Saltiness Perception Enhancement Of Fish Products By Microwave Treatment And Processing Adaptability In Salt-reduced Surimi

Table salt,as the top of all flavors,has the functions of seasoning,maintaining texture,bacteriostasis,and so on in fish products.However,excessive intake of salt is easy to increase health risks.On the premise of maintaining the taste of fish products,effectively reducing the amount of salt has become an urgent problem to be solved in the development of related industries.Microwave processing has opened up a new idea for achieving this goal.Microwave treatment could improve the gel properties of salt-reduced surimi,but whether it has the ability to enhance the salty taste of fish products has not been reported.In the current study,a representative freshwater fish processing model system was selected as the research object.By comparing the differences in texture quality and salty taste between fish cubes heated by microwave and water bath,the main factors affecting their saltiness perception were clarified.Moreover,the mechanism of enhancing saltiness perception by microwave thermal processing was illustrated from the perspectives of water and sodium mobility and microstructure changes.On this basis,the adaptability of microwave treatment to the surimi system in a salt-reduced environment was explored.The gel properties were improved through the combination of water bath heating and amino acid processing aids.Furthermore,the relationship between the microstructure of the surimi matrix,the mobility of sodium and water in the matrix and the sodium ion release and saltiness perception in oral processing were further revealed.The purpose of the current study is to provide theoretical guidance and technical support for salt reduction without weakening the salty taste of fish products.Firstly,based on the model of boiled grass carp cubes with water as medium,the differences and reasons in qulity,such as salinity,between microwave and water bath heating treatment were analyzed.The sensory evaluation results of the directional triangle test showed that the fish cubes heated by microwave were saltier(?=0.1%)than those heated by the water bath.Moreover,microwave heating promoted the formation of more free amino acids,reduced cooking loss,and improved the tenderness of fish cubes.The mass transfer law of salt during heating and the retention and distribution of sodium ions in the fish meat matrix were investigated by conductivity method,ion chromatography,and X-ray energy dispersion spectroscopy.It was found that the diffusion amount of NaCl from the fish matrix to the aqueous phase during the microwave heating process was 12%–26%lower than that of water bath process,helping to retain more sodium under the same degree of ripening.Moreover,the retaining level of sodium in fish cubes treated by microwave heating was 1.36 times that of traditional water bath heating.Through the characterization of protein surface hydrophobicity,water state,and microstructure,it was found that the mobility of immobilized water was enhanced and the corresponding content was increased in the microwave treated fish cubes with a low degree of protein denaturation,and the formation of dense and uniform microstructure was conducive to intercepting more immobilized water and sodium salt.To further clarify the relationship between microstructure changes of fish meat induced by microwave and salty taste,vacuum packaging was employed to control the cooking loss during heating treatment,so that a cooked grass carp cube model system with no significant difference in sodium retention was established.The purpose of this model system was to explore the effect of microwave heating power on the saltiness perception of fish cubes and clarify the corresponding mechanism.The results of sensory evaluation on the 9 cm scale showed that the salinity score of fish cubes treated with 10 and 12.5 W/g microwave was significantly higher than that heated in the water bath(P<0.05).Compared with water bath heating,the surface hydrophobicity and hydrophobic interaction of protein in fish cubes treated by microwave were weaker,so as to avoid the formation of large aggregates,resulting in the formation of the dense and uniform microstructure of fish cube matrix.The mobility of water and sodium in fish cubes was analyzed by low field and pulse-field-gradient stimulated echo ²³Na nuclear magnetic resonance,respectively.The results showed that microwave heating of 10 and 12.5 W/g(relatively high power in this study)significantly enhanced the mobility of water and sodium in the fish cubes.The dense and uniform microstructure and weak sodium binding ability,which induced by the low unfolding degree of protein,were conducive to the diffusion and migration of water and sodium ions in the matrix,resulting in an enhancement in the saltiness perception of fish cubes.Finally,the effect of 15%salt reduction in a grass carp system by microwave heating was determined by the triangle test and verified in the representative silver carp system.In order to explore the applicability of microwave treatment to enhance saltiness perception in fish products,the silver carp surimi system with salt reduction demand was selected to investigate the effect of microwave treatment on gel properties and the salty taste of the system.The results showed that one-stage microwave heating of 7.5 and 10 W/g and water bath heating led to the lower sensory score due to the deterioration of surimi gels,while the combination of water bath and microwave heating could improve the gel properties of surimi.Moreover,sensory evaluation based on the 9-point hedonic method showed that surimi gels treated by water bath-microwave(7.5 and 10 W/g)combined heating had a higher taste score.The effects of six kinds of heating methods on the gel properties of surimi were compared,and the protein degradation and intermolecular interaction were analyzed.The results showed that water bath-microwave heating could improve the gel strength of surimi gels by reducing the degradation of myosin heavy chain,enhancing the hydrophobic interaction between protein and protein,and promoting the formation of disulfide bonds and?-(?-Gln)-Lys covalent bonds.Scanning electron microscope observation showed that the microstructure of surimi gels treated by water bath heating was slightly rough and distributed with some larger aggregates,while the gels treated by water bath-microwave heating exhibited the dense,smooth,and uniform microstructure.Based on the analysis of the mobility of water and sodium ions,it was found that the mobility of immobilized water and sodium ions in gels treated by water bath-microwave heating was significantly more quickly than those in gels treated by two-stage water bath heating.The discontinuous sampling technique was employed to monitor the release kinetics of sodium ions from surimi gels during oral processing,and the kinetic model was used to fit the sodium release curves.In the initial stage of oral processing,the surimi gel treated by two-stage water bath heating showed the phenomenon of"burst release"of sodium ions,which was caused by the disintegration of matrix structure due to its lower gel strength.Whereas,in the later stage of oral processing,the water bath-microwave combined treatment promoted the formation of uniform and dense microstructure with no large aggregates,and enhanced the mobility of sodium ions,resulting in the higher release amount and release rate of sodium ions from the matrix,compared to the two-stage water bath treatment.The three-point sensory test showed that the salinity of the surimi gel treated by water bath-microwave(10 W/g)combined heating was significantly higher than that treated by two-stage water bath heating,achieving an effect of reducing salt by 15%.However,comparing the gel strength of surimi with 15%–30%salt reduction,it is found that lowering the salt addition level decreased the gel strength of surimi to some extent.The modulation of microstructure by heat treatment has an effect on the gel properties of surimi while reducing salt.Therefore,the fish myofibrillar protein was used as the model system of salt-reduced surimi gel,and the relationship between the concentration and type of amino acid processing aids and the solubility and gel properties of the myofibrillar protein model system were clarified.The results indicated that lysine and arginine of 15/20 mmol/L significantly improved the solubility and gel properties of myofibrillar proteins at low ionic strength(0.1 mol/L NaCl),which was comparable to those of myofibrillar proteins at 0.6 mol/L NaCl.The microstructure analysis showed that lysine and arginine promoted myofibrillar protein to form the"aggregated"gels with dense and compact microstructure.By comparing the water holding capacity and gel strength of protein gels,it was found that 15/20 mmol/L lysine could improve both of them.Finally,15 mmol/L lysine was chosen as a processing aid to improve the gel properties of salt-reduced surimi gels.Salt-reduced surimi gels(1.4%and 1.7%NaCl addition)were prepared by lysine treatment combined with water bath-microwave heating.By comparing the microstructure and gel properties of salt-reduced surimi under different heating treatments,the synergistic effect mechanism of water bath-microwave heating and lysine treatment on the improvement of gel strength was clarified.The results showed that 15 mmol/L lysine treatment combined with water bath-microwave heating promoted the surimi gel to form a dense and compact network structure,which was favorable to intercept more immobilized water and improve the water-holding capacity of the gel network.By analyzing the degree of protein degradation and intermolecular interaction,the rapid microwave heating was found to have an ability to pass over the gel deterioration zone,reduce the degradation of myosin heavy chain,enhance the intermolecular interaction such as disulfide bonds.Moreover,the lysine promoted the formation of(?)-(?-Gln)-Lys covalent cross-linking between protein molecules.Therefore,the synergistic mechanism of lysine treatment and water bath-microwave heating could improve the gel strength.Sensory analysis showed that compared with two-stage water bath(2%NaCl)and water bath-microwave(1.7%NaCl)treatment,lysine synergetic water bath-microwave heating treatment could not only significantly enhance the texture properties of salt-reduced surimi gel(1.7%NaCl),but also have an insignificant difference in saltiness perception,thus achieving double effects on saltiness perception and quality enhancement.