Research On Effect Of Lactobacillus Paracasei Exopolysaccharide On The Textural Properties Of Yogurt And Its Mechanism

Lactic acid bacteria exopolysaccharides?LAB EPS?can affect the yogurt textural properties significantly.The viscosity,water hold capacity and gelation of yogurt increased obviously during the fermentation process by using strains which can secrete EPS.This is mainly due to the interaction between EPS and the protein in yoghurt.In this paper,effect of EPS produced by Lactobacillus paracasei on the textural properties of fermented milk and the mechanism between EPS and protein were investigated.The main contents and results are shown as follows:?1?The effect of EPS secreted by L.paracasei on yogurt textural characteristics:The relation between EPS production and texture characteristics by altering the fermentation condition was investigated.The single-factor tests showed that the fermentation time,inoculum size and fermentation temperature could affect EPS release and textural formation of yoghurt significantly.The response surface methodology was performed to determine an optimal condition for yoghurt fermentation.Results showed that 1026.4mg/L EPS and 1351.9 g.sec viscosity were observed under the optimal condition including fermentation time 8 h,inoculum size 14%and fermentation temperature 40°C.In addition,we found the yogurt texture was exponentially related to the EPS production.Under the optimal fermentation condition,the hardness,cohesiveness and viscosity were time-dependently enhanced with the increase of EPS production.In order to find out how the EPS fraction affect the formation of yoghurt texture,HPLC was further employed to analyze the molecular weight distribution of EPS extraction in the yoghurt after fermentation for 4 h,5 h,6 h,7 h and 8 h.Results suggested that EPS1 and EPS2 were the most important exopolysaccharide fractions required for the formation of yoghurt texture when L.paracasei was used as the fermentation strain.?2?Isolation and purification of L.paracasei exopolysaccharides:Crude EPS were extracted from the fermented yoghurt under the optimal conditions.Then the crude EPS were purified by DEAE Cellulose DE52 ion-exchange chromatography,and three kinds of exopolysaccharides,EPS-W,EPS-S1 and EPS-S2 were obtained in the yoghurt.Among them,EPS-S1 was further purified by Sephacryl S-500 gel filtration chromatography to obtain a homogeneous exopolysaccharide?EPS-S11?,with the relative molecular weight 1.68×10⁷ Da and uronic acid content 8.26%.Monosaccharide composition analysis indicated that EPS-S11 was composed of mannose,glucose,galactose and glucuronic acid with a molar ratio of 0.87:0.92:1.00:0.24.Combined with the results of methylation analysis and NMR analysis,EPS-S11 is mainly composed with?4)-?-D-Manp-?1???6?-?-D-Galp-?1???4,6?-?-D-Glcp-(1???-D-GlcpA-(1???-D-Glcp-(1?.?3?The turbidity,particle size and zeta potential of the mixed solution of EPS-W?EPS-S1?EPS-S2 and SC were measured under different pH,exopolysaccharide concentration and ionic strength?NaCl,CaCl₂?.The results showed that EPS-S1 and EPS-S2 could interact with SC to form some soluble complexes.The mixed solution was the most stable at pH=4,which indicating a stronger interaction.While it illustrated that EPS-W got to a stable state at pH=5.The addition of EPS and lower salt ions concentration?<40 mM?could inhibit the aggregation of SC near the isoelectric point.While high concentration of salt ions blocked the interaction between EPS and SC by electrostatic shielding effect.?4?The interaction between the homogeneous fraction?EPS-S11?and SC was further explored by means of isotherm titration microcalorimetry and Laser Scanning Confocal Microscope?LSCM?.ITC showed that?H and?S were-2.307E5±1.375E4 cal/mol and-742 cal/mol/deg,respectively,indicating that the association between EPS-S11 and the protein is an enthalpy-driven spontaneous reaction.Besides,it has been found a hydrophobic interaction and a covalent bond when EPS-S11 was added to SC solution.In this process,more negative charge was produced in the solution with the increase of exopolysaccharide concentration,which was easier to interact with the positive charge on sodium caseinate micelles.The microstructure results showed that the addition of EPS-S11 could prevent the aggregation of proteins significantly.