Minimising variation in aspalathin content of aqueous green rooibos extract: optimising extraction and identifying critical material attributes

BACKGROUND High levels of aspalathin, an antidiabetic dihydrochalcone, in green rooibos underpins interest in the production of a standardised extract. Elements of a quality‐by‐design approach were applied to optimise extraction conditions, aiming at the delivery of a dry matter yield ( DMY ) ≥ 160 g kg −1 and an extract with an aspalathin content ( AC ) ≥ 80 g kg −1 . RESULTS Hot water extraction parameters, namely extraction time, extraction temperature and water‐to‐plant material ratio, were optimised for DMY and aspalathin extraction efficiency ( AEE ) using Design of Experiments. Good polynomial prediction models were obtained and multiresponse desirability plots indicated 37 min, 93 °C and 23:1 as optimal conditions. Even when using 30 min and 10:1 instead for practical reasons, the target DMY and AC values could be achieved with the caveat that plant material with an AC ≥ 30 g kg −1 is used. Particle size distribution and stem content were identified as contributing to variation in the AC of raw material. CONCLUSION By setting raw material specifications in terms of AC , as well as applying practical optimum extraction conditions, 160 g kg −1 extract with an AC ≥ 80 g kg −1 could be consistently achieved from green rooibos plant material. © 2017 Society of Chemical Industry