Feasibility study of repeated harvesting of menthol from biologically viable Mentha x piperata using ultrasonic extraction

To potentially replace the conventional destructive extraction process, we have shown the feasibility of devising a novel technique that uses ultrasound to nonlethally and repeatedly extract menthol from biologically viable peppermint plants (Mentha x piperita). Our results show that plants ultrasonicated for 1 h at 22 degrees C in a standard 40 kHz ultrasonic bath could release approximately 17.8 microg of menthol per gram of leaf tissue (2% of total product). The amount of menthol release increases with the time of treatment and is greatly affected by the temperature of the ultrasonic bath water. An increase from 2% to 12% of total product was observed when the temperature was increased from 22 degrees C to 39 degrees C. When the temperature effects were isolated, the mechanism of the product release was found to be that of cavitation. The treated plants remained viable and were ready for the subsequent ultrasound extraction after approximately 4 days of recuperation. However, the amount of product released is reduced in subsequent extractions. Scanning electron micrographs indicate that there are two mechanisms involved in extraction: (1) the diffusion of product through the cuticle of peppermint glandular trichomes and (2) the exudation of the product from broken and damaged trichomes. This study has shown the possibility of using an on-line ultrasonic, nondestructive extraction method to continuously release intracellular plant metabolites from the plants while maintaining the plant's viability.