Combined Effect Of CaCl₂ And Starch On The Gel Properties Of Low-Salt Tilapia Myosin

Moderate unfolding and aggregation of protein molecules is the key to gel formation.Myosin is the main functional component of muscle protein,which can form gels by heat treatment,and which plays a key role in the processing of surimi products.However,the solubility of myosin and degree of molecular dissociation are different under different salt conditions.It is speculated that the effect of Ca²⁺on the thermal degeneration and aggregation of myosin molecules is different,and which also results in different inter-molecular interaction and gel properties of myosin.In this study,tilapia myosin was used to study the effect of CaCl₂on thermal denaturation,aggregation and gel formation of myosin under low salt conditions.On this basis,the modified starch was added as the gelling agent,and the combined effect of CaCl₂and cassava acetate starch on the improvement and mechanism of low-salt myosin gel properties was explored.The main results are as follows:1.Under the condition of low salt concentration?1?150 mmol/L Na Cl?,the myosin solution is turbid and the molecules are in the form of filaments.During the heat treatment?30?80?,1?/min?,the myosin filaments aggregated directly,and the turbidity of the myosin solution increased significantly?P<0.05?.After adding 10 mmol/L CaCl₂,at a temperature lower than 50?,myosin molecules will unfold and expose the sulfhydryl group without forming disulfide bonds.In this state,only the aggregation of the myosin head was involved.When the temperature was 50-60?,the tail of myosin molecules started to denature and aggregate,the?-helix content was decreased significantly?P<0.05?,and the sulfhydryl groups were oxidized to form disulfide bonds.The highly dense aggregate of myosin was observed by transmission electron microscopy at 80?,and dynamic elastic modulus increased.Therefore,Ca²⁺induces partial unfolding and denaturation of myosin molecules under low-salinity conditions,causing the loss of?-helical structure and more exposure of cross-linked sites during heat treatment further promoted thermal aggregation,which lays a foundation of the formation of the low salt myosin gel.2.Heat-induced gel of myosin was prepared by heat treatment from 30 to 80?at1?/min and preservation at 80?for 30 min with a fixed protein concentration of 20 mg/m L.Under the condition of low salt concentration?1?150 mmol/L Na Cl?,there were weak opaque gels due to a rapid aggregation of myosin filaments during heating.These myosin gels exhibited a cross-linked network with coarse cross-linked strands and large cavities,and the water in the gel matrix was highly mobile and the water holding capacity was low.Hydrophobic interactions were responsible for the heat-induced gel formation of myosin on the above conditions.During the gelation process,the addition of 10 mmol/L CaCl₂promoted the unfolding of myosin and caused?-helices to partially turn into?-sheets with the reduced hydrogen bond interactions,thus resulted in stronger disulfide linkages between myosin molecules?P<0.05?,which contributed to the ordered and dense three-dimensional network with significantly improved whiteness of myosin gel?P<0.05?.The ability of the gel matrix to retain water molecules was enhanced,and the water holding capacity of the gel were significantly improved?P<0.05?.Moreover,this improvement was more obvious at 1 mmol/L Na Cl.Therefore,an appropriate amount of CaCl₂can effectively improve the gel properties of myosin under low salt conditions,and the improvement mechanism may be related to the unfolding and aggregation of Ca²⁺-induced myosin molecules and the strong disulfide linkages between protein molecules.3.Based on the addition of 10 mmol/L CaCl₂,four different starches?raw cassava starch,pregelatinized modified cassava starch,cassava acetate starch and cassava phosphate double starch?were added,and effect of starch on water holding capacity and gel strength of myosin gel at 1 mmol/L Na Cl was studied.Comparative analysis indicated that the addition of 0.5%cassava acetate starch and 10 mmol/L CaCl₂can significantly improve the water holding capacity and gel strength of low-salt myosin gel?P<0.05?.The further research showed that,under the conditions of 1?150 mmol/L Na Cl,the combined action of CaCl₂and cassava acetate starch can significantly reduce the water mobility of gel matrix?P<0.05?,and the water holding capacity,gel strength,and whiteness values of low-salt myosin gel increased significantly?P<0.05?.The proportion of hydrogen bonding and hydrophobic interaction in the chemical forces maintaining the myosin gel decreased,and the disulfide bond content increased significantly?P<0.05?.Compared with Ca²⁺-induced myosin gel,in the range of experimental salt concentration?1?600 mmol/L Na Cl?,the addition of cassava acetate starch can cause the chemical forces maintaining the myosin gel was changed obviously with stronger disulfide crosslinking?P<0.05?,and the gel properties of myosin was further improved.Therefore,the improvement of cassava acetate starch on the gel properties of low-salt myosin is mainly due to the strong disulfide crosslinking between molecules.