| Black carp(Mylopharyngodon piceusis) is one of the ―four major carps‖, which is widely distributed in the Yangtze River and south of China. In 2013, the production of black carp was 52.5 Wt. Black carp is a carnivorous fish. Due to its fast growing, high flesh rate and superior gel forming ability, it is one of the best freshwater fish materials for surimi production. Previous studies on black carp were mainly focused on the nutritional components, gel properties, collagen properties of fish skin. However, there were no systematic research on the differences of chemical compositions, protein compositions, and gel properties between black carp and other freshwater fish. In our work, the chemical compositions, protein compositions and gel properties of seven kinds of surimi(black carp, grass carp, silver carp, bighead carp, common carp, crucian and gurnard) were investigated by CD, FT-Raman, SEM, Zeta potential analyzer, and dynamic rheological analysis. The effect of Ca2+ and MTGase on denaturation, aggregation and gel properties from black carp and silver carp were investigated. The objective was to study the mechanisms of the different gelation between black carp and other freshwater fish, and establish a theoretical basis to produce high quality surimi gels. The main results are as follows:1. The chemical compositions, protein compositions of fish muscle and surimi from black carp and other freshwater fish(grass carp, silver carp, bighead carp, common carp, crucian and gurnard) were analyzed. The chemical compositions, protein compositions, amino acid compositions, TGase of fish muscle and surimi showed differences between black carp and other freshwater fish. The crude protein content of the seven freshwater fish were around 83.16~89.90%(dry matter). The content of crude fat in black carp muscle was higher while the content of ash was lower than the other six fish muscle. SDS-PAGE showed that the distributions of protein bands were similar among seven freshwater fish, however, the protein content of some species were different.Six bands showed the higher content, the molecular weight distribution was between 220-130 KDa and 43-34 Kda. The highest content of salt soluble protein, and the lowest content of water soluble and insoluble protein existed in black carp muscle among the seven freshwater fish. The highest TGase activity of fish muscle was from black carp, followed by common carp and gurnard, silver carp, bighead carp, grass carp and crucian. The appropriate rinse could reduce the content of water soluble protein, insoluble protein, mineral and TGase activity, while significantly improved content of salt soluble protein. SDS-PAGE showed that 43-28 KDa of water soluble protein were mainly eliminated by rinsing.2. The gel properties of black carp surimi and other six freshwater fish were analyzed. The gel properties of surimi gels exhibited differences from seven freshwater. Breaking force of black carp surimi gels were significantly higher than grass carp, silver carp, bighead carp and crucian, while significantly lower than common carp and gurnard. Penetration distance of black carp surimi gels were significantly higher than grass carp, silver carp and crucian, while significantly lower than bighead carp and common carp. Among seven freshwater, black carp surimi gels had higher breaking force, penetration distance, water holding capacity, brightness, and whiteness. PLSR showed that the higher content of salt soluble protein, Glu, Lys and Ca, higher TGase activity were accompanied with higher gel properties. While the higher content of insoluble protein was negatively correlated with gel properties. The higher content of Mg and water soluble protein resulted in lower brightness and whiteness.3. The difference of thermal gelation properties from black carp and silver carp myosin were investigated by CD, laser particle analyzer, and dynamic rheological analysis. The amino acid compositions, denaturation and aggregation, thermal gelation properties and secondary structure showed considerably differences. The content of cystine, tyrosine and proline had significantly differences, the proportion of hydrophobic amino acids of black carp myosin(actomyosin) was higher than silver carp. In the process of heating, the changes of turbidity, dz, solubility, and S0-ANS showed that black carp myosin had higher hydrophobic interactions than that of silver carp. Moreover, the denaturation rate of black carp myosin was faster than silver carp, its dz was nearly 2 times of silver carp myosin. Thus, the G′ of black carp myosin significantly higher than that of silver carp. The a-helical content of black carp myosin decreased and lower than silver carp with the increasing temperature, while the content of β-sheet and β-turn were higher than that of silver carp. Regression model for G’ and α-helical, β-sheet and β-turn contents of surimi indicated that the unfolding of α-helices and the formation of β-sheets facilitated the gelation of both myosin. The thermal gelation properties of black carp myosin were superior to silver carp myosin.4. Effects of CaCl2 on gel properties of black carp and silver carp surimi gels were investigated by TAPlus, FI-Raman, SEM, CD and dynamic rheological analysis. CaCl2 improved the gel properties and dense structure of black carp and silver carp surimi. Black carp surimi gels exhibited different gel properties with silver carp as affected by CaCl2. Surimi gels from black carp and silver carp exhibited their maximum breaking force and penetration distance when 80-100mmol/kg and 40mmol/kg CaCl2 were added. More ε-(γ-Glu)-Lys covalent bonds and disulfide bonds in black carp gels were promoted by Ca2+. During setting, the appropriate concentration of Ca2+ promoted the unfolding of myosin, myosin denaturation and the subsequent aggregation via hydrophobic interactions and disulfide bonds. Moreover, Ca bridges also formed in myosin aggregates. However, the high aggregation rate and salt out resulting from high Ca2+ might lead to random aggregation and formation of coarse protein networks, which were associated with poor gelling properties.5. Effects of MTGase on gel properties of black carp and silver carp surimi gels were investigated by TAPlus, FI-IR, and dynamic rheological analysis. The stress relaxation and creep properties of black carp and silver carp gels were well fitting by Maxwell and Burgers. S-E0, S-E1, C-E1, C-E2 increased while expressible moisture content and whiteness decreased indicating that the formation of more ε-(γ-Glu)-Lys covalent bonds were formed by MTGase. The shearing, compression and puncture properties increased and then decreased along with the increase TGase, they reached the highest when 28.08U/100 g surimi were added. Breaking force and penetration distance of black carp gels were higher than silver carp as affected by MTGase. The γ-carboxyamide and amino cross-links catalyzed by MTGase were constructed in suwari stage and contributed to the network formation. Meanwhile, disulfide bonds and fewer non-covalent bonds were formed, due to the unfolding of protein. When incubation at 90°C, kamaboko gels had no more γ-carboxyamide and amino groups cross-linking due to the MTGase inactivation. However, kamaboko gels had more disulfide bonds and hydrophobic interactions, which promoted the water extrusion. |
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