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Low salt brining of pre-rigor filleted farmed cod (Gadus morhua L.) and the effects on different quality parameters
LWT 41 (2008) 1167–1172 www.elsevier.com/locate/lwt
Low salt brining of pre-rigor filleted farmed cod (Gadus morhua L.) and the effects on different quality parameters Rune Larsen, Stein H. Olsen, Silje Kristoffersen, Edel O. Elvevoll Department of Marine Biotechnology, Norwegian College of Fishery Science, University of Tromsø, 9037 Tromsø, Norway Received 14 May 2007; received in revised form 10 July 2007; accepted 18 July 2007
Abstract 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 1C, in immersion solutions 2 1C, 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. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Farmed cod; Brining; Pre-rigor processing; Gaping; Fillet contraction; Pin bone removal; Yield
1. Introduction Processing of frozen white fish has declined in Norway, mainly due to less expensive operations abroad. The demand for fresh fish is rising and early processing enables products with added value. Atlantic cod is regarded as a very promising species in cold water fish farming (Rosenlund & Skretting, 2006; Tilseth, 1990). Traditionally, fish has been processed post-rigor, but the growth in aquaculture and the possibility of live storage implies that increased quantities can be processed pre-rigor. Moreover, new slaughter procedures have postponed rigor onset, and thus enabled early processing (Erikson, Hultmann, & Steen, 2006; Skjervold, Fjæra, & Christoffersen, 1996). Processing fish before resolution of rigor has been associated with some disadvantages. In cod, pre-rigor processing increased drip losses (Kristoffersen et al., 2006), and removing pin bones without damaging the flesh have been proven a difficult task. In addition, filleting pre-rigor Corresponding author. Tel.: +47 776 46311; fax: +47 776 46020.
E-mail address: [email protected] (R. Larsen).
result in a substantial shrinkage of fillet length (Kristoffersen et al., 2006; Skjervold et al., 2001b; Sørensen, Brataas, Nyvold, & Lauritzen, 1997), which from a consumer point of view, may give the fillet an unusual shape. Although there are obstacles of pre-rigor processing, there are obvious advantages of processing directly after slaughter. The products can be shipped to markets 3–5 days earlier, and an increased shelf-life is a major economic benefit (Rosnes et al., 2003; Tobiassen et al., 2006). Reports also show that fillet gaping is significantly lower in prerigor filleted fish (Kristoffersen et al., 2006; Skjervold et al., 2001a; Tobiassen et al., 2006). Additionally, Skjervold et al. (2001b) reported better color and improved texture in pre-rigor filleted salmon fillets, and they did not find significant differences in drip losses compared to fillets processed post-rigor. Brining of fish fillets can improve fillets palatability and water holding capacity (WHC) (Esaiassen et al., 2004; Esaiassen et al., 2005). However, soluble components within the fillets, such as free amino acids (FAA), vitamins and proteins, leach out during brining (Larsen, Stormo,
0023-6438/$34.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.07.015
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Dragnes, & Elvevoll, 2007). Thus, brining has both nutritional and economical implications, because it may increase both yield and palatability, but nutritional components are lost during the process. Consumer attitudes towards ready meals are increasing, and the demand for lightly salted fish fillets are growing. It is thus important to investigate how low levels of salt influence fillet quality parameters. The objectives of this experiment were to study how fillet gaping, fillet shortening, the force required to pull pin bones and weight changes was influenced after immersing pre-rigor filleted cod fillets in different salt solutions. 2. Material and methods 2.1. Raw materials Maintenance fed (Marine Feed, Skretting, Norway) farmed cod (n ¼ 24, 32047707 g round weight, 6173 cm length) was acquired from Tromsø Aquaculture Research Station (Ka˚rvika, Tromsø) in February 2006. The fish, having a temperature of 4 1C, was killed with a blow to the head, exsanguinated for 20 min in seawater (4 1C), gutted and packed in ice for transportation to the laboratory. Within two and a half hours after slaughter, 21 fish were hand filleted, skinned and labeled. The remaining 3 individuals were stored in ice for 4 days until resolution of rigor, before they were filleted and skinned. 2.2. Experimental design Pre-rigor fillets (n ¼ 42, fillet weight 385798 g) were randomly distributed among seven treatment groups, where six groups were subjected to immersion in NaClsolutions directly after filleting. One group served as prerigor control and was not immersed. Additionally, one group served as post-rigor control, being filleted after resolution of rigor and not immersed. The fillets were immersed in 10 L pre-cooled NaCl solutions for 24 h at 2 1C, and fish:brine ratio was approximately 1:4. The concentrations of solutions were 15, 20, 25, 30, 40 and 60 g/L. After immersion, fillets were put in polyethylene (PE) zip-bags and stored on ice for 14 days. The prerigor control group was stored inside PE-bags at 2 1C the first 24 h, and then stored identically as immersed fillets. On day 4 the post rigor control group was filleted and stored in ice until day 14 under same conditions as the other groups. The fillets were subjected to evaluation of fillet gaping at day 7 and 14. Measurement of the force to pull pin bones was conducted at day 1 on fillets processed pre-rigor. The weight and length of fillets were measured before immersion, after immersion, after pulling of pin bones, on day 7 and on day 14. At the end of the storage period, a muscle sample was excised from each fillet. The samples were excised from the loin in the area below the dorsal fin. Samples were frozen at 30 1C until analysis of chloride content, pH and water content.
2.3. Pulling of pin bones The force required to pull pin bones was determined using a TA-HD texture analyzer (Stable Micro Systems Ltd., Cardiff, UK) calibrated with a 25 kg load cell. The measurement procedure was performed by attaching pin bones with a surgical pincer and pulling bones with a speed of 1 mm/s over a distance of 25 mm. To facilitate the release of bones, the fillet was manually restrained by exerting an opposing pressure with the palm. The force was continually monitored and was recorded with the accompanying software package, Texture Expert Exceed. The value required to pull pin bones was determined as peak height with correction for negative force. The mean force of the 6–8 foremost bones from each fillet was used in data analysis. 2.4. Evaluation of fillet gaping Fillet gaping was evaluated as described by Andersen, Strømsnes, Steinsholt, and Thomassen (1994) by giving fillets a score from 0 to 5, where 0 indicates no gaping and 5 is extreme gaping (the fillet falls apart). Evaluation was conducted by two referees, and the mean score of both evaluations are used in data analysis. 2.5. Shrinkage, water uptake and drip loss Shrinkage was measured as the decrease in fillet length from its initial length. Water uptake was measured as increased weight during 24 h of immersion, and drip loss was measured as weight reduction of fillets after 14 days of cold storage. 2.6. Chloride Muscle tissue was homogenised with twice its weight milliQ-water with an Ultra Turrax T25 basic (Ika Werke GmbH, Staufen, Germany) at 19000 rpm for 30 s. The suspension was heated at 100 1C for 5 min, and then centrifuged at 10,000g for 15 min. Chloride content in the supernatant was determined with a Corning 925 chloride analyser (Corning, Sheffiled, UK). Results are the mean of four repeated measurements. 2.7. Water content and pH Water content was determined by drying approximately 10 g minced muscle sample at 105 1C to constant weight by using a Termaks laboratory drying oven (Termaks, Bergen, Norway). Measurement of pH was carried out with a Metrohm 744 pH meter (Metrohm Ltd., Herisau, Switzerland) after blending minced muscle samples with equal weight 0.15 mol/L KCl.
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2.8. Statistical analysis Values are presented as mean7standard deviation (SD). Statistical analysis of the data was performed with SPSS 14.0 (SPSS inc., Chicago, IL, USA). Data was analyzed with one way analysis of variance (ANOVA) and post hoc comparison with Tukey’s test. Fillet gaping was analyzed with the non-parametric Kruskal–Wallis test, and pair wise comparison between groups was made with the Mann–Whitney U-test. The level of significance was set to Po0.05. 3. Results The water uptake during immersion, drip loss during storage and the total weight change of fillets are presented in Fig. 1. All immersed fillets gained weight during immersion. Immersion in NaCl of 60 g/L resulted in significantly lower increase of weight, and immersion in 15 and 40 g/L led to the highest water uptake. The postrigor control group gained 2% weight during ice storage before filleting. The amount of drip loss was influenced by the concentration of immersion solution, as higher concentration reduced the mean drip losses. The pre-rigor control group had the highest mean drip loss, but was not significantly different from post-rigor control group and fillets immersed in NaCl of 15 and 20 g/L. Only fillets immersed in NaCl of 40 g/L had a positive weight change after immersion and 14 days of storage. All immersed fillets had significantly lower total loss than both control groups. The chloride concentration within fillets, pH, dry matter, the shrinkage of fillets and the evaluation of fillet gaping are shown in Table 1. A general trend was that fillets immersed in more concentrated solutions resulted in a more pronounced contraction. During immersion, fillet shortening ranged from 17.3% to 33.8% for fillets
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immersed in NaCl concentrations of 15 and 60 g/L, respectively. On day 7, all but the fillets immersed in 60 g/L NaCl had further shortened. All pre-rigor processed fillets contracted significantly more than the post-rigor control group, which only shortened by 6.6%. No significant difference was found in dry matter between the groups after immersion and storage. In general, the pH of the fillets decreased with increasing concentration of immersion solutions. Both the pre-rigor and the post-rigor control group were evaluated as having a higher degree of gaping compared to fillets immersed in salt solutions. Particularly, fillets immersed in NaCl concentration 60 g/L displayed little gaping, and only a slight increase in score was given from day 7 to day 14. The other groups were evaluated as having substantially more gaping on day 14 compared to day 7. The required force to pull pin bones was influenced by salt content within fillets as shown in Fig. 2. Fillets immersed in concentrations 15 and 20 g/L required a similar force as pre-rigor control, whereas fillets immersed in 40 and 60 g/L approximately halved the required force compared to pre-rigor control. Hence, an inverse relationship between required force to pull pin bones and the salt content was found for in-rigor fillets.
4. Discussion Immersion of pre-rigor processed fillets in different salt solutions influenced many of the measured parameters. Salt is well known to affect the structural proteins of muscles, including the ability of muscles to take up water during immersion, and retain the water during storage (Offer & Knight, 1988a, b). Furthermore, pH and ionic strength are known to affect proteolysis of key cytoskeletal proteins in post mortem muscle (Huff-Lonergan & Lonergan, 2005).
Fig. 1. Water uptake during immersion (’), drip loss during storage (&) and total weight change of fillets ( ). Series which do not share a common letter (a, b, c, d) was significantly different (Po 0.05).
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Table 1 Gaping score at day 7 and 14, the shrinkage of fillets from initial length at 1 and 7 days after slaughter, and pH, dry matter and chloride contents of fillets after 14 days of ice storage Treatment
Gaping score day 7
Gaping score day 14
NaCl 15 g/L NaCl 20 g/L NaCl 25 g/L NaCl 30 g/L NaCl 40 g/L NaCl 60 g/L Pre-rigor control Post-rigor control
1.370.4a,b 1.070a 1.070.3a 1.170.2a 0.970.4a 0.670.6d 1.870.4b 2.970.7c
2.370.7a 2.170.5a 2.370.7a 2.470.4a 2.270.9a 0.870.4b 3.370.7c 3.870.5c
Shrinkage day 1 (%) 17.376.2a 19.774.2a 21.876.3a 23.977.5a,b 26.875.5a,b 33.874.6b 20.273.0a
Shrinkageday 7 (%)
Chloride (mg/100 g)
Drymatter (g/100 g)
pH
24.072.6a 25.172.3a 29.273.0a,b 32.172.6b 28.474.0a,b 31.573.9b 24.872.0a 6.6v 3.5c
223717 275725 383732 521762 818747 1160765 7377 7675
22.370.5 22.070.6 22.470.6 22.370.4 21.770.7 22.971.4 22.970.6 22.470.3
6.4470.10a,b 6.4870.04a 6.3970.10a,b,c 6.3170.11b,c,d 6.2670.08c,d 6.1970.03d 6.5170.05a 6.4070.11a,b,c
a, b, c, d. Values within a column which do not share a common letter was significantly different (Po0.05).
Fig. 2. The chloride concentration within fillets and the corresponding force required to pull pin bones in in-rigor cod fillets.
Salting of fish has long traditions and been extensively studied, but the effects of immersion of pre-rigor fillets in relatively weak brines is not well documented. Compared to post-rigor filleted cod immersed in similar solutions (Larsen and Elvevoll, in press), the water uptake in prerigor fillets was relatively low, as has also been reported for saturated brines (Sørensen et al., 1997; Wang, Tang, & Correia, 2000). The differences are probably caused by the ability of pre-rigor muscle cells to regulate its cell volume since live cells to some degree can withstand osmotic and concentration differences to surrounding media. As a reaction to a hyperosmotic environment, cells will reduce their water content by pumping water out of the cell. When ATP diminishes and rigor sets in, salt will more easily diffuse into muscle fibers. In the current experiment where fillets entered rigor during immersion, the transport of salt and water can be divided into two phases; a pre-rigor phase in which muscle cells actively regulate its content of ions and water, followed by a rigor phase where passive transport applies. It is the combination of these phases that gives a different pattern of water uptake compared to fillets immersed post-rigor. Thus, the differences in salt and water diffusion between pre- and post-rigor muscle must be taken into account when establishing practices of salting fillets pre-rigor (Røra˚, Furuhaug, Fjæra, & Skjervold,
2004). The lower water uptake has also been explained by active squeezing of water caused by the strong contraction at rigor onset (Lauritzsen et al., 2004; Sørensen et al., 1997). Drip losses decreased with increasing salt concentration. Salt increases the electrostatic repulsion of muscle filaments and leads to depolymerization of thick filaments (Offer & Trinick, 1983). The overall effect is an increased distance between myofilaments in which more water can be trapped, resulting in better WHC. The pre-rigor control group had higher mean drip losses than the post-rigor control, although the difference was not significant. However, the pre-rigor control group was stored as fillets for 14 days, whereas post rigor control group were stored as fillets for 10 days. Pre-rigor filleted farmed cod have been reported to lose more drip compared to fillets produced post-rigor (Kristoffersen et al., 2006), but other studies on wild cod (Tobiassen et al., 2006) and farmed salmon (Skjervold et al., 2001b) reported insignificant differences in drip losses. The yield, or total weight change, was negative for all groups, except fillets immersed in NaCl solution of 40 g/L which had a relatively high water uptake and a low drip loss. Nevertheless, all immersed fillets had significantly better yields than the control groups. The, calculated salt content in immersed fillets ranged from 0.3 to 1.8 g/100 g fillet, and a homogenous salt concentration of 2 g/100 g fillet is regarded as desirable in rehydrated salt-cured cod (Bjørkevoll, Olsen, & Olsen, 2004). The addition of salt is likely to void the fillets in being perceived as a fresh fillet, as sensory detection levels for salt are low. However, in products that generally are lightly salted such as convenience food and ready-meals, brining of fillets before rigor onset can be applied. Curing of pre-rigor fillets in more concentrated or saturated salt solutions and injection salting has generally resulted in lower yields compared to fish processed post-rigor (Birkeland, Akse, Joensen, Tobiassen, & Ska˚ra, 2007; Lauritzsen et al., 2004; Sørensen et al., 1997). All fillets produced pre-rigor contracted not surprisingly significantly more compared to the post-rigor control group, and there was also a trend of increased contraction
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with increasing salt concentration. The increased shortening of fillets processed pre-rigor is commonly explained by the lack of mechanical restraints during rigor development, because the fillet is not attached to the vertebral column. The effect of salt on rigor contraction has been reported by others (Birkeland et al., 2007; Lauritzsen et al., 2004). A rapid rigor development have generally led to a more pronounced contraction, as evidenced by increased fillet shortening with increasing storage temperatures (Kiessling, Stien, Torslett, Suontarna, & Slinde, 2006). Also, earlier rigor onset was followed by increased fillet contraction due to ante mortem stress (Erikson, Sigholt, Rustad, Einarsdottir, & Jørgensen, 1999; Roth, Slinde, & Arildsen, 2006; Stien et al., 2005). When pre-rigor muscles were subjected to higher concentrations of salt, the cells energy stores would be more rapidly depleted, because more ATP was needed to maintain homeostasis. Thus, fillets immersed in solutions with higher concentration probably entered rigor more rapidly, and this may explain why salt seems to affect the shortening of fillets. Additionally, fillets immersed in more concentrated solution had a lower pH. Gaping is observed as holes and slits on the fillet surface, and is caused by detachment of myofiber–myocommata linkages and between myofibres. (Lavety, Afolabi, & Love, 1988; Ofstad, Olsen, Taylor, & Hannesson, 2006). A number of biochemical deteriorative processes causes the ruptures, but also physical strain and handling may lead to gaping (Rosnes et al., 2003). The pre-rigor control group had lower gaping both at day 7 and 14 compared to the post-rigor control. The developed tension of muscles entering rigor, whilst attached to the vertebral column, has been suggested as a partial explanation why fillets processed post-rigor have a higher degree of gaping (Rosnes et al., 2003). Immersion of fillets in salt solutions resulted in significantly less gaping compared to both control groups. Especially fillets immersed in NaCl solutions of 60 g/L had a low gaping score. Reasons why immersed fillets had less gaping may be plural. Salt may have an inhibiting effect of protease activity, and calpain activity have been shown to decrease with increasing ionic strength (Geesink & Koohmaraie, 2000; Kendall, Koohmaraie, Arbona, Williams, & Young, 1993; Maddock, Huff-Lonergan, Rowe, & Lonergan, 2005). Treating meat with salts generally causes ‘‘sticky exudates’’ to appear on surfaces, which is a concentrated solution of sarcoplasmic and myofibrillar proteins (Jolley & Purslow, 1988). This property is utilized industrially to create restructured meats. The extracted proteins may thus have acted as a binding agent, and reduced the amount of gaping. Removal of pin bones in pre-rigor cod has been problematic due to their rigid attachment in the fillet. Few publications are available concerning pin bone removal. However, some reports in Norwegian are available, where the numbers and sizes of pin bones have been surveyed, and differences in pulling force between pre-, inand post-rigor fillets have been measured (Akse, Tobiassen,
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& Halsebakke, 2002; Esaiassen & Sørensen, 1996; Kristiansen, 2006). To summarize their major findings, the required force was above twice as high for pre-rigor fillets compared to in-rigor and post-rigor fillets. Skinned fillets required a lower force than non-skinned fillets, and larger fillets required more force compared to smaller fillets. A higher salt concentration in fillets decreased the required force to pull pin bones. The pin bones of cod are located between the upper and lower muscle bundles, and are solidly embedded in connective tissues and the skin (Akse et al., 2002). Thus, salt seems to loosen the pin bones attachment to the fillet by disrupting linkages between the bone and connective tissue. The measurements were performed after immersion, and the fillets had entered rigor. An interesting continuance of these results would be measurements of fillets pre-rigor, salted by injection or vacuum tumbling. This would provide answers whether the effect of salt is also present in fillets pre-rigor. 5. Conclusion The current study show that relatively low levels of salt had a positive impact on many of the obstacles that is associated with pre-rigor processing of cod. At salt levels that are sensorial desired, fillet gaping and drip losses was significantly reduced and removal of pin bones required less force. Further experiments with vacuum tumbling or injection salting, which are faster and more viable techniques industrially, should be tested. Acknowledgments Funding of this research was provided by the Norwegian Research Council and The Fishery and Aquaculture Industry Research Fund. References Akse, L., Tobiassen, T., Halsebakke, H. T. (2002). Tykkfiskbein i torskefilet: Plassering (antall), trekkraft og bruddstyrke. Fiskeriforskning, report no. 15/2002. Tromsø, Norway, P. 29 (in Norwegian). Andersen, U. B., Strømsnes, A. N., Steinsholt, K., & Thomassen, M. S. (1994). Fillet gaping in farmed Atlantic salmon (Salmo salar). Norwegian Journal of Agricultural Sciences, 8, 165–179. Birkeland, S., Akse, L., Joensen, S., Tobiassen, T., & Ska˚ra, T. (2007). Injection-salting of pre rigor fillets of Atlantic Salmon (Salmo salar). Journal of Food Science, 72, E29–E35. Bjørkevoll, I., Olsen, J. V., & Olsen, R. L. (2004). Rehydration of saltcured cod using injection and tumbling technologies. Food Research International, 37, 925–931. Erikson, U., Hultmann, L., & Steen, J. E. (2006). Live chilling of Atlantic salmon (Salmo salar) combined with mild carbon dioxide anaesthesia I. Establishing a method for large-scale processing of farmed fish. Aquaculture, 252, 183–198. Erikson, U., Sigholt, T., Rustad, T., Einarsdottir, I. E., & Jørgensen, L. (1999). Contribution of bleeding to total handling stress during slaughter of Atlantic salmon. Aquaculture International, 7, 101–115. Esaiassen, M., Østli, J., Elvevoll, E. O., Joensen, S., Prytz, K., & Richardsen, R. (2004). Brining of cod fillets: influence on sensory
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Low salt brining of pre-rigor filleted farmed cod (Gadus morhua L.) and the effects on different quality parameters Rune Larsen, Stein H. Olsen, Silje Kristoffersen, Edel O. Elvevoll Department of Marine Biotechnology, Norwegian College of Fishery Science, University of Tromsø, 9037 Tromsø, Norway Received 14 May 2007; received in revised form 10 July 2007; accepted 18 July 2007
Abstract 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 1C, in immersion solutions 2 1C, 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. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Farmed cod; Brining; Pre-rigor processing; Gaping; Fillet contraction; Pin bone removal; Yield
1. Introduction Processing of frozen white fish has declined in Norway, mainly due to less expensive operations abroad. The demand for fresh fish is rising and early processing enables products with added value. Atlantic cod is regarded as a very promising species in cold water fish farming (Rosenlund & Skretting, 2006; Tilseth, 1990). Traditionally, fish has been processed post-rigor, but the growth in aquaculture and the possibility of live storage implies that increased quantities can be processed pre-rigor. Moreover, new slaughter procedures have postponed rigor onset, and thus enabled early processing (Erikson, Hultmann, & Steen, 2006; Skjervold, Fjæra, & Christoffersen, 1996). Processing fish before resolution of rigor has been associated with some disadvantages. In cod, pre-rigor processing increased drip losses (Kristoffersen et al., 2006), and removing pin bones without damaging the flesh have been proven a difficult task. In addition, filleting pre-rigor Corresponding author. Tel.: +47 776 46311; fax: +47 776 46020.
E-mail address: [email protected] (R. Larsen).
result in a substantial shrinkage of fillet length (Kristoffersen et al., 2006; Skjervold et al., 2001b; Sørensen, Brataas, Nyvold, & Lauritzen, 1997), which from a consumer point of view, may give the fillet an unusual shape. Although there are obstacles of pre-rigor processing, there are obvious advantages of processing directly after slaughter. The products can be shipped to markets 3–5 days earlier, and an increased shelf-life is a major economic benefit (Rosnes et al., 2003; Tobiassen et al., 2006). Reports also show that fillet gaping is significantly lower in prerigor filleted fish (Kristoffersen et al., 2006; Skjervold et al., 2001a; Tobiassen et al., 2006). Additionally, Skjervold et al. (2001b) reported better color and improved texture in pre-rigor filleted salmon fillets, and they did not find significant differences in drip losses compared to fillets processed post-rigor. Brining of fish fillets can improve fillets palatability and water holding capacity (WHC) (Esaiassen et al., 2004; Esaiassen et al., 2005). However, soluble components within the fillets, such as free amino acids (FAA), vitamins and proteins, leach out during brining (Larsen, Stormo,
0023-6438/$34.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.07.015
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Dragnes, & Elvevoll, 2007). Thus, brining has both nutritional and economical implications, because it may increase both yield and palatability, but nutritional components are lost during the process. Consumer attitudes towards ready meals are increasing, and the demand for lightly salted fish fillets are growing. It is thus important to investigate how low levels of salt influence fillet quality parameters. The objectives of this experiment were to study how fillet gaping, fillet shortening, the force required to pull pin bones and weight changes was influenced after immersing pre-rigor filleted cod fillets in different salt solutions. 2. Material and methods 2.1. Raw materials Maintenance fed (Marine Feed, Skretting, Norway) farmed cod (n ¼ 24, 32047707 g round weight, 6173 cm length) was acquired from Tromsø Aquaculture Research Station (Ka˚rvika, Tromsø) in February 2006. The fish, having a temperature of 4 1C, was killed with a blow to the head, exsanguinated for 20 min in seawater (4 1C), gutted and packed in ice for transportation to the laboratory. Within two and a half hours after slaughter, 21 fish were hand filleted, skinned and labeled. The remaining 3 individuals were stored in ice for 4 days until resolution of rigor, before they were filleted and skinned. 2.2. Experimental design Pre-rigor fillets (n ¼ 42, fillet weight 385798 g) were randomly distributed among seven treatment groups, where six groups were subjected to immersion in NaClsolutions directly after filleting. One group served as prerigor control and was not immersed. Additionally, one group served as post-rigor control, being filleted after resolution of rigor and not immersed. The fillets were immersed in 10 L pre-cooled NaCl solutions for 24 h at 2 1C, and fish:brine ratio was approximately 1:4. The concentrations of solutions were 15, 20, 25, 30, 40 and 60 g/L. After immersion, fillets were put in polyethylene (PE) zip-bags and stored on ice for 14 days. The prerigor control group was stored inside PE-bags at 2 1C the first 24 h, and then stored identically as immersed fillets. On day 4 the post rigor control group was filleted and stored in ice until day 14 under same conditions as the other groups. The fillets were subjected to evaluation of fillet gaping at day 7 and 14. Measurement of the force to pull pin bones was conducted at day 1 on fillets processed pre-rigor. The weight and length of fillets were measured before immersion, after immersion, after pulling of pin bones, on day 7 and on day 14. At the end of the storage period, a muscle sample was excised from each fillet. The samples were excised from the loin in the area below the dorsal fin. Samples were frozen at 30 1C until analysis of chloride content, pH and water content.
2.3. Pulling of pin bones The force required to pull pin bones was determined using a TA-HD texture analyzer (Stable Micro Systems Ltd., Cardiff, UK) calibrated with a 25 kg load cell. The measurement procedure was performed by attaching pin bones with a surgical pincer and pulling bones with a speed of 1 mm/s over a distance of 25 mm. To facilitate the release of bones, the fillet was manually restrained by exerting an opposing pressure with the palm. The force was continually monitored and was recorded with the accompanying software package, Texture Expert Exceed. The value required to pull pin bones was determined as peak height with correction for negative force. The mean force of the 6–8 foremost bones from each fillet was used in data analysis. 2.4. Evaluation of fillet gaping Fillet gaping was evaluated as described by Andersen, Strømsnes, Steinsholt, and Thomassen (1994) by giving fillets a score from 0 to 5, where 0 indicates no gaping and 5 is extreme gaping (the fillet falls apart). Evaluation was conducted by two referees, and the mean score of both evaluations are used in data analysis. 2.5. Shrinkage, water uptake and drip loss Shrinkage was measured as the decrease in fillet length from its initial length. Water uptake was measured as increased weight during 24 h of immersion, and drip loss was measured as weight reduction of fillets after 14 days of cold storage. 2.6. Chloride Muscle tissue was homogenised with twice its weight milliQ-water with an Ultra Turrax T25 basic (Ika Werke GmbH, Staufen, Germany) at 19000 rpm for 30 s. The suspension was heated at 100 1C for 5 min, and then centrifuged at 10,000g for 15 min. Chloride content in the supernatant was determined with a Corning 925 chloride analyser (Corning, Sheffiled, UK). Results are the mean of four repeated measurements. 2.7. Water content and pH Water content was determined by drying approximately 10 g minced muscle sample at 105 1C to constant weight by using a Termaks laboratory drying oven (Termaks, Bergen, Norway). Measurement of pH was carried out with a Metrohm 744 pH meter (Metrohm Ltd., Herisau, Switzerland) after blending minced muscle samples with equal weight 0.15 mol/L KCl.
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2.8. Statistical analysis Values are presented as mean7standard deviation (SD). Statistical analysis of the data was performed with SPSS 14.0 (SPSS inc., Chicago, IL, USA). Data was analyzed with one way analysis of variance (ANOVA) and post hoc comparison with Tukey’s test. Fillet gaping was analyzed with the non-parametric Kruskal–Wallis test, and pair wise comparison between groups was made with the Mann–Whitney U-test. The level of significance was set to Po0.05. 3. Results The water uptake during immersion, drip loss during storage and the total weight change of fillets are presented in Fig. 1. All immersed fillets gained weight during immersion. Immersion in NaCl of 60 g/L resulted in significantly lower increase of weight, and immersion in 15 and 40 g/L led to the highest water uptake. The postrigor control group gained 2% weight during ice storage before filleting. The amount of drip loss was influenced by the concentration of immersion solution, as higher concentration reduced the mean drip losses. The pre-rigor control group had the highest mean drip loss, but was not significantly different from post-rigor control group and fillets immersed in NaCl of 15 and 20 g/L. Only fillets immersed in NaCl of 40 g/L had a positive weight change after immersion and 14 days of storage. All immersed fillets had significantly lower total loss than both control groups. The chloride concentration within fillets, pH, dry matter, the shrinkage of fillets and the evaluation of fillet gaping are shown in Table 1. A general trend was that fillets immersed in more concentrated solutions resulted in a more pronounced contraction. During immersion, fillet shortening ranged from 17.3% to 33.8% for fillets
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immersed in NaCl concentrations of 15 and 60 g/L, respectively. On day 7, all but the fillets immersed in 60 g/L NaCl had further shortened. All pre-rigor processed fillets contracted significantly more than the post-rigor control group, which only shortened by 6.6%. No significant difference was found in dry matter between the groups after immersion and storage. In general, the pH of the fillets decreased with increasing concentration of immersion solutions. Both the pre-rigor and the post-rigor control group were evaluated as having a higher degree of gaping compared to fillets immersed in salt solutions. Particularly, fillets immersed in NaCl concentration 60 g/L displayed little gaping, and only a slight increase in score was given from day 7 to day 14. The other groups were evaluated as having substantially more gaping on day 14 compared to day 7. The required force to pull pin bones was influenced by salt content within fillets as shown in Fig. 2. Fillets immersed in concentrations 15 and 20 g/L required a similar force as pre-rigor control, whereas fillets immersed in 40 and 60 g/L approximately halved the required force compared to pre-rigor control. Hence, an inverse relationship between required force to pull pin bones and the salt content was found for in-rigor fillets.
4. Discussion Immersion of pre-rigor processed fillets in different salt solutions influenced many of the measured parameters. Salt is well known to affect the structural proteins of muscles, including the ability of muscles to take up water during immersion, and retain the water during storage (Offer & Knight, 1988a, b). Furthermore, pH and ionic strength are known to affect proteolysis of key cytoskeletal proteins in post mortem muscle (Huff-Lonergan & Lonergan, 2005).
Fig. 1. Water uptake during immersion (’), drip loss during storage (&) and total weight change of fillets ( ). Series which do not share a common letter (a, b, c, d) was significantly different (Po 0.05).
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Table 1 Gaping score at day 7 and 14, the shrinkage of fillets from initial length at 1 and 7 days after slaughter, and pH, dry matter and chloride contents of fillets after 14 days of ice storage Treatment
Gaping score day 7
Gaping score day 14
NaCl 15 g/L NaCl 20 g/L NaCl 25 g/L NaCl 30 g/L NaCl 40 g/L NaCl 60 g/L Pre-rigor control Post-rigor control
1.370.4a,b 1.070a 1.070.3a 1.170.2a 0.970.4a 0.670.6d 1.870.4b 2.970.7c
2.370.7a 2.170.5a 2.370.7a 2.470.4a 2.270.9a 0.870.4b 3.370.7c 3.870.5c
Shrinkage day 1 (%) 17.376.2a 19.774.2a 21.876.3a 23.977.5a,b 26.875.5a,b 33.874.6b 20.273.0a
Shrinkageday 7 (%)
Chloride (mg/100 g)
Drymatter (g/100 g)
pH
24.072.6a 25.172.3a 29.273.0a,b 32.172.6b 28.474.0a,b 31.573.9b 24.872.0a 6.6v 3.5c
223717 275725 383732 521762 818747 1160765 7377 7675
22.370.5 22.070.6 22.470.6 22.370.4 21.770.7 22.971.4 22.970.6 22.470.3
6.4470.10a,b 6.4870.04a 6.3970.10a,b,c 6.3170.11b,c,d 6.2670.08c,d 6.1970.03d 6.5170.05a 6.4070.11a,b,c
a, b, c, d. Values within a column which do not share a common letter was significantly different (Po0.05).
Fig. 2. The chloride concentration within fillets and the corresponding force required to pull pin bones in in-rigor cod fillets.
Salting of fish has long traditions and been extensively studied, but the effects of immersion of pre-rigor fillets in relatively weak brines is not well documented. Compared to post-rigor filleted cod immersed in similar solutions (Larsen and Elvevoll, in press), the water uptake in prerigor fillets was relatively low, as has also been reported for saturated brines (Sørensen et al., 1997; Wang, Tang, & Correia, 2000). The differences are probably caused by the ability of pre-rigor muscle cells to regulate its cell volume since live cells to some degree can withstand osmotic and concentration differences to surrounding media. As a reaction to a hyperosmotic environment, cells will reduce their water content by pumping water out of the cell. When ATP diminishes and rigor sets in, salt will more easily diffuse into muscle fibers. In the current experiment where fillets entered rigor during immersion, the transport of salt and water can be divided into two phases; a pre-rigor phase in which muscle cells actively regulate its content of ions and water, followed by a rigor phase where passive transport applies. It is the combination of these phases that gives a different pattern of water uptake compared to fillets immersed post-rigor. Thus, the differences in salt and water diffusion between pre- and post-rigor muscle must be taken into account when establishing practices of salting fillets pre-rigor (Røra˚, Furuhaug, Fjæra, & Skjervold,
2004). The lower water uptake has also been explained by active squeezing of water caused by the strong contraction at rigor onset (Lauritzsen et al., 2004; Sørensen et al., 1997). Drip losses decreased with increasing salt concentration. Salt increases the electrostatic repulsion of muscle filaments and leads to depolymerization of thick filaments (Offer & Trinick, 1983). The overall effect is an increased distance between myofilaments in which more water can be trapped, resulting in better WHC. The pre-rigor control group had higher mean drip losses than the post-rigor control, although the difference was not significant. However, the pre-rigor control group was stored as fillets for 14 days, whereas post rigor control group were stored as fillets for 10 days. Pre-rigor filleted farmed cod have been reported to lose more drip compared to fillets produced post-rigor (Kristoffersen et al., 2006), but other studies on wild cod (Tobiassen et al., 2006) and farmed salmon (Skjervold et al., 2001b) reported insignificant differences in drip losses. The yield, or total weight change, was negative for all groups, except fillets immersed in NaCl solution of 40 g/L which had a relatively high water uptake and a low drip loss. Nevertheless, all immersed fillets had significantly better yields than the control groups. The, calculated salt content in immersed fillets ranged from 0.3 to 1.8 g/100 g fillet, and a homogenous salt concentration of 2 g/100 g fillet is regarded as desirable in rehydrated salt-cured cod (Bjørkevoll, Olsen, & Olsen, 2004). The addition of salt is likely to void the fillets in being perceived as a fresh fillet, as sensory detection levels for salt are low. However, in products that generally are lightly salted such as convenience food and ready-meals, brining of fillets before rigor onset can be applied. Curing of pre-rigor fillets in more concentrated or saturated salt solutions and injection salting has generally resulted in lower yields compared to fish processed post-rigor (Birkeland, Akse, Joensen, Tobiassen, & Ska˚ra, 2007; Lauritzsen et al., 2004; Sørensen et al., 1997). All fillets produced pre-rigor contracted not surprisingly significantly more compared to the post-rigor control group, and there was also a trend of increased contraction
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with increasing salt concentration. The increased shortening of fillets processed pre-rigor is commonly explained by the lack of mechanical restraints during rigor development, because the fillet is not attached to the vertebral column. The effect of salt on rigor contraction has been reported by others (Birkeland et al., 2007; Lauritzsen et al., 2004). A rapid rigor development have generally led to a more pronounced contraction, as evidenced by increased fillet shortening with increasing storage temperatures (Kiessling, Stien, Torslett, Suontarna, & Slinde, 2006). Also, earlier rigor onset was followed by increased fillet contraction due to ante mortem stress (Erikson, Sigholt, Rustad, Einarsdottir, & Jørgensen, 1999; Roth, Slinde, & Arildsen, 2006; Stien et al., 2005). When pre-rigor muscles were subjected to higher concentrations of salt, the cells energy stores would be more rapidly depleted, because more ATP was needed to maintain homeostasis. Thus, fillets immersed in solutions with higher concentration probably entered rigor more rapidly, and this may explain why salt seems to affect the shortening of fillets. Additionally, fillets immersed in more concentrated solution had a lower pH. Gaping is observed as holes and slits on the fillet surface, and is caused by detachment of myofiber–myocommata linkages and between myofibres. (Lavety, Afolabi, & Love, 1988; Ofstad, Olsen, Taylor, & Hannesson, 2006). A number of biochemical deteriorative processes causes the ruptures, but also physical strain and handling may lead to gaping (Rosnes et al., 2003). The pre-rigor control group had lower gaping both at day 7 and 14 compared to the post-rigor control. The developed tension of muscles entering rigor, whilst attached to the vertebral column, has been suggested as a partial explanation why fillets processed post-rigor have a higher degree of gaping (Rosnes et al., 2003). Immersion of fillets in salt solutions resulted in significantly less gaping compared to both control groups. Especially fillets immersed in NaCl solutions of 60 g/L had a low gaping score. Reasons why immersed fillets had less gaping may be plural. Salt may have an inhibiting effect of protease activity, and calpain activity have been shown to decrease with increasing ionic strength (Geesink & Koohmaraie, 2000; Kendall, Koohmaraie, Arbona, Williams, & Young, 1993; Maddock, Huff-Lonergan, Rowe, & Lonergan, 2005). Treating meat with salts generally causes ‘‘sticky exudates’’ to appear on surfaces, which is a concentrated solution of sarcoplasmic and myofibrillar proteins (Jolley & Purslow, 1988). This property is utilized industrially to create restructured meats. The extracted proteins may thus have acted as a binding agent, and reduced the amount of gaping. Removal of pin bones in pre-rigor cod has been problematic due to their rigid attachment in the fillet. Few publications are available concerning pin bone removal. However, some reports in Norwegian are available, where the numbers and sizes of pin bones have been surveyed, and differences in pulling force between pre-, inand post-rigor fillets have been measured (Akse, Tobiassen,
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& Halsebakke, 2002; Esaiassen & Sørensen, 1996; Kristiansen, 2006). To summarize their major findings, the required force was above twice as high for pre-rigor fillets compared to in-rigor and post-rigor fillets. Skinned fillets required a lower force than non-skinned fillets, and larger fillets required more force compared to smaller fillets. A higher salt concentration in fillets decreased the required force to pull pin bones. The pin bones of cod are located between the upper and lower muscle bundles, and are solidly embedded in connective tissues and the skin (Akse et al., 2002). Thus, salt seems to loosen the pin bones attachment to the fillet by disrupting linkages between the bone and connective tissue. The measurements were performed after immersion, and the fillets had entered rigor. An interesting continuance of these results would be measurements of fillets pre-rigor, salted by injection or vacuum tumbling. This would provide answers whether the effect of salt is also present in fillets pre-rigor. 5. Conclusion The current study show that relatively low levels of salt had a positive impact on many of the obstacles that is associated with pre-rigor processing of cod. At salt levels that are sensorial desired, fillet gaping and drip losses was significantly reduced and removal of pin bones required less force. Further experiments with vacuum tumbling or injection salting, which are faster and more viable techniques industrially, should be tested. Acknowledgments Funding of this research was provided by the Norwegian Research Council and The Fishery and Aquaculture Industry Research Fund. References Akse, L., Tobiassen, T., Halsebakke, H. T. (2002). Tykkfiskbein i torskefilet: Plassering (antall), trekkraft og bruddstyrke. Fiskeriforskning, report no. 15/2002. Tromsø, Norway, P. 29 (in Norwegian). Andersen, U. B., Strømsnes, A. N., Steinsholt, K., & Thomassen, M. S. (1994). Fillet gaping in farmed Atlantic salmon (Salmo salar). Norwegian Journal of Agricultural Sciences, 8, 165–179. Birkeland, S., Akse, L., Joensen, S., Tobiassen, T., & Ska˚ra, T. (2007). Injection-salting of pre rigor fillets of Atlantic Salmon (Salmo salar). Journal of Food Science, 72, E29–E35. Bjørkevoll, I., Olsen, J. V., & Olsen, R. L. (2004). Rehydration of saltcured cod using injection and tumbling technologies. Food Research International, 37, 925–931. Erikson, U., Hultmann, L., & Steen, J. E. (2006). Live chilling of Atlantic salmon (Salmo salar) combined with mild carbon dioxide anaesthesia I. Establishing a method for large-scale processing of farmed fish. Aquaculture, 252, 183–198. Erikson, U., Sigholt, T., Rustad, T., Einarsdottir, I. E., & Jørgensen, L. (1999). Contribution of bleeding to total handling stress during slaughter of Atlantic salmon. Aquaculture International, 7, 101–115. Esaiassen, M., Østli, J., Elvevoll, E. O., Joensen, S., Prytz, K., & Richardsen, R. (2004). Brining of cod fillets: influence on sensory
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