Calpains- and cathepsins-induced myofibrillar changes in post-mortem fish: Impact on structural softening and release of bioactive peptides

Correlation analysis of phosphorylation of myofibrillar protein and muscle quality of tilapia during storage in ice

In this study, the correlation between protein phosphorylation and deterioration in the quality of tilapia during storage in ice was examined by assessing changes in texture, water-holding capacity (WHC), and biochemical characteristics of myofibrillar protein throughout 7 days of storage. The hardness significantly decreased from 471.50 to 252.17 g, whereas cooking and drip losses significantly increased from 26.5% to 32.6% and 2.9% to 9.1%, respectively (P < 0.05). Myofibril fragmentation increased, while myofibrillar protein sulfhydryl content and Ca2+-ATPase activity decreased from 119.33 to 89.29 μmol/g prot and 0.85 to 0.46 μmolPi/mg prot/h, respectively (P < 0.05). Correlation analysis revealed that the myofibrillar protein phosphorylation level was positively correlated with hardness and Ca2+-ATPase activity but negatively correlated with WHC. Myofibrillar protein phosphorylation affects muscle contraction by influencing the dissociation of actomyosin, thereby regulating hardness and WHC. This study provides novel insights for the establishment of quality control strategies for tilapia storage based on protein phosphorylation.

Insights into the fish protein degradation induced by the fish-borne spoiler Pseudomonas psychrophila and Shewanella putrefaciens: From whole genome sequencing to quality changes

The aim of this study is evaluating the protein degradation capacity of specific spoilage organisms (SSOs) Pseudomonas psychrophila and Shewanella putrefaciens in fish flesh during chilled storage and revealing the underlying genes by whole-genome sequencing (WGS). Biochemical and physical tests were performed on fish flesh inoculated with P. psychrophila and S. putrefaciens individually, including textural properties, myofibrillar fragmentation index, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles, free amino acid composition, total volatile basic nitrogen (TVB-N), trichloroacetic acid (TCA) soluble peptides, and muscle microstructure. Results showed that P. psychrophila and S. putrefaciens exhibited a strong capacity for decomposing the fish protein, and the deterioration of fish flesh texture was primarily attributed to P. psychrophila. The genes from SSOs associated with the production of proteases were identified by whole genome sequencing and serine protease may be the primary enzyme secreted by SSOs involved in the degradation of fish protein. Therefore, the present study has shed light on the mechanisms of protein degradation induced by SSOs, thereby offering valuable insights for the development of effective quality control strategies.

MassArray Genotyping as a Selection Tool for Extending the Shelf-Life of Fresh Gilthead Sea Bream and European Seabass

In modern aquaculture, genomics-driven breeding programs have emerged as powerful tools for optimizing fish quality. This study focused on two emblematic Mediterranean fish species, the European seabass (Dicentrarchus labrax) and the gilthead sea bream (Sparus aurata), with a primary aim of exploring the genetic basis of white muscle/fillet degradation in fresh fish following harvest. We identified 57 and 44 missense SNPs in gilthead sea bream and European seabass, respectively, located within genes encoding for endogenous proteases responsible for fillet quality. These SNPs were cherry-picked based on their strategic location within the catalytic/regulatory domains of endogenous proteases that are expressed in the white muscle. Using MassArray technology, we successfully associated differentiated enzymatic activity of those endogenous proteases post-harvest as a phenotypic trait with genetic polymorphism of six SNPs in gilthead sea bream and nine in European seabass. These findings can be valuable attributes in selective breeding programs toward the extension of freshness and shelf life of these species. The integration of MassArray technology into breeding programs offers a cost-effective strategy for harnessing the potential of these genetic variants to enhance the overall quality of the final product. Recognizing that fresh fish perishability is a challenge, extending shelf-life is pivotal in reducing losses and production costs.

Impacts of long-time transportation on whiteleg shrimp (Penaeus vannamei) muscle quality and underlying biochemical mechanisms

BACKGROUND

Shrimp is widely consumed around the world. Since muscle is the primary edible component of shrimp, muscle quality (particularly texture) has a direct impact on the economic value of shrimp products. However, reports on the shrimp muscle quality influenced by transportation are rather limited, and the underlying mechanism remains unknown.

RESULTS

During the simulated transportation, the water pH and total ammonia-nitrogen content and un-ionized ammonia contents were elevated. Furthermore, reductions in shrimp muscle water-holding capacity, hardness, and shear value with intensive myofibrillar protein degradation were detected. Simulated transportation decreased the pH and glycogen content of shrimp muscle while increasing lactic dehydrogenase activity and lactate content, resulting in an elevated level of free calcium ions and increased μ-calpain and general proteolytic activities. Water exchange could improve the water quality and reduce the mortality of shrimp during transportation, as well as decrease muscle textural softening by alleviating these stress responses.

CONCLUSIONS

Maintaining water quality and, in particular, reducing ammonia are critical to improving shrimp survival and muscle quality during live transportation. This study is of great significance for the better maintenance of the textural properties of shrimp meat. © 2023 Society of Chemical Industry.

Degradation and evaluation of myofibril proteins induced by endogenous protease in aquatic products during storage: a review

Myofibril proteins degradation constitutes an important factor in quality deterioration, procedural activation or inhibition of endogenous protease potential regulates autolytic proteolysis-induced softening of post mortem fish muscle. Based on the brief introduction of myofibril proteins degradation in fish skeletal muscle, a detailed description of the main myofibril degradation properties and the distinct role played by endogenous proteases were proposed, which reflects the limitations and challenges of the current research on myofibril hydrolysis mechanisms based on the varied surrounding conditions. In addition, the latest researches on the evaluation method of myofibril proteins degradation were comprehensively reviewed. The potential use of label-free proteomics combined with bioinformatics was also emphasized and has become an important means to in-depth understand protein degradation mechanism.

The interaction of an effector protein Hap secreted by Aeromonas salmonicida and myofibrillar protein of meat: Possible mechanisms from structural changes to sites of molecular docking

Microbial spoilage of meat products is a significant problem in the food industry. Aeromonas salmonicida is a significant microorganism responsible for spoilage in chilled meat. Its effector protein, hemagglutinin protease (Hap), has been identified as an effective substance for degrading meat proteins. The ability of Hap to hydrolyze myofibrillar proteins (MPs) in vitro demonstrated that Hap has obvious proteolytic activity, which could alter MPs' tertiary structure, secondary structure, and sulfhydryl groups. Moreover, Hap could significantly degrade MPs, focusing primarily on myosin heavy chain (MHC) and actin. Active site analysis and molecular docking revealed that the active center of Hap was bound to MPs via hydrophobic interaction and hydrogen bonding. It may preferentially cleave peptide bonds between Gly44-Val45 in actin, and Ala825-Phe826 in MHC. These findings suggest that Hap may be involved in the spoilage mechanism of microorganisms and provide crucial insights into the mechanisms of meat spoilage induced by bacteria.

Effects of cold atmospheric plasma on endogenous enzyme activity and muscle protein oxidation in Trachinotus ovatus

In this study, we investigated the effects of cold atmospheric plasma (CAP) technology on endogenous enzyme characteristics and muscle protein properties of the golden pomfret (Trachinotus ovatus) under different treatment power and time conditions. Results showed that the enzymatic activity of cathepsin B, L, and calpain in crude protease extracts (CPE) decreased significantly as the treatment power and treatment time of CAP increased (p < 0.05). Oxidative degradation of the CPE after exposure to CAP resulted in significant changes in the structure, total sulfhydryl, and carbonyl content of the CPE (p < 0.05). CAP of an appropriate intensity resulted in significant improvements in the color parameters, hydration properties, and textural property parameters of muscle proteins (p < 0.05). These results suggest that CAP, as a non-thermophysical modification technique, can inhibit the activity of endogenous enzymes as well as alter the protein function in food.

Effects of combined treatment of electrolytic water and chitosan on the quality and proteome of large yellow croaker (Pseudosciaena crocea) during refrigerated storage

The labeled quantitative proteomic method was used to study the changes in muscle proteins of large yellow croaker (Pseudosciaena crocea) treated with electrolytic water (EW) and chitosan (CHI) combined preservation during 12 days of refrigeration storage (4 °C). The analysis indicated that the freshness instructed by total viable count (TVC), total volatile basic nitrogen (TVB-N) and K value was significantly maintained after combined preservation during storage at 4 °C for 12 days (CS12). Furthermore, 46 differentially abundant proteins (DAPs) were detected in storage at 4 °C for 12 days (S12) compared to the freshness group (F), which bioinformatics confirmed were mainly skeletal proteins and enzymes. Correlation analysis showed that 19 highly correlated DAPs could be used as potential protein markers of freshness. Changes in the relation of freshness and protein were shown in further correlative analysis of F and CS12, which were caused by combined preservation. Therefore, combined preservation is promising in the quality and stability of large yellow croakers.

Effect of multiple freeze-thaw cycles on water migration, protein conformation and quality attributes of beef longissimus dorsi muscle by real-time low field nuclear magnetic resonance and Raman spectroscopy

Repeated freezing and thawing (F-T) happens during long-term storage and transportation due to the temperature variation, causing quality deterioration of beef products and influencing consumer acceptance. This study was aimed to investigate the relationship between quality attributes, protein structural changes and water real-time migration of beef with different F-T cycles. The results showed that multiply F-T cycles damaged the muscle microstructure and protein structure tended to denature and unfold, led lower population of water reabsorbed, thus triggering the decrease of water capacity, especially a decrease of T₂₁ and A₂₁ of completely thawed beef samples, finally affected the quality, such as tenderness, color and lipid oxidation of beef muscle. Beef should not be abused by F-T cycles >3 times, the quality extremely degraded when subjected to 5 or more F-T cycles, and real-time LF-NMR provided a new aspect to help us control the thawing process of beef.

Physicochemical and microstructural mechanisms for quality changes in lightly salted tilapia (Oreochromis niloticus) fillets during frozen storage

BACKGROUND

Frozen tilapia fillet has become a leading aquatic product. High drip loss, dry and fibrous mouthfeel, and an unappealing appearance are its main problems. It was hypothesized that light salting could improve the quality, and that the preparation conditions would affect the storage stability of frozen tilapia fillets.

RESULTS

The quality changes of lightly salted tilapia fillets were evaluated during frozen storage, and the underlying mechanisms were studied from the physicochemicaland microstructural perspectives. Though the salt content was 1.5% in all samples,the amount of ice crystals in frozen tissues decreased with the descending water content and freezing point (P < 0.05). No intracellular voids were observed in the samples prepared under proper salting conditions, and the myofibers were plump and smooth after freezing-thawing, which contributed to the high water-holding capacity of lightly salted fillets. After 28 days,the water-binding capacity of the salted groups was 14.69%-18.62% higher than that of their unsalted counterparts (P < 0.05). The reduced protein solubility in the salted fillets was likely to have occurred because the solubilized and unfolded proteins interacted more easily during frozen storage. The oxidation degree of myofibrillar proteins was also affected by salting condition, and the fillets with less oxidized sulfhydryl groups maintained high springiness after 28 days of frozen storage.

CONCLUSION

The salting condition of 9% NaCl solution for 1 h was recommended for the preparation of lightly salted fillets from freshwater fish, taking into account quality, processing efficiency, and storage stability. The enhanced water-holding capacity and texture of lightly salted tilapia fillets were attributed to modified physicochemical and microstructural properties. These results could provide a scientific basis for the processing and storage of high-quality, frozen, lightly salted fillets from freshwater fish. © 2022 Society of Chemical Industry.

Role of Stingray (Himantura signifier) Non-Protein Nitrogenous Fraction on the Oxidative Stability of Lipid and Myoglobin

Non-protein nitrogen (NPN) is abundant in stingray (Himantura signifier) muscle, which also has in vitro antioxidant activity. In this study, NPN from stingray muscle was further investigated for its antioxidant properties in lecithin liposome and oxymyoglobin model systems to validate its protective impact against lipid and myoglobin oxidations during storage for 120 min at various temperatures (4, 25, and 60 °C). NPN solution (10 ppm nitrogen) was added to the lecithin liposome system at different concentrations (0, 0.5, 1, 5, and 10% (v/v)) to investigate its effects on lipid stability by measuring the conjugated diene (CD), peroxide value (PV), and thiobarbituric acid reactive substances (TBARS) contents. In the oxymyoglobin system, NPN solution (10 ppm nitrogen) was also added at different concentrations (0, 0.5, 1, 5, and 10% (v/v)) to the oxymyoglobin solution in order to examine its effect on the stability of myoglobin by determining the contents of oxymyoglobin, metmyoglobin, and protein carbonyl. According to the findings, in all NPN concentrations, the system incubated at 4 °C had the lowest levels of lipid oxidation as measured by CD, PV, and TBARS values, and the lowest levels of myoglobin oxidation. At all incubating temperatures, the oxymyoglobin and lipid oxidation of all model systems tended to rise with the lengthening of the incubation duration. With the addition of 5% NPN, however, the lowest CD, PV, TBARS, oxymyoglobin oxidation, metmyoglobin formation, and protein carbonyl content were all observable, and the remarkable result was discovered during incubation at 4 °C. The results indicate that stingray NPN, especially at 5%, can be used to delay lipid and myoglobin oxidation, particularly at 4 °C. In order to prolong the shelf life of products with dark-fleshed fish and red meat, stingray NPN might be used as an alternative antioxidant to delay the oxidation of lipid and myoglobin during cold chain storage.

Effects of Modified Atmosphere Packaging with Varied CO2 and O2 Concentrations on the Texture, Protein, and Odor Characteristics of Salmon during Cold Storage

The effect of gas ratio on the growth of bacteria has been well demonstrated, but some adverse effects of modified atmosphere packaging (MAP) on seafoods have also been found. To provide a better understanding of the effects of CO₂ and O₂ concentrations (CO₂ from 40% to 100% and O₂ from 0% to 30%) in MAP on the texture and protein contents and odor characteristics of salmon during cold storage, the physiochemical, microbial, and odor indicators were compared with those without treatment (CK). Generally, MAP treatments hindered the increase of microbial counts, total volatile basic nitrogen, and TCA-soluble peptides, and decreased the water-holding capacity, hardness, springiness, and sarcoplasmic and myofibrillar protein contents. The results also indicated that 60%CO₂/10%O₂/30%N₂ was optimal and decreased the total mesophilic bacterial counts by 2.8 log cfu/g in comparison with CK on day 12. In agreement, the concentration of CO₂ of 60% showed the lowest myofibrillar protein degradation, and less subsequent loss of hardness. The electronic nose characteristics analysis indicated that 60%CO₂/20%O₂/20%N₂ and 60%CO₂/10%O₂/30%N₂ had the best effect to maintain the original odor profiles of salmon. The correlation analysis demonstrated that microbial growth had a strong relationship with myofibrillar and sarcoplasmic protein content. It can be concluded that 60%CO₂/10%O₂/30%N₂ displayed the best effect to achieve the goal of preventing protein degradation and odor changes in salmon fillets.

Protein hydrolysates derived from aquaculture and marine byproducts through autolytic hydrolysis

Autolysis technology has shown potential for protein hydrolysates production from marine and aquaculture byproducts. Viscera are a source of cheap proteolytic enzymes for producing protein hydrolysates from the whole fish or processing byproducts of the most valuable commercial species by applying autolysis technology. The use of autolysis allows economical production of protein hydrolysate and provides an opportunity to valorize downstream fish and shellfish processing byproducts at a lower cost. As a result, production and application of marine byproduct autolysates is increasing in the global protein hydrolysates market. Nevertheless, several restrictions occur with autolysis, including lipid and protein oxidation mediated by the heterogeneous composition of byproducts. The generally poor storage and handling of byproducts may increase the formation of undesirable metabolites during autolysis, which can be harmful. The formation of nitrogenous compounds (i.e., biogenic amines), loss of freshness, and process of autolysis in the byproducts could increase the rate of quality and safety loss and lead to more significant concern about the use of autolysates for human food applications. The current review focuses on the autolysis process, which is applied for the hydrolysis of aquaculture and marine discards to obtain peptides as functional or nutritive ingredients. It further addresses the latest findings on the mechanisms and factors contributing the deterioration of byproducts and possible ways to control oxidation and other food quality and safety issues in raw materials and protein hydrolysates.

Research progress on quality deterioration mechanism and control technology of frozen muscle foods

Freezing can prolong the shelf life of muscle foods and is widely used in their preservation. However, inevitable quality deterioration can occur during freezing, frozen storage, and thawing. This review explores the eating quality deterioration characteristics (color, water holding capacity, tenderness, and flavor) and mechanisms (irregular ice crystals, oxidation, and hydrolysis of lipids and proteins) of frozen muscle foods. It also summarizes and classifies the novel physical-field-assisted-freezing technologies (high-pressure, ultrasound, and electromagnetic) and bioactive antifreeze (ice nucleation proteins, antifreeze proteins, natural deep eutectic solvents, carbohydrate, polyphenol, phosphate, and protein hydrolysates), regulating the dynamic process from water to ice. Moreover, some novel thermal and nonthermal thawing technologies to resolve the loss of water and nutrients caused by traditional thawing methods were also reviewed. We concluded that the physical damage caused by ice crystals was the primary reason for the deterioration in eating quality, and these novel techniques promoted the eating quality of frozen muscle foods under proper conditions, including appropriate parameters (power, time, and intermittent mode mentioned in ultrasound-assisted techniques; pressure involved in high-pressure-assisted techniques; and field strength involved in electromagnetic-assisted techniques) and the amounts of bioactive antifreeze. To obtain better quality frozen muscle foods, more efficient technologies and substances must be developed. The synergy of novel freezing/thawing technology may be more effective than individual applications. This knowledge may help improve the eating quality of frozen muscle foods.

Effects of repeated deboning on structure, composition, and gelling properties of silver carp surimi

BACKGROUND

Surimi is produced from the various sequences of filleting, deboning, washing, dehydrating, blending with cryprotectant, and freezing. Deboning, after which fish flesh is minced and separated from bone, skin, etc., is a vital step in the surimi production. In this study, effects of repeated deboning on yield, structure, composition, and gelling properties of silver carp surimi were investigated.

RESULTS

Surimi yield increased rapidly from 10% to 23% as the cycle of repeated deboning was increased from one to three, and then slowly increased up to 26%. As the cycle increased, cellular structure and ultrastructure of muscle fibers progressively fractured. Contents of fat, cathepsins, heme proteins, and transglutaminase of surimi obviously increased and then decreased. Three-dimensional network of surimi gel without setting (NS gel) became more porous with the increase of cycles. It became more compact, and then turned to aggregated forms with lower homogeneity, for the surimi gel with setting (WS gel). Correspondently, the NS gel textural values gradually decreased with the cycles, while the WS gel textural values increased up to three cycles and then decreased. Regardless of setting, whiteness of surimi gels decreased and then increased with the cycles.

CONCLUSION

Our results suggested that structure and compositions of surimi changed with the cycle of repeated deboning, which affected gelling properties of surimi. It is recommended to debone three or four cycles in silver carp surimi production. © 2022 Society of Chemical Industry.

Effects of Weak Acids on the Microbiological, Nutritional and Sensory Quality of Baltic Herring (Clupea harengus membras)

Baltic herring (Clupea harengus membras) pickled in vinegar is a common product in the Nordic countries. Other weak acids are used to cook and preserve fish in other food cultures. The aim of this study was to evaluate the potential of weak acids to produce safe and nutritious pickled fish products with varying sensory properties. The influence of acetic, citric, lactic, malic, and tartaric acids on the preservability and quality of pickled and marinated Baltic herring was studied by measuring microbiological quality, pH, chemical composition, and lipid oxidation and by sensory profiling. Pickling with these acids with pH levels of 3.7-4.2 resulted in pickled Baltic herring products with high microbiological quality. The results of the chemical analysis of the samples indicated that pickling and storage on marinade influenced the chemical composition of fish. The most significant changes in chemical composition were the increase in moisture and decrease in protein content of the samples during storage. Fat content decreased during the storage period in acetic acid and malic acid samples. All tested acids inhibited lipid oxidation for one month, but at three and four month time points, the content of oxidation products increased except in the samples pickled with tartaric acid. The highest oxidation level was observed in the case of citric acid and the lowest with tartaric acid. The results indicate that replacing acetic acid with other weak acids frequently used in the food industry results in pickled and marinated fish products with novel and milder sensory profiles.

Changes in protease activity during fermentation of fish sauce and their correlation with antioxidant activity

BACKGROUND

Antioxidant activity has been found in fermented fish sauce. In this experiment, the properties of endogenous protease and antioxidant activity were studied in anchovy sauce during fermentation. The correlation between protease activity and antioxidant activity in fermented anchovy sauce was analyzed using the partial least squares (PLS) method.

RESULTS

The results showed that at least four proteases were present in the endogenous enzyme solution, and the optimum pH values were 2.5, 5.5, 9.0, and 12.5, respectively. The maximum inhibition rate of endogenous protease, from high to low, was: serine protease inhibitor > trypsin inhibitor > aspartic protease inhibitor (pepsin inhibitor) > cysteine protease inhibitor > metalloprotease inhibitor. At the sixth month of fermentation, fish sauce had stronger trypsin, pepsin-like activity, and antioxidant activity. At the ninth month of fermentation, the cathepsin activity was greater. A model correlating changes in protease activity with antioxidant activity suggested that the trypsin and serine protease were the main factors affecting antioxidant activity.

CONCLUSION

This study reports a model correlating changes in protease activity with the antioxidant activity of fish sauce. It lays a foundation for further exploration of the formation of antioxidant substances and antioxidant effects during the process of fish sauce fermentation. © 2021 Society of Chemical Industry.

Proteomics and metabolomics combined study on endopathic changes of water-soluble precursors in Tan lamb during postmortem aging

Tan lamb is highly recommended breed in China. It is of great significance to understand the underlying mechanism of how water‐soluble flavor precursors metabolize in Tan lamb muscles during the postmortem aging period. In this study, we investigated the muscle pH, lactate dehydrogenase (LDH) activity, and the variations in water‐soluble flavor‐related metabolites. The proteome changes were profiled to provide insights into the biochemical changes affecting accumulation of water‐soluble flavor precursors in different aging stages (days 0, 4, and 8). The results indicated that pH value considerably decreased from day 0 to day 4, and increased from day 4 to day 8 (p < .05). The activity of LDH significantly increased from day 0 to day 4, and decreased from day 4 to day 8 (p < .05). Postmortem glycolysis was activated in 4 days, which directly affected the variations in metabolic enzymes and triggered the accumulation of flavor‐related carbohydrates. The free amino acids accumulated due to hydrolysis of structural proteins, with 3‐hydroxy‐L‐proline, aspartic acid, and methionine increasing from day 0 to day 4, and aspartic acid, serine, threonine, tyrosine, phenylalanine, and D‐phenylalanine from day 4 to day 8. The inosine and hypoxanthine accumulated due to the degradation of ATP. The results of the present study provide insightful information, revealing the differences in biochemical attributes in Tan lamb muscles caused by postmortem aging.

Dominating roles of protein conformation and water migration in fish muscle quality: The effect of freshness and heating process

The quality characteristics of fish products are a key factor influencing consumer acceptance and preference. This study was aimed to investigate the relationship among quality characteristics, protein structural changes and water migration of mandarin fish with different freshness during heating process. The results showed that the protein structure tended to unfold and more loosen in low freshness fish muscle (4-5 d storage) during heating, leading to an obvious decrease in hydrogen bonds, promoting a reduction of water holding capacity in fish muscle, thus resulting in an increase of T₂₃ and a decrease of A_W, which in turn affected the hardness, stress, and springiness of fish muscle. The protein conformation and water migration could explain the textural differences after heating of different freshness mandarin fish.

Roles of Proteins/Enzymes from Animal Sources in Food Quality and Function

Animal proteins are good sources of protein for human, due to the composition of necessary amino acids. The quality of food depends significantly on the properties of protein inside, especially the gelation, transportation, and antimicrobial properties. Interestingly, various kinds of molecules co-exist with proteins in foodstuff, and the interactions between these can significantly affect the food quality. In food processing, these interactions have been used to improve the texture, color, taste, and shelf-life of animal food by affecting the gelation, antioxidation, and antimicrobial properties of proteins. Meanwhile, the binding properties of proteins contributed to the nutritional properties of food. In this review, proteins in meat, milk, eggs, and fishery products have been summarized, and polysaccharides, polyphenols, and other functional molecules have been applied during food processing to improve the nutritional and sensory quality of food. Specific interactions between functional molecules and proteins based on the crystal structures will be highlighted with an aim to improve the food quality in the future.

An Alum-Free Jellyfish Treatment for Food Applications

Jellyfish, marketed and consumed as food in The Far East, are traditionally processed using salt and alum mixtures. In recent years, the interest of Western consumers in jellyfish (JF) as a food source is increasing. In Europe [European Union (EU)], JF-derived food products are regulated by a novel food law, but methods for JF treatment and processing have not been developed yet. In this study, a protocol for the stabilization and processing of JF into semi-finished food products without the use of alum is proposed for the first time. Safety and quality parameters, together with a series of technological and nutritional traits, were used to monitor the proposed process and for the characterization of the JF-derived products. Calcium lactate (E327), calcium citrate (E333), and calcium acetate (E263), which are food thickening/stabilizing agents allowed by EU regulations, were used in order to control the presence of possible microbial pathogens and spoilage species. The use of calcium lactate and citrate led to an increase in texture values (~1.7-1.8-fold higher than in starting raw materials) and in several nutritional traits such as antioxidant activity, and protein and fatty acid content. In particular, the combination of JF treatments with calcium salts and phenolic compounds resulted in an antioxidant activity increase of up to 8-fold, protein concentration increase of up to 2.6-fold, fatty acid composition maintenance, and a ω6/ω3 ratio lower than 1. For the first time, the application of phenolic compounds to improve JF technological and nutritional features was verified. This study proposes a new procedure for JF treatment and stabilization useful for future potential food applications in Western countries.

The effect of caspase-3 in mitochondrial apoptosis activation on degradation of structure proteins of Esox lucius during postmortem storage

This study aimed to investigate the effect of caspase-3 inhibitor in mitochondrial apoptosis activation on structure protein degradation during postmortem storage. Mitochondrial dysfunction, apoptotic factors, structure protein degradation and the myofibrillar rupture index between the control and caspase-3 inhibitor groups were determined. The results show caspase-3 inhibitor repressed the mitochondrial membrane permeability and mitochondrial swelling, as well as increased mitochondrial membrane potential, causing a decrease in the release of cytochrome c from mitochondria to cytoplasm and caspase-9/3 activities (P < 0.05). Subsequently, small myofibrillar proteins (desmin and troponin-T) were susceptible to degradation, initiating texture deterioration. By contrast, giant structure proteins (titin and nebulin) were degraded during later postmortem storage, predominantly contributing to fish softening. The results further suggest that caspase-3 is involved in degradation of structure proteins during postmortem through mitochondrial apoptosis pathways.

Label-free proteomic analysis revealed the mechanisms of protein oxidation induced by hydroxyl radicals in whiteleg shrimp (Litopenaeus vannamei) muscle

The oxidative effects of hydroxyl radicals derived from a FeCl₃/ascorbic acid/H₂O₂ system on the stability of muscle proteins in peeled shrimp (Litopenaeus vannamei) were investigated. Physicochemical analysis indicated negative effects on the color (a* value), springiness, and pH of shrimp muscle, which appeared to be significantly exacerbated by higher concentrations of generated hydroxyl radicals when compared with the control. The microstructural results confirmed that a radical attack induced the incompact structure and disintegrated myofibers, thereby leading to weakened connective tissues and decreased stability of muscle proteins. Furthermore, label-free proteomic analysis revealed several differentially abundant proteins (DAPs) (i.e., ribosomal protein subunits, putative cytoskeleton proteins, and ion-binding proteins), which were detected and identified in oxidation-treated shrimp when compared with the control. The gene ontology (GO) and eukaryotic clusters of orthologous group (KOG) analyses further confirmed that the active hydroxyl radicals attacked vulnerable amino acids, modified peptide chains, and/or protein structures and/or conformations, which were responsible for a significant decrease in the muscle texture and stability of proteins in oxidation-treated shrimp. This study provides novel insight into the molecular mechanisms of muscle protein changes during oxidation development.

Effects of super-chilling storage on shelf-life and quality indicators of Coregonus peled based on proteomics analysis

The advantages of super-chilling storage at -2 °C for maintaining the quality of Coregonus peled muscle were investigated using the rigor-mortis index (RM), ATP-related compounds, K-value, muscle hardness, impedance measurement, and total viable count. The results indicated that the softening of fish muscle and increase in K-value were substantially suppressed following storage at -2 °C compared to that at 0 °C. In particular, the hardness of fish muscle stored for 6 days at -2 °C was much higher than that of the samples stored for 2 days at 0 °C. The K-value increased to 81% after 6 days at 0 °C, while increased to 57% at -2 °C. The impedance changed in a biphasic manner throughout the storage period. The initial increase accompanied by the progression of RM was followed by a gradual decrease. However, this decrease was much slower at -2 °C than 0 °C. Furthermore, proteomics analysis demonstrated that the mechanism of fish freshness changes between the two storage temperatures. Differentially abundant proteins between the samples stored at two temperatures were mainly involved in the cellular component and molecular function (GO pathway) as well as collagen digestion (KEGG pathway), which might be related to muscle textural properties. Therefore, super-chilling storage is a possible method for maintaining the freshness of Coregonus peled.

Freezing-then-aging treatment improved the protein digestibility of beef in an in vitro infant digestion model

In vitro protein digestibility of freezing-then-aged beef was investigated in an infant digestion model. The treatments were divided into freezing-then-aging (FA) and aging-only (AO) groups. Carbonyl and total free sulfhydryl contents were the same between both groups for 14-day aging. Freezing had no effect on beef myofibrillar protein tertiary structure. Although caspase-3 activity did not differ, the FA group showed higher cathepsin B activity than the AO group (p < 0.05). The 10% trichloroacetic acid-soluble α-amino content was higher in FA than AO group, on aging day 14 (p < 0.05). Post in vitro digestion of beef aged for 14 days, the FA group had a higher content, than the AO group, of α-amino groups and proteins digested under 3 kDa (p < 0.05). An electrophoretogram of the digesta showed improved digestion of actin in the FA group. Collectively, the freezing-then-aging process enhanced the protein digestibility of beef in this in vitro infant digestion model.

Estimating freshness of ice storage rainbow trout using bioelectrical impedance analysis

This study aimed to evaluate the freshness of ice stored rainbow trout by bioelectrical impedance measurements. Rigor mortis, ATP-related components, K-value, and hardness of rainbow trout muscle during storage were monitored along with impedance. The results showed that the progress of rigor mortis was accompanied by an increase in impedance. Impedance kept decreasing even in rigor state, and during the gradual resolution of rigor mortis with impedance change upon storage of fish was biphasic (r = -0.944, p < .01). Thus, when impedance decreased close to the lowest value, K-value was only about 61.57 ± 0.52%, but still exhibited a high pertinence (r = -0.959, p < .01). A gradual decrease of the hardness of fish muscle upon storage of fish showed a close correlation (r = 0.981, p < .01) with impedance decrease. These results suggested that the impedance measurement has a great potential for predicting the freshness of the rainbow trout during ice storage.

Dietary carbohydrates influence muscle texture of olive flounder Paralichthys olivaceus through impacting mitochondria function and metabolism of glycogen and protein

The present study was conducted to estimate the effects of dietary carbohydrates on muscle quality and the underlying mechanisms. Six isonitrogenous and isolipidic diets were formulated to contain graded levels of carbohydrates (0%, 8%, 12%, 16%, 20% and 24%, respectively). These diets were named as C0, C8, C12, C16, C20 and C24, respectively. After a 10-week feeding trial, results showed that the muscle pH, liquid holding capacity (LHC) and hardness were significantly decreased by the increasing dietary carbohydrate levels. Dietary carbohydrates significantly decreased the muscle fibre diameter, and the highest value was found in the C0 group. Accumulated glycogen and degenerated mitochondrial cristae were observed in the C24 group. Significantly higher contents of protein carbonyls were observed in the C20 group and C24 group (P < 0.05). There was a significant decrease of mtDNA copy number in the C24 group compared with that in the C0 and C8 groups. The AMP/ATP ratio in muscle decreased first and then increased with the increasing dietary carbohydrate levels. The dietary incorporation of carbohydrate significantly reduced the expression of opa1, pygm and genes involved in myogenesis (myf5 and myog). Meanwhile, proteolysis-related genes (murf-1, mafbx, capn2 and ctsl), pro-inflammatory cytokines (il-6 and tnf-α) and mstn were significantly up-regulated. In the C24 group, significant increase of phosphorylation of AMPK (Thr172), up-regulation of PGC-1α and GLUT4 were observed, while the phosphorylation level of S6 (Ser235/236) was significantly decreased. It was concluded that excessive dietary carbohydrate level (24%) had negative impacts on mitochondria function and promoted glycogen accumulation, and thereafter influenced the muscle quality of olive flounder. The activation of AMPK as well as the upregulation of PGC-1α and GLUT4 was the key mechanism.

Inhibitory effect of edible natural compounds with di- and tri-carboxyl moiety on endogenous protease inducing disassembly and degradation of myofibrils from grass carp (Ctenopharyngodon idella)

Inhibition of endogenous protease is a rapid and feasible approach to control the proteolysis proceeding of post mortem fish flesh. In the present study, the in vitro inhibitory effects of common edible di- and tri-carboxylic acids and salts on endogenous proteolytic activities as well as myofibrillar disassembly and degradation mediated by crude enzyme of grass carp muscle were investigated. The results showed that among the compounds tested, maleic acid, fumaric acid, tartaric acid and malic acid were the most effective inhibitor for cathepsin B, L and calpain, with IC₅₀ ranging from 7.76 to 30.13 mM, from 32.38 to 65.12 mM, from 1.06 to 6.76 mM, respectively. Also, relatively lower Ki (ranging from 1.04 to 43.21 mM) of these compounds were found towards cathepsin B, L and calpain. Incubation of myofibrillar protein with crude enzyme in the presence of di- and tri-carboxylic compounds could remarkably suppress the dissociation and degradation of myosin heavy chain (MHC), and ameliorate the loss of heat shock protein (HSP) in myofibrils, with tartaric acid and fumaric acid proved more effective than other compounds, possibly implicating their application as potential and efficient inhibitors for quality control of fish muscle products.

Comprehensive identification of native medium-sized and short bioactive peptides in sea bass muscle

Native peptides from sea bass muscle were analyzed by two different approaches: medium-sized peptides by peptidomics analysis, whereas short peptides by suspect screening analysis employing an inclusion list of exact m/z values of all possible amino acid combinations (from 2 up to 4). The method was also extended to common post-translational modifications potentially interesting in food analysis, as well as non-proteolytic aminoacyl derivatives, which are well-known taste-active building blocks in pseudo-peptides. The medium-sized peptides were identified by de novo and combination of de novo and spectra matching to a protein sequence database, with up to 4077 peptides (2725 modified) identified by database search and 2665 peptides (223 modified) identified by de novo only; 102 short peptide sequences were identified (with 12 modified ones), and most of them had multiple reported bioactivities. The method can be extended to any peptide mixture, either endogenous or by protein hydrolysis, from other food matrices.

The influence of endogenous cathepsin in different subcellular fractions on the quality deterioration of Northern pike (Esox lucius) fillets during refrigeration and partial freezing storage

The purpose of this study was to investigate the endogenous cathepsin activity in each subcellular fraction and the effect of this activity on myofibrillar protein and texture during refrigeration and partial freezing storage of northern pike (Esox lucius) fillets. The results showed that fillets stored under the refrigerated condition were more susceptible to oxidation than partial freezing. Endogenous cathepsin activity indicated that partial freezing destroys the integrity of lysosomes more effectively than refrigeration and inhibits the increase in cathepsin B and B + L in lysosomes. The activity of cathepsin B and B + L in lysosomes, mitochondria and myofibrils under the partial freezing conditions was always lower than that under refrigeration. Texture analysis showed that refrigeration had a negative impact on hardness and springiness. In conclusion, the cathepsin activity in each subcellular fraction was effectively inhibited and better textural characteristics were obtained with partial freezing than refrigeration.

Identification of a novel alkaline serine protease from gazami crab (Portunus trituberculatus) hepatopancreas and its hydrolysis of myofibrillar protein

Serine proteases are thought to play a key role in the muscle softening of gazami crab (Portunus trituberculatus) during storage. A serine protease, Pt-sp2, was purified from the hepatopancreas of gazami crab using ammonium sulfate precipitation, anion-exchange and gel filtration chromatography, and was analyzed by mass spectrometry, transcriptome and bioinformatics. It revealed that Pt-sp2 was trypsin-like, with no 100% identical proteins in the NCBI database. The molecular weight of Pt-sp2 was approximately 37.2 kDa. Its optimum pH and temperature were 9.0 and 50 °C, respectively, using t-Butyloxy‑carbonyl-Phe-Ser-Arg-4-methyl-coumaryl-7-amide as a substrate. Pt-sp2 was activated in the presence of Ca²⁺. Both soybean trypsin inhibitor and Nα-Tosyl-l-lysine chloromethyl ketone hydrochloride completely suppressed Pt-sp2 activity, while it was only partially inhibited by phenylmethylsulfonyl fluoride and EDTA. However, PMSF, Pepstatin A and cystatin inhibitor E-64 showed no inhibition on Pt-sp2 protease activity. The Km value of Pt-sp2 was 0.82 μM, and Pt-sp2 effectively hydrolyzed myofibrillar protein at 37 °C.

The differential effects of endogenous cathepsin and microorganisms on changes in the texture and flavor substances of grouper (Epinephelus coioides) fillets

Grouper is an important seafood species in China and has high economic value. However, the edible value of grouper is seriously affected by deterioration in the texture and flavor during refrigeration. The purpose of this study was to investigate the effects of endogenous cathepsin and microorganisms on texture softening and flavor changes in refrigerated grouper fillets. Iodoacetic acid and ProClin 300 were used to inhibit endogenous protease activity and microbial growth separately. Iodoacetic acid can inhibit the activity of cathepsin B, L, and calpain. Moreover, iodoacetic acid does not significantly affect the growth of microorganisms. The total amounts of bacteria and Pseudomonas spp. in the samples treated with ProClin 300 were less than 2 log CFU g⁻¹ and 1 log CFU g⁻¹ on the 18th day, and the activity of protease was not significantly affected. On the 6th day, the hardness of the iodoacetic acid treatment group decreased by 8%, while the ProClin 300 treatment group decreased by 28%, and changes in the free amino acids and volatile substances significantly exceeded those of the iodoacetic acid treatment group, indicating that endogenous protease was the main factor in the texture deterioration. A first-order exponential decay model indicated that cathepsin L was the most important protease for reducing the hardness of grouper fillets, and changes in the content of free amino acids and volatile substances indicated that microorganisms played a more important role in the deterioration of flavor substances compared to that played by endogenous protease.

This study reveals the different effects of endogenous proteases and microorganisms on the texture and flavor of grouper muscles.