Characterization of dietary fiber and soluble carbohydrates in date fruits (Phoenix dactylifera L.)

Understanding the physicochemical and structural properties of the dietary fiber present in date fruits is useful for its onward processing for food and non-food applications. This research fractionated dietary fiber from four date fruit cultivars into soluble and insoluble fractions and studied their structural features and physicochemical and thermal properties. Total dietary fibers content ranged 3.2–7.4 g/100 g with > 90% being insoluble. Insoluble fiber was crystalline and mainly composed of lignin, cellulose, and hemicellulose while the soluble fiber was amorphous and mainly contained pectin (> 50%). The lignin content was higher in Barakawi and Neghal (3.6 & 3.7 g/100 g) than in Shishi and Barhi (1.4 & 2.1 g/100 g). Guaiacyl lignin was present in the sclereid and parenchyma cells and syringyl lignin was present in the xylem vessels. The pectin in the date fruit soluble fiber is highly branched possibly by oligosaccharides such as type II arabinogalactan. Several furan and pyran-type residues with different linking patterns were observed in the soluble fiber fraction (α-L-arabinofuranose, α-L-rhamnopyranose, β-D-galactopyranose, α-L-glucopyranose). The insoluble fiber fraction was thermally more stable than the soluble fraction. Overall, this research provides a knowledge-base for onward valorization of date fruit soluble and insoluble fibers for various food applications.