Effect of salt and MTGase on the production of high quality gels from farmed sea bass

Evaluation of the texture characteristics and taste of shrimp surimi with partial replacement of NaCl by non‑sodium metal salts

Ionic strength plays a significant role in the aggregation behavior of myofibrillar proteins. The study investigated the effects of KCl or CaCl2 as substitutes for NaCl on the gel properties and taste of shrimp surimi at a constant ionic strength (IS = 0.51). Increased KCl substitution ratio resulted in a reduction in α-helix content and an increase in β-sheet content of myofibrillar proteins, thereby enhancing water holding capacity. Optimal KCl substitutions (1.5% NaCl +1.94% KCl) contributed to maintaining the desired taste and improving gel properties. CaCl2 facilitates the extraction and dissolution of myofibrillar proteins, resulting in an organized and dense gel network with significant water-holding capacity. However, excessive additions (>1.27%) resulted in a notable decrease in taste and gel strength due to excessive aggregation and precipitation of myofibrillar proteins. These findings provide a solid theoretical foundation for production of high-quality, low-salt shrimp surimi.

The Effect of Salt on the Gelling Properties and Protein Phosphorylation of Surimi-Crabmeat Mixed Gels

The effects of different salt additions (1.0%, 1.5%, 2.0%, 2.5%, 3.0%, and 3.5%) on the gelling properties and protein phosphorylation of the mixed gels (MG) formed by silver carp (Hypophthalmichthys molitrix) surimi with 10% crabmeat were investigated. The MG's breaking force, deformation, gel strength, and water-holding capacity (WHC) increased as the salt concentration increased. The intrinsic fluorescence intensity of the samples initially decreased and then increased, reaching the lowest when the NaCl concentration was 2.5%. The result of SDS-polyacrylamide gel electrophoresis indicated that large aggregates were formed by protein-protein interaction in the MG containing 2.5% or 3.0% NaCl, decreasing the protein band intensity. It was also found that with the addition of NaCl, the phosphorus content initially increased and then decreased, reaching the maximum when the NaCl concentration was 2% or 2.5%, which was similar to the changing trend of actin band intensity reported in the results of Western blot. These results revealed that the amount of salt used had a significant effect on the degree of phosphorylation of the MG protein. The increase in phosphorylation was linked to improved gelling properties, which could lead to new ideas for manufacturing low-salt surimi products in the future.

Heterologous Expression of Recombinant Transglutaminase in Bacillus subtilis SCK6 with Optimized Signal Peptide and Codon, and Its Impact on Gelatin Properties

Microbial transglutaminases (MTGs) are widely used in the food industry. In this study, the MTG gene of Streptomyces sp. TYQ1024 was cloned and expressed in a food-grade bacterial strain, Bacillus subtilis SCK6. Extracellular activity of the MTG after codon and signal peptide (SP Ync M) optimization was 20 times that of the pre-optimized enzyme. After purification, the molecular weight of the MTG was 38 kDa and the specific activity was 63.75 U/mg. The optimal temperature and pH for the recombinant MTG activity were 50°C and 8.0, respectively. MTG activity increased 1.42- fold in the presence of β-ME and 1.6-fold in the presence of DTT. Moreover, 18% sodium chloride still resulted in 83% enzyme activity, which showed good salt tolerance. Cross-linking gelatin with the MTG increased the strength of gelatin 1.67 times and increased the thermal denaturation temperature from 61.8 to 75.8°C. The MTG also significantly increased the strength and thermal stability of gelatin. These characteristics demonstrated the huge commercial potential of MTG, such as for applications in salted protein foods.

Thermal and physicochemical properties of red tilapia (Oreochromis niloticus) surimi gel as affected by microbial transglutaminase

Thermal and physicochemical properties of red tilapia (Oreochromis niloticus) surimi gel incorporated with different levels of microbial transglutaminase (MTGase) were investigated. Surimi samples mixed with various concentrations of MTGase were subjected to two-stages heating (at 45°C followed by 90°C) to prepare surimi gel. Samples formulated with 0.30 MTGase (units/g surimi) showed the highest breaking force and deformation, and lowest expressible water content among treatments. Highest storage modulus was found in the gels mixed with 0.30 MTGase (units/g surimi). Compared with control surimi gel, addition of microbial transglutaminase to levels 0.10, 0.20 and 0.30 (units/g surimi) increased the enthalpy and maximum transition temperature of myosin. Results suggest that up to 0.30 MTGase (units/g surimi) could improve texture, colour, water-holding capacity, elasticity and thermal stability of red tilapia surimi gel.

Effects of pH-treated Fish Sarcoplasmic Proteins on the Functional Properties of Chicken Myofibrillar Protein Gel Mediated by Microbial Transglutaminase

pH adjustment would be of advantage in improving the water holding capacity of muscle proteins. The objective of this study was to evaluate the addition of fish sarcoplasmic protein (SP) solution, which was adjusted to pH 3.0 or 12.0, neutralized to pH 7.0, and lyophilized to obtain the acid- and alkaline-treated SP samples, on the functional properties of the chicken myofibrillar protein induced by microbial transglutaminase (MTG). The solubility of alkaline-treated SP was higher than that of the acid counterpart; however, those values of the two pH-treated samples were lower than that of normal SP (p<0.05). All SP solutions were mixed with myofibrillar proteins (MP) extracted from chicken breast, and incubated with MTG. The shear stresses of MP with acid- and alkaline-treated SP were higher than that of normal SP. The thermal stability of MP mixture reduced upon adding SP, regardless of the pH treatment. The breaking force of MP gels with acid-treated SP increased more than those of alkaline-treated SP, while normal SP showed the highest value. The MP gel lightness increased, but cooking loss reduced, with the addition of SP. Smooth microstructure of the gel surface was observed. These results indicated that adjusting the pH of SP improved the water holding capacity of chicken myofibrillar proteins induced by MTG.

The influence of fish age, salt level, and MTGase addition on the quality of gels prepared from unwashed mince of Farmed Meagre (Argyrosomus regius)

The potential of using the unwashed mince of farmed meagre as raw material for the preparation of heat-induced gel products was assessed taking into account the effect of age (small size <1 kg meagre vs commercial size >2 kg meagre), lower salt levels (1.0%, w/w, vs 2.5%, w/w), and microbial transglutaminase (MTGase) incorporation (0.0%, w/w, vs 0.5%, w/w). Heat-induced gel products from >2 kg fish were of superior quality. Salt reduction from 2.5% to 1.0% (w/w) was detrimental for textural quality, particularly, of gels prepared from >2 kg meagre mince. MTGase addition improved texture. Moreover, MTGase incorporation led to a greater importance of non-covalent hydrophobic bonding.

Effect of seasonal changes on the gelling properties of farmed sea bass (Dicentrarchus labrax)

The effect of seasonal changes (summer versus winter) upon the quality of heat-induced gel products from farmed sea bass (Dicentrarchus labrax) was studied. Moreover, the effect of microbial transglutaminase addition (0.5%, w/w) on the quality was assessed. Fat content of gel products attained from sea bass grown during summer was higher (12.0% versus 7.6%). Textural properties of the gels from the summer fish were higher. Protein of gels from winter fish was less soluble in SDS + DTT (sodium dodecyl sulphate, SDS; dithiothreitol, DTT). Contrary to some literature, higher fat level may have played a protective role during processing. Accordingly, season via fat content variation had a strong effect on the gelling ability of heat-induced gels prepared from sea bass.

Low-salt restructured fish products from Atlantic mackerel (Scomber scombrus) with texture resembling turkey breast

Atlantic mackerel (Scomber scombrus) is a pelagic and migratory species that is usually caught with other fish as bycatch. The aim of this work was to obtain low-salt restructured fish products from Atlantic mackerel resembling turkey breast using transglutaminase (0.2 U/g) as binder. NaCl concentration (0-20 g/kg), temperature (25-40 °C) and time of incubation (30-90 min) were assayed. The texture parameters (Warner-Bratzler force and Warner-Bratzler work) and expressible water were compared to those of turkey breast. Mathematical models were obtained to determine the effect of these variables on the texture of Atlantic mackerel restructured products. Optimal conditions to obtain a similar texture than turkey breast were found. The overall optimization point out that the treatment at 31.8 °C for 63.35 min using a NaCl concentration of 8.45 g/kg allowed to obtain restructured products from Atlantic mackerel with texture and expressible water similar to those of turkey breast. Color parameters (L*, a* and b*) of the product were also similar to those of turkey breast. The results showed the feasibility of producing low-salt restructured products from Atlantic mackerel resembling turkey breast using transglutaminase.

Isoelectric solubilization/precipitation as a means to recover protein isolate from striped bass (Morone saxatilis) and its physicochemical properties in a nutraceutical seafood product

Excessive dietary intake of Na (i.e., NaCl) contributes to hypertension, which is a major risk factor for cardiovascular disease. Normally, NaOH and HCl are used to dissolve and precipitate, respectively, fish muscle proteins in isoelectric solubilization/precipitation (ISP), therefore contributing to increased Na content in the recovered fish protein isolates (FPI). Substitution of NaOH with KOH may decrease the Na content in FPI and, thus, allow development of reduced-Na seafood products. In this study, FPI was recovered with ISP using NaOH or KOH. In order to develop a nutraceutical seafood product, the FPI was extracted with NaCl or KCl-based salt substitute and subjected to cold- or heat-gelation. In addition, standard nutraceutical additives (ω-3 fatty acids-rich oil and dietary fiber) along with titanium dioxide (TiO2) were added to FPI. Color, texture, dynamic rheology, Na and K content, and lipid oxidation of the FPI gels were compared to commercial Alaska pollock surimi gels. FPI gels had greater (p < 0.05) whiteness, good color properties (L*a*b*), and generally better textural properties when compared to surimi gels. Although the ISP-recovered FPI and surimi developed similar final gel elasticity, the proteins in FPI and surimi had different gelation pattern. A reduction (p < 0.05) of Na content and simultaneous increase (p < 0.05) in K content of FPI gels was achieved by the substitution of NaOH with KOH during ISP and NaCl with the KCl-based salt substitute during formulation of the FPI paste. Although cooking and addition of NaCl during formulation of the FPI paste increased (p < 0.05) lipid oxidation in FPI gels, TBARS values were much below rancidity levels. These results indicate that KOH can replace NaOH to recover FPI from whole gutted fish for subsequent development of nutraceutical seafood products tailored for reduction of diet-driven cardiovascular disease.