Investigation Of Sodium Caseinate/Low Acyl Gellan Mixture System Rheology Property

Proteins and polysaccharides are often co-exists in the many food systems. Proteins contribute to emulsion, foam formation and stability, whereas polysaccharides are present as thickening and water-holding agents. Low acyl gellan(LA) is a new type extracellular bacterial polysaccharide, as its heat-resistant and less pH-sensitive gellan gum has been widely used in food industry as a thickening agent. By comparing with other polysaccharide gels, gellan could function at low pH(<3.2), where other polysaccharide were disassociation. This feature has a vast application prospect to improve the quality of fermented dairy products. In this thesis, sodium caseinate(SC) and gellan mixture were studied. Rheology method were adopt to investigate the mixture gelation behavior affect by sodium caseinate concentration. Gel property of mixture induced by cation, acid or combined action were also investigated.Two rheological methods were used to estimate the sol-gel transition temperature, the crossover of storage modulus G’and loss modulus G", as a convenient approach to preliminarily determined gelation temperature, with higher Cs addition crossover point move towards lower temperate, however as its obviously frequency dependent, Tc could not be taken as the sol-gel transition temperature. Accurate gel point Tgel and The relaxation critical exponent n were obtain from Winter’s criteria. Same as most physical gelation no universal n for the gelation in gellan gum/sodium caseinate solutions, indicating that this gelation belongs to the cross-linking of existing macromolecules. n increased with increasing G below0.8wt%, and decreased at Cs above1wt%. The fractal dimension df calculated from n without the screened hydrodynamic interaction nor the excluded volume effect,the structure getting looser with SC addition, which need more denser structure at lower temperature to achieve sol-gel transition. Arrhenius equation and time-temperature relationship were combined to investigate the of SC addition affected the gellan gum gelation, Ea were calculate from the slop of Arrhenius-type plot, consider as the energy barrier in two temperature range, indicate that SC addition facilitate the gellan gum gelation process. However lower sol-gel transition temperature at higher Cs may lead to high SC contents decreased network connectivity, which block the double helix LA chain further aggregation with time.The effect of LA/SC weight ratio, total polymer concentration, cations species and cation concentration on the textural properties of LA/SC mixed gels were investigated. The failure strain of mixed gels was sensitive to low levels of added cations, but less sensitive to higher cation concentrations, LA/SC weight ratio and total polymer concentrations. Mixed gels with potassium were much more deformable than with calcium. The failure stress and initial Young’s modulus of mixed gels increased with increasing total polymer concentrations and the proportion of LA. Moreover, at each LA/SC weight ratio and total polymer concentration, the failure stress and initial Young’s modulus increased with increasing cations until cations reached a critical level, after which further increases in cations resulted in a reduction of failure stress and initial Young’s modulus. At optimum cation levels, gels with calcium were stronger and firmer than gels with potassium. In general, Water holding capacity increased as cation concentrations decreased and larger total polymer concentration and LA/SC weight ratio exhibited larger water holding capacity(WHC). SC morphology in mixed gel depend on cation species,which affect gel WHC, as the result potassium induce gel got higher WHC than calcium. There is no synergistic effect between potassium and calcium.LA/SC mixture gel viscoelasticity property were investigate by stress relaxation test. The relaxation characters varied according to polymers and potassium concentration. Potassium addition increased mixture gel strength, however exceed potassium lead disintegrate of the matrix. Peleg could predict LA/SC mixture gel successfully, high correlation parameter(r2≥0.99)and low RMS. The cross-head speed had little effect on equilibrium stress, however faster crosshead speed lead to higher initial stress and initial relaxation rate, equilibrium stress increased with deformation at4-20%strain, but much lower at25%strain, this phenomenon probably due to the network collapsed.The pH effect on LA/SC mixture gel property were investigated, with the hydrogen ions increasing electrostatic repulsion between molecular chains decrease, which induce the phase separation. However phase separation were hindered by the network formation, as the result phase separation occur locally.the maximum strength of gel at the pH=4, with hydrogen ions further increase the gel strength decreased. The low filed nuclear magnetic resonance (LF-NMR) were adopt to measure the transition relaxation time of the mixture gel, built the assosiation between non-destructively detection, texture and water holding ability. Mixture gel induced by cation were exposed in different pH environment to acidized, after which the hardness toughness and deformability all received significantly increased, in addition the low field NMR could depicted the gel acidized process successfully.