Physicochemical stability of aspalathin-rich rooibos iced tea powders under accelerated storage conditions as affected by formulation

Abstract

A stable, palatable, aspalathin-rich rooibos iced tea powder in a convenient single-serve format offers a viable functional beverage for reducing sugar intake. Our initial goal was to determine the optimal ratio of green rooibos extract (GRE) to fermented rooibos extract (FRE). This was essential to create a formulation that combined high aspalathin content with the sought-after sensory profile of traditional fermented rooibos. The second goal was to ascertain how the addition of common beverage ingredients (xylitol, citric, and ascorbic acid) and moisture (6 % and 53 % relative humidity, RH) affected the physicochemical stability of the mixtures during accelerated shelf-life storage (40 °C; 12 weeks). An FRE:GRE ratio of 1:0.5 was found to have a similar sensory profile as FRE, generally accepted by consumers, with a high aspalathin content (83.5 mg L−1) compared to FRE alone (5.5 mg L−1). During storage at 53 % RH, crystalline ingredients significantly decreased the color (lower L∗ and h°) and aspalathin (based on first-order reaction rate constants) stability. Changes in the crystal structure, affected by the interaction between ingredients, were observed using powder X-ray diffraction and Fourier-transform infrared spectroscopy. Minimal changes were observed for all parameters when storing the iced tea powders at 6 % RH, regardless of formulation. Ready-to-reconstitute aspalathin-rich rooibos iced tea powders should be stored in moisture-impermeable packaging to ensure a physically (including color) and chemically stable product.