Chemical Studies on the Composition of Fiber Fractions and Lignin Determination Residues

Abstract

The objective was to define better the nature of the plant fraction identified as lignin by various assay procedures. Seven materials (alfalfa, orchardgrass, red clover, and switchgrass hays, a mixed hay, peanut hulls, and wheat straw) were used. Lignin procedures studied were as follows: acetyl bromide, 72% sulfuric acid, permanganate, chlorite, and triethylene glycol. These procedures then were applied to a variety of fiber sources (ADF, hot water-organic solvent, and extraction with 5% ammonium oxalate). Fractions were examined for CP, phenolics, and carbohydrate composition and by performing lignin assays on the residues from other lignin procedures. Results showed large differences in the values obtained by the various procedures. The most common problem was the tendency for at least some CP to be included as lignin. Each method, however, displayed unique problems. Acetyl bromide easily can overestimate lignin because of interference by carbohydrates, especially with residues from other lignin determinations. The chlorite and triethylene glycol methods did not remove all the lignin, and the 72% sulfuric acid and permanganate procedures determined some CP as lignin. Finally, although the permanganate method for lignin generally removed all the lignin, it altered carbohydrates as did the triethylene glycol method.