did not adequately decrease the undesirable “planky/pencil shavings” aroma. If it is possible to eliminate the latter by blending with good quality rooibos tea, reutilisation of the waste plant material could be feasible to address the shortages in the rooibos industry.