# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 1 Introduction  
The agri-food sector generates huge quantities of solid and liquid waste, containing a high organic load. Landfilling or disposal of such waste in an uncontrolled way can cause environmental pollution and economic loss. On the other hand, these residues containing valuable metabolites have an enormous potential for the production of sustainable additives, chemicals, biofuels, and energy [1]. In particular, high added value compounds that are still present in by-products of the food processing industry can be employed as food additives and for the formulation of new functional foods, intra-/pharmaceuticals, cosmeceuticals, and beauty products, as well as for innovative and more sustainable packaging [2]. Efforts for the systematic reuse of food waste to produce commercial products and energy provided the integrated approach proposed by the biorefinery concept. Figure 1 reports a generic example of the biorefinery solution for food waste valorization.  
Agri-food industry by-products comprise several complex biomasses. The waste originated by various branches of the food industry can be divided into two main groups (plant or animal origin) and seven subcategories: cereals, root and tubers, oil crops and pulses, fruits and vegetables, meat products, fish and seafood, and dairy products [3,4]. In agreement with the hierarchical approach proposed by the biorefinery concept [5], these by-products contain valuable compounds that can be exploited in the development and production of new functional ingredients. In particular, interesting bioactive compounds present in food industry residues include proteins, polysaccharides, lipids, polyphenols, carotenoids, tocopherols, and essential oils.  
Several extraction techniques have been proposed in the literature for their efficient recovery, from the conventional solid liquid extraction to non-conventional techniques, with the aim of improving the extraction yield of targeted compounds, avoiding deterioration and loss of functionality during processing, and allowing the food grade nature of the final product [6]. These non-conventional techniques include microwave-, ultrasound-, and supercritical fluid-assisted extraction, pressurized liquid extraction, pulsed electric field, and enzyme-assisted extraction techniques. In particular, the use of extraction performed under high-pressure conditions such as supercritical fluids and pressurized liquid extraction offers several advantages for the extraction of bioactive molecules, including short processing times, reduced use of organic solvent, and mild operative conditions able to avoid thermal degradation [7]. These technologies are considered environmentally friendly and are generally able to produce higher yields and high-quality final extracts.  
Therefore, this review provides a comprehensive introduction to the literature update on pressure-assisted extraction processes. A critical analysis of the different processes proposed in the literature is reported, with the objective of defining the most suitable process for the extraction of different categories of high added value compounds that can be find in natural waste. Moreover, the possibility of combining different extraction processes in order to complete the valorization of agri-food waste is also analyzed for some case studies.