Enzymatic/chemical analysis of dietary fiber

The Uppsala methodology for rapid analysis and characterization of total dietary fiber, defined as the sum of dietary fiber polysaccharides (DFP) and Klason lignin, was studied. A sugar- and starch-free residue was prepared by treatment with a thermostable amylase and amyloglucosidase. Neutral DFP residues were quantified by gas chromatography as alditol acetates after acid hydrolysis of this residue, and the acid-insoluble fraction, Klason lignin, was determined gravimetrically. Uronic acid residues were quantified by decarboxylation of the original sample. The efficacy of the Uppsala methodology was tested with foods varying in fiber content and composition, including heat-treated samples. The present method allowed the analysis of up to 40 samples per week. It had good repeatability and coefficients of variation of 3-5% for the main fiber components. Fiber contents determined with the method were higher than those determined with a similar method that excludes Klason lignin and starch resistant to amylases but soluble in dimethyl sulfoxide and lower than those determined with an enzymatic/gravimetric method. Important aspects of fiber analysis, like enzyme purity and the recovery of soluble fiber on ethanol precipitation, also were investigated.