# Tannin Extraction from Chestnut Wood Waste: From Lab Scale to Semi-Industrial Plant
## 1 Introduction  
Every year, all around the world, a huge amount of biowastes is generated from the agri-food production chain. Instead of considering them wastes that need to be disposed of, applying the concepts of circular economy and green extraction [1], they can result in a source of high-added-value compounds, rich in phenolics and antioxidant metabolites, concurrently saving money, recycling, and reducing residual matrices that should require disposal [2; 3]. The chestnut tree (_Castanae sativa_, [PERSON]), also known as European chestnut, is a deciduous plant belonging to the _Fagaceae_ family; it can be considered a tree of inestimable cultural and historical value for the foothill regions [4]. Present in the Mediterranean area since prehistoric times, before the import of potatoes and corn from America, chestnuts represented an indispensable source of carbohydrates in Europe [5]: this fact earned it the common name of \"breadfruit\". The chestnut tree has always been cultivated to produce timber and for its fruit [6], rich in bioactive compounds, widely extracted and used in different applications [7; 8]. Besides the common applications, chestnut wood (CW) is also attractive because of the presence of different type of extractives, in particular, polyphenols such as phenolic acids and tannins. This class of molecules, belonging to thelarger family of plants' secondary metabolites, is bioactive, with widely studied features [9]; for instance, they possess antioxidant [10], anti-inflammatory [11], antimicrobial [12], and anticancer properties [13]. In particular, the antioxidant power is generated by its capacity of scavenging reactive oxygen species (ROS) [14]. Compounds that can be derived from CW side-products lead to the provision of high-added-value substances, avoiding their synthesis and providing advantages for human health [15; 16; 17]. Furthermore, CW is a rich source of another class of complex polyphenic molecules: tannins. They are commonly used in the leather industry, in cosmetics, and in the oenological field. This group of molecules are subdivided into Condensed (CTs) and Hydrolysable (HTs) tannins. The former is represented by polymers of flavonoids, while the latter is composed of a sugar moiety linked, by an ester bond, to ellagic and gallic acid units, forming, namely, ellagitannins and gallotaninnns. The main compounds of this family include rescalagin and castalagin, positional isomers which differ only in the position of the OH- group at the C1 of the glycosidic chain [18]. Generally, the extraction protocols for polyphenol and tannin recovery involve both hot water maceration [19] and the use of organic solvents [20] or aqueous solution of them. In literature, it is possible to find a large number of works concerning the exploitation of chestnut residues aiming to extract polyphenols and other bioactive compounds, in different conditions, both by conventional solvent extraction [21] or by new green technologies. Among the most exploited green approaches, it is possible to enumerate ultrasound-assisted extraction (UAE), supercritical CO\({}_{2}\) extraction [22], the use of ionic liquid [23], and deep eutectic solvents (DESs) [24]. The extractive protocol chosen, including the type of solvent, the temperature, and the applied technologies, can greatly affect the yield and the qualitative composition of the product [25]. Typically, polyphenol extraction involves the use of organic solvents leading to a final product characterized by the distribution of different compounds. For instance, aqueous ethanol is effective for the extraction of phenolic compounds with high DPPH radical scavenging activity [26]. Methanol is commonly considered efficient in the extraction of lower-weight polyphenols, whilst for heavier molecules (such as flavanols), aqueous acetone is usually selected [16]. In addition, ethanol is also widely applied for extraction purposes, being a GRAS solvent (Generally Recognized As Safe).