Isolation of tannin as calcium salt (calcium tannate) and as free tannin from de-oiled seeds of Shorea robusta Gaertn. and evaluation of its in vitro antioxidant properties

Abstract

The present study aimed to develop a novel process which may be commercially feasible for isolating tannin as metallic salt, free from carbohydrates, proteins, minerals, etc. followed by its hydrolysis to free tannin for various potential applications. Accordingly a process was developed which involves separation of the soluble carbohydrates, proteins, minerals, and tannin together in aqueous medium at an appropriate alkaline pH from Sal (Shorea robusta Gaertn.) seed de-oiled cake rich in starch and fibre as insoluble constituents. Alkaline aqueous extract’s pH was adjusted so as to coagulate the proteins as insoluble components at their iso-electric points. The supernatant obtained from the protein precipitation step containing the soluble carbohydrate, tannin, mineral was subjected to calcium hydroxide treatment which resulted in precipitating the tannin as calcium tannate leaving the supernatant consisting of soluble carbohydrates and minerals. This calcium tannate was hydrolysed by orthophosphoric acid to collect tannin in supernatant part and subjected to dryness for utilization purpose.