In vitro mineral binding characteristics of cellulose, hemicellulose and lignin fractions isolated from five fiber sources.

Abstract

Commercially processed wheat bran (WB), rice bran (RB), oat fiber (OF), apple fiber (AF) and tomato fiber (TF) were analyzed for chemical composition, soluble fiber (SF), insoluble fiber (IF), and total dietary fiber (TDF). IF from the fiber sources were further fractionated sequentially into cellulose (CL), hemicellulose A (HCL A) and B, lignocellulose (LCL) and lignin (L). Portions of the whole fibers were first defatted (DF) and then all fibers were acid washed (AW) (1% HCL). DF, AW, and insoluble fiber fractions isolated from the various fiber sources were tested for Zn, Cu, Mg, and Ca content. The samples were further examined for their mineral binding capacity by in vitro methods. The effect of competing minerals on binding capacity of CL, HCL A, and LCL were also investigated. TDF ranged from 51% for RB to 75% for OF. Hemicellulose represented the highest percentage of TDF in all the fiber samples, followed by CL and then L. The HCL A:B ratio was high in all samples except RB. Protein content varied from 5% in OF and AF, to 25% in TF. Negligible amounts of protein were found in the insoluble fiber fractions. Variable and lower ash levels were found in the fibers. Acid washing was more effective in reducing ash than protein. Endogenous mineral content differed significantly (P < 0.05) among the whole fibers studied. The highest Cu concentration was found in WB (14 μ/g), whereas Mg was highest in RB (8825 μ/g). Most of the minerals were stripped by acid washing. At pH 6.8, significantly higher amounts of minerals than originally found were bound by the OF whole fibers. However about 90-99% of the bound minerals were released in acid re-washing. High correlations were found for DFs between protein, hemicellulose and lignin contents, and Zn and Mg bound. Fiber fractions bound more than the whole fibers from which they were extracted. Generally Zn and Cu binding of the fractions was in the order HCL A > LCL > L > CL, whereas Ca and Mg bound decreased in the order HCL A > LCL > CL > L. HCL A appeared to be the most potent binder of the minerals studied. Isolated fractions bound more minerals in single mineral treatment than in multiple. Multiple mineral treatment resulted in the following binding patterns: (i) cellulose:- Ca > Zn > Cu > Mg; (ii) lignocellulose:- Zn > Cu ≥ Ca > Mg; (iii) hemicellulose:- Zn > Ca > Cu > Mg. There appeared to be mineral-specific binding sites for each fiber constituent.