Chemical and physico-chemical characterisation of fibres from Laminaria digitata (Kombu Breton) : a physiological approach

The content and chemical composition of soluble and insoluble dietary fibres from the brown marine alga Laminaria digitata (kombu breton) were determined. Two enzymic-gravimetric methods were used to determine (1) the content of soluble and insoluble fibres according to a modification of the AOAC procedure, and (2) the distribution of the soluble fibres in saline buffer at 37-degrees-C and at pH 2.0 and 7.5 used to simulate the gastric and intestinal phases of digestion, respectively. The total dietary fibre contents obtained by the two methods were similar (37.3 and 40.0%) and of these 84.8- 87.4% was soluble. A partitioning of soluble fibres may occur during digestion since 49.3% was recovered in saline buffer at pH 2.0 whereas 50.7% was recovered in saline buffer at pH 7.5. Solubility of dietary fibres was related to the chemistry of brown algal polysaccharides. Fucans and laminarans were essentially soluble at pH 2.0 and alginate at pH 7.5, and insoluble fibres consisted primarily of cellulose. Oil adsorption and hydration properties (uptake, retention and swelling) in water, 154 mM NaCl, and 38 mM CaCl2 at 20 and 37- degrees-C of three particle sizes of L digitata were measured. Oil adsorptiion was low (0.16-0.41 g g-1) and was related to the particle size of the fibres. Hydration properties were more important with small particles except in CaCl2 solution and followed the order water > NaCl > CaCl2. Water uptake and swelling were greater at 37-degrees-C than at 20-degrees-C. The overall decrease in hydration properties observed with solutions of ionic strength approximately 0.15 was interpreted as reflecting the decrease in the electrostatic repulsion between charged polysaccharides. The lowest water uptake, water retention and swelling were obtained with solutions of CaCl2 and were related to the known selective affinity of alginate for calcium. Thus, L digitata is especially rich in soluble dietary fibres, and these have physico-chemical properties characteristic of the polysaccharides present. Water absorption and uptake and welling can be modulated according to various physical and chemical parameters.