There properties, which are characterized by the formation

There are three major groups of secondary
metabolites including terpenes, phenolics and nitrogen and sulphur containing
compounds. Saponins are a class of
chemical compounds found in particular abundance in various plant species.
They are high-molecular weight glycosides that have been known in more than 100
plant families. These compounds consist of a polycyclic aglycones attached to one or
more sugar side chains. The aglycone part, which is also called sapogenin, is
either steroid (C27) or a triterpene (C30). The foaming
ability of saponins is caused by the combination of a hydrophobic (fat-soluble)
sapogenin and a hydrophilic (water-soluble) sugar part. Saponins have a bitter
taste. Some saponins are toxic and are known as sapotoxin. They have antibacterial properties via
combining with the sterol in the cell membrane of the microorganisms, damaging
the cell wall (Morrissey and Osborne, 1999).

Phenolic compounds
such as tannins have many ecological and physiological functions (Ander et al.,
2009). Total phenol plays an important role in regulating the metabolic
processes and generally plant growth (Lewis and Yamamoto, 1990). Phenolics are
characterized by having at least one aromatic ring with one or more hydroxyl
groups attached. Tannins are poly-phenolic water soluble compounds that are generally
hard and astringent. These compounds cause the proteins to sediment, so they
can be used in tannery. Tannins also have antimicrobial properties, which are
characterized by the formation of hydrogen bonds with non-specific proteins
(such as enzymes) and cell deprivation from Fe availability. This may be the
natural defense mechanism against microbial infections of certain food plants
(Chang et al., 1993). Tannins are general
toxins that significantly reduce the growth and survival of many herbivores and
also 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 their
binding of salivary proteins. Mammals such as cattle, deer and apes, characteristically avoid plant
with high tannin contents (Oates et al., 1980).