Ultrasonic extraction of waste solid residues from the Salvia sp. essential oil hydrodistillation

Enormous quantities of aromatic plants such as Salvia sp. are used in the production of essential oils all over the world, thereby creating a huge waste solid residue. Since the spent plant material (SPM) may contain biological compounds, it could be exploited as secondary raw material for obtaining different bioactivities using solvent extraction, with possible technological, economical and ecological justification. A novel extraction technique, namely the ultrasonic extraction, and two extracting solvents of distinct polarity (petroleum ether and 70% (v/v) aqueous ethanol solution) were used to obtain extractive substances (ES) from the SPM of two Salvia sp. The investigation was focused on the ES yield, the extraction kinetics and the composition of the final extracts obtained. The maximum concentration of ES in the liquid extracts was reached after about 20–30 min of sonication. The kinetics of ultrasonic extraction was described by the unsteady-state diffusion through the plant material and the film theory. The chemical composition of the extracts depended on the extraction conditions and the type of the plant material. Many biologically active compounds were detected in the extracts. So far the “waste” SPM from the Salvia sp. essential oil hydrodistillation may be considered as a possible source of the exploitable natural products. The benefit of the ultrasound action was related to shortening of the extraction time (60 min) compared to the classical extraction (6 h).