Study of synergistic interactions between galactomannans and non-pectic polysaccharides from water soluble yellow mustard mucilage
This thesis investigated the synergistic interactions between galactomannans and non-pectic polysaccharides from water soluble yellow mustard mucilage. A non-pectic polysaccharide (NPP) fraction was isolated from yellow mustard mucilage using pectinase hydrolysis followed by ammonium sulphate precipitation. This fraction consisted mainly of a [beta]-1,4 linked glucosidic backbone. Rheological tests showed that NPP exhibited strong shear-thinning flow behaviour and a weak gel structure, acid resistance, and stable gelling properties in a wide temperature range. Detailed structure information on NPP was investigated using Nuclear Magnetic Resonance (NMR) spectroscopy. Four types of galactomannans (GMs), namely fenugreek gum (FG), guar gum (GG), tara gum (TG) and locust bean gum (LBG), were investigated for their emulsification and rheological properties. The M/G ratios of the four GMs were 1.2, 1.7, 3.0 and 3.7 respectively. The results revealed that the M/G ratio, along with molecular weight and intrinsic viscosity, played an essential role in emulsion and rheological properties. Synergistic interactions between the four GMs and NPP were investigated using rheological measurements. The four types of GMs were blended with NPP at various ratios. Results revealed that at a total polysaccharide concentration of 0.5% (w/w), the highest synergism occurred at the GM/NPP blending ratio of 3/7 for all four types of GMs. The interaction between TG and NPP showed the highest synergy, followed by LBG/NPP, FG/NPP and GG/NPP. At a higher total polysaccharide concentration (1.0% w/w), the blend of TG and NPP exhibited the highest synergy, followed by FG/NPP, LBG/NPP and GG/NPP. At a total polysaccharide concentration of 0.5% and the GM/NPP blending ratio of 3/7, neutral pH (pH=6.5) showed the strongest synergy compared to that at pH 2.0 and pH 12.O. In order to better understand the synergistic behaviour, conformations of simulated GMs with different M/G ratios were investigated using molecular modelling software (Insight II/Discover_3 and RIS program, Version 4.0.0). The results showed that the insertion of galactosyl groups could cause bending of the chains. The results could explain the synergistic interactions between GMs and cellulosic polysaccharides: a more flexible chain could penetrate through networks in a rigid structure, while the side groups can help with forming stronger "hyperentanglements". The overall synergistic behaviour of GMs and NPP is a result of a combined effect of junction zones via "smooth regions" and "hyperentanglements".