Cell wall polysaccharides of common beans (Phaseolus vulgaris L.)—composition and structure

A better knowledge of cell wall structure and composition is useful to understand the physiological properties of dietary fibre and the textural defects that affect legume seeds. Hence, cell wall polymers from cotyledon and tegument of common beans were fractionated, chemically and enzymatically degraded and analysed using GC-FID, GC-EIMS and HPLC. Results revealed that bean cotyledon is composed of 16.6% of cell wall, mostly constituted of 4 M NaOH and hot-water soluble polymers (WSP). Middle amounts of polymers were also solubilised by CDTA (1.2%) and 0.01 M NaOH (0.7%). Beans cell wall polymers are made up of arabinose-rich pectins, β-glucans and galacturonans. Endopolygalacturonase treatment in conjunction with methylation analysis revealed that galacturonans in beans cell wall are composed of xylogalacturonans and rhamnogalacturonans rather than homogalacturonans. The bulk of xyloglucan, the main hemicellulose found in cotyledon, was extracted with 4 M NaOH and showed to be fucogalactoxyloglucan. All fractions contained large ramified arabinans, highly substituted at O-3 and O-2 positions with short oligomers and terminal arabinoses. Galactans were found in low amounts probably as a linear 1,4-linked structures. Bean hulls contained high amounts of 1,4-linked xyloses and 1,5-linked arabinose, probably derived from xylans and large unbranched linear arabinans.