Study On Regulation Of Structure And Properties Of Soy Protein Isolate/Konjac Glucomannan Complex By Freezing Treatment

Both soy protein isolate(SPI)and konjac glucomannan(KGM)have significant changes in the structure and properties at low temperatures.Therefore,in this thesis,these two typical materials were chosen to fabricate a model system to explore the effects of freezing on the rheological behavior,gel properties,and microstructure of different SPI/KGM composites.The research revealed that how SPI and KGM affect the structure and properties of each other.This research could enrich the theory of interaction between protein and polysaccharide under low temperature and provide guidance for creating new type gel food.The main results are as follows:1.The SPI/KGM composite sols were prepared by using SPI and KGM as base materials.The rheological properties of the SPI/KGM composite sols showed that both G’ and G’ after freezing(-20 ℃,24h)were much larger than that of before freezing.Especially,the G’ was significantly increased.The steady-state rheological results show that the SPI/KGM complex was a pseudoplastic fluid.When the final concentrations of SPI were 0.1 wt%,0.25 wt%,and the final concentrations of KGM were 0.5wt% and1.0%,the degree of pseudoplasticity of the fluid increased after freezing.When the final concentration was 0.5% and the final concentrations of KGM were 0.5 wt% and 1.0 wt%,the degree of pseudoplasticity of the fluid decreased after freezing.The frozen composite gels showed a significant increase in apparent viscosity at the same shear rate and low shear rate range.Based on the analysis of SPI/KGM composite films,the results showed that the SPI/KGM composite films swollen better after freezing,and the microstructure became corrugate.With the increasing of SPI concentration,the diffraction peak of the unfrozen film at 2θ=12° gradually decreased.The frozen sample showed a strong diffraction peak at 2θ=9.4° compared with the untreated sample.2.The high intensity ultrasound was used to degrade KGM to different molecular weights.First,the effects of freezing on the structure and properties of different molecular weight deacetylated konjac glucomannan were studied.The intrinsic viscosity of KGM solution decreased gradually during the ultrasonic treatment closely and fitted to the first-order polymer degradation kinetics.The main repeated units of the KGM chain were not destroyed no matter how long the KGM was sonicated.Da-KGM system performed a solid-like behavior(G’ > G’)at all frequencies after freezing treatment(-20℃).With increasing sonication time,both viscosity and shear stress of unfreezing samples were decreased while had an inverse result for freezing treated samples.The modulus G’ and G’ declined for unfreezing samples but rise significantly for freezing treated samples with increase of sonication time.KGM and SPI composites were then prepared.The results indicated that the G’,G’ and apparent viscosity of the complex increased after freezing(-20 ℃).The decreases in the molecular weight of KGM decreased the G’,G’,and the apparent viscosity of the composites.The H-bond between the composite molecules was destroyed by the addition of urea and guanidine hydrochloride,leading to decline the gels’ strength.3.The effects of different freezing conditions(freezing temperature,freezing time,and number of freeze-thawing cycles)on the texture properties,water holding capacity and syneresis rate of different proportions of gels were compared.The hardness of samples increased sharply after freezing and increased with the increase of KGM concentration or freeze-thawing times.The gel hardness reached a maximum value when frozen at-20℃ or frozen for 3 d.The chewiness after freezing was significantly greater than that before freezing,and the KGM concentration or the number of freeze-thawing cycles was positively correlated with chewiness.The chewiness property was improved when the SPI-KGM ratio was 2 wt% : 2 wt% or the freezing temperature was-15℃.The effect of freeze-thawing on springiness and cohesiveness were similar,which were decreased after freezing,but resilience showed an upward trend.The water holding capacity decreased significantly after freezing.As the SPI concentration increased(SPI concentration was below 4 wt%),the syneresis rate also increased.When the SPI concentration exceed 4 wt%,the syneresis rate of gel decreased.As the number of freeze-thawing cycles increased,the syneresis rate of gel increased.The syneresis rate was relatively high when the freezing time was 1 d or the freezing temperature was-20℃.4.Different additives were added during the preparation of SPI/KGM composite gels.The texture analysis was used to evaluate the effect of freezing on the interaction between KGM and SPI.The results indicated that H-bonding,hydrophobic interactions,and disulfide bonds were important forces in maintaining the gel structure.As the SPI concentration increased,the eutectic point of the composite gel gradually decreased,and the thawing temperature of the gel decreased first and then slightly increased.The composite gel with different concentrations of SPI showed diffraction peaks around2θ=12°,indicating that freezing would affect the crystal structure of the composite gels.When the concentration of SPI in the system was low(0.5 wt%),the microstructure of the composite gel after freezing was similar to that of pure KGM gel,with fewer pores and thicker wall.With the increasing of SPI concentration,the pores of the composite gel increased gradually.When the SPI concentration increased to 2 wt%(SPI:KGM=1:1),the honeycomb structure appeared in the gel,resulting in the decrease of gel hardness.When the SPI concentration was 3 wt%,the gel also exhibited a honeycomb structure,but the gel structure is looser than that of 2 wt% SPI concentration,resulting in the reduction of hardness and water holding capacity.5.The textural properties of frozen SPI gels induced by glutamine transaminase(TG)with different concentrations of KGM at different freeze-thawing times and freezing temperature were studied.The effect of KGM on the water state of the composite gels were also investigated.It was found that the addition of KGM had a significant effect on the textural properties of frozen SPI gels.The hardness of frozen SPI gels were significantly reduced,while the springiness did not change significantly.Frozen conditions had a greater effect on the texture properties of frozen SPI gels.The best performance of frozen gel was frozen at-20℃.The hardness of the gels were significantly reduced when frozen at-40℃ and-80℃,and adding KGM can weaken this trend.The water state of frozen SPI gels with different concentration of KGM at different freeze-thawing times and freezing temperatures was studied by low field NMR.The results showed that the addition of KGM can prevent the water state from changing in the freezing process.