Use of near infrared spectroscopy (NIRS) and spectrophotometric methods in quality control of green rooibos (Aspalathus Linearis) and honeybush (Cyclopia Genistoides)

Near infrared spectroscopy (NIRS) and UV/Vis spectrophotometric methods were investigated as rapid and/or more economical methods to quantify the major monomeric phenolic compounds, soluble solid content (SSC), total polyphenol content (TPC) and total antioxidant activity (TAA) of green rooibos and Cyclopia genistoides. Principal component analysis (PCA) and linear discriminant analysis (LDA) of near infrared spectral data were applied to distinguish between green rooibos and C. genistoides and to classify green rooibos according to cultivation area and C. genistoides according to type and harvest date. NIRS was able to predict the aspalathin (SEP = 0.45 g.100 g; r = 0.92) and the dihydrochalcone contents (SEP = 0.49 g.100 g; r = 0.93) of green rooibos plant material. NIRS can be used for screening of the nothofagin content (SEP = 0.10 g.100 g; r = 0.84) of green rooibos and the mangiferin (SEP = 0.46 g.100 g; r = 0.86) and hesperidin (SEP = 0.38 g.100 g; r = 0.85) contents of green C. genistoides plant material. Poor correlations were found between the predicted and the measured values for the aspalathin content of green rooibos water extracts, mangiferin and hesperidin contents of green C. genistoides water extracts and TPC, SSC and TAA of green rooibos and C. genistoides water extracts. The aspalathin content of green rooibos water extracts determined by UV spectroscopy correlated with the aspalathin content quantified using HPLC (r = 0.97), but a slope of 1.90 was obtained (y = 1.9037x + 14.2828), indicating that UV spectroscopy overestimated the aspalathin content. A similar correlation was found between the aspalathin content determined by UV spectroscopy and the dihydrochalcone content quantified using HPLC (r = 0.97; y = 1.7084x + 14.6903). The TPC of water extracts of green rooibos can be determined using UV spectroscopy (r = 0.99; y = 1.0829x 0.6498) and aspalathin as standard. The mangiferin content of green C. genistoides methanol extracts can be determined using the aluminium chloride colorimetric method (r = 0.90; y = 1.2304x – 5.949). A good correlation (r = 0.97) existed between the TPC and the aspalathin content of green rooibos water extracts, but not between the TPC and the mangiferin content (r = 0.22) of green C. genistoides water extracts. TAA of green rooibos water extracts correlated with its aspalathin content (r = 0.96), but a moderate correlation was found between the TAA of green C. genistoides water extracts and its mangiferin content (r = 0.75). The TAA correlated with the TPC (r = 0.99) in the case of green rooibos water extracts, but not in the case of green C. genistoides (r = 0.27). NIRS together with PCA or LDA was able to distinguish between green rooibos and C. genistoides plant material and to classify green C. genistoides according to type. Although PCA could not classify green rooibos plant material according to cultivation area and C. genistoides plant iv material according to harvesting date, LDA was able to classify between 67 and 100% of the green rooibos samples and between 55 and 100% of the C. genistoides samples correctly.