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 containing 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-Hydroxymethylfurfural (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.