Changes in chemical interactions and gel properties of heat-induced surimi gels from silver carp (Hypophthalmichthys molitrix) fillets during setting and heating: Effects of different washing solutions
Graphical abstract
Introduction
Surimi is a stabilized myofibrillar protein paste mainly obtained from fish with good gel-forming ability and high nutritional value, which is generally used as a raw material for lots of highly-valued, restructured fish products and enjoys great popularity worldwide (Moreno, Herranz, Perez-Mateos, Sanchez-Alonso, & Borderias, 2016). Although traditionally produced from marine fish species, in recent years, freshwater fish surimi has arouse great interest in the aquatic processing industry because of their fast growth rate and relatively low price. With the production of 4,967,739 tons in 2014, silver carp (Hypophthalmichthys molitrix) is one of the most important freshwater fish species in the world, and it is gradually being viewed as a potential alternative to marine fish in surimi processing (FAO, 2016, p. 30).
The washing process is usually recognized as a crucial step that determines the quality of surimi for consumption. Conventional surimi manufacturing involves continuous washing of the fillet muscle with chilled water to remove blood, pigments, connective tissue, off-flavor substances, and sarcoplasmic protein, and finally increases the concentration of myofibrillar protein (Paker & Matak, 2015). To enhance the quality of surimi, different washing solutions and washing processes have been extensively studied, which included low concentration of sodium chloride (NaCl), sodium bicarbonate (NaHCO3), sodium phosphate buffer, and their combination, which resulted in variations in the microstructure and gel properties (Balange and Benjakul, 2009, Ramadhan et al., 2014). Magnesium chloride (MgCl2) and calcium chloride (CaCl2) are known to alter enzymatic activities and the conformation of protein during gelation, which leads to changes in gel properties (Barat et al., 2013, Hamzah et al., 2015). Moreover, Ca2+ plays a major role in the gelation process, because of the activation in the acyl transfer reaction between glutamine and lysine by affecting transglutaminases (TGase) and the promotion of hydrophobic interactions in actomyosin (Yongsawatdigul & Sinsuwan, 2007). Therefore, washing with MgCl2 and CaCl2 has important implications for surimi processing.
Heat treatment accelerates the formation of the protein network via the function of covalent interactions and non-covalent bonds such as hydrogen bonds, electrostatic interactions, and hydrophobic interactions in surimi gel processing (Totosaus, Montejano, Salazar, & Guerrero, 2002). In previous studies of several surimi gels from different freshwater fish species, the concentrations of non-covalent bonds, sulfhydryl groups, and disulfide bonds exhibited significant correlations with gel properties such as breaking force, expressible water content (Ding et al., 2011), cooking loss, and dynamic rheological properties (Jia, Huang, & Xiong, 2016). Nevertheless, only few studies have focused on the combined effects of different washing solutions on gel properties and chemical interactions in surimi gels and their correlations during setting and heating procedure.
The aim of the present study was to evaluate washing with MgCl2 and CaCl2 (at different concentrations) on changes in gel properties (breaking force, deformation, gel strength, whiteness, and expressible water content), protein profiles (SDS-PAGE analysis), non-covalent bonds (ionic bonds, hydrogen bonds, hydrophobic interactions, and they were expressed as the percentage of total non-covalent bonds), sulfhydryl groups, and disulfide bonds at specific setting and heating intervals (every 15 min at 40 °C and 10 min for 90 °C). In addition, we aimed to determine the main chemical interaction and their correlations to quality indicators, which could provide a comprehensive understanding in surimi gel processing.
Section snippets
Preparation of silver carp surimi gel
Thirty-six silver carp (weight 1420 ± 180 g, length 49.50 ± 1.50 cm) were purchased from a local wholesale aquatic market in Beijing, China in May 2016, and immediately transported to the laboratory alive with water. After one or two blows on the head, fish were scaled, gutted, decapitated, filleted into two pieces, washed in flowing tap water, and placed onto a stainless steel frame for draining. Then the dorsal part of the flesh fillets was manually collected and minced. After that, minced
Calcium and magnesium contents
Calcium and magnesium contents of washed surimi both increased with higher CaCl2 and MgCl2 concentrations in washing solutions. The contents of calcium and magnesium of surimi washed by distilled water were 828 mg/kg and 112 mg/kg, respectively. Higher calcium and magnesium contents were 989 mg/kg and 167 mg/kg after 0.1% CaCl2 and 0.1% MgCl2 washing, respectively. The calcium content was 1211 mg/kg of the surimi washed by 0.2% CaCl2, and magnesium content was 338 mg/kg in surimi after 0.2% MgCl
Conclusion
The present study indicated that different washing solutions significantly (P < 0.05) affected the chemical interactions and gel properties during setting and heating, especially after washing with 0.2% CaCl2, which enhanced the breaking force and gel strength, and exhibited better whiteness and expressible water content than other treatments. As for chemical interactions during gelation, ionic bonds and sulfhydryl groups decreased continuously in all washing solutions as setting and heating
Acknowledgment
This study was supported by the Earmarked Fund for China Agriculture Research System (CARS-45), the State Key Research and Development Plan “Modern Food Processing and Food Storage and Transportation Technology and Equipment” (No. 2017YFD0400200), the National Natural Science Foundation of China (Award No. 31471683), and Tianjin Project of Transformation and Extension of Agricultural Science and Technology Achievements (No. 201404020). We would like to thank Thomas A. Gavin, Professor Emeritus,
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