Effects of washing with oxidising agents on the gel-forming ability and physicochemical properties of surimi produced from bigeye snapper (Priacanthus tayenus)

The effects of washing with hydrogen peroxide (H₂O₂) and sodium hypochlorite (NaOCl) solutions on the gel-forming ability and physicochemical properties of surimi produced from bigeye snapper (Priacanthus tayenus), stored in ice for up to 14 days, were investigated. Generally, pH and the trichloroacetic acid (TCA)-soluble peptide content of washed mince varied, depending on the type of oxidizing agent and storage time of the fish. With increasing time of storage, the pHs of water- and H₂O₂-washed mince were lower than that of NaOCl-washed mince (P < 0.05). However, no differences in the TCA-soluble peptide contents of the resulting mince washed with any media were observed (P > 0.05). Washing with 20 ppm NaOCl resulted in the highest increase in both the breaking force and the deformation of mince from fish stored in ice for all the times studied (P < 0.05). Natural actomyosin (NAM) extracted from NaOCl-washed mince had higher surface hydrophobicity and disulfide bond (SS) content than that of water-washed mince (P < 0.05). With no effect on Ca²⁺-, Mg²⁺-, or Mg²⁺–Ca²⁺-ATPase activities, NaOCl washing resulted in an increase in Mg²⁺–EGTA-ATPase activity of NAM (P < 0.05). The results suggested that washing mince with the appropriate type and concentration of oxidizing agent can improve the gelling ability of surimi, particularly from low quality fish.     

Physical properties and microstructure of commercial Som-fug, a fermented fish sausage

Physical properties and the microstructure of Som-fug, a Thai-style fermented fish sausage, with seven different commercial brands were determined. The released water contents of Som-fug samples ranged from 5.81 to 15.81% and expressible water contents were different among the samples tested (p<0.05). Differences in hardness, fracturability, adhesiveness, springiness, cohesiveness and resilience were also observed. L*, a*, b* and whiteness varied with the samples. Overall, the samples had different acceptability in texture and color. Microstructural study revealed that Som-fug had a three-dimensional network. A more void and open structure correlated well with greater released water and was associated with lower hardness and cohesiveness. Therefore, the different physical properties and microstructure of Som-fug could be influenced by the differences in raw materials, ingredients, processing, and fermentation characteristics, and determined the acceptability of products.     

Isolation of antioxidative and ACE inhibitory peptides from protein hydrolysate of skipjack (Katsuwana pelamis) roe

Bioactive peptides from protein hydrolysate of defatted skipjack (Katsuwonus pelamis) roe with 5% degree of hydrolysis (DH) prepared by Alcalase digestion were isolated and characterised. Two active fractions with ABTS radical scavenging activity (973.01–1497.53 μmol TE/mg sample) and chelating activity (0.05–0.07 μmol EE/mg sample) from consecutive purification steps including ultrafiltration, cation exchange column chromatography and reverse phase high performance liquid chromatography (RP-HPLC), were subjected to analysis of amino acid sequence by LC–MS/MS. Seven dominant peptides with 6–11 amino acid residues were identified as DWMKGQ, MLVFAV, MCYPAST, FVSACSVAG, LADGVAAPA, YVNDAATLLPR and DLDLRKDLYAN. These peptides were synthesised and analysed for ACE-inhibitory activity and antioxidative activities. MLVFAV exhibited the highest ACE inhibitory activity (IC₅₀ = 3.07 μM) (p < 0.05) with no antioxidative property, whilst DLDLRKDLYAN showed the highest metal chelating activity, ABTS radical and singlet oxygen scavenging activities. Therefore, peptides prepared from skipjack roe could be further employed as a functional food ingredient.     

The effect of heating conditions on polyphenol oxidase, proteases and melanosis in pre-cooked Pacific white shrimp during refrigerated storage

Pre-cooked Pacific white shrimp (Litopenaeus vannamei) is an important shrimp product. However melanosis, especially in the cephalothorax including carapace and internal organs, is more likely caused by the remaining polyphenol oxidase (PPO) after pre-cooking. Thus, PPO from carapace and proteases from hepatopancreas of Pacific white shrimp were characterised and the remaining activities of both enzymes were monitored in pre-cooked shrimp during storage at 4 °C. Based on activity staining using L-β-(3,4 dihydroxylphenyl) alanine as a substrate, PPO consisted of two isoforms with apparent molecular weight of 210 and 220 kDa. No difference in activity band was observed when analysed under reducing and non-reducing condition. Proteases from hepatopancreas were able to activate PPO to some degree. For the in vitro study, both enzymes were quite stable when heated at temperature up to 70 °C but the loss in activities increased with increasing heating time (0-120 s). When Pacific white shrimp were pre-cooked to obtain different core temperatures (50-90 °C), different PPO and protease activities were retained. Higher core temperatures were associated with lower PPO and protease activities, but higher cooking loss. When the shrimp were pre-cooked at 80 °C, the residual PPO and protease activities were 3.9% and 5.4%, respectively and cooking yield of 95.6% was obtained. The resulting pre-cooked shrimp possessed lower melanosis score during 7 days of storage at 4 °C. Thus, pre-cooking of shrimp to obtain a core temperature of 80 °C, with a holding time of 30 s, could prevent the severe cooking loss and lower melanosis during subsequent storage.     

Hydrolysed collagen from Lates calcarifer skin: its acute toxicity and impact on cell proliferation and collagen production of fibroblasts

Acute toxicity in Wistar rat and the impact of hydrolysed collagen (HC) from seabass skin on in vitro cell proliferation and collagen production were studied using L929 fibroblasts. HC was rich in glycine (326 residue/1000 residue) and imino acids (196 residue/1000 residue). MALDI mass spectrum of HC showed several low MW peptides with MW range of 1050–1330 Da as the major components. Based on acute oral cytotoxicity test in Wistar rat, HC was considered as safe with LD₅₀ value higher than 5000 mg kg^?1 body. HC could promote L929 cell growth, especially when used in combination with vitamin C (VitC) at a ratio of 2:1. HC/VitC (2:1) mixture also exhibited the higher enhancement effect on collagen production of L929 cells, compared with HC or VitC alone. Thus, HC could be a promising candidate for biological applications, especially in combination with VitC, as nutraceuticals for skin care.     

Effect of high-temperature setting on gelling characteristic of surimi from some tropical fish

The effect of setting at 40 °C on the textural properties and the changes in myofibrillar proteins in surimi produced from threadfin bream (Nemipterus bleekeri), bigeye snapper (Priacanthus tayenus), barracuda (Sphyraena jello) and bigeye croaker (Pennahai macrophthalmus) was investigated. An increase in the time of setting generally resulted in higher breaking force and also the deformation of both suwari and kamaboko gels. Maximum increases in gel‐breaking force were obtained in 1 h for threadfin bream, 2 h for bigeye snapper, 1.5 h for barracuda and 3 h for bigeye croaker. Extended setting time caused decreases in breaking force and deformation in all surimi, except that produced from bigeye croaker. Gel strengthening was associated with an increase in non‐disulphide covalent bond formation. Degradation of proteins occurred with prolonged setting. Therefore, setting at 40 °C for an appropriate time is a promising means to improve the gelling property of surimi produced from tropical fish.     

Isolation and characterization of collagen from the cartilages of brownbanded bamboo shark (Chiloscyllium punctatum) and blacktip shark (Carcharhinus limbatus)

Acid soluble collagen (ASC) and pepsin soluble collagen (PSC) from the cartilages of brownbanded bamboo shark (BBS; Chiloscyllium punctatum) and blacktip shark (BTS; Carcharhinus limbatus) were isolated and characterized. ASC and PSC extracted from BBS cartilage showed the yields of 1.27 and 9.59 g/100 g (Based on dry weight), respectively, while yields of ASC and PSC from BTS cartilage were 1.04 and 10.30 g/100 g (based on dry weight), respectively. All collagens had protein as a major constituent with the trace amount of ash and fat. They contained glycine as the major amino acid with high contents of alanine, proline and hydroxyproline. Based on sodium dodecyl sulfate-polyacrylamide gel electrophoretic patterns and subunit compositions, all collagens more likely comprised 2 types of collagen, type I and II, and contained α- and β-chains as the major components. Peptide maps of those collagens from both species digested by V8-protease and lysyl endopeptidase were different and were completely different from those of type I collagen from calf skin. Thermal transition temperature of ASC from those collagens (36.28-36.73 °C) was slightly higher than their corresponding PSC (34.56-35.98 °C). From zeta potential analysis, isoelectric points (pI) of collagen from the cartilages of BBS and BTS were estimated to be from 6.53 to 7.03 and from 6.96 to 7.26, respectively. Fourier transform infrared (FTIR) spectra of both ASC and PSC were quite similar, suggesting that pepsin hydrolysis did not affect the secondary structure of collagen, especially triple-helical structure.     

The effects of sodium bicarbonate on conformational changes of natural actomyosin from Pacific white shrimp (Litopenaeus vannamei)

Changes in natural actomyosin (NAM) from Pacific white shrimp (Litopenaeus vannamei) treated with sodium bicarbonate (NaHCO₃) at different concentrations (0–1 M) in the absence or the presence of 2.5% NaCl were studied. Turbidity of NAM solutions decreased with coincidental increase in solubility as the concentration of NaHCO₃ increased. Surface hydrophobicity (S_oANS) and total sulfhydryl content of NAM also increased when NaHCO₃ concentration increased. Greater decreases in Ca²⁺- and Mg²⁺-ATPase activity were found in all NAM as NaHCO₃ concentration increased, suggesting the denaturation of myosin head and the dissociation of actomyosin complex. The zeta potential (ζ) analysis suggested that the surface of NAM became more negatively charged (−12.12 to −26.98) as NaHCO₃ concentration increased. Those changes were more intense in the presence of 2.5% NaCl. Transmission electron microscopy showed that the structure of actomyosin was more dissociated and lost the filamental structure when NaHCO₃ at higher levels was used.     

Effect of reducing agents on physicochemical properties and gel-forming ability of surimi produced from frozen fish

Effects of different reducing agents (cysteine, ascorbic acid and sodium bisulfite) at various levels on physicochemical properties of protein, transglutaminase activity and gel properties of surimi produced from frozen croaker, lizardfish, threadfin bream and bigeye snapper were studied. Addition of cysteine resulted in the highest increase in the breaking force and the deformation of surimi gels, compared with other reducing agents. The optimum levels of cysteine were 0.05, 0.1, 0.05 and 0.1% (w/w) for surimi from frozen croaker, lizardfish, threadfin bream and bigeye snapper, respectively. Surimi from frozen croaker with cysteine added showed a similar breaking force and deformation to that produced from fresh fish. With addition of cysteine, an increase in sulfhydryl content with a concomitant decrease in disulfide bond content was generally observed. Ca²⁺ ATPase activity also increased, indicating the renaturation of the myosin molecule. T_max of peak 1 (myosin peak) of all surimi sols in the presence of cysteine was shifted to higher temperature. The increased transglutaminase activity was observed with addition of cysteine. Therefore, reducing agents, especially cysteine, recovered the denatured muscle proteins and activated the transglutaminase in the muscle, leading to the increased gel-forming ability of surimi produced from frozen fish.     

Effect of Acetic Acid and Commercial Protease Pretreatment on Salting and Characteristics of Salted Duck Egg

Salting of duck egg pretreated with 5% acetic acid and different commercial proteases (flavourzyme, protamex, alcalase, and neutrase) was studied. After 2 weeks of salting, duck eggs soaked in 5% acetic acid for 30 min, followed by soaking in 5% (w/v) flavourzyme and neutrase had the highest hardening ratio (90.14 ± 2.43%, 90.25 ± 1.23%) with the coincidental increase in salt content in egg white and decrease in moisture content of yolk, compared with those from other treatments (p < 0.05). However, similar hardening ratio of the sample treated with alcalase was obtained to that of sample treated with either flavourzyme or neutrase (P > 0.05). After 1 week of salting, protamex showed the similar effect on hardening ratio to other proteases (p > 0.05). When eggs were pretreated with neutrase at different concentrations (0.25%, 0.5%, and 0.75%, w/v) for different times (30, 60, and 90 min), those pretreated with 0.25% (w/v) neutrase for 90 min had the shorter salting time, while soaking time did not have the impact on hardening ratio for egg treated with 0.5% and 0.75% neutrase (P > 0.05). The oil exudation of egg treated with 0.25% neutrase had the higher oil exudates than the control at week 2 and 3 of salting (P < 0.05). Treatment of neutrase had no impact on viscosity of egg white, regardless of salting time (P > 0.05). Microstructure study revealed that shell of salted egg pretreated with acetic acid had rough and porous surfaces when compared with control, whereas no changes in microstructure and FTIR spectra of shell membrane were found. Therefore, pretreatment of egg with 5% acetic acid for 30 min, followed by 0.25% neutrase for 90 min prior to salting could expedite the salting process and yielded egg yolk with hardening ratio and oil exudates, comparable to the traditionally salted egg.