The stability of aspalathin, iso-orientin and orientin in rooibos iced tea

[摘要] The change in aspalathin, iso-orientin, orientin and total polyphenol (TP) content of a commerciallyproduced fermented rooibos (FR) extract was monitored throughout production. Particular attention waspaid 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 theaforementioned parameters. Subsequently, the effect of heating and storage on the phenolic composition andcolour 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), aswell as high (660 mg/L, 0-7 days at 30ºC; UR only) and low (0.5 mg/L) concentrations of H2O2, wasdetermined 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 wellas astringency, using descriptive sensory analysis. The degree of consumer preference of the flavouredvariants was determined using the nine point hedonic scale. In all cases, changes in individual flavonoidcontent were quantified using HPLC. The TP content of the iced teas and commercial extracts wasdetermined using the Folin-Ciocalteu assay. Browning of the iced teas and reconstituted extracts wasmonitored spectrophotometrically (420 nm).Aspalathin, iso-orientin and orientin were found to be present after all stages of the FR extractproduction process. Spray-drying, specifically, also did not reduce the content of these flavonoids, or the TPcontent, in FR and UR extracts. Despite the relatively good retention during the heating and storage ofexperimental rooibos iced teas, these flavonoids were either absent or present at extremely low levels incommercial iced teas. The latter suggested that either extremely low quantities of extract, no extract at all orextracts of poor quality, were used for the production of the analysed iced teas. Increased degradation wasgenerally observed for sterilisation treatments compared to pasteurisation whilst losses during storageincreased with time. The presence of citric acid, due to its pH-lowering effect, and ascorbic acid, due to itsantioxidant activity, was integral to the retention of aspalathin, iso-orientin and orientin during heating, butless 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 storageresulted 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, withconcomitant browning. However, between pH 5 and 7, the stability of aspalathin was superior in the NEURextract formulation. The latter also greatly resisted absorbance changes at pH 3 and 4, despite a loss ofaspalathin.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 ofintermediate, colourless oxidation products. Formulations containing ascorbic acid experienced the greatestreductions. 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 phenoliccompounds. The level of iron in this extract was the highest.Despite having the greatest aspalathin and total flavonoid content, lemon flavoured unfermentedrooibos iced tea (UF/LEMON) was disliked by consumers. Preference was directed away from the plant-likecharacteristic of this tea and towards rooibos flavour, characteristic of fermented rooibos iced tea. Iced teacomprising both FR and NEUR extract produced a product that 77% of consumers rated positively. Its slighthay-like flavour did not significantly (P≥0.05) reduce the liking of this product compared to fermentedrooibos iced tea.

[发布机构] Stellenbosch University