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.     

A study of the ice crystals in vacuum-packed salmon fillets (Salmon salar) during superchilling process and following storage

The aim of this work was to study the microstructure of vacuum-packed salmon fillets superchilled in an impingement freezer at −30 °C (air temperature) and 227 W/m² K (surface heat transfer coefficient, SHTC) for 2.1 min prior to storage at a superchilling storage temperature of −1.7 ± 0.3 °C for 28 days. The microstructure of vacuum-packed salmon fillets were analysed at the surface, mid-centre and centre layers. Significant differences were observed between the ice crystals formed at the surface, mid-centre and centre layers. The size of ice crystals at the centre of the superchilled fillets was 3 times larger than those at the surface layer. Significant differences were observed between the size of ice crystals formed during the superchilling process and following storage. The results further indicated that, after temperature equalisation (1 day of storage) the growth of the intracellular ice crystal was not significant at (P < 0.05) at any storage time.     

Ice fraction assessment by near-infrared spectroscopy enhancing automated superchilling process lines

The objective of the current study was to analyze and develop some important automation steps in a superchilling process line. In order to control the product quality there is a need for online measurements of ice fraction and distribution in inhomogeneous products. Moreover, automatic handling of such superchilled products is currently commercially unavailable. The current study presents a new method for monitoring and handling superchilled product of varying form and consistency. Observation of the shift in the water absorption peak, measured by near-infrared spectroscopy (NIR) transflection mode, was used to determine the ice level in superchilled salmon, scanning approximately 1.5 cm into the fillets. The salmon fillets were stored on ice at 0 °C for 5–7 days before superchilling at −24 °C to target ice contents of 10%, 15% and 30%. Online NIR measurements of ice fraction showed promising results, with a low prediction error of 2.5%. The storage study confirmed former quality results with a microbiological shelf life of 15–17 days with only minor differences in values for drip loss and water-holding capacity between superchilled and chilled samples.     

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.     

Superchilling of rested Atlantic salmon: Different chilling strategies and effects on fish and fillet quality

Rested Atlantic salmon was superchilled in seawater slurry (−1.93 ± 0.27 °C). The chilling efficiencies of slurry and crushed ice were compared. The feasibility of using slurry to produce subzero core temperatures before packing was also evaluated. Simulated transport to market, with or without ice after initial superchilling (1 day), was also studied. Fish quality (Quality Index, fillet colour, pH, water content, water-holding capacity, hardness and bacterial loads) was evaluated at arrival to ‘market’ and after keeping the fish ‘in the market’ for 1 week. The results were compared with continuous ice (control) or slurry storage. In terms of quality, pre-chilling in slurry and continuous storage in slurry were evidently not advantageous over traditional ice storage, as evaluated after 4 days. After 11 days, both advantages and disadvantages of continuous superchilling were observed. Notably, subsequent ice storage of superchilled fish resulted in increased bacterial load and inferior fillet hardness.     

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.     

Low salt brining of pre-rigor filleted farmed cod (Gadus morhua L.) and the effects on different quality parameters

Pre-rigor processing of cod fillets may have economic benefits, but this potential has usually been overshadowed by process-linked difficulties such as pin bone removal, rapid rigor onset and higher drip losses. The aim of this work was to study the impact on fillet quality parameters after immersing pre-rigor filleted farmed cod in different NaCl solutions ranging from 15 to 60 g/L. Temperature of the fish at death was 4 °C, in immersion solutions 2 °C, and following immersion the fillets were stored in ice within plastic bags for 14 days. As controls, one group was filleted pre-rigor but not immersed, and one group was filleted post-rigor and not immersed. Immersing in salt solution resulted in better yield compared to both control groups. Higher salt content generally increased rigor contraction, but significantly reduced fillet gaping and the force required to pull pin bones. Thus, relatively low salt levels within the fillets had a positive impact on some of the problems associated with pre-rigor filleting.     

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.     

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.     

Changes in water holding capacity and drip loss of Atlantic salmon (Salmo salar) muscle during superchilled storage

Changes in water holding capacity and drip loss of Atlantic salmon (Salmo salar) fillets during superchilled storage were studied. Due to the significant differences in ice crystal sizes observed in our previous study, the liquid loss (LL) was analysed separately, at the surface and centre of the superchilled samples. No significant differences were found in LL between surface and centre parts of the superchilled samples. No significant difference in the LL was observed from surface samples between 1 and 14 days of storage. There was a significant difference in the LL at day 1 of the centre samples, but no significant differences were observed between 3 and 14 days of storage. In contrast, the LL was significantly decreased at day 21 both at the centre and the surface of the superchilled samples. No significant difference (p < 0.05) was found in drip loss between 1 and 14 days of storage for the superchilled samples. A significant increase in drip loss for the superchilled samples was observed at day 21. These findings are significant for the industry because it provides valuable information on the quality of food in relation to ice crystallisation/recrystallisation during superchilled storage.     

The effect of postmortem processing treatments on quality attributes of raw Atlantic salmon (Salmo salar) measured by sensory and instrumental methods

The main objective of this study was to compare the effects of three different processing treatments on sensory attributes and instrumental quality measurements of raw Atlantic salmon (Salmo salar) fillets. Salmon was either pre-rigor filleted and restricted or allowed to contract (Vacuum-PRE and Contracted-PRE, respectively) or post-rigor filleted (POST). Sensory evaluation (appearance, flavour, texture) and instrumental quality measurements (colour, texture, fat, astaxanthin, liquid holding capacity) were performed at 5–7 days postmortem. Sensory evaluation revealed that Vacuum-PRE fillets had less desirable quality attributes than the other treatment groups, with higher scores for tenderness and whiteness and lower scores for hardness and colour intensity. The observed changes in fillet height between the treatment groups indicated that the immediate vacuum packaging of the Vacuum-PRE fillets had limited their contraction during rigor mortis development, resulting in the negative effects observed on quality. This implicates that the well-known positive effects of pre-rigor filleting regarding colour and texture can be reduced or even abolished if the fillets are restricted from contraction during rigor mortis development.     

Effects of superchilling and cryoprotectants on the quality of common carp (Cyprinus carpio) surimi: Microbial growth,oxidation, and physiochemical properties

The objective of the present study was to investigate the effect of superchilling with cryoprotectants (a mixture of sucrose and sorbitol) on microbial growth, lipid oxidation, and proteolytic degradation in common carp (Cyprinus carpio) surimi. With increasing storage time, the microbial count of surimi increased from 4.4 log₁₀ CFU/g (0 days) to 7.2, 6.2, 5.9, and 5.5 log₁₀ CFU/g (35 days; P < 0.05) in the samples superchilled at −1 °C, superchilled at −3 °C, superchilled at −3 °C with cryoprotectants, and frozen at −18 °C, respectively. The total volatile basic nitrogen and thiobarbituric acid-reactive substances exhibited an increasing trend (P < 0.05) similar to that obtained with the microbial growth, whereas the whiteness and lightness stability decreased (P < 0.05) with increasing storage time. The SDS-PAGE analysis revealed that the degree of protein degradation increased with increasing storage time. Compared with the sample frozen at −18 °C, superchilling at −1 °C and −3 °C resulted in a marked reduction in the microstructure deterioration of the myofibrillar protein gels, and the addition of cryoprotectants further reduced this deterioration. These results suggest that superchilling with cryoprotectants offers an effective approach for reducing microbial growth and lipid oxidation and limiting proteolytic degradation in common carp surimi.     

The combined effect of superchilling and modified atmosphere packaging using CO2 emitter on quality during chilled storage of pre‐rigor salmon fillets (Salmo salar)

BACKGROUND: Pre‐rigor fillets of Atlantic salmon were either superchilled or chilled prior to packaging in air or in modified atmosphere (MAP, 60% CO₂/40% N₂) with a CO₂ emitter, in 5.3 L high‐density polyethylene trays with three to four layers of fillets (3.0–3.7 kg). All samples were stored at 0.1 °C for 28 days. RESULTS: Fillets stored in MAP had significantly lower bacterial growth compared to fillets stored in air, and MAP superchilled bottom fillets had lower bacterial counts compared to the corresponding chilled fillets. Samples superchilled prior to refrigerated storage in air had similar bacterial growth to ordinary chilled samples. Faster fillet softening during storage and higher liquid loss were observed in superchilled MAP samples. CONCLUSION: Combining short‐term superchilling and MAP with a CO₂ emitter prolonged the shelf‐life of pre‐rigor salmon fillets, which can improve sustainability throughout the value chain. The superchilling method needs to be optimized to avoid negative effects on texture and liquid loss. Copyright © 2009 Society of Chemical Industry     

Study of the quality characteristics in cold-smoked salmon (Salmo salar) originating from pre- or post-rigor raw material

Improved slaughtering procedures in the salmon industry have caused a delayed onset of rigor mortis and, thus, a potential for pre-rigor secondary processing. The aim of this study was to investigate the effect of rigor status at time of processing on quality traits color, texture, sensory, microbiological, in injection salted, and cold-smoked Atlantic salmon (Salmo salar). Injection of pre-rigor fillets caused a significant (P<0.001) contraction (-7.9%± 0.9%) on the caudal-cranial axis. No significant differences in instrumental color (a*, b*, C*, or h*), texture (hardness), or sensory traits (aroma, color, taste, and texture) were observed between pre- or post-rigor processed fillets; however, post-rigor (1477 ± 38 g) fillets had a significant (P>0.05) higher fracturability than pre-rigor fillets (1369 ± 71 g). Pre-rigor fillets were significantly (P<0.01) lighter, L*, (39.7 ± 1.0) than post-rigor fillets (37.8 ± 0.8) and had significantly lower (P<0.05) aerobic plate count (APC), 1.4 ± 0.4 log CFU/g against 2.6 ± 0.6 log CFU/g, and psychrotrophic count (PC), 2.1 ± 0.2 log CFU/g against 3.0 ± 0.5 log CFU/g, than post-rigor processed fillets. This study showed that similar quality characteristics can be obtained in cold-smoked products processed either pre- or post-rigor when using suitable injection salting protocols and smoking techniques.     

The influence of superchilling and cryoprotectants on protein oxidation and structural changes in the myofibrillar proteins of common carp (Cyprinus carpio) surimi

The objective of the present study was to investigate the effects of superchilling combined with cryoprotectants (a mixture of sucrose and sorbitol, 1:1) on protein oxidation and structural changes in common carp (Cyprinus carpio) surimi. With increasing storage time, the carbonyl content of myofibrillar proteins increased from 31.4 nmol/mg of protein (0 day) to 53.4, 46.3, and 39.7 nmol/mg protein at 35 days (P < 0.05) for −1 °C superchilled, −3 °C superchilled, and −3 °C superchilled with cryoprotectants samples, respectively. The incorporation of cryoprotectants into common carp surimi was found to significantly inhibit the formation of carbonyls (P < 0.05). The protein surface hydrophobicity increased in a similar direction, and the sulfhydryl content and Ca-ATPase stability decreased (P < 0.05). Emulsifying activities, gel textural hardness, springiness, and water binding capacity were also decreased, but they exhibited significant improvements at −3 °C when combined with cryoprotectants (P < 0.05). These results suggest that superchilling treatments at −3 °C combined with cryoprotectants offer an effective approach to reducing protein oxidation in carp surimi, thereby reducing protein structural changes known to impair the texture of surimi products.     

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.     

Pre-rigor temperature and the relationship between lamb tenderisation,free water production,bound water and dry matter

The M. longissimus from lambs electrically stimulated at 15 min post-mortem were removed after grading, wrapped in polythene film and held at 4 (n = 6), 7 (n = 6), 15 (n = 6, n = 8) and 35 °C (n = 6), until rigor mortis then aged at 15 °C for 0, 4, 24 and 72 h post-rigor. Centrifuged free water increased exponentially, and bound water, dry matter and shear force decreased exponentially over time. Decreases in shear force and increases in free water were closely related (r² = 0.52) and were unaffected by pre-rigor temperatures.     

Relevance of season and nucleotide catabolism on changes in fillet quality during chilled storage of raw Atlantic salmon (Salmo salar L.)

Farmed 5–6 kg Atlantic salmon were pre-rigor filleted, vacuum packed and subsequently analysed after 1, 9 and 13 days of storage at 4 °C. The study was repeated in February, April, August and October. The conversion of hypoxanthine (Hx) showed the highest seasonal variation among the nucleotide metabolites, with an inverse relationship with sea temperature at harvesting (R² = 0.95–0.96). The Hx content was inversely related to fresh odour and flavour (R² = 0.81–0.83), but positively to tenderness (R² = 0.87). Hence, these results suggest that salmon reared in seawater of 11–15 °C (August–October) maintain a superior sensory quality for a longer period post-mortem than salmon reared at 6–8 °C (February–April). The colour intensity increased from days 1 to 9 post-mortem, probably due to rigor contraction. The highest increase in drip loss was observed in October and the lowest in April. It is proposed that seawater temperature significantly influences the storage life of raw salmon, and that Hx is a valuable biomarker for sensory quality.     

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.     

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.