## 2 Tannin Application  
For centuries, tannins were used for the leather tanning process to improve resistance and enhance the durability and flexibility of the leather. Currently, 90% of leather production relies on the chromium tanning process, with an outstanding ease of processing and a high quality of products hardly achievable using other methods. However, the cancerogenic properties of chromium compounds, along with the challenges of recovering chromium from tanning wastewaters and solid wastes, and the risk of chromium contaminating groundwater and soils, make this process environmentally hazardous [5]. Therefore, the tanning industry is exploring more sustainable alternatives. In this context, turning back to tannins, which are non-hazardous, plant-based renewable resources, presents a promising alternative to chromium compounds (Table 1). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Plant**} & **Extraction** & **Tannin Content** & **Type of** & **Applications** & **Ref.** \\  & **Method** & **(Plant Part)** & **Tannins** & **Applications** & **Ref.** \\ \hline \multirow{4}{*}{Quebracho} & \multirow{4}{*}{C} & \multirow{4}{*}{164.3 mg/g DM} & \multirow{4}{*}{Condensed} & Oil and ceramics industry, & \multirow{4}{*}{[2,8,9]} \\  & & & & anticorrosive for metal, wood & \\  & & & & preservation, packaging animal & \\  & & & & food, leather processing & \\ \hline \multirow{4}{*}{Oak} & \multirow{4}{*}{MAE} & \multirow{4}{*}{C} & 74.82 \({}^{\text{CTC}}\) mg TAE/g DM & \multirow{4}{*}{Hydrolysable} & Wine quality & \multirow{4}{*}{[2,10,11]} \\  & & 863.07 \({}^{\text{HTC}}\) mg TAE/g DM & & & \\  & & 91 \({}^{\text{TTC}}\) mg/g DM (bark) & & Condensed & wood preservation, & \\  & & 472 \({}^{\text{TTC}}\) mg/g DM (gall) & & & antioxidants & \\ \hline \multirow{4}{*}{Coffee} & \multirow{4}{*}{C} & 5.45 mg TAE/g (pulp) & \multirow{4}{*}{Condensed} & \multirow{4}{*}{Antioxidants} & \multirow{4}{*}{[12,13,14]} \\  & & 12.2 mg GAE/g (husk) & & & \\  & & 13.2 mg GAE/g (skin) & & & \\  & & CFS & 52.07 mg CE/g & & \\ \hline \multirow{4}{*}{Tea} & \multirow{4}{*}{UAE} & 30.63 \({}^{\text{TTC}}\) mg GAE/g DM & \multirow{4}{*}{Condensed} & Antioxidants, & \multirow{4}{*}{[14,15]} \\  & & 110.96 \({}^{\text{TTC}}\) mg/g & & & \\  & & CFS & 47.4 mg CE/g & & \\ \hline \multirow{4}{*}{Olive} & \multirow{4}{*}{C} & 3.28 mg EE/g (EOCs) & \multirow{4}{*}{Congulants, adhesives, colorants, colorants, colorants} & \multirow{4}{*}{[16,17,18,19]} \\  & & 41.34–67.99 mg CE/g DE & & & \\  & & 11.1 mg/g DM & & & \\  & & SFEFS & 14.01 \({}^{\text{TTC}}\) mg GAE/g & & \\ \hline \multirow{4}{*}{Chestnut} & SE & 299.63 mg/g DM & \multirow{4}{*}{Hydrolysable} & Leather-soaking process, wood & \multirow{4}{*}{[20,21]} \\  & & 257.59 mg/g DM & & adhesives, & \\ \cline{1-1}  & & UAE & 297.24 mg/g DM & & wine production, & \\ \cline{1-1}  & & SFE & 132.59 mg/g DM & & \\ \hline \multirow{4}{*}{Mimosa} & \multirow{4}{*}{C} & 108.2 mg/g DM & \multirow{4}{*}{Condensed} & Leather tanning, wood adhesives, & \multirow{4}{*}{[8,9,22,23]} \\  & & 135 mg/g DM & & & feeding \\ \hline \multirow{2}{*}{Tara} & \multirow{2}{*}{C} & 647.5 mg EGE/g DM & \multirow{2}{*}{Hydrolysable} & Leather tanning, wood preservation, & \multirow{2}{*}{[2,9]} \\ \cline{1-1}  & & & & \\ \hline \hline Gambier & \multirow{2}{*}{C} & 169.5 mg CIE/g DM & Condensed & Leather tanning & \multirow{2}{*}{[9]} \\ \cline{1-1}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tannin content in various plant species obtained through different extraction methods, along with their applications. The polyphenolic nature resulting in reactivity with formaldehyde, other aldehydes, and non-aldehyde hardeners makes tannins a versatile reagent in plastics and adhesives production. Using tannin adhesives to produce plywood, particle board, and fiber biocomposites improved their mechanical, physical, and water resistance properties [2].