Authentication of Atlantic salmon (Salmo salar) using real-time PCR

A real-time PCR assay based on LNA TaqMan probe technology was developed for the detection and identification of Atlantic salmon (Salmo salar). Among the advantages it is worth highlighting simplicity, rapidity, highest potential for automation and minor risk of contamination of this technique. The TaqMan real-time PCR is the currently most suitable method for screening, allowing the detection of fraudulent or unintentional mislabelling of this species. The method can be applied to all kind of products, fresh, frozen and processed products, including those undergoing intensive processes of transformation.     

A study of the ice crystal sizes of red muscle of pre-rigor Atlantic salmon (Salmo salar) fillets during superchilled storage

Pre-rigor salmon fillets were superchilled in an impingement freezer and stored at −1.7 ± 0.3 °C for 29 days. The objective of this work was to study the ice crystal sizes in red muscle of pre-rigor salmon fillets that were partially frozen at fast (−30 °C, 227 W/m² K, 2.1 min) which is referred to as process F and slow (−20 °C, 153 W/m² K, 4.2 min) which is referred to as process S during superchilled storage. It was observed that the size of intracellular ice crystals in pre-rigor muscles at faster superchilling rate was significantly (p < 0.05) smaller than that at slower superchilling rate. The size of ice crystals formed in pre-rigor muscle was significant (p < 0.05) smaller than that formed in post-rigor muscle. It was also observed that the size of intracellular ice crystals formed in pre-rigor red muscles was significant smaller than that in white muscle. In addition, a large number of small ice crystals are formed within the muscle during partial freezing of pre-rigor muscle compared to post-rigor muscle. Future research should focus on tests of the quality parameters separately in red and white muscles (pre- and post-rigor) during superchilled storage of food products in order to understand more about their characteristics (quality and shelf life).     

Quality of superchilled vacuum packed Atlantic salmon (Salmo salar) fillets stored at −1.4 and −3.6 °C

The most important factor for increasing shelf life is the product temperature, and since fish is more highly perishable than meat, the temperature is even more important. In the present study, portions of fillets of farmed Atlantic salmon (Salmo salar) were superchilled at two temperature levels, −1.4 and −3.6 °C. Texture, drip loss, liquid loss, cathepsin activities and protein extractability were investigated during storage and compared to ice chilled and frozen references. Drip loss was not a major problem in superchilled salmon. Textural hardness was significantly higher in superchilled salmon fillets stored at −3.6 °C compared to those stored at −1.4 °C, ice chilled and frozen references. Cathepsins B and B + L were not deactivated at the selected storage temperatures. The storage time of vacuum packed salmon fillets can be doubled by superchilled storage at −1.4 °C and −3.6 °C compared to ice chilled storage.     

Protein expression and enzymatic activities in normal and soft textured Atlantic salmon (Salmo salar) muscle

Soft textured Atlantic salmon is a sporadic and occasionally very severe problem for the farming and processing industries. The firm and soft fillets examined in this work differed in their gelatinase activities, cross-reactivity with anti-ubiquitin and anti-cathepsin L antibodies, as well as in the in-gel α-chymotryptic peptide maps of electrophoretically isolated myosin heavy chain (MHC) bands. The immunodetections of actin, α-actinin, MHC, and the MALDI TOF MS peptide mass fingerprinting of electrophoretically isolated MHCs only showed minor differences between samples. Other analyses revealed merely individual differences. These results seem to indicate a higher level of gelatinase activation, ubiquitination and cathepsin L cross-reacting material in softer muscle. These results would be consistent with a myopathy, but also with what could be expected in the skeletal muscle of healthy salmonid fish during a normal period of hyperplastic growth.     

Effect of superchilled storage on the freshness and salting behaviour of Atlantic salmon (Salmo salar) fillets

The aim of this work has been to evaluate the effect of superchilled storage compared with ice and frozen storage on the quality of raw material and subsequent behaviour during processing of lightly salted salmon (Salmo salar), as the first step of smoked salmon production. Physicochemical parameters used as quality indicators were α-glucosidase activity, protein denaturation and degradation (as changes in protein solubility, SDS–PAGE and free amino acids), texture attributes, and mass transfer phenomena during salting. The results obtained for the raw material within the storage range studied (until 16 days) allowed us to conclude that salmon superchilled for 9 days behaved as salmon stored on ice for 2 days with regard to hardness, protein solubility and free amino acids. In general, salting minimises the effect of the different storage methods. Superchilling for 9 days obtained the highest process yield, indicating that this method is a good way to preserve freshness of the raw material before processing.     

Correlation between astaxanthin amount and a* value in fresh Atlantic salmon (Salmo salar) muscle during different irradiation doses

The amount of astaxanthin and a* value changes in fresh Atlantic salmon light and dark muscle during cold storage was studied for different e-beam doses (0, 1, 1.5, 2 and 3 kGy). Astaxanthin (mg/kg muscle) and a* value decreased with increasing irradiation dose for both fresh light and dark muscle. The level of irradiation dose gave high correlation between a* value and amount of astaxanthin. The reason for the change in colour or decrease in a* value of Atlantic salmon during irradiation could be due to the destruction of astaxanthin. The amount of astaxanthin and a* value of 1 kGy treated salmon fillets were not significantly different (p > 0.05) from that of the control but significantly different (p < 0.05) from other irradiation treatments. The colour (a* value) of salmon muscle was related to the content of astaxanthin, which decreased as irradiation increased. The amount of astaxanthin in light muscle was three to five times greater than dark muscle. This study demonstrated that irradiating salmon fillets at 1 kGy, can be successfully used and leads to no significant change in colour and amount of astaxanthin.     

High-pressure processing and water-holding capacity of fresh and cold-smoked salmon (Salmo salar)

The influence of high pressure on the water-holding capacity (WHC) of fresh and cold-smoked salmon (CSS) was investigated up to a pressure level of 200 MPa at room temperature for 10- and 20-min periods. Changes in moisture content and WHC were determined by two methods, namely filter paper and spin-spin relaxation proton nuclear magnetic resonance. Both pressure (p<0.05) and process time (p<0.05) had significant effects on the moisture content of CSS, but not on fresh Atlantic salmon. Fresh salmon had less WHC than smoked salmon and a pressure of 150 MPa for 10 min caused a 2% increase in WHC of smoked salmon (p<0.001). The spin-spin proton relaxation (T₂) values were heavily affected by high pressure in both the samples, with substantial effects seen in CSS. At 150 MPa, both fresh and smoked fish behaved differently with respect to T₂ values compared with other pressure levels used.     

The effects of carbon monoxide on Atlantic salmon (Salmo salar L.)

Atlantic salmon were exposed to carbon monoxide (CO) before the fish were percussively killed and gill cut. The fish were compared against a control group treated identically, without CO. Salmon exposed to CO expressed no adverse reactions and were easily stunned by percussion. CO-treated salmon had an earlier onset of rigor mortis and a faster decrease in muscle pH than the control group. No significant difference in drip loss was found between salmon treated with CO and the control. A significantly deeper red colour of both gills and fillets of CO-treated salmon was observed 10 days post mortem. Significantly higher levels of plasma lactate and potassium were found in CO-treated salmon compared to control, as well as a lower level of pCO₂. Exposure to CO did not increase plasma cortisol, sodium, haematocrit or glucose; however, lactate was high. Exposure of salmon or other fish to CO could improve quality and welfare when slaughtered.     

Quality changes during superchilled storage of cod (Gadus morhua) fillets

Superchilling is a method with potential for extending the shelf life of food products by partial freezing. For centuries, Atlantic cod (Gadus morhua) has been the most important commercial species in the North Atlantic fisheries and is now regarded as a very promising species in cold water fish farming. In the present work, superchilled storage at −2.2 °C of fillet portions of farmed cod was investigated. Superchilled cod showed increased shelf life with respect to reduced growth of sulphide producing bacteria compared to ice chilled. Drip loss was lower in superchilled cod. However, liquid loss by low-speed centrifugation was higher in superchilled cod fillets compared to ice chilled. This can be explained by freeze denaturation of muscle proteins, which is supported by the lower extractability of salt soluble proteins. There is a need for process optimization to minimize protein denaturation.     

A histological study of the microstructure sizes of the red and white muscles of Atlantic salmon (Salmo salar) fillets during superchilling process and storage

Atlantic salmon (Salmo salar) fillets were partial frozen in an impingement freezer at −30 °C and 227 W/m2.K for 2.1 min prior to storage at a superchilling storage temperature of −1.7 ± 0.3 °C for 28 days. The aim of this article is to study the microstructure of the red and white muscles during superchilling process and during superchilled storage. The histology and microscopic analysis of the red and white muscles were carried out. It was found that the size of the ice crystals formed in the red muscles was smaller than those formed in the white muscles. The equivalent diameters of the intracellular ice crystals obtained upon superchilling (day 0) were 17 ± 2 and 29 ± 1 μm for the red and white muscles, respectively. Significant differences were initially observed between the size of the ice crystals formed during the superchilling process and after 1 day of storage. However, after temperature equalisation (day 1), there was no significant change in the size of the ice crystals.     

Relevance of calpain and calpastatin activity for texture in super-chilled and ice-stored Atlantic salmon (Salmo salar L.) fillets

The aim of the present experiment was to measure the protease activities in ice-stored and super-chilled Atlantic salmon (Salmo salar) fillets, and the effect on texture. Pre-rigour fillets of Atlantic salmon were either super-chilled to a core temperature of −1.5 °C or directly chilled on ice prior to 144 h of ice storage. A significantly higher calpain activity was detected in the super-chilled fillets at 6 h post-treatment compared to the ice-stored fillets and followed by a significant decrease below its initial level, while the calpastatin activity was significantly lower for the super-chilled fillets at all time points. The cathepsin B + L and B activities increased significantly with time post-treatment; however, no significant differences were observed at any time points between the two treatments. For the ice stored fillets, the cathepsin L activity decreased significantly from 6 to 24 h post-treatment and thereafter increased significantly to 144 h post-treatment. There was also a significantly lower cathepsin L activity in the super-chilled fillets at 0 h post-treatment. No significant difference in breaking force was detected; however, a significant difference in maximum compression (F_max) was detected at 24 h post-treatment with lower F_max in the super-chilled fillets. This experiment showed that super-chilling had a significant effect on the protease activities and the ATP degradation in salmon fillets. The observed difference in F_max may be a result of these observed differences, and may indicate a softening of the super-chilled salmon muscle at 24 h post-treatment.     

Atlantic salmon (Salmo salar) muscle structure integrity and lysosomal cathepsins B and L influenced by dietary n-6 and n-3 fatty acids

This study had two main objectives: first, to evaluate the impact of different types and levels of dietary n-6 and n-3 fatty acids (FAs) on Atlantic salmon muscle structure integrity; second, to highlight a possible role of lysosomes and lysosomal degrading enzymes, cathepsins, in fish muscle structure integrity, in relation to dietary fatty acids. Four groups of Atlantic salmon (90 g starting weight) in fresh water tanks were fed one of four diets containing 23% crude lipids, with 100% of the added oils as either fish oil (FO), rapeseed oil (RO), eicosapentaenoic acid (EPA) enriched-oil or docosahexaenoic acid (DHA) enriched-oil. The RO diet was characterised by low levels of EPA + DHA (10% of total FAs), whereas the EPA and DHA diets were characterised by very high levels of EPA + DHA (>50% of total FAs). Fatty acid composition of the muscle crude lysosomal fraction (CLF) generally reflected the diets. Salmon fed the RO diet presented a muscle CLF FA composition close to the one of the FO group, showing moderate PUFA levels, and comparable cathepsin B and cathepsin L activities, relative gene expression of cathepsin B and cathepsin L in the muscle and rate of myofibre–myofibre detachments post-mortem. Salmon fed the EPA and DHA-enriched-oil diets presented a fairly similar muscle CLF FA composition, but different from the FO and RO groups. In the EPA and DHA groups, the percentage of PUFAs in the muscle CLF, the rate of myofibre–myofibre detachments and the relative gene expression of cathepsin B were higher than in the FO and RO groups. Cathepsin B and cathepsin L total activities in the muscle were however lower in the EPA and DHA groups 0 h post-mortem. Dietary lipids influenced the level of lysosomal degrading enzyme activity cathepsin B and cathepsin L as well as the relative gene expression of cathepsin B. Feeding Atlantic salmon with rapeseed oil and extreme levels of EPA + DHA highlighted the impact of fatty acid composition of the diet on salmon muscle integrity and the complexity of the process involving muscle lysosomes and cathepsins in relation to these dietary fatty acids.     

Fatty acids of neutral and phospholipids of three endangered trout: Salmo trutta caspius Kessler, Salmo trutta labrax Pallas and Salmo trutta macrostigma Dumeril

Total (TL), neutral (NL) and phospholipid (PL) amounts and fatty acid (FA) composition of female Salmo trutta caspius, Salmo trutta labrax and Salmo trutta macrostigma were investigated during one year. Twenty-three FAs were identified in both NLs and PLs. The principal FAs of both fractions were palmitic acid in saturated fatty acid, oleic acid in monounsaturated fatty acid, docosahexaenoic acid (DHA) in n-3 polyunsaturated fatty acids (n-3 PUFAs) and linoleic acid in n-6 PUFAs. The highest values for TLs, NLs and PLs were found in winter. As a general trend, the highest n-3/n-6 ratios and eicosapentaenoic acid (EPA) + DHA amounts were found in the winter and this coincided with the lowest gonado-somatic index.     

Partial purification and comparison of precipitation techniques of proteolytic enzymes from trout (Salmo gairdnerii) heads

Proteolytic enzymes have been detected and partially purified from trout (Salmo gairdnerii) heads, which were preserved at −20 °C. Proteolytic enzymes, either in crude extract or in partial purified samples, were stable for 15 days with an optimum temperature of 55 °C. Proteolytic activity was very high in either alkaline or acidic pH regions. A particular ratio of cold acetone to crude extract (1.25:1) was found to be best for the partial purification of proteases, with a 99% recovery, compared with the partial purifications achieved using different cold acetone ratios or ammonium sulphate. This recovery was also confirmed via measurement of the particles (particle size analyser) contained either in crude extract or in the precipitated samples. The existence of mainly Zn–serine and possibly some Zn-acidic proteases was observed.     

Effects of −1.5 °C Super-chilling on quality of Atlantic salmon (Salmo salar) pre-rigor Fillets: Cathepsin activity,muscle histology,texture and liquid leakage

The aim of this study was to evaluate the impact of super-chilling on the quality of Atlantic salmon (Salmo salar) pre-rigor fillets. The fillets were kept for 45 min in a super-chilling tunnel at −25 °C with an air speed in the tunnel at 2.5 m/s, to reach a fillet core temperature of −1.5 °C, prior to ice storage in a cold room for 4 weeks. Super-chilling seemed to form intra- and extracellular ice crystals in the upper layer of the fillets and prevent myofibre contraction. Lysosome breakages followed by release of cathepsin B and L during storage and myofibre–myofibre detachments were accelerated in the super-chilled fillets. Super-chilling resulted in higher liquid leakage and increased myofibre breakages in the fillets, while texture values of fillets measured instrumentally were not affected by super-chilling one week after treatment. Optimisation of the super-chilling technique is needed to avoid the formation of ice crystals, which may cause irreversible destruction of the myofibres, in order to obtain high quality products.     

Chilled storage of golden gray mullet (Liza aurata)

This study determined quality changes of whole ungutted golden gray mullet (Liza aurata) while stored in ice or in a refrigerator (without ice). Changes in microbiological quality (total viable and psychrophilic counts, lactic acid bacteria and Enterobacteriaceae), chemical quality (pH, total volatile basic nitrogen, trimethylamine, peroxide value, thiobarbituric acid, and free fatty acids), and raw fish sensory attributes were evaluated during 16 days of storage. The sensory attributes of golden gray mullet correlated well with the microbiological analyses (r = 0.92). Based on the overall raw acceptability sensory scores and the microbiological tests, the shelf life of the raw golden gray mullet was 10 days in ice and about 14 days in a refrigerator.     

Textural and physicochemical changes in salmon (Salmo salar) treated with commercial liquid smoke flavourings

The textural and physicochemical properties of fillets of farmed Atlantic salmon (Salmo salar) treated with two commercial liquid smoke flavourings (LS1 and LS2) were examined after 15, 30 and 45 days of storage at 2 °C in oxygen impermeable bags. Salmon flesh treated with LS1 was characterized by high water-soluble protein, fat and moisture contents, plus low hardness, fracturability, gumminess and chewiness, and a low alkali-insoluble protein content. These characteristics were similar to those of control salmon (not treated with liquid smoke flavouring). Storage time changed these properties similarly in both. Salmon flesh treated with LS2 was characterized by high hardness, fracturability, gumminess, chewiness and alkali-insoluble protein levels, plus low water-soluble protein, fat and moisture contents. Storage time appeared to have a far less important effect on salmon flesh treated with LS2.     

Low field Nuclear Magnetic Resonance on the effect of salt and modified atmosphere packaging on cod (Gadus morhua) during superchilled storage

Low field Nuclear Magnetic Resonance was used to evaluate the effect of salt and modified atmosphere packaging (MAP) on cod loins during superchilled storage. Transversal and longitudinal proton relaxation times of the cod loins were measured with Carr-Purcell-Meiboom-Gill (CPMG) and Inversion Recovery (IR) pulse sequences respectively. The relaxation parameters reflected the observed differences in the muscle caused by variation in salt concentration, the choice of salting method (brining or brine injection) and packaging (air or MAP), as well as superchilled storage temperature and storage time. Significant correlations were found between the NMR parameters and parameters describing the water dynamics of the muscle (moisture and salt content, water holding capacity, drip and cooking yield), as well as muscle pH and counts of H₂S-producing bacteria in chosen sample groups. The study showed the possibility of using low field NMR to indicate fish quality deterioration, when the spoilage mechanisms affect the water properties and muscle structure.