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.     

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.     

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.     

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.     

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     

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.     

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.     

VIS/NIR spectroscopy for non-destructive freshness assessment of Atlantic salmon (Salmo salar L.) fillets

Visible/near-infrared spectroscopy has been evaluated for use in freshness prediction and frozen-thawed classification of farmed Atlantic salmon fillets, where fresh samples were stored as whole fish in ice. A handheld interactance probe for performing rapid measurements of single fillets and an imaging spectrometer for online analysis at an industrial speed of one fillet per second, have been used. Freshness as storage days in ice is predicted with an accuracy of 2.4 days for individual fillets, whereas frozen-thawed salmon fillets are completely separated from fresh fillets. The prediction results are comparable to previous results using the Quality Index Method with trained panelists. The region between 605 and 735 nm, which excludes interference by carotenoids and water, is appropriate for both frozen-thawed classification and freshness prediction of salmon fillets. The results indicate that the spectral changes are explained mainly by oxidation of heme proteins during the freeze–thaw cycle and during chilled storage in ice.     

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.     

Lessening of high-pressure-induced changes in Atlantic salmon muscle by the combined use of a fish gelatin–lignin film

Salmon muscle is considerably affected by cooking with the resulting loss of its appealing red colour. The combined use of high pressure with fish gelatin–lignin film is proposed as an alternative to the more aggressive thermal processing procedures, with the aim of improving the appearance and overall quality of salmon fillets in ready-to-eat or semi-prepared dishes. The effects of high pressure processing (300 MPa, 10 min, 5 °C or 40 °C) and conventional heating (90 °C, 10 min) were evaluated in terms of colour changes, protein denaturation, as well as protein and lipid oxidation, by comparison with raw muscle. The stability of the processed products was assessed by monitoring changes in microbial growth and total volatile basic nitrogen and thiobarbituric acid reactive substances during 23 days of chilled storage. Fourier transform infrared spectroscopy (FTIR), apparent viscosity and dynamic oscillatory studies revealed notable differences in the overall degree and nature of protein aggregation between high pressure and heating treatments, especially when performed at 5 °C instead of 40 °C. SDS–PAGE of the protein fraction solubilised in 0.8 M NaCl showed MHC and α-actinin to be the main myofibrillar proteins denatured by high pressure processing at 40 °C, while actin was more denatured when pressurised at 5 °C. The film attenuated colour changes associated with high pressure treatment, especially at 5 °C, where redness was more preserved without jeopardising the appearance of a ready-to-eat product. High pressure processing at 5 °C in combination with gelatin–lignin film was found to improve protein quality of salmon fillets. The film reduced the levels of carbonyl groups formed immediately after processing, and prevented lipid oxidation from taking place at advanced stages of chilled storage. However, the effect on microbial growth was negligible, since total counts were similar for muscle with or without the film.     

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.     

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.     

Evaluation of Shelf Life of Superchilled Cod (Gadus morhua) Fillets and the Influence of Temperature Fluctuations During Storage on Microbial and Chemical Quality Indicators

Quality changes of aerobically packed cod fillets stored under superchilling and abusive temperature conditions were characterized by the growth of specific spoilage organisms (SSO) and the production of microbial metabolites measured by an electronic nose along with traditional sensory and chemical analysis (total volatile basic nitrogen [TVB‐N], pH). A new process based on quick contact freezing and cold air blasting was used to achieve superchilling of fillets before chilled (0.5 °C) or superchilled (‐1.5 °C) storage. Photobacterium phosphoreum dominated under temperature abusive conditions coinciding with high levels of TVB‐N and increased electronic nose responses indicating increased levels of alcohols and aldehydes at sensory rejection. Dominating growth of Pseudomonas spp. in 1 batch was associated with the origin, the catching method, and the cooling conditions during processing. The superchilling process followed by superchilled storage (‐1.5 °C) extended the sensory shelf life of the fillets for at least 3 d compared with traditional process, resulting in a total shelf life of 15 d. High content of TVB‐N was observed in superchilled fillets at sensory rejection. P. phosphoreum counts were lower under superchilling conditions (6.0 to 6.8 log colony‐forming units [CFU]/g), compared with the traditionally processed chilled fillets (7.2 log CFU/g). However, H₂S‐producing bacteria appeared to grow steadily under superchilling conditions reaching counts as high as 7.6 log CFU/g at sensory rejection.     

Superchilling of food: A review

Food preservation is very important for the safety and the reliability of the product. Superchilling as used for preserving foods, has been defined as a process by which the temperature of a food product is lowered to 1-2 °C below the initial freezing point. Fresh and high quality food products are in great demand worldwide. Temperature is a major factor determining the shelf life and quality of food products. Fish and meat are perishable food commodities, where better and more advanced preservation technology is needed. Deterioration of these foods mainly occurs as a result of chemical, enzymatic and bacteriological activities leading to loss of quality and subsequent spoilage. Storing food at superchilling temperature has three distinct advantages: maintaining food freshness, retaining high food quality and suppressing growth of harmful microbes. It can reduce the use of freezing/thawing for production and thereby increase yield, reduce energy, labour and transport costs. The study on the growth mechanism of ice crystals, modelling and computer simulation of foods during superchilling and superchilling storage is needed.     

The function of endogenous cathepsin in quality deterioration of grass carp (Ctenopharyngodon idella) fillets stored in chilling conditions

Changes of textural properties and cathepsin activity of grass carp fillets during storage in superchilling (?1.5 ± 0.2 °C) and ice (0.2 ± 0.1 °C) was investigated, and the function of cathepsin in quality deterioration of grass carp fillets was discussed. Results showed that shear force of superchilled and ice‐stored fillets decreased by 50% and 55% after 6 days of storage, respectively. The cathepsin activies in different fractions changed significantly during storage at both conditions. Cathepsin B, B+L activities in sarcoplasma, myofibrillar and heavy mitochondrial fraction significantly increased during the early 3 days postmortem, accompanied by remarkable reduction of corresponding activity in lysosome. Cathepsin D activity in sarcoplasma and myofibrillar significantly increased during 6 days of storage with corresponding decrease in lysosome and heavy mitochondria. Correlation analysis showed that changes of shear force of grass carp fillets were significantly correlated with integration of cathepsin B, D and B+L throughout storage time (r² > 0.94). As derived from the first‐order exponential decay model, the enzyme efficiencies (k_a) of cathepsin B and B+L were more than twofold higher than those of cathepsin D, suggesting the major role of cathepsin B and B+L in textural deterioration of grass carp fillets in chilling storage.     

Superchilled,chilled and frozen storage of Atlantic mackerel (Scomber scombrus) – effect on lipids and low molecular weight metabolites

Quality changes in Atlantic mackerel fillets during superchilled, chilled and frozen storage focusing on lipid quality, colour and changes in low molecular weight metabolites relevant for quality and safety were studied. Low formation of oxidation products was observed in chilled (up to 7 days), superchilled (up to 14 days) and frozen storage (up to one year). Only slight formation of TBARS was measured in the samples which correlated with the increasing fillet yellowness. The profile of low molecular weight compounds, analysed by high-resolution NMR, showed clear grouping of the different samples according to both treatment and storage time. The increase in K-value was highest in the chilled samples, reaching a K-value of 93 ± 3% on day 5, exceeding the proposed limit of human consumption (80%) while superchilling reached a K-value of ca 90 % after 14 days. Frozen samples showed acceptable quality in the whole storage period.     

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.     

Automatic freshness assessment of cod (Gadus morhua) fillets by Vis/Nir spectroscopy

VIS/NIR spectroscopy for differentiating between fresh and frozen-thawed cod fillets and for assessing the freshness as days on ice has been evaluated. Both a handheld interactance probe for doing quick measurements of single fillets and an imaging spectrometer for doing online analysis at industrial speed of one fillet per second, have been used. Results show that frozen-thawed cod fillets can be fully separated from fresh fillets using a small subset of wavelengths in the visible region. Freshness as days on ice can be determined with an accuracy of 1.6 days on individual fillets. The results indicate that oxidation of hemoglobin and myoglobin during freezing-thawing and cold storage on ice are explaining most of the variations seen in the visible region of the spectrum.     

Salt effect on heat-induced physical and chemical changes of salmon fillet (O. gorbuscha)

This research studied the effect of salt on kinetics for quality changes in pink salmon fillet, during commercial sterilisation. Sample cuts from salmon fillets were placed in sealed aluminum containers and heated at 121.1 °C for 10, 30 and 60 min. Samples with 1.5% (w/w) salt addition were compared with those without added salt. Salt addition reduced cook loss, area shrinkage and shear force of the heated fillet and resulted in a slightly darker colour. Effect of salt addition in thiamin loss, degree of lipid oxidation and fatty acid profile was not significant. Peroxide (PV) and thiobarbituric acid (TBA) values slightly increased within the first 10 min of heating, followed by a significant reduction as heating progressed. No measurable loss of polyunsaturated fatty acids (PUFA) was observed. Thermally processed shelf-stable salmon investigated should be a valuable source of ω-3 PUFA, with EPA values ranging from 52 to 71 mg/100 g product and DHA ranging from 258 to 340 mg/100 g of product.