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Purification and Removal of Condensed Tannins in Mung Bean (Vigna radiata L.) (Wilczek), Rice Bean (Vigna umbellata Thumb.), Ohwi and Ohashi and Lablab bean (Dolichos Jablab L.)
Thesis Abstract:
Mung bean (KS1, KS2, Autong 1), rice bean (native variety from Thailand and the Philippines), and lablab bean (EG Green Podded, EG Light Green Podded, Tein Roong, and Manmuyao) were analyzed for tannin content using modified vanillin, Prussian blue assay, and protein precipitation. The tannin analysis yielded different results since these detected tannin on a different basis. Several treatments reduced the chemically assayable tannins in the three legumes. Soaking seeds for 6-36 hours in water reduced tannin contents in mung bean (Autong 1), rice bean (Philippine variety), and lablab bean (Manmuyao) by 31.30 - 67.18 percent, 39.07-71.29 percent, and 24.69-53.62 percent, respectively. Use of K2CO3, NH4OH, NaOH, KOH, and CaO solutions reduced tannin content by 27-77 percent in mung bean, 29-9.27 percent in rice bean (Philippine variety), and 43-85 percent in lablab bean (Manmuyao). The amount of tannins removed depended on soaking time, seed morphology, and nature of alkali used. Low alkali concentration removed tannins more effectively than higher concentrations.
Boiling removed 70.93 percent of tannin in mung bean, 74.62 percent in rice bean, and 58.73 percent in lablab bean. In vitro protein digestibility of seeds treated with a lkali and/or heat increased with decrease in tannin content. Reduction in tannin content of treated seeds did not increase in vitro protein digestibility. During germination, chemically assayable tannins were reduced by 45, 37, and 44 percent in mung bean, rice bean, and lablab bean, respectively.
Condensed tannins were isolated and purified from the crude methanolic hull extracts from mung bean (KS2) , rice bean (Thai variety), and lablab (Manmuyao) by absorption 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, leucoanthoacyanin test, and vanillin test. UV-spectrophotometry revealed the presence of free 7-OH group in their structure of the purified tannins.