The stability of aspalathin, iso-orientin and orientin in rooibos iced tea
Date
2008-12
Authors
Viljoen, Melvi
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
The change in aspalathin, iso-orientin, orientin and total polyphenol (TP) content of a commercially
produced fermented rooibos (FR) extract was monitored throughout production. Particular attention was
paid to the effect of spray-drying on FR and unfermented rooibos (UR) extracts. The quality of commercial,
South African rooibos iced teas made with FR extract was also investigated with respect to the
aforementioned parameters. Subsequently, the effect of heating and storage on the phenolic composition and
colour of experimental iced teas containing respectively FR, UR and nano emulsified unfermented rooibos
(NEUR) extracts was investigated. The combined effect of pH (pH 3-7) and storage (5, 30 and 40ºC), as
well as high (660 mg/L, 0-7 days at 30ºC; UR only) and low (0.5 mg/L) concentrations of H2O2, was
determined on reconstituted FR, UR and NEUR extracts. Finally, eight rooibos iced teas (four variants;
unflavoured and lemon-flavoured) were analysed for plant-like, hay-like, rooibos and lemon flavour, as well
as astringency, using descriptive sensory analysis. The degree of consumer preference of the flavoured
variants was determined using the nine point hedonic scale. In all cases, changes in individual flavonoid
content were quantified using HPLC. The TP content of the iced teas and commercial extracts was
determined using the Folin-Ciocalteu assay. Browning of the iced teas and reconstituted extracts was
monitored spectrophotometrically (420 nm).
Aspalathin, iso-orientin and orientin were found to be present after all stages of the FR extract
production process. Spray-drying, specifically, also did not reduce the content of these flavonoids, or the TP
content, in FR and UR extracts. Despite the relatively good retention during the heating and storage of
experimental rooibos iced teas, these flavonoids were either absent or present at extremely low levels in
commercial iced teas. The latter suggested that either extremely low quantities of extract, no extract at all or
extracts of poor quality, were used for the production of the analysed iced teas. Increased degradation was
generally observed for sterilisation treatments compared to pasteurisation whilst losses during storage
increased with time. The presence of citric acid, due to its pH-lowering effect, and ascorbic acid, due to its
antioxidant activity, was integral to the retention of aspalathin, iso-orientin and orientin during heating, but
less so during storage. The UR iced teas generally performed better than their FR counterparts, however,
NEUR iced teas exhibited the greatest retention of the aforementioned flavonoids. Heating and storage
resulted in browning of most iced teas, whilst the TP content increased slightly or remained unchanged.
Phenolic retention in FR and UR extracts decreased with increasing pH and temperature, with
concomitant browning. However, between pH 5 and 7, the stability of aspalathin was superior in the NEUR
extract formulation. The latter also greatly resisted absorbance changes at pH 3 and 4, despite a loss of
aspalathin.
The phenolic content of UR extract was immediately reduced by high a concentration of H2O2,
however, no significant (P≥0.05) changes in absorbance were detected, suggesting the formation of
intermediate, colourless oxidation products. Formulations containing ascorbic acid experienced the greatest
reductions. This was attributed to the iron reducing ability of this compound, as reduced iron accelerates the rate of the Fenton reaction. At low levels of H2O2, only the FR extract exhibited a loss of phenolic
compounds. The level of iron in this extract was the highest.
Despite having the greatest aspalathin and total flavonoid content, lemon flavoured unfermented
rooibos iced tea (UF/LEMON) was disliked by consumers. Preference was directed away from the plant-like
characteristic of this tea and towards rooibos flavour, characteristic of fermented rooibos iced tea. Iced tea
comprising both FR and NEUR extract produced a product that 77% of consumers rated positively. Its slight
hay-like flavour did not significantly (P≥0.05) reduce the liking of this product compared to fermented
rooibos iced tea.
Description
Thesis (MSc Food Sc) (Food Science)--Stellenbosch University, 2008.
Keywords
Rooibos tea, Iced Tea, Aspalathin, Iso-orientin, Orientin, Dissertations -- Food science, Theses -- Food science