pubs.acs.org/JAFC Published on Web 10/05/2010 © 2010 American Chemical Society J. Agric. Food Chem. 2010, 58, 10965–10971 10965 DOI:10.1021/jf103552f Use of Green Rooibos (Aspalathus linearis) Extract and Water-Soluble Nanomicelles of Green Rooibos Extract Encapsulated with Ascorbic Acid for Enhanced Aspalathin Content in Ready-to-Drink Iced Teas ELIZABETH JOUBERT,* ,†,‡ MELVI VILJOEN, ‡ DALENE DE BEER, † CHRISTIAAN J. MALHERBE, † D. JACOBUS BRAND, § AND MARENA MANLEY ‡ † Agricultural Research Council (ARC) Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch 7599, South Africa, ‡ Department of Food Science, and § Central Analytical Facility, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Heat-induced changes in aspalathin, iso-orientin, and orientin content of ready-to-drink (RTD) green rooibos iced tea formulations were investigated. An organic-solvent-based aspalathin-enriched extract prepared from green rooibos was used “as-is” or encapsulated with ascorbic acid in a water-soluble nanomicelle-based carrier system. The common iced tea ingredients, ascorbic acid, and/or citric acid were added to the iced tea containing green rooibos extract. Only citric acid was added to the iced tea contain- ing the nanomicelles. Heat treatments consisted of pasteurization (93 °C/5 min and 93 °C/30 min), normal- temperature sterilization (NTS; 121 °C/15 min), and high-temperature sterilization (HTS; 135 °C/4 min). Pasteurization had little or no effect on the flavonoid content. NTS and HTS induced significant losses in the flavonoids. The addition of citric and ascorbic acids improved the stability of the flavonoids, but encapsulation of green rooibos extract with ascorbic acid in nanomicelles did not offer additional stability. The only benefit of using the water-soluble nanomicelles was the improved clarity of the RTD product. Iso- orientin and orientin contents were substantially less affected than aspalathin by the heat treatments, partially because of conversion of aspalathin to these flavones, which countered losses. 5-Hydroxymethyl- furfural (HMF), a known dehydration product of hexoses under mild acidic conditions and also a degradation product of ascorbic acid, was observed in formulations containing citric and/or ascorbic acids. KEYWORDS: Green rooibos; Aspalathus linearis; aspalathin; iso-orientin; orientin; iced tea; heat processing; nanomicelles INTRODUCTION Dietary flavonoids are considered major functional ingredi- ents, because of their potential beneficial role in the “health and wellness” of consumers. Stability of functional ingredients in food products and beverages is of the utmost importance to ensure retention of their beneficial effects. Recent years has seen a growing market for ready-to-drink (RTD) iced teas, including products containing extracts of the herbal tea, rooibos. High levels of the dihydrochalcone C-glucoside, aspalathin (Figure 1), present in the rooibos plant (Aspalathus linearis)( 1 ) has led to the production of aspalathin-enriched extracts, offering a means to deliver products with enhanced aspalathin content. Aspalathin is a potent antioxidant ( 2-5) with antimutagenic properties ( 6). In addition, recent studies demonstrated the in vivo glucose-lowering ability for aspalathin ( 7, 8), furthering interest in this compound as a nutraceutical ingredient in food and beverage products. “Fermented” (oxidized) rooibos is the most common form used by the food industry as herbal tea or in products such as RTD iced teas and yoghurt. The oxidation process, necessary for devel- opment of the characteristic rooibos flavor and color, however, greatly reduces the aspalathin content of the plant material ( 9) and leads to the formation of the two flavones, iso-orientin and orientin, as major products (Figure 1)( 10). Aqueous spray-dried extracts of fermented rooibos usually contain less than 0.5% aspalathin ( 11, 12 ). Knowledge of its stability during food and beverage processing is limited to our previous investigation on RTD rooibos iced tea, prepared with fermented rooibos extract ( 12 ). Sterilization required for RTD fermented rooibos iced tea with an extended shelf life was shown to be detrimental to the retention of aspalathin. Losses of between 10 and 78%, depending upon the severity of the heat treat- ment and the presence of other food ingredients, such as ascorbic and citric acids, were demonstrated ( 12 ). Commercial RTD rooibos iced tea products containing fermented rooibos extract were found to contain little or no aspalathin ( 12 ), indicating that the product should be improved in terms of its aspalathin content. An alternative strategy is thus required to achieve higher levels of aspalathin in the final RTD product. The obvious strategy would be to use a green rooibos extract instead of a fermented rooibos extract because substantially higher aspalathin levels could be realized in the green rooibos extract ( 13 ). However, *To whom correspondence should be addressed. Telephone: þ27- 21-809-3444. Fax: þ27-21-809-3430. E-mail: joubertl@arc.agric.za.