Studies On Silver Carp Surimi Gel And Gel-forming Mechanism

In China, Silver carp, one of the main fresh-water fish species, abundant and cheap, is a good material to develop and process surimi-based product. Compared with briny surimi, researches of silver carp surimi focused on influences of washing conditions, processing technics and additives on surimi gel properties and frozen denaturation, but little on the gel-forming mechanism. Thus, studies on affecting factors of silver carp surimi gel forming and gel forming mechanism are very important to promote the formation of surimi gel and develop silver carp surimi-based products with high quality.Setting conditions and relative ground works of silver carp surimi, influences of aggregation behavior and confirmation of myosin on the formation of silver carp surimi gel, and effects of soy protein isolate, egg albumin, MTGase, starches and some hydrocolloids on gel properties of silver carp surimi were investigated. The results were as follows:1. Studies on the basis of silver carp surimi gelation Based on the gel-forming conditions, the change of silver carp protein composition during gel forming and effects of transglutaminase activator and inhibitors on the forming of silver carp surmi gel were investigated. During the forming of silver carp surimi gel, the content of salt-soluble protein decreased gradually, while the content of insoluble protein increased correspondingly starting from smashing due to myosin heavy chain cross-linking catalyzed by transglutaminase and forming high molecule weight polymer. Three kinds of tmnsglutaminase inhibitors, NEM, EDTA, and AC, decreased gel properties of silver carp surimi. Low Ca²⁺ concentration (20mmol/kg) activated transglutaminase activity and promoted myosin heavy chain cross-linking, leading to improving silver carp surimi gel properties. However, high Ca²⁺ concentration depressed myosin heavy chain cross-linking induced by transglutaminase, and conduced to the formation of Ca-bridge between Ca²⁺ and protein, and also the hardness increased and the elasticity decreased for silver carp surimi gel. Effect of transglutaminase activator Ca²⁺ on gel properties of silver carp surimi had two kinds of mechanism. Gel strength of silver carp surimi was highest when the Ca²⁺ adding dosage was 20 mmol/kg.The setting temperature and time affected gel properties of silver carp surimi significantly. The optimum setting temperature and time were 40℃and 60min, respectively. Breaking force, deformation, and gel strength of silver carp surimi gel were 470.50g, 11.32ram and 5331.53g×mm, which were 1.29, 1.09, 1.35 times of that setting for 30min, respectively. But the longer setting time leaded to the decreasing of surimi gel strength. Setting time was decided by acyl-transfer reaction induced by transglutaminase and hydrolysis of surimi protein by endogenous protease. The chemical interactions and protein structures in order to maintain surimi protein structure and stability changed during surimi gel forming, which were correlated closely to gel forming procedure and surimi gel properties, Hydrogen bonds and ion bonds decreased significantly, while disulfide bonds and hydrophobicity increased significantly with surimi gel forming. And non-disulfide covalent bonds were formed. The main chemical interactions to maintain silver carp surimi gel network structure were disulfide bonds, non-disulfide covalent bonds and hydrophobicity. Ramman spectra results of surimi after setting 60min and then cooking 30min also showed that the band at 1664 cm^-1 indicating random coil also occurred, besides the band at 1653 cm^-1 indicatingα-helix structure of raw surimi protein; the band density near 530 cm^-1 and 540 cm^-1 indicating disulfide bonds increased; I850/830 ratio increased from 0.98 to 1.01 and 1.05, respectively indicating the increasing of hydrophobicity. This result was consistent with the change of chemical interactions.2. The aggregation behavior, configure and gel-forming mechanism of myosin from silver carp Effects of environmental factors on turbidity and solubility of myosin and myosin gel-forming conditions were investigated by turbidity method, dynamic rheology, and scanning electron microscope. Temperature, pH value, sodium paraphosphate, sodium tripolyphosphate and sodium hexa-metaphosphate could affect myosin solubility. Myosin had good solubility at 30℃or 40℃, and pH from 6.0 to 8.0. Sodium paraphosphate, sodium tripolyphosphate, and sodium hexa-metaphosphate with the concentration from 0.25% to 1.0% could increase solubility of myosin. The forming conditions of myosin gel, the critical concentration and optimum pH were 10mg/mL and 7.0, respectively. Viscoelasticity of myosin gel increased with myosin concentration being up. Dense and uniform gel structure did not form at pH higher or lower than 7.0. Low Ca²⁺ concentration enhanced gel-forming ability of myosin.The correlation among myosin denaturation, conformation change and surimi setting was investigated by CD, DSC, SEM. During setting at 40℃, the ratio ofα-helix structure of myosin reduced gradually and finally decreasing by 67.11%, which was 27.00% when setting for 60min, compared with 94.11% before setting. Meanwhile, the ratio of turn and random coil increased by 10.40% and 56.71% to 10.40% and 62.60%, respectively. A majority ofα-helix structure changed into turn. Further heating at 90℃for 30min and cooling to 4℃, the ratios of a-helix, turn and random coil of myosin were 33.70%, 12.40% and 53.90%, respectively and the main secondary structure was random coil. During the forming of myosin gel, myosin molecules extended and changed into random coil contributed to myosin heavy chain cross-linking. At the same time, the results of chemical measurements showed that -SH in myosin head was oxidized into disulfide bond, the content of which increased from 0.6 mol/10⁶g to 1.7 mol/10⁶g. And surface hydrophobicity increased from 8946.78 to 29152.33, increased by 2.26 times. Myosin denatured and aggregated into ordered aggregation after heating at 40℃for 60min and 90℃for 30min. DSC analysis showed that myosin had two phase transition temperature 43.32℃and 51.59℃. The former was corresponding to setting temperature of silver carp surimi. Setting temperature of silver carp surimi was actually the first peak temperature of myosin denaturalization..3. Effects of MTGase, soy protein isolate, and egg albumin on the formation of silver carp surimi gel MTGase could improve gel strength, water-holding ability, and whiteness of surimi gel. The optimum adding dosage of MTGase was 6-10U/g fish protein. Breaking force, deformation, and gel strength were 793.00g, 14.21mm, 11867.20g×mm, increasing by 1.12, 0.21, 1.7 times, respectively. But higher dosage decreased gel properties. When adding dosage of soy protein isolate and egg albumin were 3% and 10%, respectively, silver carp surimi gel properties and water-holding ability could be improved. However their improving mechanisms were different. Soy protein isolate improved gel properties because it might inhibit protease activity and gel by itself, while egg albumin was due to inhibiting serine protease and promoting myosin heavy chain cross-liking. These two kinds of proteins depressed the formation of MTGase-induced gel, so that gel properties of silver carp surimi were lower than that with MTGase only. Thus, soy protein isolate or egg albumin and MTGase could not use at same time during surmi-based products manufacturing.4. Effects of starches and hydrocolloids on the formation of silver carp suimi gel Effects of starches with different sources and modified methods on the gel formation of silver carp surimi were also investigated. Starch of different sources had different influences on surimi gel. Potato starch had the best improving effective among potato starch, tapioca starch, and corn starch. Breaking force, deformation, and gel strength of silver carp surimi gel with potato starch increased by 0.22, 0.54 and 0.45 times than that of control, respectively. Surimi gel properties might be improved through adding cross-linked and esterified starch, acetylated starch, phosphate estered starch, and hydropropylated starch, but were decreased by oxidized starch and micronized starch. Hydropropylated starch and cross-linked and esterified starch were suitable for processing surimi-based products. Better surimi gel properties could be obtained by starch with shorter gel length, lower solubility and bigger swelling power.Carrageenan, xanthan gum, konjac flour, and guar gum had different influences on silver carp surimi gel properties. 0.5% carrageenan or 0.1% guar gum obviously increased hardness and gel strength of surimi gel, decreased gumminess and brittleness, and improved color. However, gel strength would decrease when adding xanthan gum and konjac flour to surimi in natural pH.