| Myosin spontaneously assembles into thick filaments with bipolar structures to display functions in physiological buffers,and it is the most important functional component in gel development of surimi.The structure and stability of myosin are environment-dependent in vitro.Calcium is a common additive in surimi-based products processing.It can affect the stability of myosin,and promote formation of a "bridge"between myosin.Recently,ultrasound technology has attracted a lot of attentions in food processing.Moreover,some researchers reported that ultrasound technology could significantly improve the foaming,emulsification,solubility and stability of fish protein.Ultrasound and chopping synergistically improved the gel strength of tilapia surimi.So it is important to investigate the self-assembly behavior of myosin upon ultrasound-calcium coupling system,which can contribute to the understanding of the mechanism behind the improvement of ultrasound on gelation property of surimi.In addition,it will be beneficial for realizing the chemical and physical effect of ultrasound on the gelation property of surimi.It was anticipated to provide some new ideas for deep-processing of freshwater fish and developing new products.The results were as follows.1?The effects of ultrasound on rheological properties of myosin assemblies were investigated.The static rheological results showed that flow index(n)of all myosin assemblies were less than 1,which represented pseudoplastic fluid.The smoothness of?a-? curves and flow index(n)of ultrasound-treated myosin assemblies increased,while the consistency coefficient(K)and zero-shear viscosity(?0)decreased gradually with increasing ultrasonic power.The dynamic rheological data showed that the initial and final value of storage modulus(G')of ultrasound-treated myosin assemblies decreased significantly(P<0.05)compared with those of untreated myosin assemblies.The phase angles(?)of all ultrasound-treated myosin assemblies were larger than 88,which displayed liquid characteristics.It was speculated that ultrasound could destroy hydrogen bonds and ionic bonds among assemblies.For the untreated myosin assemblies,the storage modulus(G')increased by 13.53 times with increasing temperature from 40? to 59.2?.For the ultrasound-treated myosin assemblies,the storage modulus(G')increased sharply by 260?295 times with increasing temperature from 30? to 40?.It was speculated that ultrasonic treatment promoted myosin assemblies to expose more reactive groups,making them unfold and denature in advance.As a result,the stronger intermolecular interactions formed even at lower temperature.2?The effects of ultrasound on size,structure and intermolecular forces of myosin assemblies and molecular weight of myosin were investigated.The results showed that ultrasonic treatment decreased the average particle size of myosin assemblies,and narrowed the particle size distribution.The ultraviolet absorption spectroscopy showed that intensity of 276 nm increased and its second derivative spectroscopy showed that characteristic peaks of 282 nm,289 nm and 298 nm displayed blue shift after ultrasonic treatment,indicating that polarity of solution environment increased.Ultrasonic treatment increased the solubility and S0-ANS of myosin assemblies,however it decreased the Ca2+-ATPase activity and the sulfhydryl content.SDS-PAGE analysis showed that the ultrasonic treatment would destroy the structure of myosin molecule,producing many smaller molecular weight fractions.The correlation analysis showed that accumulative effect of ultrasonic time on myosin assemblies was more significant than that of ultrasonic power.These results suggested that ultrasonic treatment could disperse the assemblies in solution which decreased the size of assemblies and increased the contact chance between assemblies and water molecules.Moreover,ultrasonic treatment could decrease the interactions among assemblies which promoted unfolding of assembly structure.Highly reactive hydroxyl radicals formed during ultrasonic treatment,which could oxidize the free sulfhydryl groups.3?The effect of ultrasound-calcium coupling system were investigated on the size,structure and intermolecular forces of myosin assemblies.Calcium could promote cross link between myosin,which increased the turbidity and average size of myosin assemblies and decreased the solubility.The destabilized effect of calcium increased S0-ANS and reactive sulfhydryl content of myosin assemblies.However,ultrasound-calcium coupling system could significantly change the self-assembly property of myosin.Upon ultrasound-calcium coupling system,the turbidity and particle size of myosin assemblies increased with increasing calcium ion concentration,and ultrasonic treatment could further promote the increase in the indexes.Ultrasound-calcium coupling systems increased the assembly speed of myosin.The significant difference analysis showed that myosin assmblies formed more easily upon ultrasound(150 W,9 min)-calcium(40 mM)coupling systems.And the hydrophobic interaction among myosin assemblies increased significantly upon ultrasound(150 W,9 min)-calcium(?60 mM)coupling systems.Moreover,the aggregation rate of myosin was larger than unfolding rate upon ultrasound(150 W,9 min)-calcium(100 mM)coupling systems.Ultrasonic treatment decreased the size of myosin assemblies which may be more beneficial for interacting sufficiently between calcium ion and myosin. |
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