| Silver carp (Hypophthalmichthys molitrix) is one of major freshwater fish species which have the largest production in China. It is the strong earthy/musty taste and odour as well as containing too much intramuscular small bones that cause much low-acceptability. In order to increase its value and improve its utilization rate, most of the silver carp has been processed into surimi products. As high-salt food will destroy the health of human, consumers are demanding healthier foods and there is a great interest in low-salt surimi products, while the regular-salt content of the traditional surimi products is 2%?3%. In this paper, the changes of the nutrients contained in surimi products and the effects of microwave heating, carrageenan and three kinds of salts on the properties of low-salt surimi gels were investigated.Based on the nutrition ingredient analysis of silver carp mince and surimi, the moisture content of raw surimi increased compared with fresh fish mince, while the crude protein, crude lipid and ash decreased. In addition, the water soluble protein in raw surimi decreased by nearly 50%, while the salt soluble protein had a significant increased. The content of moisture and crude protein in frozen surimi slightly declined compared to raw surimi, and the content of crude lipid and ash increased relatively. The decreasing of salt soluble protein content in frozen surimi was due to the degeneration of salt soluble protein under low temperature. The test of amino acids indicated that silver carp mince and surimi contain large amounts of amino acids. The absolute content of amino acids in frozen surimi decreased compared to fresh fish mince, while the relative content of some amino acids increased.The optimum microwave heating conditions (microwave power 12.5 w/g, microwave time 70 s and NaCl concentration 1.25%) were found using response surface methodology (RSM) in a three-variable, three-level Box-Behnken experimental design (BBD). The gel heated by microwave was compared with the gel by conventional water-bath heating. Microwave heating increased gel strength and water holding capability (WHC) of the low-salt gels, significantly (p<0.05), as compared to water-bath heating, and had no effects on whiteness of low-salt surimi gels (p>0.05). The content of protein solubility, hydrophobic interactions and disulfide bonds indicated that the microwave heating decreased the protein solubility significantly (p<0.05), and it promoted the formation of hydrophobic interactions and disulfide bond during gelling as compared with water-bath heating. Microwave heating enhanced the cross-linking of MHC in surimi proteins, which was proved by SDS-PAGE. In addition, the microstructure of the gel heated by microwave showed more uniform, fine and compact structure than that heated by water-bath. In a word, microwave heating could effectively improve gel properties of silver carp surimi gels compared with conventional water-bath heating.Effects of carrageenan and three kinds of salts on gel properties of low-salt surimi gel from silver carp were studied. The results showed that effects of three kinds of salts on gel strength, whiteness and WHC of low-salt surimi gel were different. The effects of three kinds of salts on gel strength, whiteness and WHC were KCl>NaCl>CaCl2, CaCl2>KCl>NaCl and KCl>NaCl>CaCl2, respectively. When the carrageenan was added into these three kinds of low-salts surimi, the gel strength and WHC of the low-salt surimi gels were increased, significantly (p<0.05), but nonsignificant differences were observed in whiteness of low-salt surimi gels (p>0.05). SDS-PAGE and protein solubility analyses indicated that myosin heavy chains (MHC) of surimi gels could not form non-disulfide covalent bonds by adding carrageenan singly without salt, and the results of chemical forces showed that few hydrophobic interactions and disulfide bonds were contained in the surimi gels. The scanning electron microscope (SEM) observations showed the structures of surimi gels added carrageenan singly were irregular, disorderly, rough and uneven. Salts could promote the cross-linkings of MHC in surimi gels and formations of more non-disulfide covalent bonds, hydrophobic interactions and disulfide bonds. Carrageenan with CaCl2 could promote MHC of silver carp surimi gels to form non-disulfide covalent bonds, while the surimi gels had low content of hydrophobic interactions and disulfide bonds. The microstructures of the surimi gels added carrageenan and CaCl2 showed more holes, loose structures and poor qualities. Carrageenan with NaCl or KC1 could improve the gel properties of silver carp surimi gels, as well as its dense, uniform and meticulous structures, and better gel qualities. |
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