A two-stage extraction model for simultaneous extraction of essential oil and phenolics from tulsi leaves: implementing a blended mode microwave hydrodiffusion and gravity (MHG) model

The work is based on implementing a blend of high, medium, and sustained low power microwave heating for the extraction of essential oil from tulsi leaves using the principle of MHG. The blended mode was implemented to target the simultaneous extraction of essential oil and non-volatile principles (phenolics) from the same biomass through a two-stage process. The first stage dealt with the extraction of essential oil using an optimized MHG protocol comprising of a blend of high- (510 W) and medium-power (340 W) microwave surges of 5 min each, followed by the completion of the experiment with low power microwave (170 W). The yield of essential oil obtained from the optimized MHG protocol (50 min) was found to be 5% w/w. On the other hand, MHG with single-power microwaving at 170 W (60 min), 340 W (40 min) and 510 W (25 min) produced yields of 1.9%, 2.9% and 1.0% w/w, respectively. Hydrodistillation (240 min) could achieve a yield of 1.9% w/w only. As per gas chromatography results, the % area of eugenol content was found to be 16.64%, slightly higher than the 15.45% obtained from hydrodistillation. The second stage was about retention capabilities of the biomass with reference to the non-volatile components. The total phenolic content of the leftover biomass after the MHG blended mode protocol was found to be 6.1 mg GAE per g of dried extract, which was more than the control (untreated) sample that retained a phenolic content of 5.4 mg GAE per g of dried extract. However, biomass obtained after hydrodistillation showed a severe depletion of phenolic content (1.9 mg GAE per g of dried extract). Thus, MHG (blended mode) allows the extraction of essential oil in the first stage, followed by the extraction of non-volatile compounds from the same biomass in the second stage, ensuring judicious and exhaustive use of plant biomass.