| Agar is a natural water-soluble polymeric polysaccharide extracted from seaweed,which is widely used as a thickener,gelling agent and stabiliser in the food,biomedical and chemical industries.With the development of time,natural agar with easy syneresis and shrinkage and low gel strength has gradually failed to meet some commercial requirements.In order to improve the gelling properties of agar and expand the application range of agar,this research paper used sodium tripolyphosphate（STPP）and sodium pyrophosphate（SPP）as modifiers to modify agar in alkaline and non-homogeneous systems,respectively,and introduces new functional groups to change the original structure of agar and give it new properties through chemical modification.The physical and chemical properties,structural differences and gel properties were also analyzed before and after modification,and the reaction mechanism and conformational relationships were analysed.In addition,STPP-modified agar and SPP-modified agar were added into minced silver carp to prepare new composite surimi gels and investigate their basic properties,microstructure and interprotein force changes.Firstly,STPP was used as a modifying agent to prepare tripolyphosphorylated esterified agar（STAG）,and the effects of different reaction conditions（STPP addition,material-liquid ratio,reaction pH,reaction time and reaction temperature）on the gel strength and freeze-thaw stability were investigated.The physicochemical properties of STAG at different reaction temperatures were examined and analyzed.The results showed that with the increase of reaction temperature,the syneresis rate of STAG showed a trend of decreasing and then increasing,and when the reaction temperature was 30℃,the syneresis rate was as low as 1.9%.With the increase of reaction temperature,the gel strength of STAG increased continuously,and the gel strength of 60℃-STAG reached the highest value of 1529.4 g/cm2.The structural analysis showed that FT-IR observed30℃-STAG and 60℃-STAG with new absorption peaks for the P-O-C bond absorption vibration peak of the P=O bond stretching vibration peak,respectively.And for 60℃-STAG,the P-O-C bond absorption peak was enhanced and the P=O bond vibration peak was weakened,which indicated further cross-linking reaction at the reaction temperature of 60℃.The XPS results also revealed a new characteristic peak C-O-P bond and a new characteristic peak P=O bond for STAG,which proved that the agar was successfully esterified by STPP.The gel properties of esterified agar（30℃-STAG）and cross-linked agar（60℃-STAG）were further investigated,and the FE-SEM results showed that the gel network of 30℃-STAG exhibited a"thin film"structure,while the gel network of 60℃-STAG showed a more ordered,denser and thicker chain structure.The TPA and rheological results showed that 30℃-STAG and 60℃-STAG had higher hardness,chewiness,adhesiveness and cohesiveness than the native agar,and higher energy storage modulus.STAG with high freeze-thaw stability have great application potential in frozen foods.Secondly,SPP was used as a modifying agent to prepare pyrophosphate esterified agar（SPAG）,and the effects of different reaction conditions（material-liquid ratio,SPP addition,reaction pH,reaction time and reaction temperature）on the gel strength and freeze-thaw stability were investigated.The physical and chemical properties of SPAG at different reaction temperatures were investigated and analyzed.With the increase of reaction temperature,the syneresis rate of SPAG showed a trend of decreasing and then increasing,and when the reaction temperature was 40℃,the syneresis rate reduced to a minimum of 2.2%.With the increase of reaction temperature,the gel strength of SPAG increased continuously,and the gel strength of60℃-SPAG reached the maximum value of 1392.5 g/cm2.The structural analysis showed that FT-IR observed a P=O bond stretching vibration peak and a P-O-C bond absorption vibration peak for40℃-SPAG,and the P-O-C bond absorption peak was enhanced and the P=O bond vibration peak was weakened for 60℃-SPAG,which indicated that further cross-linking reaction occurred at the reaction temperature of 60℃,in addition to esterification reaction.The Zeta potential results indicated that SPAG had more net negative charge compared with the native agar.The gel properties of esterified agar（40℃-SPAG）and cross-linked agar（60℃-SPAG）were further investigated,and FE-SEM results showed that the 40℃-SPAG gel network possessed a"thin film"structure,while the 60℃-SPAG gel network was observed to have a denser,thicker chain structure.LF-NMR showed that the third peak(T23)of 40℃-SPAG and 60℃-SPAG after freeze-thawing had only a lower signal intensity compared to the native agar,and its free water content was only0.32%and 1.92%.1.92%.In addition,the sulfate content,dissolution temperature,and ash content of SPAG were lower compared to the native agar,while the hardness,chewiness,viscosity,and cohesiveness were higher.Overall,the modification with SPP conferred better functional properties to the agar.Finally,based on the characteristics of higher gel strength and higher freeze-thaw stability of STAG and SPAG,the effects of adding native agar and modified agar（30℃-STAG and 40℃-SPAG）on the gel properties of surimi were investigated.The experimental results showed that the gel strength of surimi with the addition of 30℃-STAG and 40℃-SPAG increased to 1007.2 g·cm and 1047.4 g·cm,which was about 63.9%and 70.4%higher than the gel strength of pure surimi（614.5 g·cm）,and their syneresis rates were 2.7%and 2.5%,respectively,which were also much lower than those of pure surimi（10.9%）.In addition,the addition of modified agar improved the brightness and whiteness of surimi gels.TPA of surimi gels showed that surimi gels supplemented with modified agar were higher than pure surimi in terms of hardness,chewiness,viscosity and cohesiveness.SDS-PAGE electrophoresis analysis showed that the MHC and AC bands of surimi gels supplemented with modified agar were deeper and wider.The higher gel strength of the modified agar-added surimi gels was attributed to the increased hydrophobic interactions,disulfide bond content and hydrogen bond content compared to the pure surimi,while the increased hydrogen bond content imparted better freeze-thaw stability to the surimi gels.In addition,the dense and uniformly distributed pores of the network structure of the surimi gels with the addition of modified agar indicated that the addition of modified agar not only strengthened the network structure of the surimi gels but also improved their freeze-thaw stability.In conclusion,the agar modified with STPP and SPP has excellent application prospects in surimi products. |
