Potential and challenges of tannins as an alternative to in-feed antibiotics for farm animal production

Huang, Qianqian; Liu, Xiuli; Zhao, Guoqi; Hu, Tianming; Wang, Yuxi

Tannins are naturally an occurring heterogeneous group of phenolic compounds with diverse structures that share their abilities to bind and precipitate proteins. Tannins are primarily classified into 3 major groups: hydrolyzable tannins (HT), condensed tannins (CT) also referred as proanthocyanidins, and phlorotannins (PT). The first 2 groups are found in terrestrial plants whilst PT occur only in marine brown algae (Fig. 1) . Hydrolyzable tannins are made up of a polyol core (commonly D-glucose), which is esterified with phenolic acids (mainly gallic or hexahydroxy diphenic acid). The molecular weights of HT range from 500 to 3,000 Da (Haslam, 1989) . They are susceptible to hydrolysis by acids, bases or esterases, thus can be easily degraded and absorbed in the digestive tract and may cause potential toxic effects in herbivores (Dollahite et al., 1962; Mcleod, 1974) . Condensed tannins are oligomeric or polymeric flavonoids consisting of flavan-3-ol units that include catechin, epicatechin, gallocatechin and epigallocatechin. Compared to HT, CT has more complex structures and higher molecular weights ranging from 1,000 to 20,000 Da. Unlike HT, only strong oxidative and acidic hydrolysis can depolymerize the CT structures that are also not susceptible to anaerobic enzyme degradation (McSweeney et al., 2001) . The PT, which are structurally less complex than terrestrial tannins (HT and CT), is formed as a result of the polymerization of phloroglucinol (1,3,5trihydroxybenzene) (Ragan and Glombitza, 1986) . The molecular weights of PT range from 126 Da to 650 kDa (Targett and Arnold, 1998) and can be classified into 6 classes (Phlorethols, Isofuhalos, Echole, Fucole, Fuhalols, Fucophlorethols) based on their chemical structure. They are mainly synthesized via the acetate-malonate pathway (Herbert, 1989) although the other pathway has been proposed (Chen et al., 1997) .