Methods for analysis of dietary fibre - advantageand limitations

Abstract

The two main approaches for the determination of dietary fibre (DF) in food and feedstuffs are the enzymatic-and nonenzymatic-gravimetric AOAC (Association of Official Analytical Chemists) procedures and the enzymatic-chemical Englyst and Uppsala procedures. The main analytical problems which have been shown to influence the performance of the enzymatic-gravimetric AOAC procedures are: variable starch removal, too high final phosphate buffer concentration, and hence variable ash content of gravimetric residues, problems with residual nitrogen determination, filtration problems of viscous and desugared fruit samples, and variable blank values. Standardisation and improvements of the methods have gradually improved the performance of the gravimetric procedures to acceptable levels. The enzymatic-chemical methods with gas-liquid chromatography (GLC), high performance liquid chromatography (HPLC) or colorimetry as endpoint determination are generally more complex, involve more steps and require, for the GLC and HPLC methods, more advance equipment than the gravimetric procedures. The efficiency of starch removal, the hydrolytic conditions for acid hydrolysis of DF polysaccharides, factorial corrections for hydrolytic losses of sugars and laboratory expertise are identified as factors influencing the reproducibility of the methods. Modifications and optimisation of the various steps and the build-up of expertise in laboratories participating in the collaborative trials have improved the reproducibility of the enzymatic-chemical methods to a level similar to that of the enzymatic-gravimetric AOAC methods. Technical problems which may introduce errors in the determination of DF with all methods are incomplete precipitation in 80% (v/v) ethanol, impurities in bacterial amyloglucosidases resulting in depolymerisation and potential losses of DF polysaccharides. A recent BCR Reference Material Study indicate that the various DF methods give comparable results for cereal based Part of this paper was presented at the Symposium: Dietary fibre -chemical composition and biological action, 24-25 April 1997, Radzikw, Poland BACH-KNUDSEN K.E. ET AL.