The phenolic fraction of maize bran: evidence for lignin-heteroxylan association

Maize bran heteroxylan samples were extracted in various conditions of seve rity. Their ferulate and diferulate content was investigated by GC-MS of me thyl ester-TMSi derivatives. When extracted by 0.5 M NaOH in mild condition s, the heteroxylan sample contained a low level of ferulic acid (0.032% by wt.) and the main diferulate surviving alkaline extraction was found to be the 8-8' diferulate. On peroxidase treatment, this sample nevertheless prod uced a firm and brittle gel without any change in the diferulate profile. T ypical lignin structures, mainly comprising syringyl units interconnected t hrough beta -O-4, beta -1 and beta-beta interunit bonds, were evidenced in the maize bran sample. More importantly, these lignin structures were found to be tightly associated with the alkali-extracted heteroxylans. Thioacido lysis revealed the occurrence of 0.1-0.5% (by wt.) lignin structures in het eroxplan fractions extracted in mild or severe conditions, before and after purification of the polysaccharides. The gelling potential of the heteroxy lan fractions was not only dependent on their ferulate level, but also infl uenced by associated lignin structures. These results argue for the occurre nce of covalent linkages between heteroxylan chains and lignin structures w hich could participate in the peroxidase-driven gelation of feruloylated po lysaccharides. They demonstrate the role of low lignin levels in the organi zation of native or reconstructed polysaccharide networks. (C) 2001 Elsevie r Science Ltd. All rights reserved.