Effect Of Lactic Acid Bacteria Extrapolysaccharide On The Texture Of Set Yogurt

Lactic acid bacteria extrapolysaccharides were a catergory of carbohy-drates that often secreted from cell wall into the medium during the lactic acid bacteria growth and metabolism, which could keep yogurt gel structure, prevent syneresis, increase viscosity and curd strength. Therefore, it was important for the yogurt production to study its separation, purification, characteristics and effects on the set yogurt. In this study, the separation and purification, the main characteristics of extrapolysaccharides secreted by Lactobacillus bulgaricus and its influences on the set yogurt were researched. The main results were as follows:1. The optimized extractions conditions of extrapolysaccharide were studied. Supernatant concentration ratio 1:6, ethanol concentration 70% and TCA concentration 20% when 67.8% extraction percentage could be obtained. Sephacryl gel column chromatography was selected to purifiy crude extrapolysaccharide and two major components named LBZ-1 and LBZ-2 were obtained. Results of UV spectrum and biuret reaction test of these two components indicated their good purity.2. IR identified that both LBZ-1 and LBZ-2 were polysaccharides with uronic acid and rhamnose absorption peaks. Results of HPLC analysis showed that the monosaccharide composition of them were rhamnose fructose : glucose as 1:3.12: 4.67 and 1:3.64:7.65 respectively. The content of LBZ-1 and LBZ-2 were 47.67mg/L and 342.64 mg/L, average molecular weights were 2.1×106Da and 1.7×103Da, uronic acid content were 2.61% and 1.88% respectively.3. Results of influential factors of extrapolysaccharide viscosity showed that:viscosity of extrapolysaccharide increased with the addition of metal ions and the increasing of the concentration, pH value and ionic strength. However, the rising of temperature reduced the viscosity. In addition, the viscosity was effected by the molecular weight as well. Generally, because of its larger molecular weight, LBZ-1 had higher viscosity than the LBZ-2. 4. Yogurts were fermented using different extrapolysaccharide adding methods, which were endogenous method, exogenous 1 (extrapolysaccharide adding into raw milk directly) method and exogenous 2 (before adding into raw milk directly extrapolysaccharide dissolved) method. Results showed that:with better WHC, less STS, higher viscosity, cohesiveness and resilience, and lower hardness, endogenous method and exogenous 1 method were better than exogenous 2 method and CK.5. The ratios of LBZ-1 to LBZ-2 of extrapolysaccharides (named LBZ-H, LBZ-M and LBZ-L) produced in different temperature (37℃,40℃and 43℃) were 1:6.26,1:3.05 and 1:2.37 respectively. The order of average molecular weight was LBZ-M