The physicochemical properties and stability of aspalathin in micro- and nanoencapsulated green rooibos extract formulations

[摘要] ENGLISH ABSTRACT: Green rooibos extracts with high aspalathin content have potential as nutraceutical food ingredients based on their properties relating to the prevention of metabolic syndrome. However, delivery of green rooibos extracts in convenient beverage products is a challenge due to poor stability of aspalathin in the presence of moisture. Thus, the development of alternative ingredients and convenience products is required.Microencapsulation of a green rooibos extract (GRE) with maltodextrin as control carrier and inulin and chitosan as low kilojoule functional alternatives was achieved by spray-drying. Spray-dried GRE and powders containing maltodextrin or inulin had similar yields (>76%), moisture content (<3.5%) and aspalathin retention (>95%), whereas microencapsulation with chitosan resulted in lower yields (<66%), higher moisture content (>3.4%) and lower aspalathin retention (<83%). Accelerated stability tests (40 °C/75% relative humidity (RH)) revealed similar aspalathin degradation rates (based on fractional conversion model) for GRE, inulin and maltodextrin formulations, but significantly higher degradation rates for chitosan formulations. Given the low incompatibility between GRE and inulin, inulin-microencapsulated GRE (1:1 ratio; IN50) was selected as the most suitable green rooibos nutraceutical beverage ingredient. IN50 was added to iced tea powder formulations, which contained various food grade ingredients (sucrose, xylitol, citric acid and ascorbic acid). Shelf-life trials (30 °C and 40 °C/65% RH for 5–12 months) in different packaging materials (semi-permeable vs impermeable) revealed more aspalathin degradation (based on first order reaction rates), more discolouration and clumping after the addition of crystalline ingredients. These changes were more pronounced at 40 °C and for powders stored in the semi-permeable packaging. The formulation containing IN50, xylitol and citric acid, which showed the most drastic physical and chemical changes during storage, was subjected to descriptive sensory analysis, which confirmed significant changes also in its sensory profile.Nanoencapsulation of an aspalathin-rich fraction (GRAF) prepared from green rooibos was also investigated. Combinations of natural (chitosan and lecithin) and synthetic [poly(lactide-co-glycolide) and Eudragit S100® (ES100)] polymers with suitable conventional methods and electrospraying were investigated. Overall, ES100 electrosprayed particles had the best combination of properties, i.e. encapsulation efficiency (EE, 55.4%), loading capacity (LC, 11.1%), release rate at pH 7.4 (1.67 h-1) and size (190 nm). Further optimisation of the ES100-GRAF loaded nanoparticles was achieved using a central composite design. Responses included yields between 78.2–78.3%, EE between 73.9–76.4% and LC between 9.9–12.9%. Pure aspalathin was subsequently encapsulated using the optimal conditions, resulting in a similar yield, EE, LC, particle size and particle morphology to that of GRAF loaded nanoparticles. The stability of the aspalathin and GRAF loaded nanoparticles was investigated at fixed pH-time combinations. Nanoencapsulation offered a more stable environment for aspalathin. Overall, pure aspalathin was less stable than when in GRAF. Even though intestinal permeability could theoretically be improved with nanoencapsulation, the parallel artificial membrane permeability assay and Caco-2 cell model indicated that pure aspalathin and aspalathin nanoparticles both have equally low permeability.These methods offered an alternative for the production of GRE convenience products and ingredients, whilst providing insight on the effects of encapsulation and ingredients of powder formulations.

[发布机构] Stellenbosch University