Purification and removal of condensed tannins in mungbean (Vigna radiata (L.) Wilczek), Rice bean (Vigna umbellata (Thumb.) Ohwi and Ohashi) and lab-lab bean (Dolichos lablab L.)

Mungbean (KS1, KS2, Autong 1), rice bean (native variety from Thailand and Philippines); and lablab bean (EG Green Podded, EG Light Green Podded, Tein Roong and Manmuyao) were analyzed for tannin content using three methods: modified vanillin, Prussian blue assay and protein precipitation method. The tannin analysis invariably yields different results since they detect tannin on a different basis. Several treatments reduced the chemically assayable tannins from mungbeans, rice beans, and lablab bean seeds. Soaking seeds for 6 to 36 hrs in water reduce tannin content by 31.30 to 67.18, 39.07 to 71.29 and 24.69 to 53.62% from the mungbean (Autong 1), rice bean (native variety from Philippines) and 43 to 85% from Manmuyao. The amount of tannins removed depend upon the soaking time, the morphology of the individual seeds and the nature of each alkali. Low concentrations of alkali are more effective for tannin removal than higher concentrations. With boiling the maximum amount of tannin removed was 70.93% for mungbean, 74.62% for rice bean and 58.73% for lablab bean. In vitro protein digestibility (IVPD) of seeds treated with alkali and or heat treatment increased with a decrease in tannin content. Reduction in tannin of treated seeds did not entail a corresponding increase in in vitro protein digestibility. During germination, chemically assayable tannins were reduced by 45, 37 and 44%, from mungbean, rice bean, and lablab bean, respectively. Condensed tannins were isolated and purified from the crude methanolic extracted of hulls from KS2, rice bean native variety from Thailand and Manmuyao by adsorption chromatography on Sephadex LH-20. The purified condensed tannin exhibited higher relative precipitating protein capacity compared to the crude extract. Partial characterization of each purified tannin showed differences in their reactivity toward the phenol reagents Folin-Denis, leucoanthocyanin test and vanillin test. UV-spectrophotometry revealed the presence of free 7-OH group in their structure of the purified tannins.