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Summary: Textural deterioration is a severe problem in fish industry. In order to study the role of structural proteins in refrigerated grass carp, myofibrillar proteins and collagen degradation were investigated at different stages. Results revealed that connective tissues of fish muscle disintegrated from day 1, failing to hold muscle myotomes together, with a decrease of shear force. Meanwhile, collagen solubility increased from day 1 and degradation of nebulin was observed after 3 days, while there was no obvious change in the bands of myosin heavy chain and actin. Furthermore, Pearson coefficient analysis revealed that insoluble collagen (ISC) and nebulin were significantly correlated with shear force. Negative correlation was established between drip loss and contributing factors (ISC, titin and nebulin). All results suggested that textural loss of post‐mortem grass carp fillets was mainly mediated by the degradation of collagen and to a lesser extent by myofibrillar proteins at the early storage. [ABSTRACT FROM AUTHOR]

BACKGROUND: Textural deterioration is a serious problem in chilled fish flesh. Cysteine proteinases are proposed to participate in disintegration of collagen fibers during this process, while its mechanism remains elusive. In the present study, a cysteine proteinase was purified from grass carp muscle and identified by mass spectrometry, and its effect on structural changes of collagen fibers was investigated. RESULTS: During storage at 4 °C, cysteine proteinase activity in fillets increased to 1.53‐fold at day 5 and maintained a high level later, and this variable was related to a decline in shear force and an increase in drip loss. A 29 kDa cysteine proteinase was purified through ammonium sulfate precipitation and column chromatography, and identified as cathepsin L. Cathepsin L caused collagen fibers to partly disintegrate into fibril bundles and individual fibrils at 48 h, while the triple helical structure of collagen molecules remained stable. Release of soluble proteins and glycosaminoglycans from cathepsin L‐treated collagen fibers was time dependent, coinciding with a release of 4.12 ± 0.13% and 8.57 ± 0.03% at 48 h respectively. However, 0.85 ± 0.02% of hydroxyproline was freed from cathepsin L‐treated collagen fibers at 48 h. Furthermore, scanning electron microscopy revealed that the inhibitory effect of cathepsin L could retard the destruction of intramuscular connective tissues (IMCTs). CONCLUSION: These results indicated that cathepsin L might be involved in collagen fiber breakdown by degrading collagen‐associated proteoglycans during textural deterioration of grass carp. © 2022 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the in vitro susceptibility of grass carp (Ctenopharyngodon idella) oxidized myosin, with the treatment of H 2 O 2 levels being 0, 25, 50, and 100 μmol/L, respectively. The results showed that in vitro oxidation of myosin, reactive oxidative species promoted protein modifications that propagated the conformation changes on myosin secondary structure, leading to a significant increase (P < 0.05) in the protein carbonyl content in the high oxidation treatment, when compared to the low and medium treatments. The results of the Western blot analysis and sodium-polyacrylamide amide-dodecyl sulfate gel electrophoresis demonstrated that in vitro oxidation actively promoted the degradation of protein molecular weight and Desmin (53 kDa) deterioration in the sample with the highest concentration treatment of H 2 O 2. Circular dichroism spectra showed a decrease in α-helix content from 70.1% to 29.8% and an increase in β-sheet percentage from 5.6% to 17.4%. Meanwhile, cysteine and methionine as the free amino acids were most susceptible to oxidative change due to their high sensitivity to the reaction of the sulfur group of amino acids. Protein degradation induced by oxidation resulted in significant changes to the protein secondary structure, as evidenced by the observation of oxidized myosin under different H 2 O 2 levels. [ABSTRACT FROM AUTHOR]

This research evaluates the quality and acceptability of 25%, 50%, and 75% fish protein concentrate (FPC) egg substituted cakes. Proximate chemical composition, batter and cake physical quality, microbial activity, and sensory evaluation were carried out. An impressive increase in protein, fats and ash content was noticed in GPSC. By increasing the FPC concentration, the viscosity of batters and cake volume decreased, while the specific gravity increased. In texture, the hardness, chewiness, and gumminess were increased while elasticity and cohesiveness were decreased with increase in substitution. An increase in substitution increased baking loss (BL) but reduced the water activity (aw) and the number and size of air bubbles in cake batter. By evaluating microbial activity, the total plate count (TPC) value increased, and the total fungal count (TFC) decreased while the cake crumb showed inconsistent change. The cake with 75% egg substitution was highly nutritious but displayed lower sensory attributes due to slight fishy flavor and dark color, while 25% and 50% GPSCs showed almost similar sensory attributes but nutritional value of 50% GPSC was higher than 25% GPSC. So, the cake with 50% substitution is a better option to prepare a good quality protein-enriched cake with acceptable sensory attributes. [ABSTRACT FROM AUTHOR]

The aim of the present study was to evaluate the quality of wheat flour noodles supplemented with 0%, 10%, 20% and 30% grass carp protein concentrate (GCPC). Proximate composition, color analyses, cooking properties (cooking time, cooking loss, swelling index and water absorption index), texture profile analyses (TPA), and sensory evaluations were performed. Significant (p ≤ 0.05) increases in the crude protein (18.94–35.19%), crude fat (0.88–1.06%), and ash (0.89–1.02%) were recorded for GCPC supplemented cooked noodles. Brightness (L*), redness (a*) and yellowness (b*) of GCPC 10–30% fortified cooked noodles was recorded as 77.00–69.97, 1.50–2.81 and 19.08–22.25, respectively. Cooking loss (CL) and optimal cooking time were significantly (p ≤ 0.05) increased with increased level of GCPC from 0%-30%, with readings of 6.13% −8.90% and 7.53–11.77 min, respectively. Swelling and water absorption indices showed a decreasing trend at 2.33–1.77 and 102.6–78.49%, respectively. A significant (p ≤ 0.05) decreasing trend in hardness (24.50–19.03N) was observed for cooked noodles with increased level of GCPC, while the remaining TPA parameters were significantly different at p>.05. Sensory analyses showed higher preference for control noodles and noodles fortified with 10% GCPC. [ABSTRACT FROM AUTHOR]

The effects of washing and membrane removal pretreatments on the antioxidant properties of grass carp protein hydrolysates prepared through in vitro digestion were investigated. Furthermore, antioxidant hydrolysate was fractionated using ultrafiltration membranes (10, 5, 3 and 1 kDa). Oxygen radical antioxidant capacity ( ORAC), DPPH and ABTS-scavenging activity in a gastrointestinal digest produced from pretreated minced grass carp was increased 1.74-fold, 1.08-fold and 1.72-fold, respectively, compared to untreated minced carp. Compared to the alkaline protease hydrolysate, ORAC, ferric reducing antioxidant power, ABTS- and DPPH-scavenging activity in a gastrointestinal digest prepared from pretreated minced carp were reduced by 11.5%, 60.9%, 16.3% and 78.4%, respectively. The ultrafiltration fraction (<1 kDa) displayed the highest antioxidant activity. The size of molecular weight and the amount of hydrophobic and aromatic residues in hydrolysates played an important role in antioxidant activity. Low-molecular-weight fish hydrolysates could serve as a potential source of functional ingredients for promoting health. [ABSTRACT FROM AUTHOR]

This study investigated the effect of chitosan coating (1 and 2%) on the quality of grass carp fillets stored at 4°C for 20 days. During storage, the physicochemical properties (pH, thiobarbituric acid value, total volatile basic nitrogen value, trimethylamine nitrogen value,K-value, water loss, and instrumental texture), microbiology (total viable count and psychrotrophic count), and sensory properties were evaluated. The results showed that chitosan coating could effectively inhibit bacterial growth, improve physicochemical and sensory qualities, and it reduced the deterioration of the quality of grass carp fillets. Compared with fillets without chitosan coating, the shelf life of fillets with 1 and 2% chitosan coating was extended by ~3 and 6–7 days, respectively. [ABSTRACT FROM AUTHOR]

The effects of different microbial inoculation strategies on the quality of Chinese traditional rice mash grass carp were investigated. Individual inoculation (Pediococcus pentosaceus 220, Staphylococcus xylosus 135, Saccharomyces cerevisiae 22), and mixed triple inoculation of the above strains (Pediococcus-Staphylococcus-Saccharomyces , PSS) were employed, including an uninoculated control. It was found that the inoculated microbes proliferated differently, depending on the inoculation regime. The decreases of pH, hardness, and increases of TCA-soluble peptides were accelerated, while the accumulation of TVB-N and TBARS was reduced in all inoculated groups. A significantly higher level of total free amino acids was found in PSS-inoculated grass carp (P < 0.05), as well as alcohols and esters. Moreover, the total sensory score of PSS-inoculated grass carp with 15-day fermentation was closest to the control with 30-day fermentation. Consequently, the inoculation of a mixed starter culture could help reduce the fermentation time of rice mash fish by 50% while providing the optimum texture and flavor. • Selected bacteria and yeast improved the quality of rice mash grass carp. • Co-inoculation obviously retarded the accumulation of TVB-N and TBARS. • Co-inoculation accelerated pH decrease and formation of texture. • Co-inoculation produced more total free amino acids, alcohols and esters. • Co-inoculation shortened fermentation time by 50%. [ABSTRACT FROM AUTHOR]

The role of lipids in changes of volatile organic compounds (VOCs) in grass carp during 1 month of frozen storage with different freeze-thaw cycles and subsequent heat treatment was investigated. Sixty VOCs were identified in all groups by SPME-GC–MS. Odor contents fluctuated along with the freeze-thaw cycles and heat treatment, and the highest odor content was observed in frozen sample without freeze-thaw cycles. Freeze-thaw and heat treatment significantly promoted the lipid oxidation and hydrolysis for all the groups(p<0.05). Lipid metabolites were analyzed using non-targeted lipidomics and could be well distinguished among different freeze-thaw groups and heat-treatment groups. A total of 10 key differential lipid molecules were annotated, involving 4 metabolic pathways related to lipid degradation and odor formation. Spearman correlation analysis showed that these key differential lipids were significantly related to the formation of key VOCs (p<0.05). [Display omitted] • Impact of freeze-thaw cycles and heat treatment on lipid and VOCs change was studied. • Sixty VOCs in freeze-thawed and heated grass carp were identified. • VOCs contents fluctuated dynamically with freeze-thaw cycles. • Key lipids enriched in four pathways associated with odor metabolism. • Ten key differential lipid metabolites related to odor formation were identified. [ABSTRACT FROM AUTHOR]

[Display omitted] • DIA-based proteomics revealed new texture biomarkers for grass carp fillets (GCF). • Isobutyryl-CoA dehydrogenase, mitochondrial was a key marker for chilled GCF. • [Phosphatase 2A protein]-leucine-carboxy methyltransferase marked chilled GCF. • Staphylococcal nuclease domain-containing protein 1 was a key marker for iced GCF. Consumers care about the texture of fresh fish flesh, but a rapid quantitative analytical method for this has not been properly established. In this study, texture-associated biomarkers were selected by DIA-based proteomics for possible future application. Results indicated a significant decline in texture and moisture characteristics with extended storage under chilled and iced conditions, and flesh quality was categorized into three intervals. A total of 8 texture-associated biomarkers were identified in the chilled storage group, and 3 distinct ones in the iced storage group. Biomarkers were further refined based on their expression levels. Isobutyryl-CoA dehydrogenase, mitochondrial and [Phosphatase 2A protein]-leucine-carboxy methyltransferase were identified as effective texture-associated biomarkers for chilled fish, and Staphylococcal nuclease domain-containing protein 1 for iced fish. This study provided suitable proteins as indicators of fresh fish flesh texture, which could help establish a rapid and convenient texture testing method in future studies. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study was to investigate the effect of superchilling at −3°C compared with ice storage at 0°C on the biochemical and physical properties of grass carp fillets. Fillets stored at −3°C showed significant changes in whiteness, drip loss and textural hardness, while changes in pH, total volatile basic nitrogen and TCA-soluble peptides were slowed down. Partial denaturation of myosin as demonstrated by differential scanning calorimetry differed between fillets stored at −3 and 0°C in that the transition peak showed a left shoulder at −3°C and sharpened at 0°C. Detachments between muscle cells and formation of cracks within cells were accelerated during storage at −3°C, and from 10days on, clear spaces between and within cells were observed with the concurrent appearance of white spots on the surface of fillets, suggesting the formation of both extra- and intracellular large ice crystals. [Copyright &y& Elsevier]

• IF reduced thawing loss and maintained the water holding capacity during storage. • IF sample had small and uniform distribution of ice crystals during storage. • IF sample had a shorter T 2 relaxation time during storage. • IF effectively maintained the content of unfrozen water during storage. • IF was an effective method to reduce water migration of grass carp during storage. Recently, immersion freezing (IF) has attracted the attention of many food researchers due to its potential in improving the quality of frozen food. In this study, the effects of air blast freezing (AF), IF and liquid nitrogen freezing (LNF) on the water holding capacity ice crystals, and water migration in grass carp muscles during frozen storage were evaluated. The thawing loss in AF, IF and LNF samples were 13.88 %, 5.49 % and 7.08 %, respectively, and was significantly lower in IF samples than in AF and LNF samples (P < 0.05). Besides, the water holding capacity and pH of all samples gradually decreased during frozen storage. Smaller and more regular intracellular ice crystals were observed in frozen samples during LNF and IF compared with AF. Moreover, IF and LNF had positive effects on the size and distribution of ice crystals. Low field nuclear magnetic resonance (LN-NMR) results showed that IF reduced water migration during frozen storage. In addition, with prolongation of frozen storage time, IF effectively maintained the content of unfrozen water. Therefore, IF can be used as an alternative to the rapid freezing of grass carp with better restriction of water in tissues. [ABSTRACT FROM AUTHOR]

• Intramuscular connective tissue (IMCT) was mainly degraded in first 10 days. • IMCT degradation was mainly attributed to decorin. • IMCT was hydrolyzed by endogenous collagenase rather than cathepsins. • IMCT and myofibrils (MF) contributed equally to fish texture maintenance. • IMCT failed to protect MF in the process of texture degradation. Although intramuscular connective tissue (IMCT) is low in fish, its impact on texture cannot be ignored due to its special location. Therefore, this study was aimed to investigate the contribution of IMCT degradation to fish softening and its mechanism induced by endogenous proteases. Results showed that IMCT honeycomb-like structure collapsed entirely on the 10th day of ice storage, along with a decrease of shear force by 36.5%. Meanwhile, IMCT and myofibrils (MF) degradation accelerated softening by 25.1% and 15.3% during 10 days of ice storage, respectively. Next, IMCT deterioration was indicated to be highly correlated with decorin degradation (0.956**), followed by elastin (0.928**) and collagen (0.904**). Ulteriorly, endogenous collagenase was shown to degrade IMCT crucial components, while endogenous cathepsins had little effect. In conclusion, this study confirmed that IMCT played an essential role in maintaining fish texture and was mainly degraded by endogenous collagenase. [ABSTRACT FROM AUTHOR]

Abstract The effects of a chitosan-based coating on the inhibition of microbial spoilage of grass carp (Ctenopharyngodon idellus) fillets were studied during refrigerated storage for 15 days in terms of pH, total volatile base nitrogen (TVB-N), trimethylamine (TMA), adenosine triphosphate (ATP)-related compounds, K value, microbial enumeration and high-throughput sequencing. The results indicated that the fillets treated with chitosan-based coating enriched with 0.3% glycerol monolaurate and 0.5% clove essential oil had lower values of TVB-N, TMA, hypoxanthine riboside (HxR), hypoxanthine (Hx) and K value with the significant reductions (P < 0.05) of nearly 34, 73, 32, 74 and 38%, respectively, when compared to the control at day 15 of storage. Using high-throughput sequencing analysis, the major bacteria phyla of Firmicutes , Cyanobacteria and Bacteroidetes and the bacteria family of Lachnospiraceae and Bacteroidaceae were observed in fresh grass carp. As storage time increased, the coated samples retained bacterial diversity. However, Shewanellaceae , Pseudomonadaceae and Flavobacteriaceae increased and became the predominant microbiota in spoiled control samples. The significant difference between the bacteria species in the control and coated samples showed that the coating had the potential to inhibit microbial growth, especially spoilage microorganisms, and reduced quality deterioration caused by bacteria during refrigerated storage of grass carp fillets. Highlights • Effect of chitosan coating on bacterial composition and quality changes of grass carp was assessed • Lachnospiraceae and Bacteroidaceae were the predominant species in fresh grass carp • Shewanellaceae , Pseudomonadaceae and Flavobacteriaceae became the main microbiota in spoiled samples • Chitosan-based coating effectively inhibited the growth of spoilage bacteria in refrigerated fillets. [ABSTRACT FROM AUTHOR]

The study evaluated the inhibitory effects of chitosan-based coatings on endogenous enzyme activities, proteolytic degradation and texture softening of grass carp ( Ctenopharyngodon idellus ) fillets stored at 4 °C for 15 days. In general, the inhibitory effects of chitosan coating were worse than that of chitosan composite coatings, which significantly inhibited cathepsin B, B + L and calpain activities of fillets, with the most reductions of 58%, 65% and 41%, respectively when compared to the corresponding values of control fillets. Besides, the treatments also retarded the proteolytic degradation through quantifiable indicators including salt-soluble proteins (SSP), TCA-soluble peptides and SDS-PAGE pattern. After storage of 15 days, the shear forces of coated fillets were 28–53% higher than that of control samples. These results indicated that chitosan composite coating combined with glycerol monolaurate and clove essential oil improved the quality of refrigerated grass carp fillets by alleviating endogenous enzyme induced proteolysis and softening. [ABSTRACT FROM AUTHOR]

This study investigated the effects of chitosan-based coatings on flavor retention of refrigerated grass carp fillets by using various indicators: free amino acids (FAA), nucleotides, trimethylamine (TMA), volatile profile, sensory quality, and electronic nose analysis. The results indicated that chitosan-based coatings contributed to the significant reduction of off-flavor compounds, such as TMA, hypoxanthine (Hx) and histidine, and accumulation of inosine monophosphate (IMP) and umami-associated FAA. GC–MS analysis showed 23 volatile organic compounds, including many C 5 –C 9 aldehydes and alcohols in the fresh fillets. The coating treatments, especially chitosan-clove bud essential oil composite coatings, sharply reduced the relative content of off-odor volatiles, such as hexanal, octanal and 1-octen-3-ol. According to the results of the sensory evaluation and electronic nose analyses, chitosan coating combined with glycerol monolaurate and clove bud essential oil was a promising method to improve the edibility of grass carp fillets by maintaining flavor quality during refrigerated storage. [ABSTRACT FROM AUTHOR]

Maximizing the waste valorization of renewable resources to develop biocomposite materials is highly desirable regarding environmental sustainability and food chain security. Herein, lignin nanoparticles (LNPs) obtained using deep eutectic solvent (DES) by anti-solvent strategy were employed as nanofillers in the chitosan matrix to fabricate novel bioactive packaging films. The results showed that adding LNPs reduced the swelling degree and water vapor transmittance of chitosan films. Upon introduction of LNPs content, the tensile strength of the composite films significantly increased by about 40%, whereas the elongation at break showed an opposite trend. When the content of LNPs was 10 wt %, the UV shielding and free radical scavenging ability of the composite films reached the maximum point. However, the diameter of the inhibition zone only increased at the addition of 1 wt%. Furthermore, the refrigerated (4 ± 1 °C) grass fish wrapped with composite films revealed better quality and the shelf life was extended to 10 days. Therefore, nanocomposite films provide an attractive and promising pathway for high value-added commercial utilization of lignin and chitosan for food packaging applications. [Display omitted] • Chitosan-lignin nanoparticles (LNPs) films developed with edible solvents. • Resultant films displayed enhanced water resistance, mechanical properties. • Even 1 wt% LNPs improved UV-blocking, antibacterial and antioxidant activity. • The nanocomposite films were firstly clarified to extend the shelf life of filets. [ABSTRACT FROM AUTHOR]

• Chitosan-GTE coating inhibited texture softening of refrigerated fillets. • Lower levels of enzyme activities and proteolysis were shown in coated fillets. • Active coating stabilized proteome of fillets during refrigerated storage. • The down-regulation of structural proteins was effectively retarded after coating. • Protective patterns of flesh texture stabilization by the coating were decoded. Herein, the protective pattern of chitosan coating enriched with green tea extract on texture stabilization of refrigerated grass carp fillets was explored. In general, higher shear force and lower endogenous enzyme activities were observed in coated fillets, with the max. level of cathepsins and calpain decreasing by 30.2 ∼ 39.6 % when compared to the control during storage. Meanwhile, the coating reduced protein composition changes and accumulation of protein degradation products. According to label-free proteomic analysis, the proteome closer to fresh sample was observed in coated fillets than that of the control, as supported by PCA and hierarchical cluster analysis. Meanwhile, 33 differential proteins involved in tissue structure, protein phosphorylation and protein turnover were further screened out, and most DAPs showed up-regulation in coated fillets compared to the control on day 12. Presumably, the coating modulated endogenous enzyme-induced myofibrillar protein degradation and protein phosphorylation level, thereby stabilizing the texture properties of refrigerated fillets. [ABSTRACT FROM AUTHOR]

Softening, causing by the degradation of collagen, leads to serious quality deterioration of fish fillets during post-mortem storage. In order to study the role of endogenous enzymes in softening of fish muscle, collagen fibers from grass carp (Ctenopharyngodon idellus) were degraded by serine proteinase. The structural and biochemical changes of collagen fibers with serine proteinase digestion were investigated in terms of scanning electron microscopy, fourier transform infrared spectroscopy, electrophoresis and chemical analysis. In the present study, an endogenous proteinase (approximately 250 kDa) with gelatinolytic activity was isolated from grass carp muscle by ammonium sulfate precipitation and column chromatographies, and identified as serine proteinase. Collagen fibers were partly disaggregated into collagen fibrils after serine proteinase treatment for 48 h, coinciding with a release of hydroxyproline (5.56 ± 0.02%) and glycosaminoglycans (9.04 ± 0.03%). However, almost all gelatin and 40% of acid-soluble collagen were degraded within 1 h and 6 h, respectively. Meanwhile, serine proteinase-treated collagen fibers maintained the triple-helical structure of collagen molecule, while their thermal stability was gradually decreased with digestion time. All results indicated that disintegration of collage fibers by serine proteinase might be due to both the degradation of proteoglycans and collagen molecules during textural deterioration of chilled grass carp fillets. • Collagen fibers were partly degraded into collagen fibrils by serine proteinase. • Serine proteinase freed hydroxyproline and glycosaminoglycan from collagen fibers. • Collagen fibers were more difficultly degraded than collagen monomer and gelatin. • Serine proteinase involves in textural deterioration of grass carp fillets. [ABSTRACT FROM AUTHOR]

Fish texture is important for consumers and degradation of myofibrillar structural proteins could cause softening. However, the role of endogenous proteases in degrading was not clear. In this study, protease inhibitors (Ac-DEVD-CHO, E−64 and EDTA-2Na) were used to distinguish the role of endogenous caspase-3, cathepsin B and L, and calpain. Results showed that caspase-3 and calpain had relatively high activity at initial ice-storage and decreased quickly afterwards, while cathepsins increased firstly and then fluctuated. Titin, nebulin and troponin-T were partially degraded by caspase-3 firstly. However subsequently, only titin and troponin-T were affected by cathepsins and calpain. Desmin showed a different degradation pattern. It was influenced by calpain significantly but not by caspase-3. Less than 2.5% of desmin remained after storage. Myofibril fragmentation index and shear force indicated that caspase-3-induced degradation occurred at initial ice-storage, while cathepsins had the greatest importance in texture softening during the whole storage. • Role of endogenous caspase-3, cathepsins and calpain were specified in ice storage. • Caspase-3 initiated degradation of titin, nebulin and troponin-T in fish. • Calpain mainly degraded desmin and desmin remained lower than 2.5% finally. • Cathepsins contributed the most for the fish softening during whole ice storage. [ABSTRACT FROM AUTHOR]

• Ultrasound was an effective auxiliary measure for COS coating preservation. • Ultrasound assisted COS (COS-UA) coating retarded the quality loss of fillets. • Microbial composition of refrigerated fillets treated with coatings were analyzed. • Spoilage bacteria including Aeromonas and Shewanella were inhibited after coatings. • The shelf life of refrigerated fillets was prolonged by 5 days after COS-UA coating. Effects of ultrasound-assisted chitooligosaccharides (COS-UA) coating on the quality attributes and microbial composition of refrigerated grass carp fillets were evaluated. The results showed that COS and COS-UA coatings retarded quality deterioration of fillets during storage. Compared to COS coatings, COS-UA treated samples had lower contents of BAs, simultaneously their levels of total volatile base nitrogen (TVB-N), K value and total viable counts (TVC) were further decreased by 13.6%, 4.2% and 7.8% on day 12, respectively. High-throughput sequencing showed that Aeromonas and Shewanella increased rapidly in control samples and became the main microbiota at day 12. By contrast, both coatings changed the microbial composition and reduced the proportion of spoilage organisms. Based on multiple evaluations, COS-UA extended shelf life of fillets by another 2 days when compared to COS. Therefore, ultrasonic treatment could be considered as an effective supplementary to improve the preservation effect of COS-based coatings for fresh preprocessed fish. [ABSTRACT FROM AUTHOR]

• Small myofibrillar structural proteins degradation initiated fish softening. • More giant myofibrillar structural proteins degraded during later storage. • Desmin and titin contributed more to softening comparatively. Softening is always a problem in fish preservation. This study was aimed to investigate the role of myofibrillar structural proteins degradation in fish softening. The changes of myofibrillar structural proteins, muscle ultrastructure, myofibril fragmentation, and shear force were studied. The results indicated that during the superchilled preservation of grass carp (Ctenopharyngodon idella), small (low-molecular-weight) myofibrillar structural proteins like desmin and troponin-T initiated textural deterioration, leading to Z-disk weakening and actin loosening. In contrast, giant (high-molecular-weight) myofibrillar structural proteins like titin and nebulin were degraded in more amount in the later storage, contributing to Z-disk and M-band disassembly and vague of light and dark regions (I and A bands). Compared to each other, desmin and titin played more important part in softening. All these changes were involved in the increase of muscle fibril segments and the sharp decrease of shear force. [ABSTRACT FROM AUTHOR]