Quantitative determination of hydroxycinnamic acids in wheat, rice, rye, and barley straws, maize stems, oil palm frond fiber, and fast-growing poplar wood

A new method has been developed for the quantitative determination of hydro xycinnamic acids participating in ester or ether linkages to the cell wall polymers. The method is based on mild alkaline hydrolysis followed by acid, hydrolysis or mild alkaline hydrolysis, which partially removed esterified phenolic acids, and high-temperature concentrated alkaline treatment, whic h cleaved both the ester and ether linkages. It was found that traditional mild alkaline hydrolysis and acid hydrolysis released only part of the este r- 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 tota l esterified ferulic acid remained ester-linked to the mild alkali-soluble lignin polymers, and 55.0-72.0% of the total ether-linked p-coumaric acid a nd 37.5-53.8% of the total ether-linked ferulic acid remained ether-linked to the solubilized lignin molecules after the acid. hydrolysis. To correct this, a second mild alkaline hydrolysis, of the alkali-soluble lignin prepa rations and acid hydrolysis of the solubilized lignin fractions, obtained f rom the first acid hydrolysis of the cell wall materials, was investigated. On the basis of this new method, a majority of the cell wall p-coumaric ac id (55.8-81.5%) was found to be ester-linked to cell wall components, mainl y to lignin, and about half of the cell wall ferulic acid is etherified thr ough its phenolic oxygen to the cell wall lignin component, whereas the rem ainder is esterified to the cell wall hemicelluloses and/or lignin in diffe rent plant materials.