# Recent Advances in the Development and Application of Green Extraction Techniques
## 2 Green Extraction Techniques  
In this Special Issue, various green techniques were addressed encompassing subcritical water, microwave-assisted, enzyme-assisted, and ultrasound-assisted extraction and also addressing recent trends in extraction approaches, i.e., combined extraction techniques covering the current state-of-the art in this field. Mass transfer from a sample matrix to a solvent is an extremely complex process, accompanied by many limiting factors. The release of target compounds from a natural matrix depends on many factors, such as bond strength and the type of target compounds with matrix constituents, porosity of the matrix, i.e., the diffusivity of the target compounds, first through the pore filling matrix, and in later stages transfer into the extraction solvent. In the majority of extraction processes, convective mass transport is of utmost importance, since it reduces the thickness of the Prandtl diffusive layer at the matrix/solvent surface, reducing the necessary extraction time. The solubility of compounds in the selected solvent, but also viscosity, surface tension, and viscosity of the solvent, are highly dependent on the processing temperature and pressure. Some green extraction techniques, such as subcritical water extraction, modify the properties of conventional solvents by applying high pressures and temperatures. In these techniques high pressures and temperatures improve not only the penetration of the solvent into the pores of a solid matrix, but they also modify the overall physico-chemical properties of the solvents, such as polarity, surface tension, viscosity, and density. Thus, in these conditions, solvents act as completely distinct solvents, according to their mobility, polarity, solvating properties, and diffusivity. Taking advantages of the green properties of water, several studies explored it as an extraction solvent [2; 3; 4]. [PERSON] et al. [2] proposed a new strategy to valorize oilseed cake biowastes by subcritical water extraction. These authors [1], for the first time, comprehensively characterized the extraction of relevant compounds from pumpkin, flax, and hemp seed cases when exposed to different gaseous environments and homogenous catalysis. The presented data unequivocally demonstrated the chemical and nutritional value of the aqueous oilseed cake extracts and the high interest of their incorporation in the food, pharmaceutical, and cosmetic sectors. In addition, in the context of the valorization of natural resources, [PERSON] et al. [3] applied subcritical water for the recovery of bioactive compounds from _Paulovonia elongata x fortune_ (bark, leaves, and petioles) with the final aim of using the extracts for the development of enriched biopolymer-based hydrogels. The results indicated that the extraction temperature had a crucial influence on the color, rheological, and textural properties of the hydrogels, which displayed high potential applicability in food and other related industries [3]. Moreover, the adequacy of the extraction technique for valorization of underexplored natural resources in a green and sustainable approach was proven. [PERSON] et al. [4] also successfully used water as a solvent for extraction of tannins from _Pinus sylvestris_, but in this case in a conventional solid-liquid extraction. Some of the modern extraction techniques, such as microwave-assisted extraction aim to overcome slow heat transfer by direct absorption of the microwave energy, avoiding slow convection and conduction. Not all solvents are efficiently heated in a microwave field, since heating is closely related to the dielectric properties that must be adjusted for this extraction approach, either by using a solvent mixture, where one solvent is serving as an indirect heating medium, or by assuring that the matrix itself has adequate dielectric properties for efficient heating in the microwave field. [PERSON] et al. [5] selected ethyl acetate as optimal solvent for the microwave-assisted extraction of antioxidants from koji rice fermented with _Aspergillus flavus_.