Proteolytic processing of Atlantic mackerel (Scomber scombrus) and biochemical characterisation of hydrolysates

Research on fish composition has demonstrated that they constitute a promising source of molecules with health benefits. In the present study, Atlantic mackerel ( Scomber scombrus ) was processed using Protamex® and the distribution of nutrients in various fractions obtained following membrane filtration was analysed. The fish starting material was approximately 84% water and the dry matter contained 42.7% proteins, 45.2% lipids and 5.6% minerals. The recovery of fish dry matter in the liquid hydrolysate was 77.8%. Most of the fractions were protein-enriched and characterised by a well-balanced amino acid composition, notably in terms of the most essential amino acids and by molecules of relatively low molecular weight (≤42 kDa). Even before process optimising, the biochemical and nutritional analyses indicate that the Atlantic mackerel may provide high-value products for future applications in the health and food sectors.     

Influence of feeding state and frozen storage temperature on the lipid stability of Atlantic mackerel (Scomber scombrus)

Lipid deterioration of mackerel caught in Icelandic waters was studied, as affected by different frozen storage temperatures (?18 °C vs. ?25 °C) and seasonal variation (August vs. September). The lipid stability was investigated by analyses of hydroperoxide value (PV), thiobarbituric acid reactive substances (TBARS), free fatty acids, as well as changes in fatty acid composition. Results showed significant lipid deterioration with extended storage time, where the lower storage temperature showed significantly more protective effects. Furthermore, a higher lipid oxidation level was recorded for fish caught in September than in August, although lipid hydrolysis occurred to be greater for fish in August than in September. Moreover, results indicated a rather stable level of omega‐3 fatty acid during the whole frozen storage period. The analysis indicated that both lipid oxidation and hydrolysis were affected by the frozen storage temperature and the stability differed with regard to season of catch.     

Ultrasonic characterization of Atlantic mackerel (Scomber scombrus)

The influence of frequency (1–6 MHz), temperature (5–25°C) and composition on the ultrasonic velocity and attenuation coefficient of Atlantic mackerel (Scomber scombrus) was investigated. The ultrasonic velocity was insensitive to frequency, whereas the attenuation coefficient increased with frequency. The fat, water and solids-non-fat content of mackerel tissue was determined by analyzing the temperature dependence of the ultrasonic velocity in the tissue using a semi-empirical equation. There was good agreement between the fat, water and solids-non-fat content determined by ultrasonic velocity and proximate analysis methods. In particular, there was a high correlation between the two methods for the determination of fat content (r²=0.949). There was no correlation between the composition of mackerel tissue determined by ultrasonic attenuation and proximate analysis methods. The increase in ultrasonic attenuation of mackerel with temperature, especially above about 20°C, was attributed to protein denaturation and tissue disruption.     

Evolution of the indigenous microbiota in modified atmosphere packaged Atlantic horse mackerel (Trachurus trachurus) identified by conventional and molecular methods

A combination of conventional methods and genetic identification (PCR sequencing) was used to study the dynamics of the bacterial population during the spoilage of modified atmosphere packaged (MAP) Atlantic horse mackerel (Trachurus trachurus) fillets. The cultivable microflora in Atlantic horse mackerel samples packaged in a modified atmosphere (48% CO₂, 50% N₂ and 2% O₂) at refrigeration temperature (6 °C) was measured on days 1, 5 and 7 using non-selective (Long and Hammer agar) and selective media (Kligler's iron agar, STAA and MRS). The microflora was genetically characterised using partial amplification of 16S rRNA gene sequences from 309 bacterial isolates obtained from Long and Hammer agar. At the end of the shelf life (5 days), the total viable counts (TVC) on Long and Hammer agar were not significantly different to the LAB counts on MRS agar (p > 0.05). The molecular approach showed that Photobacterium, Arthrobacter, Chryseobacterium and Pseudoclavibacter (44.5% of total) dominated the microbial composition of the fish at the beginning of storage. However, Serratia, Shewanella and Yersinia dominated at the late spoilage stages (over 57.2% of the total). Carnobacterium was the most important species of the lactic acid bacteria (LAB) and was identified at the beginning and end of the storage period. Vibrio spp. was only found at the end of the shelf life. This research demonstrates that the microbial biodiversity in MAP Atlantic horse mackerel is enormous and the dominant species change over the storage time. The results presented here on the dominant communities in fish products will make it possible to accurately select the best preservation practices.     

Non-dioxin like polychlorinated biphenyls in mackerel (Scomber scombrus) available on the Serbian market

Mackerel is rich in omega-3 fatty acids, but this fish may be contaminated with environmental pollutants. The aim of this study was to evaluate concentrations of non-dioxin like polychlorinated biphenyls (ndl-PCBs) in 160 samples of mackerel available on the Serbian market. Average and maximum ndl-PCB concentrations were 7.9 ng/g and 74 ng/g, respectively. A higher average level of ndl-PCBs was found in mackerel from Spain, but this was still three times lower than the permitted level. We conclude that levels of ndl-PCBs in mackerel available on the Serbian market are acceptable.     

Thermostable Water Dispersions of Myofibrillar Proteins from Atlantic Mackerel (Scomber scombrus)

A process for preparation of a thermostable, low viscosity water dispersion of myofibrillar proteins from Atlantic mackerel (Scomber scombrus) was developed. The comminuted meat was washed sequentially with cold water, a bicarbonate solution, and cold water. It was then homogenized in ice-cold water. Apparent viscosity of the dispersion depended on protein concentration and temperature. Lowering pH to 3.7 by acetic acid reduced viscosity of the dispersion which remained unchanged irrespective of temperature. The proteins did not precipitate upon heating to 100° followed by centrifugation at 5000 xg for 15 min, even in the presence of 50 mM NaCl solution. However, increasing pH of the acidified solution resulted in precipitation of proteins. Acetic acid also prevented separation of water upon centrifugation of heated dispersion.     

The effect of frozen storage of mackerel (Scomber scombrus) on its quality when hot-smoked

Whole mackerel (Scomber scombrus) were frozen using a horizontal plate freezer, wrapped in plastic bags and frozen stored at - 20°C. At suitable intervals (11, 22 and 33 weeks) the fish were removed from the cold store and subsequently hot smoked (in gutted from) using an AFOS-Torry Mini Kiln. All smoked mackerel samples, despite their different previous histories, were assessed by the panellists as moderately acceptable products in- terms of their texture and flavour, even after 33 weeks frozen storage prior to smoking. Protein denaturation, as related to salt-soluble protein, was influenced by the frozen storage history (24% drop after 33 weeks frozen storage) and seemed to be affected by the free amino acids formed during frozen storage. After smoking the denaturation was extensive (above 80%) in all mackerel samples. Lipid oxidation was quite extensive (PV 108 meq kg^?1) in the 22 and 33 weeks frozen stored mackerel samples. However, no rancid flavour in the latter smoked mackerel samples was detected by taste panellists. A 58% increase in free amino acids during frozen storage was observed. Extensive losses of 74% in available lysine were observed in the 22 and 33 weeks frozen stored mackerel samples after smoking which could be due to aminocarbonyl reactions with the products of lipid oxidation. A 40% loss of thiamine was observed in the 33 weeks frozen stored samples after smoking. The histamine contents did not exceed 94 mg kg^?1 and would not be expected to cause symptoms of scombrotoxin poisoning.     

Quality of Atlantic Mackerel (Scomber scombrus L.) Fillets during Modified Atmosphere Storage

Sensory, physical, biochemical and microbial quality were investigated in mackerel fillets stored under CO₂ modified atmosphere packaging (MAP) at -2°C for 21 days. Packaging gas composition remained constant during storage. An experienced sensory panel found seaweedy and cucumber-like odors (raw) decreased whereas sourness corresponded with lactic acid bacteria increase. Both sensory (raw) fillet ‘yellowness’ and Hunter ‘b’ values increased. Visual (cooked) moistness and expressible fluid decreased but no difference was found in cooking losses. Small increases in total volatile base-nitrogen and trimethylamine-nitrogen paralleled aerobic plate counts; ethanol increased linearly. Coliforms, yeasts and molds were detected at < 10 CFU/g. CO₂ MAP storage at – 2°C can maintain Grade 1 shelf-life extension for ≥ 21 days     

The effects of freeze-drying and storage on the FT-Raman spectra of Atlantic mackerel (Scomber scombrus) and horse mackerel (Trachurus trachurus)

FT-Raman spectroscopy was undertaken for quantitative characterization of lipids in horse mackerel (Trachurus Trachurus) and Atlantic mackerel (Scomber scombrus). Peroxide value of fish lipids and protein extractability of the fish lipids and muscles were also measured by titration and Bradford assay, respectively.     

Effect of a flax seed (Linum usitatissimum) soaking treatment on the frozen storage stability of mackerel (Scomber scombrus) fillets

The effect of a flax seed (Linum usitatissimum) soaking on the development of rancidity in frozen mackerel (Scomber scombrus) was studied. Fresh mackerel fillets were soaked in an aqueous flax seed extract for 20 min and then kept frozen (?20 °C) for up to 7 months. A parallel experiment with non‐soaked fillets was carried out under the same conditions. The development of rancidity was measured by biochemical (free fatty acids, peroxides, conjugated dienes and trienes, secondary oxidation products, fluorescent and browning compounds and lipoxygenase activity) and sensory (general aspect, odour and colour) analyses. An inhibitory effect of the soaking treatment on rancidity development was observed according to the peroxide content and the formation of fluorescence and browning. A lower lipoxygenase activity was detected at 1 month in the soaked fillets; after this, no differences were obtained between either type of sample, whose activities at month 7 were negligible. According to the sensory analyses, non‐soaked fillets had fair quality at 1 month and were rejectable at 3 months, while the soaked ones were still of good quality at 1 month and rejectable at 5 months. According to the present results, soaking in an aqueous flax seed extract could be useful for inhibiting the development of rancidity in fatty fish fillets. Copyright © 2006 Society of Chemical Industry     

Effect of lipid oxidation and frozen storage on muscle proteins of Atlantic mackerel (Scomber scombrus)

The effect of storage on the lipids and proteins in Atlantic mackerel stored for up to 24 months at ?20 and ?30 °C was studied. Traditional methods including the peroxide value, thiobarbituric acid‐reactive substances (TBARS) and a reverse phase HPLC method were used to determine the primary and secondary lipid oxidation products. All tests showed an increase in lipid oxidation products with storage time and at a higher storage temperature of ?20 °C compared with samples stored at ?30 °C. Antioxidants had a significant effect (P < 0.01) on the inhibition of lipid oxidation, as shown by the reduction in peroxide value and hydroxides, and malondialdehyde formation. Similarly, deterioration of protein structure and functionality in mackerel stored for 3, 6, 12 and 24 months was greater at ?20 than ?30 °C. ATPase activity in the myosin extract of Atlantic mackerel showed a significant decrease (P < 0.01) with progressive frozen storage. Protein solubility in high salt concentration (0.6 M NaCl) decreased (P < 0.01) during storage at both ?20 and ?30 °C but was greater at ?20 °C. Interestingly, antioxidants BHT, vitamin C and vitamin E protected the proteins against complete loss of ATPase activity and protein solubility to a significant level (P < 0.01) for up to 1 year at ?20 °C compared with samples stored without antioxidants. This study confirms the deleterious effect of lipid oxidation products on protein structure and function in frozen fatty fish. © 2002 Society of Chemical Industry     

Effect of pre‐treatment and smoking process (cold and hot) on chemical,microbiological and sensory quality of mackerel (Scomber scombrus)

The sensory, hygienic, toxicological and nutritional profiles of hot‐ and cold‐smoked mackerel samples were studied with various pre‐treatments. The panellists assessed all smoked samples as barely to quite acceptable products whilst the product immersed in 120 g kg^?1 sodium chloride and 60 g kg^?1 fructose prior to smoking was assessed as very acceptable regarding its sensory characteristics. The available lysine in all hot smoked samples was reduced to the same extent (32%) whilst a very good correlation (r = 0.912) was observed between loss of available lysine and colour formation of the cold‐smoked products, indicating the high contribution of lysine in the interactions with carbonyls. Histamine was found in highly unacceptable levels even in the unprocessed samples (600 mg kg^?1) and strongly increased (2220 and 2250 mg kg^?1) in the cold‐ and hot‐smoked samples, respectively, due to all treatments. These are levels which would be expected to cause symptoms of scombrotoxin poisoning. Benzo(a)pyrene, fluoranthene and perylene were at high levels both in cold‐ (2.1, 4.3 ± 0.04 and 7.2 ± 0.05 µg kg^?1) and hot‐smoked samples (9.2, 7.8 ± 0.03 and 9.4 ± 0.14 µg kg^?1, respectively) and were, as expected, influenced by the temperature. The aerobic bacteria remained at acceptable levels, since salt and high temperature prevent bacterial growth. Copyright © 2004 Society of Chemical Industry     

PCR-based methodology for the authentication of the Atlantic mackerel Scomber scombrus in commercial canned products

A multiplex-PCR assay for the authentication of the Atlantic mackerel Scomber scombrus in commercial canned products has been developed. This novel method consists of a S. scombrus-specific fragment (123 bp) corresponding to the mitochondrial NADH dehydrogenase subunit 5, and a positive amplification control corresponding to the small rRNA 12S subunit (188 bp). The system was assayed using six different canned products labeled as S. scombrus. We observed a positive identification in all samples, revealing this methodology as a potential molecular tool for direct application in the authentication of S. scombrus canned products.     

Inhibition of mackerel (Scomber scombrus) muscle lipoxygenase by green tea polyphenols

The high polyunsaturated fatty acid content of oily fish such as mackerel (Scomber scombrus) makes it particularly susceptible to oxidative degradation. We have shown previously the presence of lipoxygenase (LOX), a lipid oxygenase, in mackerel muscle. In the current study, commercially available green tea polyphenols were shown to effectively inhibit the LOX activity of mackerel muscle. EGCG (epigallocatechin gallate) was the strongest inhibitor tested with an IC₅₀ (concentration for half maximal inhibition) value of 0.13 nM. All the tea catechins showed a mixed non-competitive type inhibition. In addition, antioxidants such as BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), esculetin, caffeic acid, ascorbic acid, and ethylene diamine tetraacetic acid (EDTA) were effective to varying degrees (IC₅₀ values between 0.02 and >50 μM) in the inhibition of mackerel muscle LOX. Nordihydroguaiaretic acid (NDGA), a classical LOX inhibitor and potassium cyanide (KCN), a heme protein inhibitor were assayed for their inhibitory activities for comparison.Post harvest spoilage of fish account for loss of as much as 10% of the world’s catches of cultured fish. This data indicates that the green tea polyphenols, nature’s very potent antioxidants, may be used as an effective and natural means of reducing post harvest spoilage in fish.     

Rheological and differential scanning calorimetry studies on structural and textural changes in frozen Atlantic mackerel (Scomber scombrus)

The viscoelastic behaviour and thermal stability of Atlantic mackerel fillets stored at ?20 and ?30 °C for up to 2 years were investigated. An increase in elastic (G′) and viscous (G″) modulus values, reflecting protein aggregation, was observed in samples stored at ?20 °C compared with those stored at ?30 °C, as well as with storage time. The results indicate that toughening on frozen storage is not just limited to lean gadoid fish but also occurs in fatty fish, leading to texture deterioration. Differential scanning calorimetry of fillets stored at ?20 °C showed a shift to a lower transition temperature (T_m) and a decrease in enthalpy (ΔH) compared with control fillets stored at ?30 °C; this change was enhanced when fillets were stored for a longer period of time, confirming protein denaturation and the formation of aggregates reported previously by the authors (J Sci Food Agric 82: 579–586 (2002)). The contribution of lipid oxidation to protein aggregation was shown by storing minced mackerel with or without the antioxidant vitamin E at ?10 °C. The G′ and G″ values were higher in samples stored without vitamin E than in samples stored with vitamin E; thus antioxidants may be used to minimise protein aggregation in fatty fish. The role of lipid oxidation in promoting protein aggregation and deterioration in the texture of fatty fish has not been reported hitherto. Antioxidants such as vitamin E may be used not only to prevent lipid oxidation but also to minimise protein damage in order to prolong the shelf‐life of fatty fish. Copyright © 2004 Society of Chemical Industry     

Histidine metabolism in fish. Urocanic acid in mackerel (Scomber scombrus)

Urocanic acid, isolated from perchloric extracts of the flesh of mackerel (Scomber scombrus) using an anion exchange resin Dowex-1 acetate, was identified by thin-layer chromatography, u.v. absorption and mass spectra. During storage of sterile and non-sterile tissue the concentration of urocanic acid increased at similar rates, indicating that L-histidine ammonia-lyase is not associated with bacterial spoilage.     

Supercritical CO2 Extraction of Oil from Atlantic Mackerel (Scomber scombrus) and Protein Functionality

Supercritical carbon dioxide (SC-CO₂) extraction temperature and pressure for oil removal from freeze-dried, fall Atlantic mackerel muscle were optimized. The effect of extraction conditions on pH and water-binding potential (WBP) of the protein residue was evaluated. For the temperature range (35–55°C) and pressure interval (20.7–34.5 MPa), 34.5 MPa/35°C gave highest oil yield and concentration of ω-3 fatty acids. SC-CO₂ extraction at higher pressure levels caused slight decreases in protein residue pH. WBP of residual proteins was maximal at 1.49g H₂O/g protein for 34.5 MPa/45°C. The high lipid removal and retention of protein functionality in the residue after oil extraction may make such processes useful for future food applications.     

12‐Lipoxygenase activity in the muscle tissue of Atlantic mackerel (Scomber scombrus) and its prevention by antioxidants

Lipoxygenase was prepared from Atlantic mackerel muscle using differential centrifugation, ammonium sulphate precipitation and gel permeation (phenyl Sephadex G‐50) column chromatography. The crude lipoxygenase enzyme preparation was characterised by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS‐PAGE), which showed two prominent bands with molecular weights of 119 and 125 kDa. Fractions collected from the chromatography column were tested for enzyme activity by reacting with arachidonic acid and determining the production of hydroxyeicosatetraenoic acid (12‐HETE) using reverse phase HPLC and GC–MS. The 12‐HETE peak was absent from the fresh arachidonic acid control sample and from arachidonic acid treated with heat‐inactivated lipoxygenase. Esculetin, a known inhibitor of lipoxygenase, inhibited the production of 12‐HETE from the reaction of lipoxygenase with arachidonic acid, thus confirming that the enzyme was lipoxygenase. The HETE peak was partially reduced in the presence of antioxidants, namely synthetic butylated hydroxytoluene (BHT) and natural antioxidants vitamins C and E. The presence of lipoxygenase in Atlantic mackerel muscle indicates the possibility that the lipid oxidation mechanism is initiated enzymatically in chilled and frozen stored fillets of mackerel and that this oxidative deterioration could be inhibited by antioxidants (BHT, vitamins C and E) which are used widely in the food industry. © 2001 Society of Chemical Industry     

Effect of Smoking on the Proximate Composition of Atlantic Mackerel (Scomber scombrus)

Fall (fat) and spring (lean) Atlantic mackerel (Scomber scombrus L.) were both used in a study of the effects of “hot” smoking on proximate composition. The AFOS-Torry Mini Kiln and the method described in Advisory Note No. 82, Torry Research Station, Aberdeen, Scotland were employed. For particular comparisons both nonsmoked and smoked fish were taken from the same identical catch. The overall process resulted in an approximately 10% loss of protein. This was offset by the moisture loss, twice as much in the lean fish as in the fatter fish. The much higher fat content in and under the skin of fall fish acted as a protective barrier during smoking and helped diminish the moisture loss. The overall process, which included brining, led to an elevation of salt content.