Production of dihydrochalcone-rich green rooibos (Aspalathus linearis) extract taking into account seasonal and batch-to-batch variation in phenolic composition of plant material

Abstract

Randomly selected rooibos plants from three plantations were harvested (a few branches per plant) at intervals from early summer to late spring. The aspalathin and nothofagin content of the plant material was highest at the first harvest date (3.76% and 0.54%, respectively) in summer whereafter their content decreased gradually to the lowest levels in winter and increased to 3.31% and 0.36%, respectively in mid-spring. Commercial green rooibos production batches (n=47), produced over a period of 49days (end of January until mid-March) from plant material harvested from one plantation were analysed to provide further insight into expected batch-to-batch variation in aspalathin, nothofagin, orientin and iso-orientin content. The aspalathin content of green rooibos varied between 2.50% and 4.49%, even though the plant material was harvested from the same plantation. The other compounds were present in the plant material in much lower quantities (<0.5%). The iso-orientin and orientin contents of the green rooibos samples did not vary substantially. The variation observed in the plant material was subsequently also observed in the freeze-dried hot water extracts prepared from the commercial green rooibos production batches, with the theoretical dried extract yield varying between 15.07% and 22.89%. The aspalathin content and total polyphenol content of the dried extract varied between 8.11% and 12.33% and between 29.7 and 43.5g gallic acid equivalents/100g, respectively. The mean content values for the other compounds were less than 1.2%. Combined these major dihydrochalcones and flavones comprised between 10.22% and 14.88% of the freeze-dried extract. Aspalathin contributed between 13.1% and 22.8% of the total antioxidant capacity (2873–4286μmol Trolox equivalents/g) of the dried extract as determined by the DPPH radical scavenging assay.