Kinetics of ultrasound-assisted extraction and concentration of rosmarinic acid as bioactive fraction

Abstract

Malaysia’s herbal industry is growing rapidly over the years. Orthosiphon aristatus, or locally named as “Misai Kucing”, is one of the herbs in Malaysia that possesses remarkable therapeutic potentials. Rosmarinic acid is the biochemical marker of O. aristatus that is of interest in this study. This study was conducted to determine the effectiveness of ultrasound-assisted extraction in extracting rosmarinic acid, as well as solid-phase extraction in concentrating rosmarinic acid from O. aristatus. The aqueous extraction of rosmarinic acid assisted by ultrasound was performed for 30 minutes at temperatures ranged from 27℃ to 60℃, whereas extraction without ultrasound was performed at temperatures of 27℃ and 40℃. The samples were analysed for their yield, rosmarinic acid content, antioxidant, and antiinflammatory activities. The aqueous extract from 27℃ produced high yield and rosmarinic acid content. The experimental data of rosmarinic acid extraction was fitted into seven kinetic models. The two-sites kinetic model was the best-fit kinetic model with the highest R2 and the lowest RMSE values. The estimated rate constants were used to calculate the activation energy and the pre-exponential factor using the Arrhenius equation. The activation energies for extraction with ultrasound were 10.1065 kJ/mol for the washing stage and 0.7373 kJ/mol for the diffusion stage, whereas the pre-exponential factors for extraction with ultrasound were 12.1642 min- 1 for the washing stage and 4.1371 min-1 for the diffusion stage. Consequently, the aqueous extract with high rosmarinic acid content at 27℃ was selected and subjected to solid-phase extraction with methanol. The eluted fractions were collected from the solid-phase extraction and analysed for their rosmarinic acid content, antioxidant, and anti-inflammatory activities. The solid-phase extraction succeeded in increasing rosmarinic acid yield from 1.36% in the crude extract to 11.14% in the methanol fraction. The antioxidant and anti-inflammatory activities of the extracts reduced after ultrasound-assisted extraction and solid-phase extraction due to the degradation and selective adsorption of phytochemicals. The findings for the antioxidant and antiinflammatory activities were also affected by the immediate reaction of mixtures and the colour intensity of the extracts. This study concluded that the combination of ultrasound-assisted extraction and solid-phase extraction methods could increase the rosmarinic acid content in O. aristatus extract.