## 4 Extraction Methods  
The diversity of tannins in chemical composition and structures, along with their sources and technical requirements linked to their applications, drives a search for the optimal extraction method. The efficiency of tannin extraction is strongly influenced by solvent choice, temperature, time, the solid-to-solvent ratio, biomass raw material preparation, particle size, and moisture content [5, 34]. Industrial biomass processing to obtain tannins uses aqueous solutions of sodium sulfite or bisulfite (1.5-2.0%) and sodium bicarbonate (0.5%). The process is usually conducted with hot water at 50 to 100 \({}^{\circ}\)C [53]. Before extraction, biomass is ground or milled to the desired particle size. The post-treatment of tannin extract is typically performed through spray- or vacuum-drying, resulting in a powdered or solid final product. Water extraction stands out due to its simplicity, versatility, cost-efficiency, and sustainability, mainly when water is used instead of other organic solvents [34]. However, aqueous extraction leads to the co-extraction of other water-soluble compounds. Therefore, tannin extracts may contain non-phenolic impurities, including saccharides, hydrocolloids, and stilbenes, requiring post-treatment procedures to purify the final product based on its application restrictions [53, 54, 55, 56]. The content of impurities can also be controlled by selecting the appropriate extraction parameters [5]. Alternative solvents such as ethanol, methanol, and acetone are utilized to enhance process efficiency and selectivity. These solvents are typically used as co-solvents with water in varying concentrations ranging from 50% to 85% [5, 34, 57]. In many other studies, other solvents were also employed: disopropyl ether, ethyl acetate ethyl ether, n-hexane, chloroform, ionic liquids, and deep eutectic solvents [5, 6, 46]. Furthermore, new techniques were used to enhance the efficiency of the process, such as microwave-assisted, ultrasound-assisted, and enzyme-assisted extraction [58, 59] and extraction with supercritical fluids [5, 34]. A general schematic of the extraction process is presented in Figure 3.  
Ultrasound-assisted extraction (UAE) enhances the recovery of tannins and other polyphenols by employing acoustic cavitation, a phenomenon through which sound waves (typically 20-100 kHz) induce the formation and violent collapse of microbubbles in a liquid medium. This collapse generates localized high pressures (>1000 atm) and temperatures (estimated up to \(\sim\)5000 K), disrupting plant cell walls and enhancing solvent penetration and mass transfer rates [60; 61]. The UAE significantly reduces extraction time and allows the use of low temperatures (25-60 \({}^{\circ}\)C), protecting thermolabile compounds like hydrolyzable tannins from oxidative degradation [62]. The technique also lowers solvent consumption and supports the use of GRAS solvents (e.g., ethanol and water), aligning with green chemistry principles [63]. However, UAE is not without limitations. The cavitation process can generate reactive oxygen species (ROSs) such as hydroxyl radicals, potentially leading to the degradation of sensitive phenolics if parameters (amplitude and time) are not optimized [64]. Moreover, UAE often lacks selectivity, extracting both desired bioactives and unwanted compounds like sugars, chlorophyll, or proteins [62]. Industrial scaling is also challenging due to uneven energy distribution in larger volumes, which affects reproducibility and efficiency [65]. Enzyme-assisted extraction (EAE) is an environmentally friendly and efficient method for recovering tannins and other polyphenic compounds from plant materials. The method uses specific enzymes, such as cellulases, hemicellulases, pectinases, and tannases,  
Figure 3: Scheme of conventional production of tannins [6; 25]. to hydrolyze structural polysaccharides and protein-phenolic complexes in the plant cell wall, thus enhancing the release of bioactive compounds into the solvent.