Acid-soluble Peptides Research Articles

Uncovering quality changes of shrimp (Penaeus vannamei) during solar drying and its relationship with protein-related properties

This study explored the intrinsic relationship between the quality traits and protein-related property changes during the solar drying of shrimps (Penaeus vannamei). During drying, the shrimp exhibited a gradual decline in L*a*b* values and a notable increase in the hardness and chewiness. Proteins were degraded and oxidized. Especially, the trichloroacetic acid-soluble peptide and carbonyl contents increased, whereas the total sulfhydryl content decreased. The proportions of different types of intramolecular bonds were significantly changed. The ionic and hydrogen bonds greatly decreased to 10.72% and 9.05% and the hydrophobic and disulfide bonds notably increased to 19.38% and 28.19%, indicating the changes in the spatial structure of the protein and its denaturation during the drying process. The Pearson's correlation analysis showed that protein degradation and denaturation greatly affected the textural properties and protein oxidation caused color changes. The result of this work provides a theoretical support for improving the quality of shrimp products.

Effect of Lactobacillus plantarum and flavourzyme on protein degradation and flavor development in grass carp during fermentation

This study examined the effect of Flavourzyme and Lactobacillus plantarum (L. plantarum) on protein degradation and flavor development during grass carp fermentation. The control groups comprised natural fermentation and fermentation with L. plantarum. Compared with the two control samples, those exposed to combined Flavourzyme and L. plantarum fermentation exhibited lower moisture content and enhanced protein hydrolysis, which accelerated the production of water-soluble taste substances (trichloroacetic acid-soluble peptides and free amino acids). The electronic tongue and electronic nose results indicated that the grass carp subjected to combined fermentation way displayed a more intense umami taste and aroma. Moreover, the sensory evaluation results confirmed that the combined fermentation method significantly improved the taste and odor attributes of fermented grass carp. In conclusion, combined fermentation with Flavourzyme and L. plantarum may effectively reduce fermentation time and enhance the flavor of fermented grass carp products.

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.

Application of Collagenolytic Proteases from Bacillus subtilis B13 and Bacillus siamensis S6 for Tenderizing Goat Meat during Wet Aging.

This research aimed to assess the effect of collagenolytic proteases from Bacillus subtilis B13 and Bacillus siamensis S6 for tenderizing goat meat during wet aging. Collagenolytic proteases B13 and S6 were prepared at 5 U/mL of collagenolytic activity before injecting into goat meat with 10% (v/w) of initial weight. The control sample was injected with distilled water and used as a negative control. The injected meats were placed in vacuum-sealed bags and wet aged at 4°C for 0, 3, 5, 7, 14, and 21 days. Thereafter, total aerobic count and physicochemical quality were elucidated. Both enzyme-treated samples from B13 and S6 aged for 5 days showed an acceptable microbial quality with lower than 5.7 Log CFU/g. These conditions produced the tender meats by the reduction in shear force accounting for 30% for B13 and 26% for S6 as compared to the control. Moreover, the enzyme-treated samples showed lower values of hardness, gumminess, and chewiness, with higher springiness and trichloroacetic acid-soluble peptides than the control (p<0.05). The detrimental impact on cooking loss and lipid oxidation was not found. Enzyme-injected meat had a lower cooking loss than the control (p<0.05) with no significant difference in lipid oxidation (p>0.05). Notably, meats treated with B13 and S6 were lower in CIE L* value as compared to the control (p<0.05) with no significant impact on CIE a* and CIE b* (p>0.05). These results suggested that these two collagenolytic proteases could enhance the quality of goat meat in terms of tenderness and reduce the aging time for meat tenderization.

Formation of advanced glycation end-products in minced pork during frozen-then-chilled storage and subsequent heating.

The influences of frozen-then-chilled storage of minced pork on the formation of advanced glycation end-products (AGEs) including Nε-carboxymethyllysine and Nε-carboxyethyllysine, and their corresponding α-dicarbonyl precursors (α-DPs; glyoxal and methylglyoxal) during storage and subsequent heating were investigated in comparison with chilled storage. During cold storage, the levels of AGEs, trichloroacetic acid-soluble peptides, and Schiff bases in minced pork continuously increased while α-DPs decreased. The 30min heating (100°C) resulted in 64-560% increase of AGEs in pork, corresponding with an increase of Schiff bases and decreases of α-DPs. Compared to the chilled storage, the frozen-then-chilled storage led to no significant difference (P>0.05) on the levels of AGEs and α-DPs in raw or heat-treated pork, implying that the formation and thawing of ice crystals in pork during the frozen-then-chilled storage had minor to no effects on the formation of AGEs and their α-DPs.

Surimi Production from Tropical Mackerel: A Simple Washing Strategy for Better Utilization of Dark-Fleshed Fish Resources

Mackerel (Auxis thazard), a tropical dark-fleshed fish, is currently a viable resource for the manufacture of surimi, but the optimal washing procedure for more efficient use of this particular species is required right away. Washing is the most critical stage in surimi production to ensure optimal gelation with odorless and colorless surimi. The goal of this study was to set a simple washing medium to the test for making mackerel surimi. Washing was performed three times with different media. T1 was washed with three cycles of cold carbonated water (CW). T2, T3, and T4 were washed once with cold CW containing 0.3%, 0.6%, or 0.9% NaCl, followed by two cycles of cold water. T5, T6, and T7 were produced for three cycles with CW containing 0.3%, 0.6%, or 0.9% NaCl. For comparison, unwashed mince (U) and conventional surimi washed three times in cold tap water (C) were employed. The maximum yield (62.27%) was obtained by washing with T1. When varying quantities of NaCl were mixed into the first washing medium (T2–T4), the yield decreased with increasing NaCl content (27.24–54.77%). When washing with NaCl for three cycles (T5–T7), the yield was greatly decreased (16.69–35.23%). Conventional surimi washing (C) produced a yield of roughly 40%, which was comparable to T3. Based on the results, treatments that produced lower yields than C were eliminated in order to maximize the use of fish resources and for commercial reasons. The maximum NaCl content in CW can be set at 0.6% only during the first washing cycle (T3). Because of the onset of optimal unfolding as reported by specific biochemical characteristics such as Ca2+-ATPase activity (0.2 μmol inorganic phosphate/mg protein/min), reactive sulfhydryl group (3.61 mol/108 g protein), and hydrophobicity (64.02 µg of bromophenol blue bound), T3 washing resulted in surimi with the greatest gel strength (965 g.mm) and water holding capacity (~65%), with fine network structure visualized by scanning electron microscope. It also efficiently removed lipid (~80% reduction), myoglobin (~65% reduction), non-heme iron (~94% reduction), and trichloroacetic acid-soluble peptide (~52% reduction) contents, which improves whiteness (~45% improvement), reduces lipid oxidation (TBARS value &lt; 0.5 mg malondialdehyde equivalent/kg), and decreases the intensity of the gel’s fishy odor (~30% reduction). As a result, washing mackerel surimi (A. thazard) with CW containing 0.6% (w/v) NaCl in the first cycle, followed by two cycles of cold water washing (T3), can be a simple method for increasing gel-forming capability and oxidative stability. The mackerel surimi produced using this washing approach has a higher quality than that produced with regular washing. This straightforward method will enable the sustainable use of dark-fleshed fish for the production of surimi.

Effects of static magnetic field (SMF) and alternating magnetic field (AMF) assisted freezing on the microstructure and protein properties of channel catfish (Ictalurus punctatus) fillet

The effect of static and alternating magnetic fields assisted freezing with intensity of 1, 2, and 3 mT on the microstructure and protein properties of channel catfish fillet were investigated. The results showed that the magnetic field treatment shortened the phase transition time of freezing, and significantly reduced the size of the formed ice crystals. The changes of trichloroacetic acid-soluble peptide, Ca2+-ATPase activity, particle size, and Zeta potential, which represented solubility, denaturation and aggregation of protein, indicated that magnetic field treatment could improve the protein stability. The chemical force analysis, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and Fourier transform infrared spectroscopy (FTIR) results proved that the magnetic field could change the structure of protein. Furthermore, these changes had effects on the thermal stability of catfish meat protein which reflected by increasing of the transition temperature and enthalpy. However, the waveform and intensity of magnetic field affected the stability of protein structure.

Moderate protein degradation and lipid oxidation induced by cold plasma and its effect on the quality of dried fish products

In this study, atmospheric cold plasma (CP) was applied to treat dried fish products at 65 kV and 60 Hz, and its effects on the composition associated with the physicochemical properties were investigated. The CP treatment for 5 min induced the fracture and contraction of fish muscle fibers and increased the microporous structure, and the moisture content of the samples reduced from 39.88 % to 37.88 %. Compared to the control group, the 5-min CP treatment decreased the puncture hardness from 10.42 to 8.43 N (p < 0.05), and increased the final rehydration ratio from 20.86 % to 28.84 %. In addition, the 5-min CP treatment increased cathepsin B + L by 58 %, and increased trichloroacetic acid (TCA) soluble peptides content by 12 % (p < 0.05), which indicated that CP contributed to protein degradation. Besides, the CP treatment also promoted lipid oxidation (TBARs value increased from 1.12 to 1.96 mg/Kg) and the production of dicarbonyl compounds (3-deoxyglucuronide and 2,3-butanedione were increased from 0.56 and 0.51–0.95 and 1.75 μg/g, respectively) (p < 0.05). In conclusion, CP is promising for improving storage stability, chewiness and rehydration of dried fish products, which associated with moderate protein degradation and lipid oxidation in fish muscle.

Dynamic Changes of the Microbial Community and Volatile Organic Compounds of the Northern Pike (Esox lucius) during Storage.

In this study, the quality (sensory evaluation, microbial enumerate, color, tvb-n (total volatile basic nitrogen), tca-soluble peptide (trichloroacetic acid-soluble peptide), muscle glucose, lactate, total sugar, Bas (Biogenic amines), VOCs (volatile organic compounds) and the microbial dynamic structure in samples stored at 4 °C were evaluated, and the relationship between VOCs and the diversity structure of microorganisms was also discussed. It was determined by sensory evaluation that the shelf life of samples was around 8 days. Protein and sugar were detected in large quantities by microorganisms in the later stage. At the same time, this also caused a large amount of Bas (biogenic amines) (tyramine, cadaverine, and putrescine). According to high-throughput amplicon sequencing, the initial microbiota of samples was mainly composed of Pseudomonas, Acinetobacter, Planifilum, Vagococcus, Hafnia, Mycobacterium, Thauera, and Yersinia. Among them, Pseudomonas was the most advantageous taxon of samples at the end of the shelf life. The minor fraction of the microbial consortium consisting of Vagococcus, Acinetobacter and Myroides was detected. The substances 3-methyl-1-butanol, ethyl acetate, and acetone were the main volatile components. The glucose, lactic acid, and total sugar were negatively correlated with Yersinia, Hafnia-Obesumbacterium, Thauera, Mycobacterium, and Planifilum; the proportion of these microorganisms was relatively high in the early stage. TVB-N and TCA-soluble peptides were positively correlated with Pseudomonas, Shewanella, Brochothrix, Vagococcus, Myroides, and Acinetobacter, and these microorganisms increased greatly in the later stage. The substance 3-methyl-1-butanol was positively correlated with Pseudomonas and negatively correlated with Mycobacterium. Ethyl acetate was associated with Hafnia-Obesumbacterium, Thauera, and Yersinia. Acetone was positively correlated with Acinetobacter.

Shelf life extension of chilled blunt snout bream fillets using coating based on chia seed gum and Oleum ocimigratissimi

The edible active coating is a popular trend for fish preservation. A hybrid coating composed of chia seed gum (CSG) and Oleum ocimi gratissimi (OG) was investigated, aiming to evaluate its ability to extend the shelf life of chill-stored fish. Physiochemical changes such as sensory analysis, color, moisture state, trichloroacetic acid-soluble peptides, total volatile basic nitrogen content, lipid oxidation indicators including peroxide value and thiobarbituric acid reactive substances, as well as microbial indicators including microbial enumeration and microbiota composition about fillets were assessed to evaluate the antimicrobial and antioxidant capacities of OG/CSG coating. The results showed a reduction of 1–2 log CFU/g of microbial enumeration was achieved during storage. The Illumina-MiSeq high throughput sequencing results showed spoilage bacteria Pseudomonas and Shewanella were effectively inhibited (p < 0.05). OG/CSG coating retarded protein degradation, lipid oxidation, water loss, and discoloration at both lower and higher concentrations (0.5% and 1%) compared with the control group (p < 0.05). In the meantime, there were no differences between the initial color of OG/CSG-coated fillets and the control group (p > 0.05), indicating OG/CSG coating did not affect the initial color of fish fillets. Based on sensory characteristics and microbial enumeration, the shelf-life of the control, 0.5% OG/CSG coated, and 1% OG/CSG coated fillets could be suggested as 5, 7, and 9 days, respectively. This study provides a scientific basis for the applicability of OG/CSG coating in fish preservation.

Photodynamic inactivation of spoilers Pseudomonas lundensis and Brochothrix thermosphacta by food-grade curcumin and its application on ground beef

This study evaluated the bactericidal effects of curcumin-mediated photodynamic inactivation (PDI) combined with citric acid on meat spoilage bacteria, and their preservative effects of refrigerated beef. After treatment of PDI (60 μM curcumin and 33.01 J/cm2) combined with 0.5 mg/mL citric acid, the populations of Pseudomonas lundensis and Brochothrix thermosphacta in PBS solution were reduced by 3.07 and 6.14 log CFU/mL, respectively, compared to 0.31 and 1.39 log CFU/mL for PDI. Combined treatment enhanced reactive oxygen species (ROS) activity in the two strains, and damaged their cell morphology, especially B. thermosphacta. With increasing curcumin concentration, the growth rate of spoilage bacteria was significantly inhibited (P < 0.05), and the changes of dominant spoilage species in bacterial community were retarded in aerobic-packaged and vacuum-packaged ground beef at 4 °C. Compared to the control, trichloroacetic acid-soluble peptide and total volatile basic nitrogen production were greatly restrained, and changes in pH values and brown color were apparently reduced for the combined treatment, which could extend the shelf life by 2 days. These results suggested the synergistic potency of PDI combined with citric acid against spoilage bacteria in meat products.Industrial relevance: The proliferation and spread of some psychrotrophs are major causes of quality deterioration and cross-contamination in fresh meat during cold chain. This study revealed that photosensitized curcumin combined with citric acid significantly decreased the survival of two psychrotrophic spoilage bacteria and extended the shelf-life of ground beef. Thus, curcumin-based PDI provides a new insight for the development of efficient and economic preservation in the meat processing industry.

Effect of Citrus aurantium juice as a disinfecting agent on quality and bacterial communities of striped catfish steaks stored at -20 °C.

Sodium hypochlorite is generally used as a disinfectant in washing of freshwater fishes where the safety aspect of health is of concern. Although plant-based essential oils and synthetic chemical agents have been applied, they might contain toxic substances, are expensive and can cause undesirable quality. This research aims to fill the knowledge gap necessary to validate Citrus aurantium juice as a disinfecting agent for preserving striped catfish steaks at -20 °C for 28 days. Fifty (50) ppm sodium hypochlorite was used as a commercial disinfectant (control). The results showed that a negative color characteristic (higher a* and increased b*) was found in the control but not in striped catfish steaks immersed in C. aurantium juice (TM) on days 14 and 28. No significant differences were found in the peroxide value among the treatments on days 14 and 28 (P > 0.05). A lower accumulation of trichloroacetic acid soluble peptides was detected in TM but not in control, while total volatile basic nitrogen of all treatments was up to standard of fish quality during storage. Contrastingly, the total viable count of both treatments increased to >7.0 log CFU/g on day 28 which did not meet the edible limit of standard for freshwater fishes. The spoilage microbial community was observed on days 0 and 28 of storage which showed a decrease in relative abundance of Acinetobacter, Pseudomonas, Brochothrix, Lactococcus, Carnobacterium, Psychrobacter, and Vagococcus as found in TM on day 28, when compared to the control. Thus, these results implied that C. aurantium juice could replace sodium hypochlorite as an alternative disinfecting agent to control the microbiological spoilage and physico-chemical quality of striped catfish steaks.

A promising insight into the inhibition of lipid oxidation, protein degradation and biogenic amine accumulation in postmortem fish: Functional glazing layers of modified bio-polymer

This study focused on a glazing layer made by modified bio-polymer to inhibit chemical variations in super-chilled postmortem fish. Essential oils were extracted from tangerine peel and their major antioxidant and antimicrobial components were identified. The essential oils were introduced into chitosan material as functional agents to improve the performance as freshness-keeping glazing layers for super-chilled tilapia. During the storage, antimicrobial effects of glazing significantly decreased the bacterial growth, as the growth of total bacterial count (TBC) decreased from ∼12.8 log CFU/g to ∼ 3.5 log CFU/g and that of total aerobic count (TAC) decreased from ∼11.1 log CFU/g to ∼ 3.3 log CFU/g. Antioxidant effects of glazing led to decreases of peroxide value (PV) from 6.41 meq/kg to ∼2.5 meq/kg and thiobarbituric acid reactive substances (TBARS) content from 1.09 mg MDA/kg to ∼ 0.5 mg MDA/kg at the end of the storage. Glazing inhibited the change of corruption degree reflected by K-value and protein decomposition reflected by the contents of trichloroacetic acid (TCA) soluble peptides and total volatile basic nitrogen (TVB-N). Glazing also limited the contents of main biogenic amines (histamine, putrescine, cadaverine, spermine, spermidine, tyramine, tryptamine) within relatively low levels.

Role of Microorganisms in the Development of Quality during the Fermentation of Salted White Herring (Ilisha elongata).

Salted white herring (Ilisha elongata) is a popular fish product in the coastal region of China. The complex endogenous enzymes and microbial action determine the quality of a traditionally salted herring. In order to investigate the role of microorganisms in the quality formation of salted herring, three groups for different salting processes were established: traditional salted (TS), non-starter salted (NS), and starter culture salted (SS). The predominant microorganism in each processing group was Staphylococcus spp., as inferred by next-generation sequencing data. Different physicochemical parameters were obtained in each of the three processing groups (TCA-soluble peptide (trichloroacetic acid-soluble peptide), TVB-N (Total volatile basic nitrogen), and TBA values (thiobarbituric acid-reactive substance)). The TS group had the maximum level of total biogenic amines, while the SS group had the lowest. A strong positive correlation was found between Staphylococcus and 14 aromatic compounds, of which 5 were odor-active compounds that created fishy, grassy, fatty, and fruity flavors. Shewanella may produce trimethylamine, which is responsible for the salted herrings' fishy, salty, and deteriorating flavor. The findings demonstrated that autochthonous strains of Staphylococcus&amp;nbsp;saprophyticus M90-61 were useful in improving product quality because they adapted quickly to the high osmotic environment.

Influence of repeated freeze-thaw treatments on the oxidation and degradation of muscle proteins from mirror carp (Cyprinus carpio L.), based on myofibrillar protein structural changes

The effects of repeated freeze-thaw (F-T) treatments on the oxidation and degradation of muscle proteins from mirror carp (Cyprinus carpio L.) were investigated. The myofibrillar fragmentation index, trichloroacetic acid-soluble peptides, total volatile basic nitrogen, amino nitrogen, and carbonyl content of the samples significantly increased (P < 0.05). However, the samples showed a significant reduction in the fragmentation index, myofibrillar protein (MP) solubility, and total sulfhydryl content after five F-T cycles (P < 0.05). Moreover, the sodium dodecyl sulfate-polyacrylamide gel electrophoresis bands of the MP faded because of the oxidation and degradation of the protein with increasing F-T cycles. By the fifth F-T cycle, the α-helix and β-turn content significantly decreased by 10.41 % and 5.72 %, respectively (P < 0.05), whereas the β-sheet and random coil content significantly rose by 7.66 % and 8.47 %, respectively (P < 0.05). Furthermore, the intrinsic fluorescence of the MP showed a substantial decrease in intensity and a redshift. In summary, iterative F-T cycles destroyed the MP structure and caused the oxidation and degradation of muscle proteins from mirror carp.

Effects of ultra high pressure-magnetic field treatment on protein properties and quality characteristics of stored shrimp (LitopenaeusVannamei)

The preservation of ultra high pressure (UHP) and magnetic field (MF) technologies on food have been reported, but the effect of combined treatment (UHP-MF) on aquatic product quality is rarely explored. The purposes of this study were to investigate the effects of UHP-MF treatment on protein properties and quality deterioration of stored Litopenaeus Vannamei. The vacuum-packed shrimps were treated at 200 and 300 MPa for 5 min, then stored in 5 mT MF for 12 days. Results showed the hardness, springiness, chewiness, shear force, L*, a*, b*, and W of muscle were improved by UHP, whereas the texture softening, a* and b* increase, water holding capacity were inhibited during MF storage. On the 12th day, compared with CON group, the cathepsin B, D, H, L activities, TBARS, carbonyl and dityrosine content in UHP300-MF group were reduced by 46.67%, 27.26%, 43.10%, 52.00%, 41.30%, 32.86%, and 30.42%, respectively. Furthermore, UHP-MF treatment slow down the accumulation of surface hydrophobicity, myofibril fragmentation index, and trichloroacetic acid-soluble peptide content. Generally, UHP-MF treatment inhibited lipid oxidation, weakened cathepsin activities, delayed protein oxidation and degradation of stored shrimp, retarded quality deterioration and prolonged shelf life of stored shrimp.

Control on moisture distribution and protein changes of Antarctic krill meat by antifreeze protein during multiple freeze-thaw cycles.

Control on the moisture distribution, protein structure changes, and protein degradation of Antarctic krill meat during freeze-thaw (F-T) cycles by presoaking with antifreeze protein (AFP) was investigated. The results from the thawing loss rate and cooking loss rate indicated that 0.1% was the optimal AFP concentration. Magnetic resonance imaging and low-field nuclear magnetic resonance results showed that AFP inhibited the changes in moisture distribution and maintained the moisture in Antarctic krill meat. The contents of nonprotein nitrogen and trichloroacetic acid-soluble peptides indicated that AFP reduced protein degradation. Further, SDS-PAGE showed that AFP reduced the degradation of actin, troponin T, and myosin light chain. The results of fluorescence spectra, circular dichroism, and chemical bond contents indicated that AFP reduced the damage of the protein tertiary and secondary structures of Antarctic krill meat by holding it in a weak polar environment. This study supplied basic theory for the quality control of Antarctic krill meat. PRACTICAL APPLICATION: Protein degradation, moisture distribution, and protein structure changes occurred to Antarctic krill meat during freeze-thaw cycles due to ice crystal growth and recrystallization, which leads to the decrease in quality. Antifreeze protein has been proven to avoid ice crystals' growth and inhibit ice recrystallization. During freeze-thaw cycles, the moisture distribution of Antarctic krill meat treated with antifreeze protein was more uniform, the degree of protein degradation was lower, and the protein structure was protected. This study demonstrated the potential of antifreeze protein as a water and protein protectant of Antarctic krill meat during freeze-thaw cycles.

Characteristics of silver carp surimi gel under high temperature (≥100 °C): quality changes, water distribution and protein pattern

AbstractThe effects of high‐temperature treatments (100–125 °C) on the properties of silver carp surimi gel were studied. The results showed that the gel strength and water holding capacity (WHC) of surimi gel decreased significantly (P &lt; 0.05), with the increase of heating temperature and the prolongation of heating time, and the higher the temperature, the greater the decrease trend. Hydrophobic interaction and disulphide bonds decreased significantly, while ionic bonds increased significantly (P &lt; 0.05), as well as β‐sheet increased while β‐turn and coil decreased significantly, which confirmed the changes of protein secondary structure caused by high‐temperature treatments. Myosin heavy chain (MHC) and actin bands gradually narrowed, and the production of new bands larger than 250 kDa by SDS‐PAGE showed the aggregation and cross‐linking reaction of proteins. However, the increased band intensity of 50–75 kDa and the increase of trichloroacetic acid (TCA) soluble peptides showed the degradation of proteins. The decrease of PT22 and the increase of PT23 indicated the conversion of immobilised water to free water in the gel matrix. In conclusion, high‐temperature heating caused the changes of protein secondary structure as well as chemical forces and water conversion, resulting in protein denaturation, aggregation, cross‐linking and degradation, forming a loosen and irregular gel network.

Preparation of Magnetic Chitosan Beads as Carriers for Papain Immobilization

Papain, a well‐known cysteine protease, is predominantly found and extracted from papaya fruit. Its broad hydrolyzing activity and stability at different temperature and pH, make it a widely used enzyme in food, and pharmaceutical industries. For example, it is used in various food processing procedures and in the production of bioactive peptides. However, under extreme industrial conditions, the activity of this and other enzymes, are significantly decreased. This loss of enzyme activity could be overcome by immobilizing the enzymes on a solid support. The immobilization of enzymes also allows them to be reused. Chitosan is a non‐toxic, biodegradable polymer derived from the deacetylation of chitin, the second most abundant polysaccharide in nature. Chitosan offers multiple functional groups for binding enzymes and other molecules and has myriad enzyme immobilization applications on a wide variety of materials, including nanoparticles, nanocomposites, and nanofibers. In this study, we have prepared mechanically stable magnetic chitosan beads as carriers for immobilized papain. Chitosan beads were prepared using a phase inversion technique. Chitosan (2.0 gm) was dissolved in 50mL diluted acetic acid and mixed thoroughly with 0.3M FeCl3 solution. The homogenous mixture was placed in a syringe and extruded dropwise into a beaker with 0.5M sodium hydroxide with contestant stirring. The resulting beads were stirred for one hour and washed with water. The washed beads were incubated with 0.5% glutaraldehyde solution for 10 hours with constant shaking. This step was carried out to stabilize the beads by crosslinking the chitosan beads while also creating additional free aldehyde groups for papain binding. Following washing, the beads were immersed in 20mL of 10% ammonium hydroxide and 0.2M FeCl2 solution and stirred for 2 hours to create magnetic chitosan beads. The beads were separated using an external magnet and washed thoroughly. The beads were stored in 50mM phosphate buffer (pH 7.5). For immobilization of papain, the beads were incubated at room temperature in 0.1% papain solution for 20 hours with constant shaking. The immobilized papain was washed and stored in buffer. The enzyme activity of free and immobilized papain was determined by measuring the acid soluble peptides generated by hydrolyzed casein. The thermal stability studies shows that the immobilized papain retained 93% activity after 60 minutes incubation at 80oC while the soluble enzyme retained only 40% of its initial activity. There was no loss of immobilized papain activity after six weeks of storage at 4oC. In addition, a significant amount of immobilized papain activity is retained after multiple usage. These results show that attachment of papain on the magnetic chitosan beads could broaden its application in industrial use. Moreover, immobilization on magnetic chitosan beads can impede losses in activity of other industrially important enzymes.

Unraveling the Effects of Biochemical Drivers on the Bacterial Communities and Volatile Profiles in Refrigerated Sturgeon Filets at 4°C.

Spoilage bacteria seriously influence the flavor and quality of fish meat. In this study, we investigated the quality characteristics, bacterial community, and volatile profiles of refrigerated (4°C) sturgeon filets during 10-day storage. On day 10, the refrigerated samples showed the lowest bacterial diversity and the largest difference in microbiota and biochemistry. The dominant genera in the fresh samples were Macrococcus, Acinetobacter, Moraxella, Brucella, and Pseudomonas, while the dominant bacteria changed into Acinetobacter, Carnobacterium, Macrococcus, Pseudomonas, and Psychrobacter at the end of storage. Our results suggest that these dominant taxa contribute to the spoilage of the refrigerated sturgeon filets. Meanwhile, during the storage, total viable counts, total volatile basic nitrogen, thiobarbituric acid-reactive substances, and trichloroacetic acid-soluble peptide significantly increased (P < 0.05), while the sensory score decreased steadily. Additionally, the ATP-related compounds and the K-value showed similarly increasing trends. The shelf-life of the refrigerated sturgeon filets was less than 8 days. The gas chromatography–ion mobility spectrometry results suggest that hexanal, ethyl acetate, ethanol, butanal, 1-propanol, isopentyl alcohol, 2-pentanone, 2-heptanone, ethyl propanoate, and propyl sulfide are potential chemical spoilage markers. The predicted metabolic pathways indicated an abundant carbohydrate metabolism and amino metabolism in the refrigerated sturgeon filets. This study provides insight into the determinants of sturgeon shelf-life and the spoilage process involved in refrigerated fish.