There are three major groups of secondarymetabolites including terpenes, phenolics and nitrogen and sulphur containingcompounds. Saponins are a class ofchemical compounds found in particular abundance in various plant species.They are high-molecular weight glycosides that have been known in more than 100plant families. These compounds consist of a polycyclic aglycones attached to one ormore sugar side chains. The aglycone part, which is also called sapogenin, iseither steroid (C27) or a triterpene (C30). The foamingability of saponins is caused by the combination of a hydrophobic (fat-soluble)sapogenin and a hydrophilic (water-soluble) sugar part.
Saponins have a bittertaste. Some saponins are toxic and are known as sapotoxin. They have antibacterial properties viacombining with the sterol in the cell membrane of the microorganisms, damagingthe cell wall (Morrissey and Osborne, 1999).Phenolic compoundssuch as tannins have many ecological and physiological functions (Ander et al.,2009). Total phenol plays an important role in regulating the metabolicprocesses and generally plant growth (Lewis and Yamamoto, 1990).
Phenolics arecharacterized by having at least one aromatic ring with one or more hydroxylgroups attached. Tannins are poly-phenolic water soluble compounds that are generallyhard and astringent. These compounds cause the proteins to sediment, so theycan be used in tannery.
Tannins also have antimicrobial properties, which arecharacterized by the formation of hydrogen bonds with non-specific proteins(such as enzymes) and cell deprivation from Fe availability. This may be thenatural defense mechanism against microbial infections of certain food plants(Chang et al., 1993). Tannins are generaltoxins that significantly reduce the growth and survival of many herbivores andalso act as feeding repellents to a great diversity of animals. In mammalian herbivores,they cause a sharp, astringent sensation in the mouth as a result of theirbinding of salivary proteins.
Mammals such as cattle, deer and apes, characteristically avoid plantwith high tannin contents (Oates et al., 1980).