Chemical composition, physico-chemical properties, enzymatic inhibition and fermentative characteristics of dietary fibres from edible seaweeds

The content, chemical and physico-chemical properties of dietary fibres in seaweed based food ingredients were investigated as well as their effects on digestive enzymes activity and their in vitro fermentation by human faecal flora. Powders from the red algae Palmaria and Gracilaria and those from the brown algae Ascophyllum and Himanthalia contained 278-474 g.kg(-1) of fibre. These contents increased to 567-694 g.kg(-1) after blanching, acid or alkaline washes of the brown algal powders, or extraction and purification of soluble fibres from Palmaria (PP). Alginate and partially purified carrageenan (PC) contained about 570 g.kg(-1) fibres. Dietary fibres contained in the different ingredients were rich in soluble fibres particularly those from the brown algal ingredients where they represented 56-75% of the total fibres. The soluble fibres chemical composition and structure were in agreement with that of alginate, fucan, laminaran for the brown algal powders, and agar, carrageenan and xylan for the red algal powders. Cellulose was the main insoluble fibre. Palmaria, Gracilaria, Ascophyllum, Himanthalia powders and that of the green algae Ulva sp have high water binding capacities. Among soluble fibres or ingredients, only alginate and PC gave high intrinsic viscosities (10962 and 5912 L.kg(-1), respectively). The cation exchange capacity ranged between 0.4 eq.kg(-1) for Palmaria and Gracilaria powders containing neutral polysaccharides and 2.7 eq.kg(-1) for Himanthalia powder containing alginates and fucans. Unidentified non-fibre compounds contained in Ascophyllum and Himanthalia powders increased pepsin activity whereas others in Ascophyllum and Ulva powders decreased that of trypsin. PP was well fermented by faecal bacteria into short chain fatty acids (SCFA; 81.5% of the initial sugar content were degraded and 5.8 mol SCFA were produced per kg of substrate), but the fermentation of the other algal ingredients was low (15.4-53.7% of the initial sugar content were degraded for 2.8-3.7 mol.kg(-1) SCFA produced). The degradation of PC and alginate ingredients (30.0 and 49.6%) and the low SCFA production (2.4 and 3.0 mol.kg(-1)) indicated that some of the released sugars were not metabolized into SCFA. This study demonstrated that the chemistry, physico-chemical properties and fermentative characteristics of dietary fibres of algal-based food ingredients vary markedly depending on the botanical origin and the process applied.