## 3 Tannin Classification and Sources  
The diverse group of tannin compounds can be classified in various ways. The most common classification considers their functional groups and chemical structures. According to this system, tannins are primarily divided into two groups--hydrolysable and condensed tannins [3]. Hydrolyzable tannins can be fractionated into their components through hydrolysis [44] since they are esters of monosaccharides (mainly D-glucose) and phenolic acids [3]. This group consists of two subgroups: gallotannins and ellagitannins (Figure 2), which differ in their hydrolysis products. Gallotannins break down into gallic acid and its derivatives, whereas ellagitannins decompose into ellagic acid and its derivatives [45]. The most common monomer in the latter group is hexahydroylphenoyl (HHDP) which can spontaneously convert to ellagic acid during hydrolysis. Ellagitannins are the most well-studied tannins, with over 500 naturally occurring compounds identified [45]. Figure 1: Applications of tannins (based on data from [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]). Condensed tannins (Figure 2), also known as proanthocyanidins, are the primary product of global tannin extraction, exceeding 90% of worldwide annual tannin production. They consist of at least three repeating units of flavonoid structures, mainly flavan-3-ols such as catechin and epicatechin. Depending on the number of repetition units, condensed tannins are classified as oligomeric or polymeric flavonoids with molecular weights ranging from 500 to over 20,000 Da [2; 46]. Although tannins are water-soluble compounds, the increase in the molecular weight of the condensed tannins limits their solubility in water. Condensed tannins may also contain amino acids and imino acids [47]. Unlike hydrolyzable tannins, they do not contain any saccharide components [48] and do not readily undergo hydrolysis. During degradation, they may break down into anthocyanins, a class of plant pigments responsible for the red, purple, and blue colors observed in many fruits, flowers, and vegetables [49]. Beyond these two main groups, other tannins, such as complex tannins and phlorotannins, are also reported (Figure 2). Complex tannins are hybrid structures combining hydrolyzable and condensed tannins, where flavan-3-ols link with gallotannins and el-agitannins via C-C bonds [4]. In turn, phlorotannins comprise phloroglucinol units, whose derivatives form oligomeric or polymeric structures with molecular weights ranging from 126 to even 625,000 Da [50]. These tannins are primarily found underwater in brown macroalgae, including species from the _Fucaceae_, _Sargassaceae_, and _Alariaceae_ fam  
Figure 2: Selected examples of tannin structures representing main tannin groups. ities [4, 45, 51]. In 2022, phlorotannins derived from brown algae comprised the largest revenue share of 78.4%, with the fastest forecasted annual growth of 6.8% from 2023-2030 [43]. Tannins are commercially extracted from different resources and various parts of many plants, depending on their application. Tannins used in the tanning industry are generally condensed tannins extracted from quebracho (_Schinopsis balansae_ or _Schinopsis lorentzii_) and mimosa (_Acacia mearnsii_) trees. Proanthocyanidins, are also derived from the wood of acacia (_Acacia mollissima_, _A. mearnsii_), the bark of pine and oak, and sorghum seeds [48]. In turn, hydrolyzable tannins are obtained from gall-producing plants such as sumac (_Rhus semialata_), which yields tannic acid (Chinese gallotannin), and oak (_Quercus infectoria_), which produces Turkish gallotannin. Other sources include the fruits of chebulic myrobalan (_Terminalia_ sp. ), which contain chebulinic acid, as well as the wood of chestnut (_Castanae sativa_), oak, the pods of divi-divi (_Cesalpinia coriaria_), and sumac leaves [48]. Additionally, tannins are abundant in plants cultivated for food, such as coffee, tea, cocoa, nuts, fruits, vegetables, herbs, spices, and cereals [52].