STRUCTURE AND PROPERTIES OF THE POLYSACCHARIDES FROM PEA HULLS .2. MODIFICATION OF THE COMPOSITION AND PHYSICOCHEMICAL PROPERTIES OF PEA HULLS BY CHEMICAL-EXTRACTION OF THE CONSTITUENT POLYSACCHARIDES

Fibres, prepared from pea hulls as the alcohol insoluble residue (AIR) , were modified by two sequences of chemical treatments. In one, chela ting agent (CDTA), acid (HCl) then increasing concentrations of alkali (0.05, 1 and 4 M KOH), removed firstly the pectins, and then the xyla ns, whereas in the second (alkali alone: 0.05, 1 and 4 M KOH) only sig nificant quantities of xylans were extracted. The AIR was rich in tota l polysaccharides and was characterised by low water holding capacity (WHC; 3.2 ml/g), low swelling capacity (SC; 5.2 ml/g), and high cation exchange capacity (CEC; 0.52 meq/g)/. The fibre preparations CSPR, HS PR and OHSPR-I, recovered after CDTA, HCl and 0.05 M KOH, gave an augm entation in WHC and SC, respectively to 5.8 and 8.3 ml/g in CSPR and 6 .6 and 12.6 ml/g in OHSPR-I, while the CEC decreased (to 0.18 meq/g in OHSPR-I) in proportion to extraction of pectins. After depectination, extraction with 1 and 4 M alkali caused a breakdown in cell wall stru cture of the preparations, followed by changes in average particle siz e, and microscopic examination of the pea hull particles. As the struc ture broke down, a concomitant decrease in the hydration properties wa s measured in these fibre preparations. In contrast, the residues prod uced by extracting the AIR with 0.05, 1 and 4 M alkali, respectively, showed a marked increase in hydration capacities and also in CEC. This phenomenon was due to the fact that almost no pectin was extracted by alkali-only treatment. Examination of the residues by light microscop y confirmed that significant breakdown of particles only occurred afte r depectination and extraction with alkali. These results show that ch emical treatments which extract specific classes of polymers can provi de fibre preparations with increased hydration properties and with eit her increased or decreased CEC.