Optimising Supercritical Carbon Dioxide Extraction of Rosmarinic Acid from Rosmarinus officinalis L. and Enhancing Yield Through Soxhlet Coupling

Rosmarinic acid (RA) is a bioactive phenolic compound prevalent in various medicinal plants, renowned for its significant pharmacological properties. This study aims to optimise the extraction conditions of this compound from Rosmarinus officinalis L. using the response surface methodology (RSM) with a three-variable, three-level Box–Behnken design. Optimising the parameters for supercritical CO2 (scCO2) extraction focused on pressure (150 to 350 bar), temperature (40 to 80 °C), and co-solvent weight percentage (5 to 15% ethanol), evaluating their impact on overall yield and RA content. The optimal conditions determined were a pressure of 150 bar, a temperature of 80 °C, and 15% ethanol, yielding a total extract of 21.86 ± 1.55%, with an RA content of 3.43 ± 0.13 mg/g dry matter (DM). Scanning electron microscopy revealed that the scCO2 treatment induced microcracks on the surface of the rosemary powder, enhancing the fluid’s ability to penetrate the plant matrix. By employing the combined scCO2-Soxhlet method, the RA content increased to 5.78 mg/g DM. Furthermore, the final extract obtained using the Soxhlet post-scCO2 treatment contained only trace amounts of carnosic acid (0.38 ± 0.10 mg/g DM) and carnosol (0.38 ± 0.20 mg/g DM), compared to the crude extract obtained solely with Soxhlet, which exhibited significantly higher concentrations of 8.45 ± 2.98 mg/g DM of carnosol and 16.67 ± 0.94 mg/g DM of carnosic acid. This work highlighted an innovative extraction strategy based on the coupling of scCO2 and Soxhlet, which significantly increased RA content while reducing concentrations of other compounds such as CA and CAR. This approach makes it possible to produce RA-enriched extracts, offering considerable potential for future large-scale applications and commercialisation.