Fermentation characteristics of polysaccharide fractions extracted from the cell walls of soya bean cotyledons

Full-fat soya beans were separated into hulls and cotyledons. After separat ion the cell wall fraction was extracted from the cotyledons. These purifie d cell walls were sequentially extracted with 0.05 M cyclohexane-trans-1,2- diamine-N,N,N',N'-tetraacetate (CDTA) + 0.05 M NH4 oxalate (extract 1), 0.0 5 M NaOH (extract 2), 1 M KOH (extract 3) and 4 M KOH (extract 4) to fracti onate the cell wall into its polysaccharide fractions. The extraction proce dure was designed to first extract the pectic fraction (extracts 1 and 2), after which more hemicellulose was extracted in subsequent steps (extracts 3 and 4). In addition to the polysaccharides solubilised during extraction, the residues after each extraction step were collected. Extracts and resid ues were analysed for their fermentation characteristics using an in vitro gas production procedure. Fermentability of the sequential extracts increas ed for each subsequent extraction step, as witnessed by an increasing rate of fermentation (from 2.2 to 10.0%h(-1)) and decreasing half-time of gas pr oduction (from 56.0 to 18.7 h). Fermentability of the residues increased af ter pectins had been removed in the first two extraction steps, as witnesse d by a shorter half-time of gas production (from 47.0 to 29.7 h). Fermentab ility was similar for residues 2 and 3, but decreased again for residue 4, which was deemed to consist mainly of cellulose. The different cell wall su gars were degraded at different rates, with a rapid rate of degradation for galactose and arabinose, an intermediate rate for xylose and uronic acids and a slow rate for glucose. The sugar degradation rates for extract 1 had a similar ranking. These results are discussed in light of the concept that pectins determine the pore size of the cell wall matrix (Carpita NC and Gi beant DM, Planta J3: 1-30 (1993) and the model of the cell wall architectur e of legumes of Hatfield (Hatfield RD, in Forage Cell Wall Structure and Di gestibility, Ed by Jung HG, Burton DR, Hatfield RD and Ralph J. American So ciety of Agronomy/Crop Science Society of America/Soil Science Society of A merica, Madison, WI, pp 285-313 (1993)). (C) 2000 Society of Chemical Indus try.