Submitted to: Bean Improvement Cooperative Annual Report
Publication Type: Proceedings
Publication Acceptance Date: May 15, 1999
Publication Date: N/A
Interpretive Summary: In the USA, color is a major criterion for classifying beans into the various commercial (market) classes. However, color may not be a good indicator of the kinds and amounts of the compounds that are found in the seed coats, many of which are classified as health chemicals, also called "nutriceuticals." Flavonoids are naturally occurring compounds that are classified as "nutriceuticals" but are important to bean plants because they impart color to bean seed coats. Many flavonoids are important in health because they protect the body from the effects of cancer-causing agents. We extracted flavonoids from four bean genotypes varying from brown to yellow-brown in seed coat color. One flavonoid called astragalin was found in all four genotypes although its concentration varied. This result pointed to certain genetic loci which, acting independently or in concert, regulate the amount of astragalin present. Although astragalin imparts the yellow color to the yellow and yellow-brown genotypes, other genes control tannin compounds in the seed coats. Tannins are a class of flavonoids that can mask the yellow color of astragalin and give the brown seed coats their distinctive color. Tannins also complex with bean seed protein and contribute to indigestibility and gastrointestinal discomfort. Knowing which genes give bean seed coats their color is important to breeders because they now can more easily breed for particular colors that define the various market classes. Farmers may be able to market beans with flavonoids that are easier to digest than beans with large amounts of tannin. Pharmaceutical companies may also benefit from our work because of the potential to extract particular flavonoids from bean seed coats and market them as nutritionally important food supplements.
Technical Abstract: Flavonoid compounds were extracted and purified from four dry bean (Phaseolus vulgaris L.) genotypes differing in seed coat color. Freeze- dried and ground seed coats of yellow-brown (P,[Cr], D,J,G,b,v,Rk,Asp), mineral brown (P,[Cr],D,J,G,B,v,Rk,Asp), pale greenish-yellow (P,[Cr],D,J,g,b,v,Rk,Asp), and matt mineral brown (P,[Cr],D,J,B,v,Rk,asp) were extracted sequentially with hexane, ethyl acetate, and methanol (MeOH). Flavonoids were purified from the crude MeOH extracts with a preparative HPLC system. The purified flavonoids were dissolved in DMSO and identified on a Varian 500 MHZ Nuclear Magnetic Resonance (NMR) instrument. The flavonoid monomer, astragalin (kaempferol 3-O-glucoside) was the principal compound in all four genotypes. We were not able to determine how much astragalin contributed to the seed coat color of the four genetic lines. Although purified astragalin is a yellow pigment, the methanol extract is usually dark brown before and after chromatographic removal of astragalin. The methanol extract, from which the astragalin was removed, tested positively for the presence of proanthocyanidins. Proanthocyanidins themselves are colorless, but in the sorghum seed coat the different shades of brown that darken with time are the result of proanthocyanidins that have complexed with other phenolics. The brown colors in dry bean that occur over time in storage are probably due to phenolic-proanthocyanidin complexes. Whether or not astragalin itself complexes with proanthocyanidins over time is not known.