Effect Of Salts And Polysaccharides On The Formation Of Surimi Gel And The Mechanism

With the rapid increase of the demand for surimi products,freshwater surimi tends to be an important material for producing surimi products.However,it presents poor gel properties due to the degradation.As a result,it is urgent to find an effective process technology to improve the gel properties of freshwater surimi.Gelation is one of the most important procedures for the formation of surimi gel.Molecular forces between proteins are the foundation of the gelation.Although many researchers at home and abroad have studied the changes of molecular forces and protein secondary structure during the formation of surimi gels.However,the relationship between molecular forces,gel properties and protein conformation is still not clear,and researches about the effect of metal salt and polysaccharide on the reduced-salt surimi gel are also scarce.As a result,this paper mainly aimed at investigating the relationship between molecular forces,protein secondary structure and gel properties of heat-induced freshwater surimi gels,and defining the relative molecular force for the change of texture.It also tailored the gel properties based on the effect of metal salt ions and polysaccharides on the protein molecular forces.This study is of significant importance for revelation of the effect of protein interaction on the formation of freshwater surimi gels.In addition,it also provides the theoretical support and technological guidance for the development of freshwater surimi products with desired texture,which are high-protein,low-salt and the texture was suitable for eating.Firstly,the influence of molecular forces on the formation of surimi gels was investigated.Urea,sodium dodecyl sulfate（SDS）and N-ethylmaleimide（NEM）were employed to monitor the contributions of molecular forces to heat-induced surimi gels by texture profile,gel strength,rheological and Raman analysis.The correlation between molecular forces,protein conformation and textural properties was also studied.With the destruction of hydrogen bonds,hydrophobic interactions and disulfide bonds,hardness and gumminess of surimi gel presented a dramatical decrease.Gel strength significantly decreased as the concentration of urea and NEM increased,while it was improved by 2% SDS.The rheological analysis demonstrated that remarkable changes of storage moduli（G′）presented below the critical temperature（Tc）,especially for SDS treatments,which were significantly different with the classical rheological property of myosin.The results from Raman spectrum indicated that bands in the range of 1600cm-1-1700 cm-1,which represented the amide I region,shifted to a low wavelength.It also demonstrated that protein secondary structure tended to transform to random coil,with content of α-helix and β-sheet decreasing.The correlation analysis indicated that less of random coil led to improvement of gel strength,and springiness was mainly contributed by β-sheet and β-turn.In the next study,the impact of metal salt ion type（NaCl,KCl and CaCl₂）and ionic strength（0.51,0.34 and 0.17 mol/kg）on the gel properties and molecular forces of surimi gels was investigated.The results showed that surimi gels with NaCl and KCl presented better water-holding capacity（WHC）than with CaCl₂.With the increasing content of CaCl₂,WHC significantly decreased from 93% to 87%.Surimi gels with addition of high ionic strength of CaCl₂ presented higher hardness and gumminess,while the springiness was lower.When the ionic strength was 0.17 mol/kg,the gel strength of surimi gel with CaCl₂ was 1.55 times higher than with NaCl.As the temperature increased from 25°C to 90°C,surimi gels with NaCl and KCl presented similar rheological properties.However,surimi gels with CaCl₂ presented obviously different rheological properties.When the temperature was higher than 49°C,the G′ of treatments with CaCl₂ was significantly higher than with NaCl and KCl.As the concentration of CaCl₂ increased,the content of hydrogen bonding and sulfhydryl group gradually reduced,while the content of the hydrophobic interaction,nonspecific associations and solubility increased.Furthermore,microstructure observed from scanning electronic microscopy（SEM）demonstrated that surimi gels prepared with monovalent salt presented more compact and denser network,while more aggregated particles existed in gels with CaCl₂.Then the effect of CaCl₂ on the formation of reduced-salt surimi gel was investigated when the total ionic strength was 0.51 mol/kg.In order to decrease the usage of NaCl,the textural properties,gel strength,rheological properties,water distribution,molecular forces and protein conformation of surimi gels were analyzed to obtain suitable content of CaCl₂ to replaced NaCl.The results indicated that when 1.5% NaCl was replaced by 0.95% CaCl₂,the textural properties,gel strength and G′ were improved.However,as excess NaCl was substituted by CaCl₂,the gel properties decreased.As the content of CaCl₂ increased from 0 to 1.58%,the WHC of surimi gels decreased 15.6% and the proportion of T23 increased from 0.37% to 5.31%.With the temperature increasing,the G′ of surimi gel with higher content of CaCl₂ first decreased gradually then increased dramatically until it kept stable.The final value of G′ was higher then the contrast.When the content of CaCl₂ increased to 1.58%,Tc decreased from 59°C to 47°C.The content of SH group decreased as more NaCl replaced by CaCl₂.Low content of CaCl₂ promoted the formation of（γ-Glu）-Lys bond.The addition of CaCl₂ induced the transformation of protein secondary structure from α-helix to β-sheet and random coil,while it impacted the β-turn non-significantly.Finally,the effect of three kinds of anionic polysaccharide（ι-carrageenan,κ-carrageenan and sodium alginate）on the calcium-surimi system was investigated.The results indicated that ι-carrageenan improved the gel strength significantly,κ-carrageenan increased the hardness of surimi gels,while decreased the springiness.Sodium alginate enhanced the breaking force,and impacted the distance to rupture and springiness non-significantly.WHC could be improved by the three kinds of polysaccharide,especially for sodium alginate.With the addition of polysaccharide,the G′ of calcium-surimi system increased during the heating process.Compared with the treatment without polysaccharide,Tc increased with the addition of ι-carrageenan and sodium alginate,while there was no change for κ-carrageenan.The results from frequency sweep indicated that in the linear region,the value of G′ of surimi with ι-carrageenan was higher than others.ι-carrageenan could impact the surimi gel properties by in cooperation with calcium.The results from turbidity and surface hydrophobicity indicated that ι-carrageenan reduced the hydrophobic interaction and inhibited the excessive aggregation between proteins.The results from Fourier transform infrared spectroscopy（FT-IR）showed that there was no significant change of protein secondary structure.As a result,the addition of ι-carrageenan improved gel properties of surimi by affecting the tertiary and quaternary structure of proteins.In addition,CaCl₂ could also improve the gel properties of ι-carrageenan-surimi system.With the content of CaCl₂ increased,the gel strength improved.The synergistic effect of ι-carrageenan and CaCl₂ could improve the gel properties of reduced salt surimi gel and tailor the gel formation to obtain surimi gels with suitable textural properties.