In recent years, almost all extraction processes in the perfume, cosmetic, pharmaceutical, food ingredients, nutraceuticals, biofuel and fine chemical industries rely massively on solvents, the majority of which have petroleum origins. The intricate processing steps involved in the industrial extraction cycle makes it increasingly difficult to predict the overall environmental impact; despite the tremendous energy consumption and the substantial usage of solvents, often the yields are indicated in decimals. The ideal alternative solvents suitable for green extraction should have high solvency, high flash points with low toxicity and low environmental impacts, be easily biodegradable, obtained from renewable (non-petrochemical) resources at a reasonable price and should be easy to recycle without any deleterious effect to the environment. Finding the perfect solvent that meets all the aforementioned requirements is a challenging task, thus the decision for the optimum solvent will always be a compromise depending on the process, the plant and the target molecules. The objective of this comprehensive review is to furnish a vivid picture of current knowledge on alternative, green solvents used in laboratories and industries alike for the extraction of natural products focusing on original methods, innovation, protocols, and development of safe products.

Highlights Recent advances in ultrasound combined processes are detailed. Hybrid combination of ultrasound with conventional techniques. Innovative combinations of ultrasound with modern techniques. Ultrasound combination techniques as efficient tool for food processing.

There are more than 1300 articles in scientific literature dealing with positive impacts of Ultrasound-Assisted Extraction (UAE) such as reduction of extraction time, diminution of solvent and energy used, enhancement in yield and even selectivity, intensification of diffusion, and eliminating wastes. This present study aims to understand what are the mechanism(s) behind these positive impacts which will help to design a decision tool for UAE of natural products. Different microscopic observations (Scanning Electron Microscopy (SEM), Environmental Scanning Electron Microscopy (e-SEM), Cyto-histochemistry) have been used for spacial and temporal localization of metabolites in rosemary leaves, which is one of the most studied and most important plant for its antioxidant metabolites used in food industry, during conventional and ultrasound extraction. The study permits to highlight that ultrasound impacted rosemary leaves not by a single or different mechanisms in function of ultrasound power, as described by previous studies, but by a chain detexturation mechanism in a special order: local erosion, shear forces, sonoporation, fragmentation, capillary effect, and detexturation. These detexturation impacts followed a special order during ultrasound treatment leading at the end to the total detexturation of rosemary leaves. These mechanisms and detexturation impacts were identified in glandular trichomes, non-glandular-trichomes and the layer adaxial and abaxial cuticle. Modelling metabolites diffusion phenomenon during conventional and ultrasound extraction with the second Fick's law allowed the estimation of diffusivities and solvent penetration into the inner tissues and in meantime to accelerate the release of valuable metabolites.

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