The Study On The Structure And Physicochemical Properties Of Water-soluble(1-3)(1-6)-α-d-glucan

Polysaccharides are the main kind of carbohydrate and have gained increasing research interests recently. The polysaccharide can be applied in food system as an ingredient to improve the texture, rheological properties and flavor of products. They have also shown some beneficial effects, such as anti-tumor, improving immunity, antioxidation and probiotic effects. Generally, the physicochemical and physiological properties of polysaccharides are closely related to its molecular weight, structure and conformation. Hence, it is meaningful to find suitable and effective way to modify and alleviate polysaccharides’ molecular weight, structure in order to expand their applications. This thesis focused on the study of physicochemical characters of water-soluble(1-3)(1-6)-α-D-glucans bioengineered by alternansucrase from Leuconostoc citreum SK24.002. Besides, enzymatic and ultrasonic methods were used as green technology to improve physicochemical and physiological properties of this glucan.Firstly, the characterization of native water-soluble(1-3)(1-6)-α-D-glucans was studied. Surface morphology revealed that the glucan had a porous structure, the particle sizes of the glucan in water solution ranged from 40 to160 nm. The glucan had a narrow and single weight distribution and showed a relatively high thermal stability decomposing mainly at 265~345 o C. The viscosity of this glucan increased exponentially with concentration in the range of 5 to 10 g/100 m L, concentrated solution demonstrated typical shear-thinning behavior and could display an obvious weak gel-like character.In the second part, the structure and properties of bioengineered(1-3)(1-6)-α-D-glucans subjected to endodextranase treatment were investigated. Upon enzyme treatment, the molecular weight and high molecular fraction decreased substantially. After modifying for 8 h(50 U/g), the molecular weight of glucan reduced from 2.11′107 g/mol to 5.84′106 g/mol. The chain hydrolysis reaction during enzyme treatment followed a combination of 0th- and 1th-order processes, and the viscosity of glucan solutions were reduced significantly. Structure analysis showed that the backbone structure of glucan was preserved, and some specific α-1,6 linkages of glucan chains were cleaved with enzyme treatment. The enzymatic modified glucan manifested better probiotic effect and could produce more short-chain fatty acid(SCFA) in vitro fermentation than native one. The total SCFAs of enzymatic modified glucan after fermentation was 3.2 times higher than that of native one.Finally, in the third part, ultrasonication was applied to modify the structural, physicochemical and physiological properties of the(1-3)(1-6)-α-D-glucans from Leuconostoc citreum SK24.002. Upon ultrasound treatment, the UV absorption at 220 nm and the number-average molecular weight decreased substantially in the first 20 min, and then incrementally decreased to a plateau value after 160 min. During ultrasonic treatment for totally 160 min in the power of 15%, the molecular weight of this glucan decreased from 2.11′107 g/mol to 5.0′105 g/mol and the chain degradation reaction followed the random scission model. The viscosity of modified glucan solution dropped remarkably and Newtonian plateaus for sonicated glucan solutions were observed at the high shear rates(1~100 1/s). Structure analysis indicated that the partial consecutive α(1-6) linkages was reduced by 2.4%. In vitro fermentation, modified glucan, due to its lower molecular weight and shorter chain length, was more accessible and can be utilized to produce more SCFAs when compared with native ones.