Quantification of the Major Phenolic Compounds, Soluble Solid Content and Total Antioxidant Activity of Green Rooibos (Aspalathus Linearis) by Means of near Infrared Spectroscopy

Abstract

Near infrared (NIR) spectroscopy was used to develop calibration models to predict aspalathin, nothofagin and dihydrochalcone contents of dried, green rooibos plant material, as well as to predict the aspalathin content of the respective water extracts. Calibration models were also developed to predict the total polyphenol content (TPC), soluble solid content (SSC) and total antioxidant activity (TAA) of the water extracts. Results showed that NIR spectroscopy can effectively predict the aspalathin content of dried, green rooibos with a standard error of prediction ( SEP) and coefficient of determination ( r2) of 0.45 g 100 g−1 and 0.85, respectively and the dihydrochalcone content of rooibos with an SEP of 0.49 g 100 g−1 and r2 of 0.87. After extending the aspalathin content range of the sample set, by adding varying amounts of a dried rooibos extract powder with high concentrations of aspalathin (15.95 g 100 −1) and nothofagin (1.94 g 100 g−1) to some samples, slightly less accurate, but more robust, models were obtained for the aspalathin ( SEP = 0.53 g 100 g−1; r 2 = 0.87) and dihydrochalcone ( SEP = 0.57 g 100 g−1; r 2 = 0.88) contents. The NIR calibration model developed for nothofagin content ( SEP = 0.10 g 100 g−1; r 2 = 0.71) (extended range: SEP = 0.10 g 100 g−1; r 2 = 0.77) of dried, green rooibos is suitable for screening purposes. The current transmittance NIR calibration models developed for aspalathin content, TPC, SSC and the TAA of green rooibos water extracts did not give acceptable results.