Component Analysis Procedures

Unavailable Carbohydrate Method

Southgate (17) had developed an analytical procedure to fractionate DF into its various components. Hot water digestion removes the soluble starches and pectins, the enzyme takadiastase dissolves the insoluble starch, dilute acid breaks down the hemicelluloses, and finally strong acid hydrolyzes the cellulose. The residue remaining is lignin. The insoluble starch and hemicelluloses can be quantified either colorimetrically or by liquid chromatography. The method has been criticized for both the incomplete removal of starch and the nonspecific colorimet-ric methods used for quantification of sugar components (3).

Uppsalla Gas-Liquid Chromatography (GLC) Method

The Uppsalla method (18) (AOAC method 994.24) involves gelatinization followed by treatment with Termamyl, an a-amylase, and amyloglucosidase to remove starches. After precipitation of the soluble fiber with 80% ethanol, the dried residues are subjected to acid hydrolysis for liberation of neutral sugars and uro-nic acids. Neutral sugars are determined as alditol acetate derivatives by GLC, and uronic acids are determined by either decarboxylation or colorimetric methods. The acid-insoluble fraction, called Klason lignin, can be determined gravi-metrically. An important point to note is that this method does not use any protein digestion step (18). Marlett (19) has established fiber tables using the Uppsalla GLC method in the U.S. Although the AOAC enzymatic-gravimetric method and

Dietary Fiber Intake

the Uppsalla GLC method will give comparable TDF values in most cases, the GLC method tends to be lower.

The DNS Reduction Method for Soluble and Insoluble NSP

The NSP method, developed by Englyst and colleagues (20), has gone through several modifications since its inception for evaluation of NSP alone, NSP plus resistant starch, resistant starch alone and/or lignin determination (21-23). Like the Uppsalla method, neutral sugars are quantified by GLC after derivitization with alditol acetate. Colorimetric techniques are also used. The soluble NSP content is calculated as the difference between TDF and IDF to avoid the problems of incomplete precipitation of SDF (23). The NSP method has been chosen to replace the unavailable (CHO) carbohydrate method for fiber labeling purposes in the United Kingdom based on the superior precision in comparison to other component analysis procedures. Although NSP are major components of dietary fiber, the values do not represent all the DF values. For high starch foods, grains, and cereals, NSP values are significantly lower than TDF values obtained by AOAC methods. Differentiating NSP from resistant starch does not sufficiently address the issue of the physiological effects of these fractions. The physiological function of DF as a quantified entity is more acceptable. Major improvements are required to measure TDF, resistant starch and lignin if separate labeling is the goal with the Englyst methods. The employment of dimethylsulfoxide (DMSO) in this method to solubilize starch makes the soluble and insoluble NSP value interpretation complicated since DMSO is a known solvent for hemicellu-loses (24,25).

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  • arvi
    Why would the unavailable carbohydrates not be hydrolysed in the DNS colorimetric determination?
    8 years ago

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