Evaluation of rooibos waste plant material for the development of a high-value herbal tea product

Sishi, Manzimase (2018-03)

Thesis (MSc Food Sc)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Rooibos (Aspalathus linearis) is ‘n Suid-Afrikaanse fynbos plantspesie wat grotendeels gebruik word as ‘n kruie-tee, en aanvraag daarvoor het oor die jare aansienlik toegeneem tesame met ‘n groeiende bedryf. Dalende opbrengste en uitvoer-volumes, gedeeltelik toegeskryf aan heersende droogte, bedreig egter die industrie. Verwerking van rooibos plantmateriaal vir tee produksie lewer ‘n aansienlike hoeveelheid afvalmateriaal in die vorm van fyn stof en growwe stingels (“stok”) op. Die doel van hierdie studie was om die sensoriese profiel van gefermenteerde rooibos afvalmateriaal, apart en as mengsels, te ondersoek om die vatbaarheid van hul moontlike hergebruik as rooibos produkte vir die kruie-tee mark te bepaal. Eerstens is drie verskillende kommersiële ensieme (Rapidase, Validase en Filtrase) getoets vir hul effektiwiteit om die oplosbare vastestofinhoud van rooibos stof-ekstrak te verhoog. Ensiembehandeling het die ekstrakopbrengs van rooibos stof effe verhoog. Rapidase teen ‘n dosering van 10% (1000× meer as die vervaardiger se aanbeveling) het die hoogste toename in opbrengs (8.4%) bewerkstellig. Ensiembehandeling van rooibos stof is dus onprakties. Gevolglik is warm water ekstraksie (WWE) van rooibos afvalmateriaal geoptimiseer deur toepassing van respons-oppervlak metodiek. Voorlopige enkel faktor eksperimente het getoon dat ekstraksietyd, -temperatuur en plantmateriaal-tot-water verhouding ‘n beduidende effek (P ≤ 0.05) op ekstrakopbrengs het. ‘n Sentraal saamgestelde ontwerp is eers gebruik om die drie veranderlikes te optimiseer. Die optimale ekstraksie toestande is gevolglik deur middel van multi-respons optimisering geïdentifiseer, inaggenome praktiese oorwegings en koste-doeltreffendheid. Die goeie voorspellingsvermoeë van die kwadratiese model vir ekstrakopbrengs is geverifieer (R2adj = 0.988), wat op die toepaslikheid van die model dui. Ekstrakopbrengste van verskillende lotte rooibos stof (n = 20) het gewissel tussen 16.4 en 27.9% toe die prakties optimale ekstraksie parameters (94 °C, 20 min en 1:20 plantmateriaal-tot-water verhouding, m/v) daarop toegepas is. Tweedens is die geassosieerde sensoriese eienskappe (aroma, geur, smaak en mondgevoel) van verdunde stof-ekstrak (teen die sterkte van ‘n koppie rooibostee) en stok-infusie, apart, sowel as mengsels van 50/50 en 75/25 verdunde stof-ekstrak/stok-infusie, bepaal m.b.v. beskrywende sensoriese analise. Die sensoriese profiele van die verdunde stof-ekstrak en stof-ekstrak/stok-infusie mengsels was soortgelyk aan normale gefermenteerde rooibostee. Daarenteen was stok-infusies flou, wat dui daarop dat die gebruik van slegs rooibos stok ‘n flou tee van ‘n laer kwaliteitsgraad sal lewer. Verder was buitengewone “plankagtige/potloodskaafsel”, “rosyntjie” en “amandel” aromas teenwoordig in die stok-infusies. Die “plankagtige/potloodskaafsel” aroma is beskou as nie-tipies en onwenslik. Dit is ook waargeneem in beide die stof-ekstrak/stok-infusie mengsels. Verlaging van die hoeveelheid stok het die intensiteit van die onwenslike “plankakgtige/potloodskaafsel” aroma voldoende verlaag nie. Indien laasgenoemde geëlimineer kan word deur die byvoeging van goeie kwaliteit rooibostee, sou die hergebruik van rooibos afvalmateriaal moontlik vatbaar wees om sodoende tekorte in die rooibosbedryf aan te spreek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103273
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