Effect of Sodium Hydroxide Treatment of Rice Straw on the Composition of Lignin of Cell Wall

Abstract

In order to characterize the features of cell wall lignins of untreated and 4% NaOH-treated rice straw, analyses of phenolics of cell wall were done. And, infrared (IR) spectra of Bjorkman lignins prepared from rice straw were examined. The amounts of nitrobenzene oxidation products of the untreated rice straw was higher than those of the NaOH-treated rice straw and the most abundant phenolic aldehydes and acids released from rice straw by the 4% NaOH treatment which come in the following order were vanillin, p-coumaric acid, ferulic acid and p-hydroxybenzaldehyde. In the untreated rice straw, the total amount of esterified and etherified p-coumaric acids was much more abundant than that of the ester-and ether-forms of ferulic or syringic acids. About 50% of the esterified p-coumaric acid was released from rice straw by the 4% NaOH treatment. Some of the esterified ferulic acid also was released from rice straw by the 4% NaOH treatment. IR spectroscopy showed that both bands at 1, 730cm-1 attributable to cinnamic acid esters and at 1, 270cm-1 due to the methoxyl groups of guaiacyl units clearly decreased in the spectrum of the lignin prepared from NaOH-treated rice straw. These results conclusively indicate that p-coumaric and ferulic acids are covalently linked to carbohydrates and to lignin through ester linkages, and they are mainly released from rice straw by 4% NaOH treatment. Also a considerable amount of hemicelluloses and lignin are solubilized from rice straw by the treatment.