Inhibition of protease in intact fish fillets by soaking in or injection of recombinant soy cystatin or bovine plasma

Injection of Natural Protease Inhibitors and Evaluation of Their Impact on Cooked Pacific Whiting (Merluccius productus) Fillets

Brine injection of natural proteases, dried egg white (EW), and dried potato extract (PE), was investigated as a means to prevent softening during cooking of Pacific whiting fillets. Treatments included fillets injected with Water (W), 3% sodium chloride and 3% sodium tripolyphosphate brine (B), B with 0.1% xanthan gum (B_XG ), B with 1%, 2%, or 3% EW (B_1-3EW ), B with 1%, 2%, or 3% PE (B_1-3PE ) or non-injected (NI). Non-injected Pacific cod (Cod) was also utilized as a reference for texture analysis. Fillets were subsequently cooked using cooking protocol 1 (90 °C for 20 min) or cooking protocol 2 (60 °C for 30 min then 90 °C for 20 min). Cooked fillet pH, moisture, total protein, total extractable protein (TEP), total non-extractable protein (TNEP), texture profile analysis (TPA), and electrophoretic pattern (SDS-PAGE) were measured. Cooking protocol 2 significantly reduced moisture, increased total protein, TEP and TNEP. Loss of moisture suggested cooking protocol 2 promoted proteolysis. Electrophoretic evaluations of the fillets treated with the cooking protocol 2 confirmed proteolysis was enhanced by cooking protocol 2 and myosin band integrity was protected in fillets containing natural protease inhibitors. For TPA, there was no significant difference between protease inhibitor types although means for B_EW did trend higher than B_PE . However, PCA analysis clearly demonstrated B_2EW and B_3EW fillets were most similar to Cod. Results demonstrated brine injection with protease inhibitors prevented fillet softening during cooking.

PRACTICAL APPLICATION

Texture softening in fish has typically been addressed by diverting product to lower-value minced applications where ingredients can be blended-in to counteract softening caused by enzymes. Investigations demonstrate feasibility of brine injection of protease inhibitor ingredient, dried egg white, to preserve protein integrity during cooking. Dried potato extract required a suspension aide, xanthan gum, in order to become sufficiently suspended in the brine for injection. Dried potato extract was able to inhibit protease activity, however, the suspension aide negatively impacted protein-protein interactions during cooking. An alternative suspension aide is therefore required for dried potato extract.

Proteolysis and Its Control Using Protease Inhibitors in Fish and Fish Products: A Review

Texture is one of the food quality attributes affecting the consumer's acceptability and the market value. Fish and shellfish undergo weakening or softening of muscle, particularly during extended storage under inappropriate conditions. The phenomenon is governed by endogenous proteases, both digestive and muscle proteases. Proteases present in the gastrointestinal tract that leach out to muscle tissue can induce proteolysis of myofibrillar and collagenous proteins. Furthermore, the muscle proteins present in gels fabricated from fish or shellfish meat also encounter degradation during thermal processing. Endogenous heat-activated proteases strongly bind to muscle proteins and are activated during heating, thereby degrading myofibrillar proteins, which are abundant in muscle tissue. This deterioration of the proteins directly leads to a weakened gel with poor water-holding capacity. Both cysteine and serine proteases are responsible for the degradation of myofibrillar proteins in several aquatic animals. Effective pretreatment of fish and shellfish, as well as the use of food-grade protease inhibitors (PIs), have been implemented to inactivate endogenous muscle and digestive proteases. For this review, proteolysis of muscle proteins and its control by food-grade PIs are revisited. Improved and effective lowering of proteolysis should be gained, thereby maintaining the quality of fish and their products.

Preventing Soft Texture Fish Fillets through Brine Injection Technology

An exploratory study was conducted to determine if multineedle injection technology could deliver protease inhibitor ingredients into fish fillets at sufficient levels to inhibit protease activity. Pacific whiting is used as a model in this study. Fillet treatments (n=8/treatment) included noninjection (C), injection of base brine containing 3% salt and 3% sodium tripolyphosphate (B), injection of base brine and 3% egg white(BEW), and injection of base brine, 0.1% xanthan gum, and 3% dried potato extract(BPE). Xanthan gum was used as a suspension aid. Actual brine incorporation was12.2±0.5%. Cathepsin L activity was evaluated at pH 5.5 (optimal pH) and ultimate pH. Quality measures evaluated included CIE Lab color, shear force, and lipid oxidation. Fillets injected with BEWand BPEwere significantly lower in cathepsin L activity when measured at pH 5.5. BEWand BPEfillets were darker in appearance than B or C fillets. Untreated fillets (C) had higher variability in shear force value than treated fillets. There was no effect of treatment on lipid oxidation. Results suggested that injection technology can be utilized to incorporate protease inhibitor ingredients (3% EW or 3% PE) at levels sufficient to reduce cathepsin L activity in Pacific whiting fillets.

Differential role of endogenous cathepsin and microorganism in texture softening of ice-stored grass carp (Ctenopharyngodon idella) fillets

BACKGROUND

Texture deterioration often negatively affects sensory attributes and commercial values of ice-stored fish fillets. The mechanism of softening of fish fillets during chilling storage is not fully resolved. Grass carp is a predominant freshwater fish species in China. The objective of the present study was to investigate the differential role of endogenous cathepsin and microorganisms in texture softening of ice-stored grass carp fillets.

RESULTS

The fillets were immersed in either NaN3 solution to reduce microbial activity or in iodoacetic acid solution to exclude cathepsin activity before ice storage. Treatment with NaN3 reduced microbial load of fillets below 2 log CFU g(-1) muscle during the entire storage period, and had no significant influence on the cathepsin activity and proteolysis. But the shear force of fillets treated with NaN3 decreased by 66% after 21 days of storage. Meanwhile, treatment with iodoacetic acid inactivated cathepsin B and B + L but did not significantly affect the microbial growth of fillets. Compared to NaN3 treatment, iodoacetic acid effectively alleviated softening and inhibited the increase in TCA-soluble peptides during storage.

CONCLUSION

This study demonstrated that proteolysis induced by endogenous cathepsins, rather than microorganisms, plays an important role in texture softening of ice-stored grass carp fillets. © 2015 Society of Chemical Industry.

Plant cystatins

Plant cystatins have been the object of intense research since the publication of a first paper reporting their existence more than 20 years ago. These ubiquitous inhibitors of Cys proteases play several important roles in plants, from the control of various physiological and cellular processes in planta to the inhibition of exogenous Cys proteases secreted by herbivorous arthropods and pathogens to digest or colonize plant tissues. After an overview of current knowledge about the evolution, structure and inhibitory mechanism of plant cystatins, we review the different roles attributed to these proteins in plants. The potential of recombinant plant cystatins as effective pesticidal proteins in crop protection is also considered, as well as protein engineering approaches adopted over the years to improve their inhibitory potency and specificity towards Cys proteases of biotechnological interest.

Diffusion of active proteins into fish meat to minimize proteolytic degradation

Proteases in fish muscle often cause undesired softening of intact meat pieces during refrigerated storage or slow cooking. Several food-grade proteinaceous inhibitors can overcome this softening if properly delivered to the intracellular sites where proteases are located. Fluorescence recovery after photobleaching (FRAP) and laser scanning confocal microscopy (LSCM) were used to measure the translational diffusion of fluorescein isothiocyanate (FITC)-labeled protease inhibitors into intact muscle fibers of halibut. Diffusion coefficients (D) of alpha-2-macroglobulin (720 kDa), soybean trypsin inhibitor (21 kDa), and cystatin (12 kDa) were measured in both muscle fibers and dilute aqueous solutions. On the time scale of the observation (35 min), cystatin and soybean trypsin inhibitor diffused through the cell membrane (sarcolemma) and sarcoplasm, but at a considerably slower rate (>10-fold difference) than in dilute aqueous solution. alpha-2-Macroglobulin did not diffuse into muscle cells within the time frame of the experiment, but did completely penetrate the cell during overnight exposure. The present study thus shows a clear dependence of D on protein inhibitor size when moving within intact skeletal muscle fibers. Low molecular weight protease inhibitors such as cystatin can be effectively diffused into intact fish muscle cells to minimize proteolytic activity and meat softening.

Compositional characteristics of materials recovered from whole gutted silver carp (Hypophthalmichthys molitrix) using isoelectric solubilization/precipitation

Isoelectric solubilization/precipitation (ISP) at acidic and basic pH was applied to whole carp, yielding proteins, lipids, and insolubles. The objective was to characterize composition of recovered materials. Crude protein was concentrated to 89-90% in proteins recovered at acidic pH and to 94-95% at basic pH. Basic pH yielded proteins with more (P < 0.05) essential amino acids (EAAs). EAA content in recovered proteins met FAO/WHO/UNO requirements. ISP did not affect fatty acid (FA) composition. Lipids recovered at acidic pH contained 88-89% of total fat and at basic pH, 94-97%. Total fat in recovered proteins was low, with EPA and DHA at the highest (P < 0.05) percentage for pH 11.5. ISP, particularly basic pH, effectively removed impurities such as bones and scales from whole carp. This is indicated by 3.8-5.8% of ash in recovered proteins compared to 11.2% for whole carp and 5.4% for boneless/skinless carp fillets. Basic pH yielded less (P < 0.05) Ca, P, and Mg in recovered proteins. These minerals were more (P < 0.05) concentrated in insolubles recovered with basic pH. This study indicates that materials recovered from whole carp using ISP have high nutritional value and may be useful in the development of human food and animal feeds.

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.

Gelation of protein recovered from whole Antarctic krill (Euphausia superba) by isoelectric solubilization/precipitation as affected by functional additives

This study demonstrated that the novel isoelectric solubilization/precipitation can be applied to recover functional muscle protein in a continuous mode from whole Antarctic krill. Protein recovered from whole krill had a much lower ash content than whole krill, suggesting good removal of inedible impurities (shell, appendages, etc.). Lipids were retained to a higher degree with krill protein solubilized at acidic rather than basic pH. The viscoelastic modulus (G') showed that recovered krill protein failed to form heat-induced gel unless beef plasma protein (BPP) was added. Therefore, protease inhibitors are suggested for development of krill-derived products. Even with BPP, the G' decreased between 45 and 55 degrees C. However, krill protein solubilized at acidic pH had a higher decrease of the G' than the protein solubilized at basic pH, likely due to krill endogenous cathepsin L. Krill protein-based gels developed from protein solubilized at basic pH, especially pH 12.0, had better texture (torsion and Kramer tests and texture profile analysis) than acidic counterparts, possibly due to higher proteolysis and denaturation at acidic pH. Gels made from protein solubilized at acidic pH were brighter and whiter likely due to a higher lipid content.