The influence of different acids and pepsin on the extractability of collagen from the skin of Baltic cod (Gadus morhua)

Solutions of (0.5 M) citric, lactic and acetic acids and 0.15 M HCl were used for the extraction of collagen from the whole skins of Baltic cod (Gadus morhua). The extractions were performed at a temperature of 4 °C for 24, 48 and 72 h using a solid/solution ratio of 1:6 (w/v). Of the acids used, HCl was the least effective solvent for collagen. The maximal yield of collagen extracted with citric acid was 60%. Collagen extraction with acetic or lactic acid give a maximal yield of about 90% with HCl yielding of only 18%. After enzymatic treatment of cod skin the yield of protein extracted with HCl and citric acids increased to 40% and 20%, respectively. Collagen was completely solubilized under the same conditions in acetic and lactic acids. Electrophoretic analysis of collagens extracted in HCl and citric acids with enzymatic treatment proved that the isolated protein was denaturated. The solutions of acetic and lactic acids are solvents for native collagen.     

Marine and farmed fish in the Polish market: Comparison of the nutritional value

The proximate composition and fatty acids profiles of the muscle tissues of nine fish species that are popular on the Polish market were examined. The nine studied fish species were: Baltic fish (cod, herring, salmon), fish farmed in Poland (carp, trout), oceanic fish imported from China (walleye pollock, sole), and farmed fish imported from Vietnam and China (sutchi catfish, tilapia). The lowest lipid content (below 0.1%) was noted in the muscle tissues of Baltic cod and walleye pollock caught in the Pacific. The muscle tissue of walleye pollock also had the lowest protein content (12.2 ± 2.0%). The highest lipid content was noted in the muscle tissues of Baltic salmon (13.1 ± 2.4%). The highest percentage content of eicosapentaenoic (C20:5 n − 3 – EPA) and docosahexaenoic (C22:6 n − 3 – DHA) acids (over 40%) was noted in the fat extracted from the oceanic fish and Baltic cod. However, due to the low fat content, the concentrations of EPA + DHA in these fish species and in imported farmed fish expressed in mg/100 g of muscle tissues are the lowest and range on average from 24.8 ± 5.7 mg/100 g (sutchi catfish) to 207.4 ± 125.4 mg/100 g (sole). This is why the consumption of these fish species has no significant meaning for coronary heart disease prevention. Consumers with symptoms of cardiovascular diseases should include the following fish species, which have high concentrations of EPA + DHA: Baltic salmon (3807.2 ± 666.3 mg/100 g); Polish farmed trout (1804.0 ± 279.2 mg/100 g); and Baltic herring (940.9 ± 306.6 mg/100 g) in their diets. However, the consumption of Baltic salmon must be limited on account of the levels of persistent organic pollutants found in it.     

Characterization of fish gelatin from surimi processing wastes: Thermal analysis and effect of transglutaminase on gel properties

Fish gelatin extraction from wastes of fish Herring species (Tenualosa ilisha) was carried out by a series of pretreatment with 0.2 M Ca(OH)₂ followed by 0.1 M citric acid and final water extraction at 50 °C for 3 h. The resulting fish gelatin preparation was evaluated for its dynamic viscoelastic properties, gelling and melting temperatures and gel strength. The gelling and melting temperatures of gelatin samples (at 6.67%, w/v) were obtained from differential scanning calorimetry and rheological studies. The melting temperature of extracted fish gelatin (EFG) obtained ranged from 16.2 to 16.7 °C compared to that of commercial fish gelatin gel (CFG), from 23.7 to 25.6 °C and halal bovine gelatin (HBG), from 26.5 to 28.7 °C. On the other hand, gelling temperatures of EFG, CFG and HBG ranged from 5.1 to 5.2 °C, 11.9 to 17.46 °C, and 12.6 to 19.33 °C, respectively. EFG gave gels with a considerably lower G′ values than CFG and HBG. The bloom strength of EFG gels at 6.67% (w/v) was 69.03 g which was much lower than HBG (336.2 g) and CFG (435.9 g). Enzyme transglutaminase was added in the amounts of 0.5, 1.0, 3.0 and 5.0 mg/g gelatin to modify the gel properties of the extracted fish gelatin. The modified EFG gels obtained had higher gel strengths of 101.1 g and 90.56 g with added transglutaminase of 1.0 and 3.0 mg/g, respectively. However with addition of 5.0 mg/g enzyme the gel strength increased only up to 75.06 g. SDS-PAGE of extracted gelatin gel showed protein band intensities for α1-chains and 53 kDa but in gels added with higher concentration of transglutaminase, these protein band intensities seemed to disappear.     

Rheological and functional properties of gelatin from the skin of Bigeye snapper (Priacanthus hamrur) fish: Influence of gelatin on the gel-forming ability of fish mince

The rheological and functional properties of gelatin from the skin of bigeye snapper (Priacanthus hamrur) fish were assessed. The protein content of dried gelatin was 94.6% and moisture content was 4.2%. The amino acid profile of gelatin revealed high proportion of glycine and imino acids. The bloom strength of solidified gelatin was 108 g. The average molecular weight of fish skin gelatin was 282 kDa as determined by gel filtration technique. The emulsion capacity (EC) of gelatin at a concentration of 0.05% (w/v) was 1.91 ml oil/mg protein and with increase in concentration, the EC values decreased. The gelling and melting temperatures of gelatin were 10 and 16.8 °C, respectively as obtained by small deformation measurements. The flow behavior of gelatin solution as a function of concentration and temperature revealed non-Newtonian behavior with pseudoplastic phenomenon. The Casson and Herschel–Bulkley models were suitable to study the flow behavior. The yield stress was maximum at 10 °C with the concentration of 30 mg/ml. Thermal gelation behavior of threadfin bream (Nemipterus japonicus) mince in presence of different concentration of gelatin was assessed. Gelatin at a concentration of 0.5% yielded higher storage modulus (G′) value than control. Frequency sweep of heat set gel with gelatin revealed strong network formation.     

Effect of freezing on the physicochemical, textural and sensorial characteristics of salmon (Salmo salar) smoked with a liquid smoke flavouring

This study reports the effect of different refrigeration/freezing treatments on the physicochemical, textural and sensorial properties of farmed Atlantic salmon (Salmo salar) treated with a commercial liquid smoke flavouring. Observations were made on three groups of fillets - group RFS: salted, smoked and stored at 4 °C; group BFS: frozen at −25 °C for 24 h, thawed, salted, smoked and stored at 4 °C; and group AFS: salted, smoked and frozen at −25 °C for 24 h and stored at −18 °C - over a period of 45 days. Scores (on a scale of 1-9) were provided for different sensorial attributes by a panel of 10 trained tasters. Sixty percent of the panellists consistently preferred the AFS fillets. The maximum shelf life associated with each treatment was defined as the last sampling day on which a mean score of ≤5 was awarded for the fillet sensorial attributes by ≥50% of the panellists. Freezing the salmon for 24 h before smoking (BFS) did not increase its shelf life (30 days) over that of refrigerated smoked salmon (RFS). In addition, the former treatment had a negative effect on the adhesiveness, cohesiveness, smoke odour intensity and colour intensity of the flesh. However, maintaining the fish frozen at −18 °C (AFS) increased its shelf life (>45 days) and invested the flesh with greater firmness, cohesiveness and colour intensity.     

Optimisation of gelatine extraction from hoki (Macruronus novaezelandiae) skins and measurement of gel strength and SDS–PAGE

The extraction of hoki (Macruronus novaezelandiae) gelatine was optimised using Response Surface Methodology (RSM) and gel strength and SDS–PAGE were evaluated. The optimum conditions for extraction were 0.75 M NaCl for 9 min of pre-treatment time and hot water extraction at 49.3 °C for 60 min. Results showed that the predicted yield by RSM (17.4%) closely matched the experimental yield of 17.6%. SDS–PAGE showed that hoki gelatine contained higher molecular weight subunits (∼191 kDa) but lower gel strength (197 ± 5 g) than those from porcine (307 ± 8.4 g) or bovine (273 ± 16.1 g) gelatine determined at 7 °C. However, hoki gelatine gel strength was significantly higher than those from other cold-water fish species reported in literature, which could account for the slight differences in methodologies reported.     

Extraction of fish oil from the skin of Indian mackerel using supercritical fluids

The total oil was extracted from the ground skin of Indian mackerel (Rastrelliger kanagurta) using various techniques of supercritical fluid extraction (SFE) at 20–35 MPa and 45–75 °C and by the Soxhlet method for comparison. The oil yield increased with pressure and temperature and the highest yields were 24.7, 53.2, 52.8, and 52.3/100 g sample (dry basis) for the continuous, cosolvent, soaking, and pressure swing techniques, respectively, at 35 MPa and 75 °C. The yield from the Soxhlet extraction was 53.6/100 g sample (dry basis). The CO₂ consumption was 581.8, 493.6, 484.9 and 290.9 g for the continuous, cosolvent, soaking and pressure swing techniques, respectively, at 35 MPa and 75 °C. The largest recoveries of PUFA, especially the ω-3 family, were achieved from the soaking and pressure swing techniques at 35 MPa and 75 °C. Thus, the pressure swing and soaking techniques are the most effective at extracting the oil from fish skin.     

Comparison of the properties of multi-composite fish gelatin films with that of mammalian gelatin films

Four types of films viz. gelatin, gelatin–MMT, gelatin–chitosan and gelatin–MMT–chitosan prepared from redsnapper and grouper bone gelatin were compared with the mammalian gelatin films, for their mechanical and barrier properties. Grouper gelatin films had higher tensile strength (TS) and Young’s modulus (YM), but lower elongation at break (EAB) than redsnapper films. Incorporation of MMT and chitosan improved the TS (p < 0.05) of the films. Water solubilities were lower (p < 0.05) in films incorporated with chitosan compared to simple gelatin film. Protein solubilities were lower in gelatin–MMT films, irrespective of the type of solvent used. The water vapour transmission rates (WVTR) of fish and mammalian gelatin films were similar, but addition of MMT had reduced WVTR (p < 0.05). SEM micrographs depicted smoother surface for gelatin–MMT and gelatin–MMT–chitosan films. Thus, composite fish gelatin films made with MMT and chitosan could be the good natural biodegradable films due to their better mechanical and barrier properties.     

Optimisation of hydrolysis conditions and fractionation of peptide cryoprotectants from gelatin hydrolysate

The inhibition of ice crystal growth in an ice cream mix by gelatin hydrolysate produced by papain action was studied. The ice crystal growth was monitored using a cold stage by thermal cycling between −14 and −12 °C at a rate of one cycle per 3 min. Optimum conditions for producing cryoprotective peptides from bovine gelatin (Type 225B40) was investigated by incubating gelatin with papain at pH 7.0 for various times (10–60 min), temperatures (37, 45, and 55 °C), and enzyme-to-gelatin (E/S) ratios (1:10 to 1:100). The hydrolysate obtained after 30 min at 37 °C and pH 7.0 (E/S ratio of 1:10) substantially inhibited ice crystal growth in an ice cream mix. The molecular weight range of ice crystal growth inhibiting peptides was determined by matrix-assisted laser desorption ionisation – time of flight (MALDI-TOF) mass spectrometry after fractionation on Sephadex G-50 and SP-Sephadex C-25 chromatography columns. Cationic gelatin peptides with average molecular mass in the range of 700–1400 Da significantly reduced ice crystal growth in an ice cream mix by a factor of ten under identical experimental conditions.     

Preparation and characterisation of gelatins from the skins of sin croaker (Johnius dussumieri) and shortfin scad (Decapterus macrosoma)

Gelatins were prepared from the skins of the tropical fish, sin croaker (Johnius dussumeiri) and shortfin scad (Decapterus macrosoma). Visual appearance, colour, pH, bloom strength, viscoelasticity, melting point and amino acid profiles of the fish gelatins were evaluated. Shortfin scad gelatin had higher melting and gelling temperatures than those of sin croaker gelatin. The bloom strengths of gelatins from sin croaker and from shortfin scad were 125 and 177 g, respectively, compared to 240 g for commercial bovine gelatin. The pH values were significantly different between the solutions of the two fish gelatins. The elastic modulus (G′) of the fish gelatin gels increased by more than 10-fold and the viscous modulus (G″) of fish gelatin solution increased sixfold after holding at 5 °C for 2 h. These viscoelastic properties of bovine gelatin only increased by less than twice.     

Enzymatic hydrolysis of by‐products from the fish‐filleting industry; chemical characterisation and nutritional evaluation

Fish frames without heads from Atlantic cod and Atlantic salmon were proteolysed with the industrial enzymes neutrase®, alcalase® and pepsin for 1, 15, 30, 45, 60, 90 and 120 min. After 120 min of hydrolysis, salmon treated with alcalase and cod treated with pepsin yielded significantly (p < 0.05) higher protein recoveries (67.6 and 64% respectively) as compared to salmon treated with neutrase or pepsin and cod treated with neutrase or alcalase (53–62%). To minimise bitterness in the fish hydrolysates, kojizyme™ was added after 120 min of pre‐hydrolysis with alcalase, and the hydrolysis was run for additional times of 120, 240, 360, 480, 600 and 720 min. Protein recovery did not change significantly during the hydrolysis with kojizyme, but the degree of hydrolysis increased significantly (p < 0.01) in both the cod and salmon hydrolysates. A hydrolysate from cod treated with alcalase (150 min) followed by treatment with kojizyme (510 min) was produced. The final hydrolysate was freeze‐dried to a fish protein hydrolysate (FPH) and chemically characterised. The nutritional value of the FPH was established in an experiment with rats. Inclusion of 10% FPH‐N showed significantly (p < 0.05) higher nutritional value as compared to rats fed higher inclusion levels of FPH. © 2000 Society of Chemical Industry     

Effect of high-pressure versus conventional thawing on color, drip loss and texture of Atlantic salmon frozen by different methods

Atlantic salmon (Salmo salar) samples were frozen by conventional air freezing, plate freezing and liquid nitrogen (LN) freezing, and subjected to different thawing treatments: water immersion thawing (WIT) (4°C and 20°C) and high-pressure thawing (HPT) at 100, 150 and 200 MPa with water (containing 2 g oil/100 g) as pressure medium at 20°C. Temperature and phase change behavior of fish samples were monitored during freezing and thawing. The phase change point of frozen salmon was lowered to −14°C, −19°C and −25°C for the HPT processes at 100, 150 and 200 MPa, respectively. These phase change temperatures were lower than for pure ice at the same pressures possibly due to the presence of solutes in salmon. The HPT times were 22.6±1.4, 18.1±1.4 and 17.0±1.3 min at 100, 150 and 200 MPa, respectively, as compared with 26.6±2.1 and 94.3±3.4 min for the WIT process at 20°C and 4°C, respectively. Employing pressures above 150 MPa caused noticeable color changes in salmon during the HPT process and the product texture was significantly modified during HPT at 200 MPa. Different freezing rates prior to thawing resulted in differences in drip loss in salmon samples, but they did not induce specific color and texture changes. A significant (P<0.05) reduction of drip loss by the HPT process was observed only for the LN frozen samples in which mechanical cracking occurred and much of the drip appeared after WIT process. Drip loss formed during pressure thawing seems to be a complicated process, for which further studies are needed.     

Protein hydrolysates from meriga (Cirrhinus mrigala) egg and evaluation of their functional properties

Protein hydrolysates from underutilised meriga (Cirrhinus mrigala) fish egg were prepared by using commercial Alcalase and papain enzymes. The degree of hydrolysis was 62% for Alcalase and 17.1% for papain, after 90 min digestion at 50–55 and 60–65 °C, respectively. The protein content of Alcalase-produced hydrolysate was higher (85%) than that of papain hydrolysate (70%) (p < 0.05). Hydrolysis by both enzymes increased protein solubility of fish egg protein hydrolysates to above 72.4% over a wide pH range (2–12). Results showed that the hydrolysates had good fat absorption capacity (0.9 and 1.0 g/g sample), foam capacity (70% and 25%) and emulsifying capacity (4.25 and 5.98 ml/g hydrolysate), respectively for Alcalase and papain protein hydrolysates. Gel filtration chromatograms and SDS–PAGE analysis indicated the distribution of smaller peptides. These results suggested that fish egg protein hydrolysates could be useful in the food industry.     

PROPERTIES OF DIFFERENT FISH PROCESSING BY-PRODUCTS FROM POLLOCK, COD AND SALMON

Individual fish processing waste stream components can be used to make feed ingredients or other products. Waste stream components obtained from commercial fish processing plants included heads, viscera, frames, and skins from Alaska pollock (Theragra chalcogramma) and Pacific cod (Gadus macrocephalus); and heads, and viscera from pink salmon (Oncorhynchus gorbuscha). The protein content of heads from all three species ranged from 13.9 to 16.4%; and the fat content ranged from 0.9 to 10.9%. Viscera protein content ranged from 13.0 to 15.3%, and the fat content from 2.0 to 19.1%. After heating to 85C the percent soluble protein in salmon heads was different (P < 0.05) from pollock or cod heads. Percent soluble protein of pollock and cod skin increased 8fold (P<0.05) after the 85C heat treatment. Connective tissue content was calculated from chemical determination of hydroxyproline content, and large differences in percent connective tissue content were found (1% for pollock viscera to 46% for skin). Estimated rat PER values ranged from a low of 2.1 for skin to a high of 3.1 for viscera and fillet samples (P<0.05).     

Effects of pan-frying in margarine and olive oil on the fatty acid composition of cod and salmon

Effects on the fatty acid composition of cod (Gadus morhua) and salmon fillets (Salmo salar) after pan-frying in margarine and olive oil were determined. The fatty acids of the margarine used were 55.5% saturated (SFA), 33.0% mono-unsaturated (MUFA) and 11.5% polyunsaturated (PUFA). The olive oil used contained 15.4% SFA, 76.1% MUFA and 8.5% PUFA. Using margarine or olive oil increased the SFA and MUFA percentages, respectively, in both species. For cod fillets (lean), pan-frying increased the fat content (0.55–4.15 g/100 g and 0.55–2.30 g/100 g before and after pan-frying, with margarine and olive oil, respectively), whereas, for salmon fillets (fat), it decreased (13.91 to 10.57 g/100 g and 15.35 to 12.95 g/100 g before and after pan-frying with margarine and olive oil, respectively). In conclusion, the culinary fat selection affects the total fatty acid content and composition of the prepared fish fillet.     

Fatty acid compositions of fish oil extracted from different parts of Indian mackerel (Rastrelliger kanagurta) using various techniques of supercritical CO2 extraction

Fatty acid compositions of fish oil extracted from different parts of Indian mackerel (Rastrelliger kanagurta) using various techniques of supercritical carbon dioxide (SC-CO₂) at optimised conditions (35 MPa, 60 °C, 2 ml/min) were analysed and compared to the results of Soxhlet extraction. The amount of polyunsaturated fatty acids (PUFA) recovered (as a percentage of total extracted fatty acids) were within the ranges of 73.24–74.68% in the skin, 68.36–69.37% in the flesh, 56.20–57.3% in the viscera and 61.21–62.09% in the heads. The greatest amount of the ω-3 fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), were found in fish skin followed by flesh, heads and viscera. The greatest amounts of EPA (9–12%) and DHA (10–14%) were obtained using the soaking and pressure swing techniques. The pressure swing and soaking techniques are the most effective techniques for extracting the ω-3 family of fatty acids from fish samples.     

Histamine levels in seventeen species of fresh and processed South African seafood

Histamine levels were determined in fresh and processed seafood from a representative range of 10 outlets after several incidents of scombroid seafood poisoning occurred. Species included seventeen fresh and processed scombroid- and non-scombroid fish, marine mollusks and crustaceans. Histamine levels in fresh seafood were generally low (0–9 ppm) with the exception of one sample of snoek (scombroid fish; >50 ppm) and one sample of yellowtail (non-scombroid fish; >50 ppm). Both species are rich in free histidine (1.5–5.3 ppb), a precursor of histamine. Processed seafood had, in general, low histamine concentrations (0–3 ppm) with the exception of fish meal (76 ppm), salted herring (47 ppm), one sample of smoked snoek (>50 ppm) and dried tuna (8000 ppm). In total, 5 of 80 examined samples (6%) contained histamine concentrations above the legal limit of 50 ppm. Experimental formation of histamine was demonstrated to be strongly temperature- and time-dependent. Samples were not contaminated with Vibrio spp., Pseudomonas spp., Klebsiellas spp. or Enterobacteria.     

High resolution two-dimensional electrophoresis as a tool to differentiate wild from farmed cod (Gadus morhua) and to assess the protein composition of klipfish

Tris and CHAPS–urea extracts from wild and farmed cod muscle and from rehydrated cod klipfish fillets were analyzed by one (1DE) and two-dimensional electrophoresis (2DE). 2DE maps of tris extracts from farmed cod differed from the wild in a series of spots of Mw 35 and 45 kDa. The CHAPS–urea extracts from farmed cod had a several spots of Mw between 100 and 45 kDa, which were hardly detectable in wild cod and very prominent in klipfish. Klipfish was clearly different from the other samples: the myosin heavy chain was hardly detectable in these samples, and the tris extracts contained fewer, and the CHAPS–urea more spots than the corresponding extracts from the raw muscles. Further identification of these potentially diagnostic spots will make it easier the differentiation of farmed from wild cod and the evaluation of klipfish processing on the protein content of the product.     

Characteristics of gelatin from the skin of unicorn leatherjacket (Aluterus monoceros) as influenced by acid pretreatment and extraction time

Gelatins from the skin of unicorn leatherjacket (Aluterus monoceros) pretreated with different acids (0.2 M acetic acid or 0.2 M phosphoric acid) and extracted with distilled water at 45 °C for various times (4 and 8 h) were characterized. Yields of 5.23–9.18 or 6.12–11.54% (wet weight basis) were obtained for gelatins extracted from the skin pretreated with 0.2 M acetic acid or 0.2 M phosphoric acid, respectively. Extracted gelatins contained α₁ and α₂ chains as the predominant components and some degradation peptides. The absorption bands of gelatins in FTIR spectra were mainly situated in the amide band region (amide I, amide II and amide ???) and showed the significant loss of molecular order of triple helix. Gelatin samples had a relative solubility greater than 90% in the wide pH ranges (1–10). The gel strength of gelatin from skin pretreated with phosphoric acid (GPA) was higher than that of gelatin from skin pretreated with acetic acid (GAA). Both GPA and GAA had the lower gel strength than that of commercial bovine gelatin (P < 0.05). Net charge of GAA and GPA became zero at pHs of 6.64–7.15 and 6.78–7.26, respectively, as determined by zeta potential titration. Emulsifying and foaming properties of GAA and GPA increased with increasing concentrations (1–3%, w/v). Those properties were governed by pretreatments and extraction time. Thus gelatin can be successfully extracted from unicorn leatherjacket skin using the appropriate acid pretreatment and extraction time.     

Microwave-assisted extraction of phenolic antioxidant compounds from peanut skins

A microwave-assisted extraction system was used to extract phenolic antioxidants from peanut skins. The effects of microwave power (10%, 50%, 90% nominal), irradiation time (30, 90, 150 s) and sample mass (1.5, 2.5, 3.5 g) on total phenolic content (TPC), ORAC (oxygen radical absorbance capacity) level and resveratrol content of peanut skin extracts (PSE) were investigated. Peanut skins were extracted with 37.5 ml of 30% ethanol (EtOH) in water. A response surface method was used to estimate optimum extraction conditions, based on TPC, ORAC level and resveratrol content. The maximum predicted TPC, under the optimised conditions (90% microwave power, 30 s irradiation time and 1.5 g skins), was 143.6 mg gallic acid equivalent (GAE)/g skins. The highest ORAC value was 2789 μmol trolox equivalents (TE)/g skins, which occurred at 90% power, 150 s and 1.5 g of skins. Resveratrol was identified in PSE by LC–MS–MS analysis.