Green rooibos neutraceutical : optimisation of hot water extraction and spray-drying by quality-by-design methodology

[摘要] ENGLISH ABSTRACT: Unfermented Aspalathus linearis, otherwise known as green rooibos (GR), contains high levels of aspalathin, a potent C-glucosyl dihydrochalcone antioxidant with antidiabetic bioactivity, unique to rooibos. Inherent variation in the phenolic composition of rooibos is likely to cause significant variability in the aspalathin content of different GR production batches and thus also the batch-to-batch quality of a nutraceutical green rooibos extract (GRE). The aim of this study was to optimise hot water extraction and spray-drying for the production of a shelf-stable GRE. A quality-by-design (QbD) approach was applied, entailing a preliminary risk assessment step, one-factor-at-a-time analysis, and analyses according to a central composite design (CCD) to determine the effects of process parameters on responses. Response surface methodology (RSM) was applied to identify suitable control spaces, i.e. ranges of process input factors in which optimal responses, i.e. product quality, could be expected. Significant variation in aspalathin content of different GR production batches (n = 47; 2.5–4.5%) was demonstrated. The CCD for extraction included three independent variables: extraction time (10–40 min), extraction temperature (41–93 °C) and water-to-plant material ratio (6.6:1–23.4:1; v.m-1). Prediction models and response surfaces for extract yield (EY; g.100 g-1 plant material), aspalathin extraction efficiency (Asp_EE; g.100g-1 in plant material) and aspalathin content (g.100 g-1 soluble solids) were generated. Verification of the prediction models showed good predictive ability for EY and Asp_EE. Multi-response optimisation was applied to identify levels of the independent variables which would maximise EY and Asp_EE. Optimal conditions were identified based on these results, along with considerations of cost-efficiency and practicality: extraction time, 29–31 min; extraction temperature, 90–95 °C and water-to-plant material ratio, 9:1–11:1 (v.m-1). Validation of the optimal extraction conditions using the 47 commercial GR production batches, (aspalathin content >2.5%) showed that at least 15% EY and 8% aspalathin content in the extract could be achieved. Standardisation of the maximum particle size by sieving out of large particles could potentially improve the overall process efficiency. The CCD for spray-drying included three independent variables: inlet air temperature (150–220 °C), feed concentration (5–35%) and feed flow rate (0.12–0.64 L.h-1). Powder yield (g powder recovered per 100 g solids in feed) was the only response for which a statistically significant prediction model was generated. Optimal spray-drying conditions (inlet air temperature of 210–230 °C, feed concentration of 34–36% and feed flow rate of 0.62–0.67 L.h-1) were identified and applied in the spray-drying of a pure GRE as well as GRE in a 1:1 mass ratio blend with the carriers, inulin and maltodextrin, respectively. Amorphous powders with low moisture content (<2.2%) and water activity (<0.13) and >89% retention of aspalathin were obtained. The hygroscopic character of the powders was confirmed by moisture sorption analysis, and storage conditions of <25 °C and <40% relative humidity are therefore recommended in order to maintain optimal quality. Inulin improved the flowability and wettability of the powder as compared with maltodextrin. An inulin-GRE formulation was identified as a good candidate for further development as a high-value antidiabetic nutraceutical.

[发布机构] Stellenbosch University