Structure-Function Relationship of Flaxseed Gum from Flaxseed Hulls
Soluble dietary fibre with low viscosity can be included in the diet in a significant amount to show health benefits without over-texturization. Soluble flaxseed gum has recently been the subject of increasing research attention due to its low viscosity, which is favored as a potential fibre fortifier. In this work, soluble flaxseed gum (SFG) extracted from flaxseed hulls was fractionated into a neutral (NFG) and an acidic (AFG) fraction gum using ion exchange chromatography. The physicochemical properties, structure and conformation of both fractions were investigated. The protein and uronic acid content were 11.8 and 38.7 % in SFG, and 8.1 and 23.0 % in AFG, respectively. NFG contained no protein or uronic acid. NFG and AFG exhibited pseudoplastic and Newtonian flow behavior, respectively. The ranking of intrinsic viscosities (mL g-1) in a decreasing order was: SFG (446.0) >NFG (377.5) >AFG (332.5). AFG was expected to have higher chain flexibility due to its lower value of Huggins constant (0.16) compared to that of NFG (0.54) and SFG (0.48). AFG had a highly branched rhamnogalacturonan-I backbone substituted iii at O-3 of →2)-α-L-Rhap-(1→ which was mostly terminated by a monomeric sugar unit (19.6 % T-α/β-D-Galp, 4.5 % T-α-L-Fucp, and 3.1 % T-β-D-Xylp) or occasionally by a longer side chain with a similar structure as its backbone. NFG contained β-1,4-linked xylose backbone being mono-, di- or unsubstituted at O-2 and/or O-3 positions by one to three linearly- linked sugar residues (24.8 % arabinose, 15.5 % xylose and 9.4 % galactose/glucose). Static and dynamic light scattering results showed that NFG exhibited random coil conformation with a molecular weight of 616±19 kDa. The star-like conformation of AFG (with a molecular weight of 285±25 kDa) also coincided with the presence of longer side chains along its rather flexible RG-I backbone. The random coil and star-like conformation of NFG and AFG, as well as their relatively small molecular sizes, explained the overall low viscosity of flaxseed gum extracted from flaxseed hulls.