ROLE OF pH IN SOLUBILITY AND CONFORMATIONAL CHANGES OF PACIFIC WHITING MUSCLE PROTEINS

Insight into Ionic Strength-Induced Solubilization of Myofibrillar Proteins from Silver Carp (Hypophthalmichthys molitrix): Structural Changes and 4D Label-Free Proteomics Analysis

In this study, changes in the physical, structural, and assembly characteristics of silver carp myofibrillar proteins (MPs) at different ionic strength (I) values were investigated. Moreover, the differential proteomic profile of soluble MPs was analyzed using 4D proteomics based on timsTOF Pro mass spectrometry. Solubility of MPs significantly increased at high I (>0.3), and the increase in I enhanced the apparent viscosity, fluorescence intensity, surface hydrophobicity, and α-helix content of MPs solution. Particle size and sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns also supported the solubility profiles. Transmission electron microscopy and atomic force microscopy observations revealed the morphological assembly and disassembly of MPs under different I conditions. Finally, proteomic analysis revealed the evolution law of salt-induced solubilization of MPs and the critical molecular characteristics in different I environments. The number of differentially abundant proteins (DAPs) decreased with the increase of I, and most DAPs related to the muscle filament sliding, contraction and assembly, actinin binding, and actin filament binding. The soluble abundance of myosin and some structural proteins was dependent on I, and structural proteins in the Z-disk and M-band might contribute to the solubilization of myosin. Our findings provide insightful information about the impact of common I on the solubility pattern of MPs from freshwater fish.

Physicochemical characteristics and gel-forming properties of mandarin fish (Siniperca chuatsi) protein during the fish fermentation with Lactobacillus sake SMF-L5: The formation of garlic-cloves shaped protein gel

Mandarin fish (Siniperca chuatsi) during fermentation presents a unique elastic texture. In this investigation, the physicochemical and gel-forming properties of fish proteins were evaluated to explain the formation of elastic characteristics. During fermentation, the combined effects of acidification by Lactobacillus sake SMF-L5, increased sodium chloride, and decreased moisture content in the fish protein generated a suitable microenvironment for gelation. The mass transfer of sodium chloride was accompanied by NMR relaxation of the immobilized water. The ripening fermented fish had a functionally available MHC, a higher fractal dimension, and a stable α-helical structure. Also, it exhibited excellent gel-forming performances, mainly including garlic-cloves shaped protein gel, stronger springiness, and enhanced L* and whiteness. Correlation analysis showed that the gel's physical properties were differently related to the protein's physicochemical characteristics except for total free amino acids. These results could lay a theoretical foundation for the gel formation mechanism of fermented mandarin fish.

Effects of Potato Protein Isolated Using Ethanol on the Gelation and Anti-Proteolytic Properties in Pacific Whiting Surimi

Pacific whiting is a primary species utilized for surimi processing in the Pacific Northwest of the US. However, endogenous protease in Pacific whiting surimi deteriorates the quality during slow cooking. The demand for clean-labeled and economically competitive protease inhibitors has been increasing. In the present study, the anti-proteolytic effect of potato protein isolate (PPI), a by-product from the potato starch industry, prepared using 20% ethanol on the endogenous protease activity of Pacific whiting (PW) surimi was investigated. The ohmic heating method was carried out for a better assessment of the anti-proteolytic activity of inhibitors. A factorial design was carried out in which the independent variables were the four types of inhibitors and their concentration (0, 0.5, 1, 2, and 3% w/w) at two heating conditions. The heating condition was used as a blocking factor. All experiments were randomized within each block. The addition of 2% PPI which demonstrated the highest anti-proteolytic activity among five different concentrations significantly increased the breaking force, penetration distance, and water retention ability of PW surimi gel as the endogenous proteases were effectively inhibited when heated ohmically at 60 °C for 30 min prior to heating up to 90 °C. In addition, SDS-PAGE disclosed that PPI successfully retained the intensity of myofibrillar heavy chain (MHC) protein of PW surimi gels even under the condition at which proteases could be activated at 60 °C. The whiteness of gels was not negatively affected by the addition of PPI. Comparing all samples, a denser and more ordered microstructure was obtained when PPI was added. A similar trend was found from the fractal dimension (Df) of the PPI-added gel’s microstructure. Therefore, PPI could be an effective and non-allergenic protease inhibitor in PW surimi leading to retaining the integrity of high gel quality.

Trace the difference driven by unfolding-refolding pathway of myofibrillar protein: Emphasizing the changes on structural and emulsion properties

The effective strategy of pH-shifting to improve the emulsifying properties of myofibrillar proteins (MPs) extracted from pale, soft and exudative (PSE)-like chicken was investigated. To determine the mechanism of improvement, changes on structural and physicochemical properties were clarified by tracing the difference driven by unfolding-refolding process. According to the results of tryptophan fluorescence intensity and circular dichroism spectroscopy, it is found that unfolding-refolding process markedly changed MPs secondary and tertiary structure. The atomic force microscopy images showed MPs appeared to have fibrous-like appearance at pH 7.0, however, exhibited as spherical shape after pH-shifting. Both emulsifying activity index and emulsifying stability index increased after pH-shifting. These results systematically illustrated the changes on structural and emulsion properties of MPs during unfolding-refolding process. It proved that the strategy pH 11.0-7.0 could more effectively promote MPs emulsifying properties, whose mechanism was simultaneously the transformation in MPs structure and potentially formation of highly-soluble particle.

Influence of the Isolation Method on the Technofunctional Properties of Protein Isolates from Lupinus angustifolius L

The technofunctional properties of 2 protein isolates from Lupinus angustifolius L. Vitabor isolated by different procedures were investigated. The lupin protein isolate prepared by aqueous alkaline extraction with subsequent isoelectric precipitation (ILP) showed a significantly higher degree of protein denaturation and lower denaturation temperatures than the one obtained by aqueous salt-induced extraction followed by dilutive precipitation (MLP) as determined by differential scanning calorimetry. Rheological investigations revealed higher firmness and a viscoelastic solid-like behavior of ILP, in contrast to MLP that showed viscoelastic, liquid-like properties. Protein solubility of MLP was higher compared to ILP and solubility minima were slightly different for both lupin protein isolates. The protein isolates exhibited different technofunctional properties with ILP showing higher water binding capacity, lower oil binding capacity and lower emulsifying capacity than MLP. This reflects the different putative application of both lupin protein isolates as food ingredients, for example for ILP as a moisture enhancer and for MLP as a "natural" emulsifier in mixed food systems.

Chemical and Functional Characterization of Sarcoplasmic Proteins from Giant Squid (Dosidicus gigas) Mantle

Modification of pH and NaCl concentration changed the physicochemical properties of sarcoplasmic proteins (SP) from jumbo squid mantle and consequently their functional properties. Better results of emulsifying activity index (EAI) and foam capacity (FC) were exhibited at pH 11 in NaCl absence due to higher solubility. But better emulsifying stability index (ESI) was obtained at pH 11 in 0.5 M NaCl, while, foaming stability (FS) was better at pH near to isoelectric point (pI). These results suggest that SP from jumbo squid may be a promising ingredient, whose functional properties can be manipulated by changing pH and NaCl concentration.

Chemical composition and functional properties of mrigal (Cirrhinus mrigala) egg protein concentrates and their application in pasta

Protein concentrates were prepared from underutilized mrigal (Cirrhinus mrigala) roe to produce value added by-products for food applications. Chemical composition and physicochemical properties of protein concentrates prepared from mrigal (Cirrhinus mrigala) roes were determined. The effects of pH and salt concentration on protein solubility were investigated. The protein content of the concentrates was found to be 75, and 91%, respectively for dehydrated and defatted powders. The maximum protein solubility was observed at pH 12, while minimum protein solubility was observed at pH 5, for both dehydrated and defatted protein concentrates. Salt concentration (0.1 to 1 M Nacl) significantly affected the solubility of protein concentrates. The mineral analysis revealed substantial amounts of iron and phosphorus. The emulsifying capacities of dehydrated and defatted protein concentrates were noted as 5.9, and 7.1 ml/g protein, respectively. SDS-PAGE analysis of fresh, dehydrated and defatted roe proteins has revealed the presence of major protein with a molecular weight (MW) of 97 kDa. Addition of defatted fish egg protein concentrate to pasta preparations had improved taste and texture. The results indicated that protein concentrates from underutilized mrigal fish egg may be useful for preparing protein rich food supplements.

Structural changes and dynamic rheological properties of sarcoplasmic proteins subjected to pH-shift method

Structural changes and dynamic rheological properties of sarcoplasmic proteins from striped catfish ( Pangasius hypophthalmus ) treated by various pH-shift processes were investigated. Isoelectric precipitation of acid-extracted sarcoplasmic proteins led to the lowest solubility in water. Sarcoplasmic proteins were unfolded after extremely acidic and alkaline extraction, exposing tryptophan and aliphatic residues. The alpha-helical structure was converted to beta-sheet following acidic extraction, whereas alkaline treatment did not disturb the alpha-helical structure of sarcoplasmic proteins. Disulfide formation, hydrogen bonding via tyrosine residues, and hydrophobic interactions occurred under extreme pH extraction. Acidic extraction induced denaturation and aggregation of sarcoplasmic proteins to a greater extent than did alkaline treatment. Hydrophobic interactions via aliphatic and aromatic residues were formed during isoelectric precipitation. Sarcoplasmic proteins were partially refolded after isoelectric precipitation followed by neutralization. Sarcoplasmic proteins prepared from an alkaline pH-shift process readily aggregated to form a gel at 45.10 degrees C, whereas higher thermal denaturation temperatures (>80 degrees C) and gel points ( approximately 78 degrees C) were observed in acid-treated sarcoplasmic proteins. The pH condition used for extraction, precipitation, and neutralization greatly affected structural changes of sarcoplasmic proteins, leading to different thermal and dynamic rheological properties.

Protein recovery from rainbow trout (Oncorhynchus mykiss) processing byproducts via isoelectric solubilization/precipitation and its gelation properties as affected by functional additives

Solubility of rainbow trout proteins was determined between pH 1.5 and 13.0 and various ionic strengths (IS). Minimum solubility occurred at pH 5.5; however, when IS = 0.2, the minimum solubility shifted toward more acidic pH. Isoelectric solubilization/precipitation was applied to trout processing byproducts (fish meat left over on bones, head, skin, etc.), resulting in protein recovery yields (Kjeldahl, dry basis) between 77.7% and 89.0%, depending of the pH used for solubilization and precipitation. The recovered protein contained 1.4-2.1% ash (dry basis), while the trout processing byproducts (i.e., starting material) 13.9%. Typical boneless and skinless trout fillets contain 5.5% ash, and therefore, the isoelectric solubilization/precipitation effectively removed impurities such as bones, scales, skin, etc., from the trout processing byproducts. The recovered proteins retained gel-forming ability as assessed with dynamic rheology, torsion test, and texture profile analysis (TPA). However, the recovered proteins failed to gel unless beef plasma protein (BPP) was added. Even with BPP, the recovered protein showed some proteolysis between 40 and 55 degrees C. Addition of potato starch, transglutaminase, and phosphate to the recovered proteins resulted in good texture of trout gels as confirmed by torsion test and TPA. Higher ( P < 0.05) shear stress and strain were measured for gels developed from basic pH treatments than the acidic counterparts. However, proteins recovered from acidic treatments had higher ( P < 0.05) lipid content than the basic treatments. This is probably why the gels from acidic treatments were whiter ( L* - 3 b*) ( P < 0.05) than those from the basic ones. Our study demonstrates that functional proteins can be efficiently recovered from low-value fish processing byproducts using isoelectric solubilization/precipitation and subsequently be used in value-added human foods.