Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2004
Publication Date: 10/20/2004
Citation: Bunzel, M., Ralph, J., Lu, F., Hatfield, R.D., Steinhart, H. 2004. Lignins and ferulate-coniferyl alcohol cross-coupling products in cereal grains. Journal of Agricultural and Food Chemistry. 52:6496-6502. Interpretive Summary: Cereal grains are the most important source of dietary fiber in many industrialized countries. Dietary fiber components are 'polysaccharides, oligosaccharides, lignin, and associated plant substances'. It has marked effects on gut function, and may afford protection against colon cancer. In the literature, cereal grains are frequently described as being highly lignified. In most studies lignin was determined as an acid insoluble residue. However, the determination does not prove that lignin, according to its accepted definition, is present; plant material residues such as structural proteins or waxes may contribute to acid-insoluble residues. The aim of this study was to authenticate the presence of lignin in cereal grain dietary fiber and to estimate its composition, as a prelude to understanding the role of fiber in the health effects noted above. Furthermore, we wished to survey commercially important cereal grains for presence of products implicated in cross-linking the lignin polymer to the polysaccharide components. The results of this study demonstrate rather unambiguously that cereal grains are indeed lignified. Using the 'DFRC-method' developed in our laboratories on rye, maize and wheat dietary fibers, diagnostic compounds from the major lignin structural units were liberated. Furthermore we found cross-products that implicated active cross-coupling mechanisms between lignins and polysaccharides in the wall. In future projects a more detailed structural characterization of cereal grain lignins is needed to explain their possible effects, especially binding of carcinogens in the human gastrointestinal tract.
Technical Abstract: Plant cell walls containing suberin or lignin in the human diet are conjectured to protect against colon cancer. To prove the existence of real lignin (not only 'Klason-lignin') in cereal grain dietary fibres, the DFRC (derivatization followed by reductive cleavage) method was applied to different cereal grain dietary fibres. By cleavage of diagnostic arylglycerol-b-aryl (b-O-4) ether linkages, and identification of the liberated monolignols, we ascertained that lignins are truly present in cereal grains. From the ratios of the liberated monolignols coniferyl alcohol and sinapyl alcohol we suggest that lignin compositions vary among cereals. Furthermore, dimeric cross-coupling products, comprising of ferulate and coniferyl alcohol, were identified in most cereal fibres investigated. These ferulate 4-O-b- and 8-b-coniferyl alcohol cross-coupled structures indicate active cross-coupling of polysaccharides to lignin precursors via ferulate.