Dietary fiber pectic substances: Source of discrepancy between methods of fiber analysis

Abstract

A survey of methods for analysis of dietary fiber suggested that a number of differences between the methods could result in large discrepancies between them in extraction of pectic substances, which are a major component of cell walls and soluble fiber from most fruit and vegetables. Accordingly, the influence of buffer (phosphate versus sodium acetate), buffer concentration, pH, the presence of Ca 2+, and sample/buffer ratio on pectic polyuronide extraction was investigated. Within the ranges used in published methods, phosphate extracted much more polyuronide than acetate buffer. Extraction by acetate but not phosphate buffer was dependent on pH, buffer molarity, and sample/buffer ratio, and Ca 2+ suppressed polyuronide extraction in water but not in acetate buffer. Comparison of Englyst and Cummings' (1988, J. Assoc. Off. Anal. Chem. 71, 808–814) method, which uses phosphate buffer, with that of Theander and Westerlund (1986, J. Agric. Food Chem. 34, 330–336), which uses sodium acetate buffer, showed a large discrepancy (>400% in some cases) in polyuronide extraction. As soluble fiber may contain more than 70% polyuronide the results indicate an urgent need for standardization in fiber methodology. But (it is argued), standardization or precision with existing methods may be of questionable benefit as the extraction conditions they use have limited nutritional relevance.