| Abstract
| - A new method has been developed for the quantitative determination of hydroxycinnamic acidsparticipating in ester or ether linkages to the cell wall polymers. The method is based on mild alkalinehydrolysis followed by acid hydrolysis or mild alkaline hydrolysis, which partially removed esterifiedphenolic acids, and high-temperature concentrated alkaline treatment, which cleaved both the esterand ether linkages. It was found that traditional mild alkaline hydrolysis and acid hydrolysis releasedonly part of the ester- and ether-linked phenolic acids, respectively. Approximately half (44.0−47.9%) of the total ester-linked p-coumaric acid and 18.2−32.6% of the total esterified ferulic acidremained ester-linked to the mild alkali-soluble lignin polymers, and 55.0−72.0% of the total ether-linked p-coumaric acid and 37.5−53.8% of the total ether-linked ferulic acid remained ether-linkedto the solubilized lignin molecules after the acid hydrolysis. To correct this, a second mild alkalinehydrolysis of the alkali-soluble lignin preparations and acid hydrolysis of the solubilized ligninfractions, obtained from the first acid hydrolysis of the cell wall materials, was investigated. Onthe basis of this new method, a majority of the cell wall p-coumaric acid (55.8−81.5%) was found tobe ester-linked to cell wall components, mainly to lignin, and about half of the cell wall ferulic acidis etherified through its phenolic oxygen to the cell wall lignin component, whereas the remainderis esterified to the cell wall hemicelluloses and/or lignin in different plant materials. Keywords: Ferulic acid; p-coumaric acid; lignin; wheat straw; rice straw; rye straw; barley straw;maize stems; oil palm frond fiber; fast-growing poplar wood
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