Studies On Aggregation Of Myofibrillar Protein And Properties Of Surimi Gel From Silver Carp And Grass Carp

Silver carp (Hypophthalmichthys molitrix) and grass carp (Ctenopharyngodon idellus) are the two most commonly cultured fish species for the Chinese fresh water fisheries, but their application in surimi food is limited because of their unique protein structure and characteristics. How to utilize the conventional low-value freshwater fish to produce surimi food of high quality has been becoming meaningful in aquatic products processing research field. In order to solve the main basic research problems, we first need to clearly understand and elaborate the quality changes of surimi gel for obtaining the optimal conditions and the effective way to improve the quality. The environmental effects on aggregation of myofibrillar protein and the influence of processing conditions on rheological properties and texture of surimi gel were investigated in this paper using silver carp and grass carp as raw materials. The image processing techniques was applied to obtain surimi gel microstructure parameters, and a relationship between texture and microstructure parameters was established. Dynamic rheometer and low-field nuclear magnetic resonance (NMR) were applied to determine gelation kinetics and water state during surimi processing, respectively. These studies laid the foundation for the better understanding of mechanism of surimi gelation and the controll of surimi product quality. The main results were as follows:1. Studies on the effects of environment condition on the functional properies of myofibrillar protein from sliver carp. An increasing of protein solubility, foaming capacity, and a decreasing in solution turbidity were observed with the increasing of pH and sodium chloride concentration. The effect of calcium chloride concentration on myofibrillar protein functional properties varied with temperature. CaCl2 enabled increasing of turbidity of myofibrillar protein solution, especially at 40℃.2. Studies on the effects of environment condition on myosin aggregation. Concentration-effect relationship existed during myosin aggregation. The aggregation of silver carp myosin is reversible when temperature is below 40℃. The pH of solution had less effect on the turbidity of grass carp myosin. CaCl2 can promote the aggregation of myosin. Intrinsic viscosity of myosin decreased with increasing of temperature, and the intrinsic viscosity of silver carp myosin was higher than that of grass carp. The effect of CaCl2 concentration on the intrinsic viscosity of myosin had a concentration-effect relationship. The intrinsic viscosity and the relative viscosity of the silver carp fish myosin was higher than that of grass carp with the pH range of 6-7.5 and CaCl2 concentration of 20-60mmol/L.3. Studies on the stress relaxation characteristics of freshwater fish surimi gel. Maxwell model can be used to describe the process of stress relaxation of surimi gel, and decay coefficient E1, balance coefficient of elasticity E2 gradually increased with the increasing of water content, and relaxation timeτreached the minimum at water content of 84.0%. The setting temperature had greater effects on the stress relaxation properties of silver surimi gel than on those of grass surimi gel, the three parameters reached the maximum at the setting temperature of 40℃. Cooling method had less effect on the relaxation properties. When CaCl2 addition was 80mmol/Kg, both of E1 and E2 reached the maximums. Both of E1 and E2 gradually decreased with the increasing of NaCl addition, and relaxation timeτreached the minimum at NaCl addition of 2.5%. Storage period could affect the stress relaxation characteristics of freshwater fish surimi gel.4. Studies on the creep characteristics of freshwater fish surimi gel. Burgers model can be used to describe the creep characteristics of surimi gel. The creep characteristics of silver carp and grass carp surimi gel differed from each other. The setting temperature affected the creep properties of surimi gels, and the suitable setting temperature was 40℃. Cooling method had greater impact on the relaxation time than on other parameters. The creep characteristics of silver carp and grass carp surimi gel were affected by the addition of calcium chloride and sodium chloride. The general elastic deformation modulus of silver carp and grass carp surimi gel decreased with the increasing of water content, and other creep parameters almost unchanged. During the period of storage, the creep characteristics had little change.5. Thermorheological behavior of silver carp and grass carp surimi was evaluated during gelation at different water contents. There are three stages of changes of elastic modulus G’ during heating process. The G’ of surimi decreased with the increase of water content, and the G’ of silver carp surimi is higher than that of grass carp surimi at water content from 82.5% to 92.5%. The gelation temperature (Tgel) of surimi increased with the increase of water content, and Tgel of silver carp surimi was higher than that of grass carp surimi. The enthalpy of cross-links formation (H) of silver carp and grass carp surimi were estimated to be 502.2 kJ/mol and 375.1kJ/mol according to an Arrhenius-type kinetic model. The activation energy (Ea) for aggregation after gelation temperature varied from183.1 to 262.0 kJ/mol with water content for silver carp surimi, and from 113.6 to 155.5 kJ/mol for grass carp surimi. The Ea of silver carp surimi is higher than that of grass carp surimi, which implied that the gelation of grass carp surimi is easier than that of silver carp surimi.6. Studies on the effects of the processing conditions on surimi gel properties and microstructure. The hardness and water holding capacity (WHC) decreased with the increasing of water content, and the hardness of silver carp surimi gel was higher than that of grass carp surimi gel. The dispersion of protein aggregation increased with the increasing of water content. The hardness and WHC increased with the increasing of NaCl addition from 1.5% to 3.5%, and the springiness of surimi gel firstly increased and then decreased. Protein aggregates microstructure varied with NaCl addition. The hardness, springiness and WHC of surimi gel reached the maximum at the setting temperature of 40℃, and the compact gel was obtained. The hardness, springiness and WHC of surimi gel reached the minimum at the setting temperature of 60℃, and the stripped gel was obtained. The CaCl2 addition affected the quality of surimi gel and microstructure. The springiness of gel reached the maximum with the CaCl2 addition of 20mmol/kg, and the uniform gel was obtained. Texture characteristics of surimi gel correlated with gel microstructure parameters (fractal dimension, lacunarity, pore diameter) at different significant level.7. Low-field magnetic resonance (NMR) and scanning electron microscopy (SEM) were applied to determine water state and microstructure of surimi gel, respectively. In surimi gel, the spin-spin relaxation time of water]H (T2) is affected significantly by water content; T2 relaxation measured has been shown to linearly correlate with the water content of surimi. In the heating process, the T2 value started to decrease firstly and then to increase. The NMR data was fitted to one component at heating temperature of 40℃and two components at heating temperature of 90℃. Both of T21 and T22 increased with the increase of water content, and so for the populations for the component with the longer relaxation time. Scanning electron micrographs (SEM) showed the water state of the protein gel network was related to gel microstructure.