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Complex microbial communities have an important impact on the flavor of low salt dry-curing (LSD)-pretreated grass carp blocks. Here, the flavors, metabolites, and bacterial diversity of LSD-pretreated fish during cold storage were analyzed using flavor analysis, metabolomics, and high-throughput sequencing to investigate their correlations in detail. LSD promoted the volatile flavor deterioration of grass carp blocks under 6 days of refrigeration but inhibited it under 15 days of refrigeration. Furthermore, 924 metabolites were identified in the refrigerated grass carp blocks, and LSD inhibited the growth of Psychrophilic dominant spoilage microorganisms (Proteobacteria) and promoted microbial abundance (Actinobacteriota , Firmicutes , Bacteroidota, and Cyanobacteria). Correlation analysis revealed that the degradation of phosphatidylcholine connected with the monomonas genus in LSD-pretreated fish blocks played a vital role in inhibiting the key volatile flavor (esters, aldehydes, and alcohols) deterioration. This information is useful for elucidating the inhibition mechanism of LSD on flavor deterioration in refrigerated fish blocks. [Display omitted] • The volatiles, metabolites and microorganisms in the LSD fillets were studied. • LSD promoted volatile flavor deterioration in early cold stage but inhibited it later. • LSD inhibited the dominant microorganism growth and promoted microbial abundance. • Dominant microorganisms inhibited flavor deterioration by mitigating metabolites. [ABSTRACT FROM AUTHOR]

The effect of NaCl content on the protein structures, water status and thermal properties of grass carp flesh, along with its relationship with energy required for freezing was investigated to improve the quality of frozen flesh and to reduce energy consumption for freezing. Adding salt prompted the shifting of the secondary structure of α-helix to β-sheet, β-turn and random coil. The interaction between water molecules and hydrophilic groups in the unfolded protein structure increased nonfreezing water content, which decreased specific heat capacity, thermal conductivity of sample during freezing. The lowest energy required (235.69 kJ/kg) for freezing was found in 7% NaCl salted sample, indicating a 38.39% reduction compared to the unsalted sample. Few pores between muscle fibers were observed in the 3% and 5% NaCl salted sample. Therefore, an optimal salt concentration improved quality of frozen flesh and reduced the energy required for freezing, promoting energy-efficient freezing of aquatic products. • Salt addition increased nonfreezing water content (NFWC) of fish meat • The higher NFWC of fish meat reduced the thermal properties during freezing • The lowest energy required for freezing was observed in 7% NaCl salted fish meat • The tissue microstructure of fish meat was improved when salted in 3–5% NaCl solution [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]

In this study, the electrospinning technique was used to co-encapsulate Quercetin (Qu) and Lactiplantibacillus plantarum 1–24-LJ in PVA-based nanofibers, and the effect of bioactive films on fish preservation was evaluated at the first time. The findings indicated that both Lpb. plantarum 1–24-LJ and Qu were successfully in the fibers, and co-loaded fibers considerably outperformed single-loaded fiber in terms of bacterial survival and antioxidant activity. Following fish preservation using the loaded fibers, significant reductions were observed in TVB-N, TBARS, and microbial complexity compared to the control group. Additionally, the co-loaded fibers more effectively reduced the counts of H 2 S-producing bacteria and Pseudomonas. In the future, fibers with both active substances and LAB hold promise as a novel approach for fish preservation. [Display omitted] • Co-loading of flavonoids quercetin (Qu) and Lpb. plantarum by electrospinning technology. • Qu and Lpb. plantarum synergize provide strong antioxidant and hyper-bacterial activity. • Qu or/and Lpb. plantarum films inhibit the growth of spoilage bacteria. • Active films improved fish's physiology, raised sensory appeal and extended storage time. [ABSTRACT FROM AUTHOR]

The protective mode of PostbioYDFF-3 (referred to as postbiotics) on the quality stability of refrigerated fillets was explored from the aspects of endogenous enzyme activity and the abundance of spoilage microorganisms. Compared to the control group, the samples soaked in postbiotics showed significant reductions in TVC, TVB-N and TBARS values by 39.6%, 58.6% and 25.5% on day 5, respectively. In addition, the color changes, biogenic amine accumulation and texture softening of the fish fillets soaked in postbiotics were effectively suppressed. Furthermore, the activity of endogenous enzyme activities was detected. The calpain activities were significantly inhibited (p < 0.05) after soaking in postbiotics, which declined by 23%. Meanwhile, high throughput sequencing analysis further indicated that the growth of spoilage microorganism such as Acinetobacter and Pseudomonas were suppressed. Overall, the PostbioYDFF-3 was suitable for preserving fish meat. • The quality of grass carp fillet was maintained using PostbioYDFF-3. • The inhibitory of PostbioYDFF-3 on Acinetobacter and Pseudomonas was confirmed. • The activity of calpain was reduced by PostbioYDFF-3. [ABSTRACT FROM AUTHOR]

Highlights • Gelation of myofibrillar protein-Na 4 P 2 O 7 ·10H 2 O or Na 5 P 3 O 10 (MP-SPP or STP) was studied. • Phosphate weakened MP network as instantaneous and retarded compliance rose at 4 and 40 °C. • MP changed from weak gel into concentrated sol as 280 mg/kg SPP or 440 mg/kg STP added. • Phosphate contributed to smaller pore size and finer structure for MP gel network. • Phosphate more tightly trapped immobile water into smaller-sized pores of MP network. Abstract Effect of phosphates on the heat-induced gel characteristics of myofibrillar protein (MP) from grass carp was investigated. Both heating and phosphates exerted significant influences. Heating induced more elastic, water-holding and less flowing gel. But phosphates had diverse effects at different temperatures. At 4 °C and 40 °C, phosphate as a dominant factor reduced the gel elasticity and resistance and increased flowability with increasing levels of phosphates. Furthermore, 280 mg/kg sodium pyrophosphate (SPP) or 440 mg/kg sodium triphosphates (STP) transformed MP from weak gel into concentrated solution. It clarified phosphates disentangled MP macromolecules and inhibited their aggregation and pre-gelation at low temperature. At 80 °C, heating accompanied with phosphates governed MP gelation. The appropriate level of phosphates (SPP superior to STP) endowed MP-phosphate gels with the lowest flowability and greatest elasticity, textural properties as well as finest microstructures. Besides, phosphates entrapped a portion of weak immobile water more tightly into smaller-sized pores of protein network. [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]

[Display omitted] • More down-regulated phosphosites of structural proteins were found at d6 than d0. • Dephosphorylation of structural proteins was related with texture softening. • Myosin, actin, collagen and cytoskeleton were major structural dephosphoproteins. • Kinases, phosphatases and assembly regulators tended to be dephosphorylated. • Dephosphorylation mostly appeared at Serine site in structural proteins of muscle. Texture is an important sensory attribute of fish affected by modifications of structural proteins in muscle architecture. To investigate the changes in protein phosphorylation during texture softening of fish, the proteins of grass carp muscle after chilling storage of 0 day and 6 days were compared by phosphoproteomics, and their association with texture was analyzed. Totally 1026 unique phosphopeptides on 656 phosphoproteins were identified as differential. They were mainly classified as intracellular myofibril and cytoskeleton, and extracellular matrix, of which the molecular function and biological process were binding into supramolecular assembly and myofilament contraction. The concomitant dephosphorylation of kinases and assembly regulators indicated dephosphorylation and disassembly tendency of sarcomeric architecture. Correlation analysis defined the relation between texture and dephosphorylation of myosin light chain, actin, collagen and cytoskeleton. This study revealed that protein phosphorylation may affect the texture of fish muscle through regulating sarcomeric assembly of structural proteins in muscle architecture. [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]

Flesh quality, muscle antioxidant status and related signalling molecule expressions were investigated in young grass carp fed six levels of tryptophan (Trp) for 8 weeks. The results indicated that fish fed 0.7 (deficiency) and 6.1 g Trp g/kg (excess) diets exhibited lower muscle water-holding capacity, tenderness, cathepsin activity, protein levels, lipids and collagen contents. Optimal Trp reversed these negative effects, which were related to enhanced glutathione (GSH) content and glutathione peroxidase (GPx) activities regulated at gene transcription levels, rather than to superoxide dismutase (SOD) or catalase (CAT). The expression of signalling molecules [Kelch-like ECH-associated protein 1, target of rapamycin (TOR) and ribosomal S6 protein kinase 1] involved in the NF-E2-related factor 2 (Nrf2) pathway revealed a potential method of Trp-enhanced antioxidant defence. Collectively, the present study indicated that appropriate Trp levels improved flesh quality partly related to the enhancement of antioxidant ability through Nrf2 and TOR signalling. [ABSTRACT FROM AUTHOR]

The changes of actomyosin, proteolytic activities and myofibril fragmentation during the postmortem aging of grass carp were studied. The study revealed dramatically increased actomyosin dissociation within 6 h of storage postmortem in grass carp, and it was associated with the drop of pH from 6.9 to 6.7, while liberated actin remained almost unchanged after 6 h postmortem. The myofibril fragmentation also increased significantly with the storage time in 6 h, and a highly positive correlation ( P < 0.01) existed between MFI and cathepsin B, D, H activities. The study indicated both actomyosin dissociation and cathepsin B, D, H played a role in postmortem tenderization and textural changes in grass carp. [ABSTRACT FROM AUTHOR]

The potential use of feature wavelengths for predicting drip loss in grass carp fish, as affected by being frozen at −20 °C for 24 h and thawed at 4 °C for 1, 2, 4, and 6 days, was investigated. Hyperspectral images of frozen–thawed fish were obtained and their corresponding spectra were extracted. Least-squares support vector machine and multiple linear regression (MLR) models were established using five key wavelengths, selected by combining a genetic algorithm and successive projections algorithm, and this showed satisfactory performance in drip loss prediction. The MLR model with a determination coefficient of prediction ( R 2 P ) of 0.9258, and lower root mean square error estimated by a prediction (RMSEP) of 1.12%, was applied to transfer each pixel of the image and generate the distribution maps of exudation changes. The results confirmed that it is feasible to identify the feature wavelengths using variable selection methods and chemometric analysis for developing on-line multispectral imaging. [ABSTRACT FROM AUTHOR]

• Low frequency EMF changed Ca2+ATPase activity of grass carp in cold storage. • The EMF between 100 and 400 Hz inhibited protein oxidation during cold storage. • Lipid oxidation of grass carp was affected during low frequency EMF exposure. • Low frequency EMF reduced the relaxation of water molecules of grass carp. This work studied the effects of electromagnetic fields (EMF) with frequencies between 100 and 400 Hz and a fixed strength of 12 mT on cold storage of grass carp at 4 °C for 30 min, and Ca2+ATPase enzyme activities, and lipid and protein oxidations in samples were measured to assess changes in intracellular Ca2+ concentration and oxidative stability. Results showed higher Ca2+ATPase activities in samples treated with EMF frequencies. Significant (p < 0.05) decreases occurred in protein oxidation for samples treated between 100 and 300 Hz, but an increase was observed for treatment with 400 Hz. However, the lipid oxidation increased for samples treated up to 200 Hz and decreased with further increase in frequency to 300 and 400 Hz. Nuclear magnetic resonance analysis showed that exposure to different frequencies of EMF could reduce the association of water molecules with protein for both bound and immobilized water. Overall, treatments of EMF between 100 and 400 Hz could improve grass carp quality during cold storage. [ABSTRACT FROM AUTHOR]

K value is an important freshness index widely used for indication of nucleotide degradation and assessment of chemical spoilage. The feasibility of hyperspectral imaging (400–1000 nm) for determination of K value in grass carp and silver carp fillets was investigated. Partial least square (PLS) regression and least square support vector machines (LS-SVM) models established using full wavelengths showed excellent performances and the PLS model was better with higher determination coefficients of prediction ( R 2 P = 0.936) and lower root mean square errors of prediction (RMSEP = 5.21%). The simplified PLS and LS-SVM models using the seven optimal wavelengths selected by successive projections algorithm (SPA) also presented good performances. The spatial distribution map of K value was generated by transferring the SPA-PLS model to each pixel of the images. The current study showed the suitability of using hyperspectral imaging to determine K value for evaluation of chemical spoilage and freshness of fish fillets. [ABSTRACT FROM AUTHOR]

This study investigated the effects of alkaline pretreatments and acid extraction conditions on the production of acid-soluble collagen (ASC) from grass carp skin. For alkaline pretreatment, 0.05 and 0.1 M NaOH removed non-collagenous proteins without significant loss of ASC at 4, 10, 15 and 20 °C; while 0.2 and 0.5 M NaOH caused significant loss of ASC, and 0.5 M NaOH caused structural modification of ASC at 15 and 20 °C. For acid extraction at 4, 10, 15 and 20 °C, ASC was partly extracted by 0.1 and 0.2 M acetic acid, while 0.5 and 1.0 M acetic acid resulted in almost complete extraction. The processing conditions involving 0.05–0.1 M NaOH for pretreatment, 0.5 M acetic acid for extraction and 4–20 °C for both pretreatment and extraction, produced ASC with the structural integrity being well maintained and hence were recommended to prepare ASC from grass carp skin in practical application. [ABSTRACT FROM AUTHOR]

The suitability of hyperspectral imaging technique (400–1000 nm) was investigated to determine the thiobarbituric acid (TBA) value for monitoring lipid oxidation in fish fillets during cold storage at 4 °C for 0, 2, 5, and 8 days. The PLSR calibration model was established with full spectral region between the spectral data extracted from the hyperspectral images and the reference TBA values and showed good performance for predicting TBA value with determination coefficients ( R 2 P ) of 0.8325 and root-mean-square errors of prediction (RMSEP) of 0.1172 mg MDA/kg flesh. Two simplified PLSR and MLR models were built and compared using the selected ten most important wavelengths. The optimised MLR model yielded satisfactory results with R 2 P of 0.8395 and RMSEP of 0.1147 mg MDA/kg flesh, which was used to visualise the TBA values distribution in fish fillets. The whole results confirmed that using hyperspectral imaging technique as a rapid and non-destructive tool is suitable for the determination of TBA values for monitoring lipid oxidation and evaluation of fish freshness. [ABSTRACT FROM AUTHOR]

The effects of frying temperature and the number of frying cycles on the formation of heterocyclic amines (HAs) and trans fatty acids (TFAs) in grass carp were investigated. 9 t -18:1 FAs was detected in all samples. The TFA contents of samples fried at 150–210 °C were not significantly different ( P > 0.05). The content and number of different types of HAs increased with increasing frying temperature. 9H-pyrido[3,4- b ]indole (Norharman), 1-methyl-9H-pyrido [3,4- b ]indole (Harman), and 2-amino-3,7,8-trimethylimdazo[4,5- f ]quinoxaline (7,8-diMeIQx) were detected in most of the tested samples. The differences in the surface colour (Δ E ) increased with frying temperature, and Δ E of samples fried at 170 °C was significantly higher than that of 150 °C ( P < 0.05). The analysis of different cycle times revealed that the TFA levels increased with an increase in the usage period of the frying fat, and 9 t ,12 t -18:2 FAs was detected after the 40th frying cycle. As the number of frying cycles increased, the number of different types of HAs increased, seven types of HAs were detected after the 25th frying cycle. [ABSTRACT FROM AUTHOR]

Highlights: [•] A gelatinolytic proteinase was purified from grass carp muscle. [•] The purified proteinase was metalloproteinase according to its characters and PMF analysis. [•] The purified metalloproteinase could hydrolyse gelatin and type I collagen. [ABSTRACT FROM AUTHOR]

[Display omitted] • Monascus treatment promoted the release of free amino acid from fish protein. • Monascus treatment enhanced the decomposition of fish protein during digestion. • Monascus treatment reduced ammonia and indole during fermentation of fish protein. • Monascus -treated fish protein improved the SCFA release and microbial community. The aim of this study was to investigate the digestion and fermentation properties of fish protein fermented by Monascus. Semi-dried fish was fermented by applying Monascus purpureus Went M 3.439. Our results show that the Monascus fermentation of the fish protein enriched the free amino acids and achieved a relatively higher glutamate content than the control group. The Monascus treatment promoted the decomposition of the fish protein during in vitro digestion, reduced the ammonia and indole content and tended to increase the propionic acid content during in vitro fermentation. The Monascus treatment considerably changed the gut microbiota composition, and particularly increased the relative abundance of Parabacteroides in the in vitro fermentation model of human distal colon. Consumption of Monascus fermented fish protein could result in positive changes in fermentation metabolites and gut microbiota, which brings potential health benefits. [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]

Abstract: The cryoprotective effect of konjac glucomannan (KGM) on myofibrillar protein from grass carp (Ctenopharyngodon idella) during frozen storage at −18°C and the influence of five levels of KGM (0%, 0.5%, 1%, 1.5%, and 2%) on texture properties, water-holding capacity, and whiteness of grass carp surimi gels were investigated. KGM as a novel cryoprotectant could significantly mitigate the decrease in salt extractable protein (SEP), Ca2+-ATPase activity, and total sulphydryl and active sulphydryl contents of myofibrillar protein during frozen storage. KGM at the level of 1% showed the same good cryoprotective effect as a conventional cryoprotectant (10% sucrose–sorbitol, 1:1, w/w). As the levels of KGM increased, breaking force and deformation of grass carp surimi gels increased significantly. Water-holding properties of the surimi gels are improved with the increasing addition of KGM, but the whiteness decreased and the colour became darker. The optimum addition level of KGM was suggested to be 1%. [Copyright &y& Elsevier]

Abstract: Hot air drying and microwave drying were applied to dry fish fillets made of grass carp (Ctenopharyngodon idellus), and the correlative influences on nutritional and odorous properties were evaluated by proximate composition, protein solubility, amino acid composition, fatty acid composition, peroxide value, anisidine value, and odour evaluation. Drying resulted in a significant increase of protein but reduced fat content. There was lack of negative influence of the drying process on the amino acid composition of grass carp fillets. Both saturated and monounsaturated fatty acid contents decreased, while polyunsaturated fatty acids increased by an average of 23.8% after drying. Microwave-dried samples showed lower fat loss, higher protein solubility, and lower anisidine values than hot air-dried samples. There was no significant difference in odour quality between hot air-dried and microwave-dried samples. The present study provides a possible application of microwave drying as an efficient drying process for fish fillets. [Copyright &y& Elsevier]

Abstract: Protein hydrolysates from grass carp skin were obtained by enzymatic hydrolysis using Alcalase®. Hydrolysis was performed using the pH-stat method. The hydrolysis reaction was terminated by heating the mixture to 95°C for 15min. At 5.02%, 10.4%, and 14.9% degree of hydrolysis (DH), the hydrolysates were analyzed for functional properties. The protein hydrolysates had desirable essential amino acid profiles. Results demonstrated that the hydrolysates had better oil holding and emulsifying capacity at low DH. The water holding capacity increased with increased levels of hydrolysis. Enzymatic modification was responsible for the changes in protein functionality. These results suggest that grass carp fish skin hydrolysates could find potential use as functional food ingredients as emulsifiers and binder agents. [Copyright &y& Elsevier]

Abstract: Pepsin-soluble collagen was extracted from the skin of grass carp (Ctenopharyngodon idella) with a yield of 46.6%, on a dry weight basis. Electrophoretic patterns showed that the collagen contained α1 and α2 chains, similar to those of calf skin collagen. The imino acid content of the collagen from grass carp skin was much lower than those of mammalian’s collagens, as also were the transition temperature and denaturation temperature which were only 24.6°C and 28.4°C respectively. Peptide maps of the collagen digested by trypsin and V8 protease showed different peptide fragments from those of calf skin collagen and other fish skin collagens, suggesting differences in amino acid sequences and collagen conformation. In addition, the lyophilized collagen sponge from grass carp skin had a uniform and regular network structure, just like calf skin collagen sponge. These results suggest that grass carp skin has potential for use as a supplementary source of collagen. [Copyright &y& Elsevier]

• 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]