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The two main approaches for the determination of dietary fibre (DF) in food and feedstuffs are the enzymaticand 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, Radzików, Poland 186 BACH-KNUDSEN K.E. ET A L . materials when appropriate corrections are made for lignin and resistant starch. The same was the case with full fat soya. It is concluded that the techniques for measuring DF are as reproducible as other analytical methods for feed and food labelling and for research. The choice of analytical method depends therefore primarily on the purpose of the analytical work. K E Y WORDS: dietary fibre, analysis I N T R O D U C T I O N The need for appropriate analytical methods for the determination of dietary fibre (DF) in foodand feedstuffs is apparent. It is well recognised that DF is an important component of the diets for humans and animals and numerous studies have documented important gastrointestinal and metabolic effects of fibre-rich diets. Furthermore, dietary guidelines in human nutrition advocate increasing the consumption of plant foods. This have led to the demand for reproducible analytical DF methods for control and labelling as well as for nutritional research. Unfortunately, however, the lack of consensus on the definition of DF and the wide range of methods, which do not measure the same dietary entities, have led to inconsistent and confusing data. The crude fibre method, invented in the middle of the last century, and the neutral detergent fibre (NDF) method, based on the pionering work of Van Soest, have been used earlier to determine the DF value in foodand feedstuffs (Henneberg and Stohmann, 1859; Van Soest, 1963; Van Soest and Wine, 1967). Both methods, however, have their limitations. In the crude fibre method only a small and variable fraction of the total DF is measured, while the water-soluble NSP and water-insoluble pectic substances are lost in the N D F procedure (Bailey and Ulyatt, 1970; Reichert, 1981; Carre and Brillouet, 1986). Moreover, starch and protein may contaminate the NDF residue (Theander and Aman, 1980). Over the last two decades there has been a rapid growth in the development of robust and reproducible DF methods. The two main approaches are the enzymatic or non-enzymatic gravimetric AO AC (Association of Official Analytical Chemists) procedures and the enzymatic-chemical Englyst and Uppsala procedures. In the first approach, all non-fibre components are removed from the sample by selective extraction and enzymatic degradation and the residue weighed. The second approach is to measure the dietary fibre constituents directly after extraction of low-molecular weight sugars, enzymatic removal of starch, acid hydrolysis of DF polysaccharides and determination of their monosaccharide residues by gas-liquid chromatography (GLC), high-performance liquid chromatography (HPLC) or colorimetry. These methods were in most cases developed for the analysis of foodstuffs but they can also be applied to feedstuffs. ANALYSIS OF DIETARY FIBRE 187 The present summary will primarily deal with advantage and limitations of the enzymatic-gravimetric and enzymatic-chemical DF methods commonly in use. ENZYMATIC-GRAVIMETRIC METHODS The original AOAC enzymatic-gravimetric method for the determination of total dietary fibre (TDF) was developed on basis of the joint experience of Asp et al. (1992). The origins of these methods can be tracked back to the biochemical approach of measuring the indigestible residues in human foods introduced by Williams and Olmsted in the 1930s, (1935) and to the work of Hellendrom and collegues (1975). These methods use enzymes to remove the digestible components, which was considered more physiological than extraction with chemical reagents as used in the detergent method. The main steps in the AOAC procedure (Prosky et al., 1988) and the recent modification proposed by Lee et al. (1992) are outlined in Table 1. Briefly, the procedure consists of the following steps: (a) weighing of -1 g in a beaker, (b) incubation with enzymes to remove starch and protein, (c) precipitation of soluble polymers with four volumes of ethanol, (d) quantitative transfer of sample to a pre-weighed crucible, (e) filtration, (f) weighing of the dry crucible and (g) correction for ash and protein in residues. Blanks are carried through the procedure in parallel. In the modification of Lee et al. (1992) the phosphate TABLE 1 Main steps of the original and modified enzymatic-gravimetric AOAC methods AOAC original Prosky et al. (1988) AOAC modified Lee et al. (1992) Sample ~ 1 g ~ 1 g Buffer Na-phosphate pH 6 MES/TRIS pH 8.2 Enzyme step 1 Termamyl 100°C, 15-30 min Termamyl 95-100°C, 35 min pH adjustment pH 7.5 Enzyme step 2 Protease 60°C, 30 min Protease 60°C, 30 min pH adjustment pH 4.0-4.6 pH 4.1-4.7 Enzyme step 3 Amyloglucosidase 60°C, Amyloglucosidase 60°C, 30 min 30 min Volume of 96% EtOH required to precipitage soluble DF 280 ml 225 ml Filtering aid Celite 545 Celite Protein correction N x 6.25 N x 6.25 Ash correction Incineration 525°C Incineration 525°C 188 BACH-KNUDSEN K.E. ET A L . buffer is replaced with a MES/TRIS (2(N-morpholino) ethanesulphonic acid/tris hydroxymethyl amino-methane) buffer, one of the pH adjustment is eliminated and the total volume is reduced.

Methods for analysis of dietary fibre - advantage and limitations