Processing of Salted Cod (Gadus spp.): A Review

Authors

  • Helena Oliveira,

    1. Author Oliveira is with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal; ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal; ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Authors Pedro and Nunes are with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal. Author Costa is with ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal. Author Vaz-Pires is with ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Direct inquiries to author Oliveira (E-mail: helaoliveira@gmail.com).
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  • Sónia Pedro,

    1. Author Oliveira is with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal; ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal; ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Authors Pedro and Nunes are with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal. Author Costa is with ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal. Author Vaz-Pires is with ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Direct inquiries to author Oliveira (E-mail: helaoliveira@gmail.com).
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  • Maria Leonor Nunes,

    1. Author Oliveira is with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal; ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal; ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Authors Pedro and Nunes are with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal. Author Costa is with ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal. Author Vaz-Pires is with ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Direct inquiries to author Oliveira (E-mail: helaoliveira@gmail.com).
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  • Rui Costa,

    1. Author Oliveira is with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal; ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal; ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Authors Pedro and Nunes are with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal. Author Costa is with ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal. Author Vaz-Pires is with ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Direct inquiries to author Oliveira (E-mail: helaoliveira@gmail.com).
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  • Paulo Vaz-Pires

    1. Author Oliveira is with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal; ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal; ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Authors Pedro and Nunes are with U-VPPA, Research Unit of Upgrading of Fishery and Aquaculture Products, Natl. Inst. of Biological Resources (INRB I.P./L-IPIMAR), Avenida de Brasília, 1449-006 Lisboa, Portugal. Author Costa is with ESAC, College of Agriculture of the Polytechnic Inst. of Coimbra, Food Science and Technology Dept., Bencanta, 3040-316 Coimbra, Portugal. Author Vaz-Pires is with ICBAS-UP, Abel Salazar Inst. for the Biomedical Sciences, Univ. of Porto, Rua Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal; and CIIMAR-UP, Interdisciplinary Center of Marine and Environmental Research, Univ. of Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal. Direct inquiries to author Oliveira (E-mail: helaoliveira@gmail.com).
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Abstract

Abstract:  The main objective of this review is to summarize the present state of knowledge of different ways of processing cod, giving emphasis to salting, drying, and desalting steps. As an introduction, a description of the main characteristics of the Gadidae family and cod species, the general chemical composition of fresh cod, and a reference to farmed cod is included. Statistics on world fishery and aquaculture cod production are also reviewed. It is expected that this review will contribute to build a current picture of the technical and scientific knowledge on the cod processing situation, helping to move forward and to support future developments in this important seafood industry.

Introduction

Drying and salt-curing of fish have been used as preservation methods since ancient times for the purpose of product storage stability (months or even years under the right conditions). However, these methods have lost importance for preservation purposes due to the widespread use of cold-based technologies in developed countries. Nevertheless, cod has long been highly appreciated as dried and/or salted product due to its high nutritional value (high protein and low fat content) and specific sensory properties (color, texture, aroma, and a characteristic taste) imparted by these preservation methods (Borgström 1968; Bjørkevoll and others 2003; Lauritzsen and others 2004b; Martínez-Alvarez and others 2005b; Heredia and others 2007). So, the aim of salting and drying fresh cod is not only to get a shelf stable product (low moisture and high salt content) able to be stored for several months, but also to promote important sensory changes that remain during desalting and cooking and make this product very valued by consumers (Andrés and others 2005b). The different salting and drying processes, used for a long time now as a method of fish preservation, have been reviewed by Ismail and Wooton (1992).

Before consumption, the products are subject to rehydration, which involves soaking the fish in water, resulting in water uptake and salt leaching out of the muscle tissue. After rehydration, the fish may be either consumed immediately, stored chilled for a number of days, or stored frozen and then used for the preparation of various cod dishes (Lauritzsen and others 2004a). The particular texture and flavor make it extremely appreciated and, certainly, if a food could represent a country, Portugal would be culturally and historically typified by its hundreds of salted cod recipes from all over the country (Rodrigues and others 2003).

Nowadays, salted cod is still considered a highly popular product due to high demand and simplicity of processing (Gallart-Jornet and others 2003). This important and traditional product is highly appreciated in many countries, is mainly produced in Norway and Iceland, and is primarily consumed in Mediterranean countries such as Spain and Portugal, as well as in Latin America (Thorarinsdóttir and others 2002; Lauritzsen and others 2004a; Andrés and others 2005a; Martínez-Alvarez and others 2005a; Muňoz-Guerrero and others 2010) under the name bacalhau or bacalao (Bjørkevoll and others 2003; Fernández-Segovia and others 2007), with the United States rapidly becoming an increasingly growing market (Vilhelmsson and others 1996; Thorarinsdóttir and others 2001; Rodrigues and others 2003). Already in 1999, Spain was the second country, behind Portugal, with the highest consumption of salted cod (around 15% of the market). The salted cod consumed in Spain comes mainly from Spanish factory ships and from imports (mostly from Norway and Iceland). The derived products of salted cod, wet or dried, are of great economic and cultural interest in Spain (Barat and others 2004b,c, 2006) and in Portugal.

The annual consumption of salt-cured products based on cod from the North-Atlantic fisheries can be estimated to be more than 150000 metric tons (Norwegian Export Council 2009).

Salted cod has attracted increasing attention in recent years because several companies are interested in producing ready to-cook cod products, desalted directly from salted cod instead of from dried salted cod since it is more economic and convenient (Urch 1998). In fact, dried salted cod requires an additional processing stage: the drying stage (Rodrigues and others 2005). Thus, despite traditionally drying the fish after curing (Figure 1), most of it is now stored refrigerated and consumed “wet.” Already in 1996, the annual world production of cod was estimated at around 250000 tons (Vilhelmsson and others 1996) and only 10% of the world production of cod was dried (Vilhelmsson and others 1997).

Figure 1–.

Stages of traditional cod processing with the main intermediate products (adapted from Ismail and Wootton 1992; Di Luccia and others 2005; and Rodrigues 2006).

In the Mediterranean countries, cod is commercialized mainly as salted-dried cod with different moisture contents depending on the extension of the drying step (Andrés and others 2005b).

A decrease in the salting and drying periods, the extension of the ‘‘water-horsing’’ stage in order to save drying energy, and use of unsuitable conditions for preservation (Freixo 1947; Botelho 1952; van Klaveren and Legendre 1965) are some processing changes that have happened. Moreover, cod with low levels of salt can sometimes reach 40% of a production batch. This cod, classified as being of inferior quality, can also be dried and sold in the market (Botelho 1952) (Figure 1).

Many factors, including quality and condition of the raw material, the type, concentration and quality of salt, as well as the method used for salting the cod, are believed to influence the quality and characteristics of the final product (Thorarinsdóttir and others 2001, 2004; Andrés and others 2002).

Fresh raw materials and good manufacturing practices during salting, drying, and desalting are essential to improve cod quality and shelf life (Pedro and others 2002a).

Raw Materials

Fish

Species.  In the production of dried salted cod, different species are used: Gadus morhua, Gadus macrocephalus, and Gadus ogac, being the first the most important in commercial terms and therefore the most studied. These species belong to the Gadidae family, to the order of Gadiformes and to the class of Actinopterygii (ray-finned fishes). Gadids are characterized by having 3 dorsal fins and 2 anal fins, with the first dorsal behind the head, no spines, pelvic fins before pectorals, teeth present on vomer, usually with barbell, no otophysic connection between swim bladder and auditory capsules, eggs without oil globules, and up to about 2 m maximum length in Gadus morhua. Members of the family are found in circumpolar and temperate waters, mainly of the northern hemisphere. So, it is possible to find members of this family in the Arctic, Atlantic, and Pacific oceans. Gadids are typically marine fish, but a number of species (like Gadus morhua, among others) tolerate low salinities, and hence, also inhabit estuaries and occasionally even freshwaters. Most species are demersal or benthopelagic, feeding on fish and invertebrates. Schooling and long-distance migrations are known for several gadid species (Cohen and others 1990).

Gadids are divided into 3 subfamilies that are rather different from each other. The subfamily Gadinae, which is the more important one in this study, with approximately 22 species divided into 12 genera, includes some of the most abundant and important fishes in the sea, the true cods, genus Gadus, the haddock, Melanogrammus, and the Alaska pollock, Theragra. The other subfamilies are Lotinae and Phycinae (Cohen and others 1990) or, according to Nelson (2006), Lotinae and Ranicipitinae. Following Eschmeyer (1990), Lotinae and Phycinae are not considered subfamilies of Gadids (Cohen and others 1990).

Atlantic cod (Gadus morhua) has become scarce and is protected by strict management systems designed to limit overexploitation (Warm and others 2000). The main cod fishing area lies along the coast of Newfoundland-Labrador, Iceland, Greenland, and Lofoten Island (Norway) (Di Luccia and others 2005). Most of the cod landings are from traditional cold-water trawl fisheries of the northern hemisphere (Cohen and others 1990) and in the case of Iceland trawlers provide over 40% of the annual catch of cod (Margeirsson and others 2007).

The capture of cod has decreased gradually between 1970 and 2000, from approximately 3000000 to 1000000 tons per year. In 2010, the global capture production for Gadus morhua was 950950 tons. A sharp increase has occurred in the production of farmed cod between 2000 and 2003, from 169 to 2565 tons. However, this increase continued and in 2010, the global aquaculture production for Gadus morhua was 22558 ton. In the near future, most of the increase in fish production is expected to come from aquaculture (FAO 2010).

Raw material from aquaculture.  Farming of cod is currently a fast-growing industry and has received much attention (Jobling 1988; Tilseth 1990; Puvanendran and Brown 1999; Morais and others 2001; Bjørnevik and others 2003; Hemre and others 2003, 2004; Gildberg 2004; Lauritzsen and others 2004b; Kristoffersen and others 2006; Rosenlund and Skretting 2006; Solberg and others 2006, Duun and Rustad 2007; Herland and others 2007, 2010, 2011; Mørkøre and others 2007; Esaiassen and others 2008; Solberg and Willumsen 2008; Gudjónsdóttir and others 2010; Åsli and Mørkøre 2012; Björnsson and others 2012; Hultmann and others 2012). With the introduction of farmed cod, now there is a need for understanding the changes in the product that occur during processing (Skipnes and others 2007).

Compared to wild-caught cod that has limited access to feed, farmed cod is heavily fed with formulated feed during the production cycle, which results in a high liver somatic index and fast growth (Jobling 1988; Herland and others 2007). Other prominent differences of farmed cod are the higher condition factor, the smaller head (Gildberg 2004), and the higher carbohydrate level in muscle tissue (Rustad 1992). Farmed cod also differs from its wild counterparts by having other chemical and physical muscle properties, such as lower water content (Table 1) and lower pH (Gudjónsdóttir and others 2010). Also, the TMAO-N level found in farmed cod was only 5% to 10% of that found in wild cod (Herland and other 2007) (Table 1). This large difference was attributed to different diets and different fish age (Ágústsson and Strøm 1981; Herland and others 2007).

Table 1–.  Chemical quality parameters of wild and farmed cod.
  Wild cod Farmed cod References
Water content (%) 78.2 Åsli and Mørkøre (2012)
  78.6 to 79.1 Esaiassen and others (2008)
 82.180.5 Gudjónsdóttir and others (2010)
 83.280.1 Herland and others (2007)
  79.6 to 79.9 Herland and others (2011)
  78.5 Hultmann and others (2012)
 79 to 88  Lambert and Dutil (1997)
  78.9 to 79.0 Mørkøre and others (2007)
  78.8 to 81.3 Solberg and others (2006)
 81.8  Thorarinsdóttir and others (2011a)
 80 to 83  Waterman (2001)
Protein content (%) 19.2 to 19.7 Herland and others (2011)
  17.6 to 19.4 Solberg and others (2006)
 16 to 19  Waterman (2001)
Fat content (%)≈ 0.1 to 0.4  Ingolfsdóttir and others (1998)
  0.9 Mørkøre and others (2007)
 0.1 to 0.9  Murray and Burt (1969)
 0.0 to 0.4  Waterman (2001)
TVB-N (mg/100 g)12.813.9 Gudjónsdóttir and others (2010)
TMAO-N level (mg/100 g)69.83.6 Herland and others (2007)
TMA (mg/100 g)00 Gudjónsdóttir and others (2010)
 <1<1 Herland and others (2007)

Atlantic cod reared at optimal temperature and unrestricted food supply may reach a weight of 2.5 to 3 kg in 2 y after hatching, while the mean weight of wild cod at the same age could be less than 10% of this weight (Björnsson and others 2001). Faster growth will probably affect the texture and structure of the muscle (Johnston 1999). Furthermore, it has been observed that the quality of farmed cod may differ greatly from the quality of wild cod: soft texture and gaping are frequently seen in fillets from well-fed cod as compared with fillets of wild fish (Ang and Haard 1985; Love 1988; Rustad 1992; Ofstad and others 1996; Kristoffersen and others 2006). Moreover, to some extent, the heavy feeding is probably also causing the low ultimate muscle pH seen in farmed cod (Ang and Haard 1985; Kristoffersen and others 2006; Herland and others 2007). The muscle pH of farmed cod 24 h postmortem is usually 0.5 to 0.9 units lower than in wild cod (Rustad 1992). It is shown that low pH is related to lower water-holding capacity (WHC) and increased myofibrillar shrinkage during heating of cod (Ofstad and others 1996), and consequently, reduced juiciness and changed texture in farmed cod compared to the wild. Therefore, it has been suggested that farmed cod is inferior in quality to wild-caught cod (Nyvold and Landfald 1996) and, for this reason, factors that affect the quality of farmed cod have been explored to reach a tasty and overall acceptable consumer product (Stien and others 2005).

One clear advantage of farmed fish is the availability of raw material, but also the opportunity to process the fish under hygienic conditions prior to on set of rigor mortis (Sivertsvik 2007). Farming of cod is making prerigor fish more available for processing (Lauritzsen and others 2004b).

Furthermore, some consumers perceive farmed fish to have lower sensory quality than wild-caught fish (Kole 2003; Verbeke and others 2007; Kole and others 2009), which could threaten the development of economically viable aquaculture. Also, Sveinsdóttir and others (2009) analyzed the sensory quality of 8 cod products with differences in origin (wild/farmed). At the same time, 378 consumers in 4 European countries tasted and scored the cod products. This study confirmed that farmed cod products were considerably different from wild cod, with more light and uniform color, meaty texture, odor, and taste. But some studies have reported no clear sensory differences between farmed and wild cod. The differences that may have existed in color (whiteness) and firmness were not conclusive (Kole and others 2009). Moreover, in another study, farmed cod was slightly more appreciated by consumers and, when evaluated by a trained sensory panel, received higher scores for dull and white appearance, fibrousnesses, and cod taste, but lower scores for juiciness (Luten and others 2002). Recently, Kole and others (2009) concluded that it is unlikely that nonexpert consumers will detect significant differences in taste between wild and farmed cod. The same authors also demonstrated that information about product type, freshness, price, and the advantages of fish-farming could influence product evaluation. For example: cod labeled as wild, highly priced, or recently caught was judged more favorably than the unlabeled product. Interaction of the information provided with product perception in a realistic situation demonstrated that farmed fish was associated with less favorable product characteristics (Kole and others 2009).

Chemical composition.  Atlantic cod is classified as a lean species, with a lipid content of the body musculature representing less than 1% of the muscle wet weight (Table 1 and 2) (Bogucki and Trzesinski 1950; Dambergs 1964; Jangaard and others 1967; Murray and Burt 1969; Eliassen and Vahl 1982a; Holdway and Beamish 1984; Jobling 1988; Lie and others 1988; Kjesbu and others 1991; Lambert and Dutil 1997), and therefore considered negligible (Barat and others 2004c). The body musculature is the main protein depot, while lipid reserves are primarily stored in the liver ranging between 2% and 75% of its weight (Bogucki and Trzesinski 1950; Jangaard and others 1967; Love 1970; Eliassen and Vahl 1982b; Holdway and Beamish 1984; Black and Love 1986; Lie and others 1988; Jobling and others 1991; Kjesbu and others 1991; Dos Santos and others 1993; Lambert and Dutil 1997; Kristoffersen and others 2006).

Table 2–.  Nutritional data of cod (Gadus morhua) (Bandarra and others 2004; Nutrition Data 2012).
Nutritional data (/100 g)
Energy value (kcal/kJ)84.7/354.5; 82/343
Edible part (%)79.2
Total fat (g)0.4; 0.7
Saturated fat (g)0.1
ω3 (g)0.20; 0.21
ω6 (g)0.005; 0.02
Cholesterol (mg)43; 52
Protein (g)17.8; 19
Vitamin A (μg)3.8; 40 IU
Vitamin E (mg)0.3; 0.6 (Alpha Tocopherol)
Potassium (mg)362; 413
Phosphorus (mg)116; 203

Cod composition varies according to gender, age, season, water temperature, type, and abundance of available food (Ironside and Love 1958; Love 1958, 1960, 1962; Dambergs 1964; Ross and Love 1979; Ingolfsdóttir and others 1998; Waterman 2001).

Freshness.  Freshness, although being a controversial concept (Bremner and Sakaguchi 2000), is one of the most important parameters of fish quality in most markets of fresh and lightly preserved fish, so the role of this attribute in cod processing has been widely studied. Moreover, as mentioned above, the growing industry of farmed cod makes prerigor fish (which means highly fresh) more available for processing.

With respect to weight yield and transport kinetics, the results obtained by Barat and others (2006) showed that the main influence on cod freshness occurs during the salting process, as the freshest raw material gives the lowest salt uptake and the lowest overall yield. According to sensory quality and microbial status evaluations after desalting, the freshest raw material tended to give a harder fish with less flakiness, while the oldest raw material gave a higher flakiness (Barat and others 2006).

Frozen fish may also be used for salting, but only after being thoroughly thawed and inspected for suitability. In fact, products exposed to repeated freezing and thawing cycles show evidence of considerable quality loss associated with protein denaturation (Mackie 1993; Hurling and McArthur 1996).

The effects of the rigor state and freezing of cod prior to salting on the mass transfer during production and the quality of heavily cured cod were investigated by Lauritzsen and others (2004b). They found that prerigor salting resulted in higher water losses, larger reduction in weight, and lower uptake of NaCl than in fish salted postrigor or salted after frozen storage. The cause of this is probably the simultaneous influx of NaCl and rigor contractions in the fish muscle. Furthermore, these authors revealed that the fish should be salted in the prerigor state, if the main objectives are simultaneously to decrease the waste of protein from the raw material and to increase the lightness values of the salt-ripened product. However, if the focus is on weight yield, the fish should be salted postrigor after chilling or freezing pre- or postrigor. Freezing and thawing of cod prior to salting postrigor increased the firmness of the salt-cured product compared to chilled fish salted postrigor. This increased firmness and the lower WHC are believed to be caused by protein denaturation that happens during the salting and during freezing.

The absence of such rigor contractions probably explains the development in weight, water loss, and uptake of NaCl during salting of chilled and/or frozen fish postrigor and during salting of fish frozen prerigor (Lauritzsen and others 2004b). Such prerigor salting resulted in a lower weight yield of the product and unfavorable changes in appearance and texture compared to salting of postrigor fillets (Sørensen and others 1997).

Furthermore, Akse and Joensen (1996) noted that prerigor raw materials produced a better quality product after desalting, compared with raw materials stored in ice for 8 and 12 d before salting. Thus, the greater weight loss (higher water loss) during salting of prerigor raw materials (Lauritzsen and others 2004b), compared to postrigor, was partly regained during desalting (Akse and Joensen 1996).

A study by Larsen and others (2008) proved that after immersing prerigor filleted farmed cod at acceptable salt levels, fillet gaping, and drip losses were significantly reduced and removal of pin bones required less force. It is important to know that higher salt content generally increases rigor contraction (Larsen and others 2008).

Parasites.  Atlantic cod has an exceptionally rich and varied parasite fauna (Pseudoterranova decipiens, Anisakis simplex, and others) compared with most other species of marine fish. This diversity is apparently linked to the omnivorous diet of cod, its occurrence at low salinities where it is exposed to infection by euryhaline parasites that most marine fish do not encounter, and its status as one of the most abundant and widespread piscivorous species in the North Atlantic (Hemmingsen and MacKenzie 2001).

Parasitic nematodes or seal worms are a problem in cod fisheries (Margeirsson and others 2007) because when they are present in the cod flesh, they contribute to an unpleasant appearance. This affects the overall quality of the fish, but it does not contribute to the freshness changes throughout storage (Bonilla and others 2007). Moreover, parasites are one of the most expensive quality defects for cod processing because of the cost of cleaning the fillets and the decrease in yield and value (Margeirsson and others 2007). In 1998, Ramos studied the presence of parasites in dried salted cod, noting that only 12 out of 88 (13.6%) analyzed samples were parasite-free (Ramos 1998).

Controlling parasites.  Parasite larvae in fish are located mainly in the intestines. It has been reported that if fish is kept chilled with viscera after catching, rupture of viscera can happen and parasites can migrate to the muscle and infect it. Thus, immediate removal of the viscera (gutting) after catching is fundamental to reduce the number of parasites in cod flesh (Rodrigues 2006). After that, candling procedures, a brief visual inspection of parasites on a light table, have been used by the fish processing industry (Levsen and others 2005). Nevertheless, this method is not totally effective (FDA 1999) because sometimes larvae are whitish making it difficult to differentiate them from the muscle of cod or they are located so deep in the tissues that it is not possible to detect them (Rodrigues 2006). Recently, transillumination hyperspectral imaging was implemented as a method for automatic nematode detection in cod fillets, and was evaluated on industrially processed cod fillets. The detection rate (71% and 46% for dark and pale nematodes, respectively) was comparable to what is reported by manual inspection under industrial conditions. However, this method has the potential to reduce the manual labor required for cod fillet inspection, and hence, reduce the cost (Sivertsen and others 2011). It seems that there is no foolproof method for physical removal of parasite larvae, so they should be killed before the consumption of fish. There is some evidence that parasites only threaten human health if they are alive in the product (Alonso and others 1999; Sastre and others 2000; Rodrigues 2006). However, other studies have shown that even inactivated parasites can cause allergic reactions in some consumers (Fernández de Corres and others 1996).

The question of whether or not the parasites survive the salting has been very controversial (Ramos 1998). According to Wootten and Cann (2001)Anisakis larvae are resistant to salting, but immersion in brine with 21% of salt for 10 d will kill all larvae. Also, Rodrigues (2006) showed that cod salting for 13 d or more was sufficient to inactivate parasites. Thus, heavy salting seems to be a solution as a basic procedure. Brining and pickling may also reduce the parasite hazard in fish, but they do not eliminate it, nor do they minimize it to an acceptable level (FDA 2011). Rodrigues (2006) proved that the total number of parasites in salted cod was generally lower than in fresh cod due to manipulation and removal by drag with salt water of the parasites that are located superficially.

The process of heating raw fish has been considered sufficient to kill all parasites (Huss 1994; FDA 2011). However, some cod is still consumed raw or semi-raw (partly cooked), which represents a threat to human health.

Moreover, freezing of fish at −20 °C or lower or at −35 °C or lower for 7 d or 15 h, respectively (Huss 1994; FDA 1999, 2011) destroys 99% of the larvae and it has been stated that those that survive may be so damaged that they do not represent a danger (Rodrigues 2006). Also, Wootten and Cann (2001) have published that freezing fish at −20 °C for 60 h kills all worms. The processing of green salted cod sometimes does not imply a prior freezing and these fish products are considered unsafe (Rodrigues 2006). Therefore, a short period of freezing either of the raw material or the final product must be included in the processing as a means of control of parasites (Huss 1994).

It is important to note that no matter how carefully fish is inspected by processors, caterers, and retailers, some worms will occasionally be found in fish by the consumer. Also, it should be emphasized that the presence of parasites in fish offered for sale does not imply carelessness or bad practice by the processor or retailer (Wootten and Cann 2001).

Salt

One of the major factors affecting the quality of salted fish is the salt composition (Rodrigues and others 2005). Its composition differs according to the salt origin, namely, the sea, underground rock salt deposits, or vacuum-processed, and refined brine. Mine salt and solar salt of marine origin contain several other salts besides sodium chloride, like calcium sulfate, magnesium sulfate, and chloride, as well as impurities. Vacuum-processed and refined salt are almost pure sodium chloride (NaCl) (Codex Alimentarius 2003).

Generally, marine salt is used and sodium (Na) is the predominant component. NaCl is considered essential to produce the desired texture and flavor and guarantee the safety of the product (Ismail and Wootton 1992). However, salt produced from marine sources may contain halophilic bacteria and mold, which survive in the salt and in the dry-salted fish, and could contribute to spoilage (Codex Alimentarius 2003). The content of calcium and magnesium ions in solar salts may vary from 1 to 15 g·kg−1 (Lauritzsen and others 2004a). One advantage of these compounds is that they prevent the yellow coloration that occurs in fully cured cod, improving the color, but they also improve the texture of cod (Penso 1953; van Klaveren and Legendre 1965; Horner 1992; Lauritzsen and others 2004a; Rodrigues and others 2005). Dry-salted cod produced with salt containing magnesium chloride (MgCl2) and/or calcium chloride (CaCl2) was whiter, and consequently, more attractive, more opaque, and more resistant than fish only salted with pure NaCl (Penso 1953). The results obtained in the work of Rodrigues and others (2005) for fish immersed in brine with 50% NaCl, 0.4% MgCl2, and 49.6% KCl (potassium chloride), at pH 6.5, were similar to those above of Penso (1953).

According to Lauritzsen and others (2004a), calcium ions are responsible for increased lightness and firmness, whereas magnesium ions only increased the lightness of the cured muscle. This may happen due to the cross-linking effect of Ca2+ ions and their ability to denature and precipitate proteins. The increased protein precipitation or interaction between the divalent cations and the muscle surface perhaps may explain the nature of the increased whiteness. So, to obtain a salt-cured product of white color and firm texture, with minimal protein loss during the process, the calcium and magnesium contents of the salt should be high (800 and 400 mg·kg−1, respectively) (Lauritzsen and others 2004a; Martínez-Alvarez and Gómez-Guillén 2005, 2006). However, other authors have commented that calcium salts promote firmness of salted fish up to a concentration of 0.3%, while higher concentrations make the fish excessively hard and compact (Beatty and Fougère 1957; van Klaveren and Legendre 1965). Thus, very large concentrations of both salts may produce hardening of cell walls, reducing the rate of salt penetration to an extent that makes spoilage possible (Wheaton and Lawson 1985). Nevertheless, small amounts of calcium and magnesium can also be beneficial in that they halt enzymatic processes that can spoil the product (Wheaton and Lawson 1985; Martínez-Alvarez and others 2005a). Soudan (1955) suggested the use of salt with 0.15% to 0.3% CaCl2 and 0.05% to 0.15% MgCl2. Later, Codex Alimentarius (1989) recommended the use of the same concentrations of CaCl2 and a limit of 0.1% for Mg, and noted that the limits established for the concentrations of CaCl2 and MgCl2 are not always satisfactory. The utilization of magnesium salt concentrations of more than 0.15% can lend a bitter taste to the product (Gillette 1985).

On the other hand, partial replacement of 50% NaCl with 50% KCl has been shown to reduce penetration of Na+ into the muscle of cod fillets, as did the addition of small amounts of CaCl2 (0.8%) and/or MgCl2 (0.4%) to pH 6.5 brines (Martínez-Alvarez and others 2005a; Martínez-Alvarez and Gómez-Guillén 2005). In the case of MgCl2, one possible explanation is the fact that the Mg2+ cation is the most electronegative of all those assayed it binds strongly to the protein polar groups, strengthening protein interactions (Xiong and Brekke 1991) and thus hindering the penetration of salt.

According to Larsen and Elvevoll (2008), a reduction in salt levels from the industry standard of 2% to 3% to around 1.2%, and the replacement of some of the sodium by potassium, do not significantly influence the technological parameters of yield and drip loss and would render a product with higher acceptability. Nevertheless, Hand and others (1982a, b, c) and Seman and others (1980) proved that the presence of more than 50% KCl can attenuate the flavor and produce bitterness. Moreover, metallic tastes have been associated with KCl-based salt substitutes (Gillette 1985).

The study of Rodrigues and others (2005) concluded that CaCl2 (0.8% w/w), MgCl2 (0.4% w/w), and KCl (around 50%) can be used in the brining of cod without adversely affecting the microbiological and sensory quality of the salted cod. Only brines containing MgCl2 and KCl seemed to produce a very slight increase in microbial growth (at pH 6.5) (Rodrigues and others 2005). Hess (1942a) also showed that MgCl2 added to the brine (0.129 g/100 mL) stimulated microbiological growth and unpleasant trimethylamine production. When Mg was replaced by Ca, the salt mixture (50% NaCl; 0.8% CaCl2; and 49.2% KCl), at pH 6.5, did not exhibit the same effect. The different pattern could be due to the antimicrobial effect of CaCl2 (FDA 2003), which possibly inhibits the microbial growth-inducing effect of KCl (Rodrigues and others 2005).

Concluding that differences in composition and pH of brines used prior to dry-salting of Atlantic cod slightly affected the composition of the muscle as regards the major constituents and the functional quality of the muscle protein (Martínez-Alvarez and others 2005a; Martínez-Alvarez and Gómez-Guillén 2005, 2006), such as the degree of protein denaturation (Kinsella 1982; Morrissey and others 1987), which can also influence the protein quality of the desalted product. Moreover, the salt composition and pH of the brines mainly affect taste (cooked cod), odor (desalted cod), and color (salted and cooked cod) (Rodrigues and others 2005). In addition, the effect of salt concentration is dependent, in turn, on the pH of the medium (Martínez-Alvarez and others 2005a). So, the extension of the changes induced by salting may affect properties during further drying and desalting operations, in terms of mass transfer kinetics and sensory properties of the final product (Yashoda and Rao 1998; Andrés and others 2005b).

Salt granulometry is also an important issue. The use of very fine salt granules could result in the formation of clusters, which is not favorable for ensuring the uniform distribution of salt on the fish. The use of very coarse salt granules could result in damage to the fish flesh during salting and may reduce the rate of maturation (Codex Alimentarius 2003). Therefore, and to obtain a more efficient salting, a mixture of salts with different granulometry is often used. The smaller crystals dissolve more easily and the coarse crystals allow a better flow of liquids. Thus, besides dissolving and penetrating inside the tissues of fish, salt will absorb water from the same tissues, allowing an effective conservation, meaning that salt will induce both salting and drying of the fish at the same time. A mixture of 1/3 of fine salt granules and 2/3 of coarse salt granules has been considered adequate (Rodrigues 2006).

Other ingredients

Another alternative to traditional cod processing of particular interest where the process duration can be reduced and water loss and salt entrance better controlled is dewatering and salting (simultaneous) of cod fillets by soaking them in mixed concentrated solutions (sucrose/salt) at low temperature (Collignan and Raoult-Wack 1994). The presence of sugar can enhance water loss and hinder salt uptake. Moreover, the use of high-molecular-weight sugar prevents sugar entrance and allows good dehydration (Collignan and Raoult-Wack 1994).

Nowadays, some producers are interested in producing salted cod with addition of polyphosphates. The main interest is to maintain the quality and appearance (lighter color) of the product through salting and desalting, mostly due to retardation of lipid oxidation (Nguyen and others 2012). Thorarinsdóttir and others (2001), studying the use of polyphosphates during the processing of salted cod, submerged cod fillets in brine with 2% or 2.5% sodium polyphosphate. It was shown that the addition of polyphosphates resulted in poorer quality when considering the entire process (salting and desalting). However, differences were not observed by sensory evaluation after soaking and steam boiling, and improvements in yield were seen in the fillets containing polyphosphate, after dry-salting and storage. Nevertheless, the increase in weight during rehydration was far less in fillets containing polyphosphate than in the control group (where no polyphosphate was used).

Processing

Salting is one of the oldest methods of preserving fish, especially cod (Martínez-Alvarez and others 2005a), by lowering the water activity. Water activity (aw) is lowered in the cod muscle to inhibit bacterial growth and enzymatic spoilage. The aw value of heavily salted cod generally lies in the range of 0.7 to 0.75 (Lupín and others 1981; Gomez and Fernandez-Salguero 1993).

Additionally, drying of salted cod can be carried out to obtain semi-dried or extra-dried products.

Heavily salt-matured products have a water content of 550 to 600 g·kg−1 (approximately 55% w/w) and a salt content of 200 to 250 g·kg−1 (about 20% w/w) (Borgström 1968; Zaitsev and others 1969; Bjørkevoll and others 2003). According to van Klaveren and Legendre (1965), Bogason (1987), and Akse and others (1993), the final salted fish product contains 55% to 58% water and 18% to 21% salt, compared with approximately 80% water and 0.3% salt in the raw material. The water content, however, can be further reduced by drying, and when it becomes less than 500 g·kg−1 (50% w/w), the dried salt-cured cod (klipfish) is obtained (Bjørkevoll and others 2003; Lauritzsen and others 2004a). Water and salt contents in wet salted, dried salted, and desalted cod (Gadus morhua) are shown in Table 3.

Table 3–.  Water and salt contents (%) in wet salted, dried salted, and desalted cod (Gadus morhua).
Gadus morhua Wet salted Dried salted Desalted
  1. a IPCP-2 1991.

  2. b Rodrigues and others 2003.

  3. c Bandarra and others 2004.

  4. d Lauritzsen and others 2004b.

  5. e Thorarinsdóttir and others 2004.

  6. f Andrés and others 2005b.

  7. g Martínez-Alvarez and others 2005b.

  8. hRodrigues and others 2005.

  9. I Barat and others 2006.

  10. j Heredia and others 2007.

  11. k Lorentzen and others 2010a, 2011.

  12. LThorarinsdóttir and others 2010.

  13. m Jónsdóttir and others 2011.

  14. nThorarinsdóttir and others 2011b.

Water content/51 to 58a≤46 (unpacked)a79.2 to 81.6b
 moisture (%)56.8b≤47 (prepacked)a76.2c
 52.8 to 57.9d47.4b83.6 to 84.2e
 57.6 to 58.7e47.2 to 48f80.1 to 82.7f
 56 to 60.9f43I73I
 56.7 to 57.3g48.1j75.4j
 55.4 to 57.3h 82.2 to 85.4L
 53.3I 82.5 to 84.9m
 59.2j 84n
 55.8 to 59.5L  
 56.2 to 59.5m  
Salt content16 to 20a16 to 20a0.8 to 0.9b
  (% NaCl)18.5b20.6b1 to 1.2e
 18.4 to 20.0d23.3 to 26.1f2.4 to 3.9f
 20.2 to 20.6e20I2.2I
 20 to 21.8f 2 to 3k
 23 to 25g 1.3 to 1.7L,m
 19.2 to 20.2h 2.3 to 2.9n
 18.5I  
 20.6 to 21.4L  
 19.5 to 21.4m  

The curing time, and whether or not further processing such as drying is applied, depends on the market requirements. Traditional processing comprises a salting step, resulting in a wet product, which is usually followed by drying (Martínez-Alvarez and others 2005a; Martínez-Alvarez and Gómez-Guillén 2005). The curing method for cod and the time vary according to region and producer, but are characterized by the development of a yellowish color, stockfish-like flavor, and tough consistency of the fish flesh (Vilhelmsson and others 1997). In some regions, the process of heavy salting still relies on the main traditional stages reported some decades earlier (Beatty and Fougère 1957; van Klaveren and Legendre 1965). These stages include heading, gutting, filleting/splitting, salting, curing, and packaging of the fish (Rodrigues and others 2005).

Preparing for salting

Generally, cod is subjected to bleeding and gutting after catching, then is chilled or frozen onboard and stored. At the processing unit, the fish is beheaded, trimmed, filleted, or split. This later step consists of cutting cod along the ventral median line and opening to remove the vertebral column for about 3/4 of its length (Di Luccia and others 2005). Immediately after splitting, fish is washed in running clean water or clean seawater, to remove blood and, according to the intended presentation, the “black membrane” (peritoneum) might be removed from the belly walls before salting (Codex Alimentarius 2003).

Salting

Salting is a process in which mass transfer, basically salt and water, between cod and its surroundings (Barat and others 2004a) occurs by diffusion: the fish muscle takes up salt and loses water (Thorarinsdóttir and others 2001; Andrés and others 2002; Martínez-Alvarez and Gómez-Guillén 2006). The rate of salt penetration into the cod muscle may increase up to 100-fold by splitting, filleting, and skinning the fish.

Salting of cod may be carried out by dry-salting, wet-salting, brining, brine injection, or a combination of these techniques. The water content of the cod muscle is usually reduced from approximately 82% to about 54% during the traditional salt-curing process.

In dry-salting (or kenching), cod is placed with the skin side down in stacks with dry salt crystals interspersed between the layers until fully cured (pile-salting) (Barat and others 2003). Commonly and first fine-grained salt is used and after 1 week, another salting process is carried out, this second time using coarse-grained salt (Di Luccia and others 2005). The resultant brine, which forms by solution of salt in the water leached out from the fish tissue, is allowed to drain away continuously (Beatty and Fougère 1957; Borgström 1968; Zaitsev and others 1969; Horner 1992; Thorarinsdóttir and others 2004; Rodrigues and others 2005). During this stage, usually 2 to 8 weeks, depending on the degree of desired curing in the resulting products (Burgess and others 1987) and on the thickness of the fish, cod acquires its characteristic appearance, flavor, and consistency (Rodrigues and others 2005). Moreover, the use of overpressure on the fillets during dry-salting can improve salting kinetics (Filsinger 1987), and this is what occurs in stack-salting because of the pressure exerted by the weight of the piled fillets and the salt crystals (Andrés and others 2005b).

Wet-salting (or pickling) is the process whereby cod is mixed with salt, layered alternately and stored in watertight containers under the resulting brine, designated as “pickle” (Codex Alimentarius 2003), which is formed within 1 d (Lauritzsen and others 2004a). New brine may be added to the container. The fish can be removed from the container and stacked so that pickle drains away.

Brine-salting (or brining) is carried out by immersing fish into a brine solution prepared with coarse salt and water. The diffusion of salt into the muscle depends on several factors, such as concentration and composition of the brine, the shape and thickness of the product, ratio of brine to product, and duration of brining.

In traditional processing, cod (split or filleted) is most often pickle-salted or brine-cured for a week (5 to 7 d), and then salt-matured by kenching in stacks (dry-salting) for at least 10 d, producing salt-cured cod (Bjørkevoll and others 2003, 2004; Lauritzsen and others 2004a). Brine-salting has gained popularity in the Icelandic fish industry. This method is as follows: the fish is submerged for 1 to 4 d in a solution of water and salt (brine), with salt concentration normally of 17 ± 1% NaCl. Then, the fish is removed from the brine and the butterfly fillets are placed with alternate thin layers of salt into stacks approximately 1-m-high or in plastic tubs where the stacks are only 30 to 40 cm high. The fish is kept stacked for 10 to 12 d for dry-salting. Individual fish may be rotated during storage to even out the pressure exerted on each fish. The major difference between the 1-m-high kench and the 30 to 40 cm stacks is the pressure applied to the fish in the bottom layers. Dry-salting is followed by packaging and storage (Thorarinsdóttir and others 2004).

Soaking the fish in brine first, giving sufficient time for the muscle to absorb a significant amount of salt, is considered to be an advantage (Gallart-Jornet and others 2003). This decreases the time and increases the weight yield of the salting process (Bogason 1987; Thorarinsdóttir and others 2004).

The industrial cod-salting process is carried out in rather high piles, which means that the process is similar to that followed in pressed salted cod. This implies that higher process yields would be obtained if pile height is reduced in the manufacturing process (Barat and others 2003).

For example, the osmotic mechanism and diffusion transport of salt can be enhanced during vacuum-pulsed-brine-salting, which leads to a lower water loss and higher yield. Unlike dry-salting, the coupling of osmotic pressure and suctioning and the dry salt crystals (more so in the case of pressured dry-salting) favors water losses and limits salt gain (Andrés and others 2002). Therefore, the salting method has an influence on the structural and mechanical properties of the fish muscle (Andrés and others 2002; Barat and others 2002, 2003; Thorarinsdóttir and others 2004). Moreover, quality graders of commercial salted fish have indicated that the appearance of the fish may be related to the salting method, with adverse effects on color and appearance if the initial salt concentration has been too high (Thorarinsdóttir and others 2004).

The effect of the salt concentration in the brine has been a matter of controversy, with some indication that higher weight yield and quality may be obtained by using lower salt concentrations than by using a fully saturated brine solution (Hamm 1961; Offer and Trinick 1983; Bligh and Duclos-Rendell 1986; Wilding and others 1986; Barat and others 2002; Thorarinsdóttir and others 2004; Gallart-Jornet and others 2007; Nguyen and others 2010). The process temperature should not exceed 25 °C (Torry Research Station 1962).

Brine concentration showed a very important effect on cod weight changes throughout the salting operation (Barat and others 2002) because the concentration of salt in the brine affects the rate of salt diffusion into the muscle and the quantity of water and proteins extracted (Thorarinsdóttir and others 2004). When salt concentrations in the brine exceed about 13% to 15% (Fougère 1952; Deng 1977), especially in the early stages (Hamm 1961), the fish not only takes up salt, but also loses water. Nguyen and others (2010) also reported that salt concentration in the brine of 15% (w/w) seems to be a critical concentration separating hydration from dehydration regimes for brining of cod loins. The rates of the salt and water diffusion were shown to be positively correlated with increasing salt concentration of the brine by Poernomo and others (1992), Lawrie (1998), and Nguyen and others (2010).

First, the presence of high concentrations of salt in muscle gradually increases the WHC, obtaining a maximum at an ionic strength of 1 M (about 5.8% salt) (Offer and Knight 1988), but at higher ionic strengths, WHC decreases, apparently by a salting-out effect due to water-binding by the salt and concurrent dehydration of the protein (Martínez-Alvarez and others 2005a; Heredia and others 2007). Thorarinsdóttir and others (2002) showed that the salting process significantly decreased the heat stability of both myosin and actin, with the detection of changes in the transition peaks by differential scanning calorimetry. Changes in myofibrillar proteins during cod salting were detected by electrophoresis, showing that the myosin heavy chain (MHC) was cleaved into smaller subfragments in the salting process, with the 2 heavy meromyosin fractions and the light meromyosin fraction being the most abundant. Moreover, the conformational stability of myosin and actin was lower than in the fresh material (Thorarinsdóttir and others 2002). Another explanation to the observed decreases of MHC on heavy salting can be the gross aggregation of myosin at the high salt concentrations (Thorarinsdóttir and others 2011a).

Thus, salting induces changes in the muscle proteins (including some precipitation) resulting in changes in texture, such as hardness increase (Dunajski 1979; Yashoda and Rao 1998; Barat and others 2002; Gallart-Jornet and others 2007), weight, and WHC (Bligh and Duclos-Rendell 1986; Thorarinsdóttir and others 2002, 2004; Sannaveerappa and others 2004; Martínez-Alvarez and Gómez-Guillén 2006). Therefore, it is now well-known that high salt concentration, close to saturation (25%, w/w) in brining and dry-salting, denatures the proteins and reduces their WHC (Duerr and Dyer 1952; Cheftel and others 1989; Sannaveerappa and others 2004; Gallart-Jornet and others 2007), while for more diluted brines, protein denaturation is lower and an increase in the WHC due to salt uptake is observed, as reported by Hamm (1961), Barat and others (2002), and Thorarinsdóttir and others (2002). During heavy salting of cod, the activity of cathepsin B/L remains unchanged, whereas the activity of acidic proteases declines as the salt concentration in the muscle increases (Stoknes and others 2005).

A reasonable alternative to saturated brine-salting may be the use of increasing concentrations of brine in order to obtain higher processing yields (Barat and others 2002). However, increased control of the brine concentration did not show significant effects on weight yield, WHC, or composition of salted, rehydrated cod fillets (Thorarinsdóttir and others 2004).

Brine-salting of cod may offer better control over the rate of changes in water and salt contents in the muscle than the other salting methods, and thus increase the weight yield (Bogason 1987; Barat and others 2003) and the overall quality of the salted fish (Thorarinsdóttir and others 2004). The literature shows that some other advantages of brining may exist, with protection against oxidative rancidity by preventing contact with air (Wheaton and Lawson 1985) and faster salting due to a higher rate of salt penetration into the fish muscle (Akse and others 1993).

In spite of the large influx of salt into the cod muscle during the curing process, weight is lost mainly due to extensive dehydration (Akse and others 1993) and protein leaching from the muscle (Barat and others 2003; Lauritzsen and others 2004a). Beyond soluble muscle proteins, vitamins and free amino acids may also be lost during the process (Larsen and others 2007; Larsen and Elvevoll 2008), and it has been recommended that this loss be reduced as much as possible (FAO 1981). According to Lauritzsen and others (2004a), most of the weight loss occurs in the kench-curing steps. However, in industrial production, the fillets in the lower parts of the kench-curing stacks will be exposed to a heavier weight from the upper layers, and the protein and liquid losses from the fillets may therefore be increased even further. It has been observed that lost protein ends up in the brine or in the dry salt used and represents both a waste of valuable fish protein and a possible environmental problem (Lauritzsen and others 2004a).

Over the last 2 decades, processes used for salt-cured cod production have significantly changed. The changes in salting procedures and curing conditions have altered the characteristics of the products, increased weight yields, and improved some quality parameters (Lindkvist and others 2008). Shorter curing times, lower temperatures during curing, and better storage conditions can result in milder curing flavors and whiter appearance (Barat and others 2003; Lindkvist and others 2008).

Also in the case of salting, vacuum-tumbling or injection-salting will reduce the time of brining needed to obtain the desired salt content and increased yield, and have therefore a higher potential to retain water-soluble components (proteins, vitamins, and free amino acids) within the muscles. Thus, a different technique with a shorter brining period is more suited for retaining such components (Larsen and Elvevoll 2008).

The vacuum osmotic dehydration (VOD) method has been tested for cod salting and has given a faster uptake of NaCl and lower water loss from the muscle than ordinary dry-salting methods. The weight yield of salted cod was higher by the VOD method, but the muscle surface of the fish had a more yellow color than by ordinary dry-salting methods (Joensen and others (1997), as pointed out by Lauritzsen (2004)).

Injection-salting consists of automatic needle injections of saturated brine solution into the muscle. Salt is forced mechanically and at high pressure into the muscle tissue prior to passive salt diffusion, therefore increasing salting speed. Several points of brine entrance ensure uniform salt concentration throughout the muscle. Usually, the injection needles are pierced into the flesh from the open fillet surface down to the inner side of the skin. When the needles reach the skin, the brine flows continuously out of the needles into the flesh as they are retracted through the fillet back to the initial position. The injection pressure and the number of injections into the fish can be adjusted for the automatic injection machine. Usually, the salt concentration increases from 0.15% in fresh cod muscle to 2% to 5% in ready-salt-injected fish muscle. After this step, the cod may be brined, pickle-salted, and/or kench-cured (Lauritzsen 2004).

Other recent studies that analyzed the different salting techniques (combined or not), as well as its influence on chemical, physical, and sensory parameters of cod, are shown in Table 4.

Table 4–.  Some recent studies that analyzed the different salting techniques (combined or not), as well as its main results/conclusion (S = salt, P = polyphosphates).
Salting techniques studied Parameters analyzed Main results/conclusions References
Injection (S or S+P) + brining (2 d);Weight yield, chemical composition, andInjection and brining – Increased weight yields (during both salting and rehydration) Thorarinsdóttir and others
Only brining (2 d); protein aggregation of salted cod fillets. compared to brining only and pickling. (2010, 2011a)
Pickling (3 d); Dissimilarities in yield depend on the degree of protein denaturation and aggregation: 
(all followed by dry-salting Injection and brining (stronger salting-in effects on proteins) < brining < pickling. 
 (23 to 26 d)); Differences in denaturation/aggregation were assigned to both myosin and collagen. 
Dry-salting (26 d) (reference group). The yield of nitrogenous compounds tended to be lower for injected and brine-salted fillets (higher losses of nonprotein nitrogen). 
Injection + brining (2 d) (S+P) compared with brining (2 d) (S+P) only;Microstructure and water retention of heavy-salted cod products.Salting resulted in shrinkage of fiber diameter and enlargement of intercellular space probably due to myofibrillar protein aggregation and enzymatic degradation of the connective tissue. Thorarinsdóttir and others (2011b)
Followed by dry-salting. The intercellular space tended to be larger in the injected and brined muscle than in the brined only. 
  The main water changes occurred during dry-salting (salt content of the muscle increased from 7% (brined) and 9% (injected and brined) to 27%). 
Injection (S+P) + brining (2 d);Flavor and quality characteristics of saltedThe used of presalting improved the appearance of the salted products (increased Jónsdóttir and others
Only brining (2 d); and desalted cod. lightness and reduced yellowness). In the same products, the intensity of curing (2011)
 (both followed by dry-salting (25 to  flavors was milder. 
  26 d)); Derivatives from lipid and protein degradation contribute to the characteristic flavor 
Kench-salting (25 to 26 d).  of the salted products. 
Brining (4% (w/w)) (2 d);Drip loss in cod loins.Brine injected loins showed a significantly higher drip loss than those brined by immersion. Gudjónsdóttir and others
Brining (4% (w/w)) (8 min); During brine injection, there is an increased risk of puncturing cells with the needles. (2011a)
Injection (2% (w/w) S + 2% (w/w) P);  Also, a possible destruction of the muscle due to too high injection pressure can lead 
Injection (4% (w/w) S + 2% (w/w) P).  to reduce water-holding properties and thus increased drip. 
Injection (S or S+P) + brining (2 d);Water distribution and protein denaturationInjection of S+P did not have a significant effect on the water distribution compared to Gudjónsdóttir and others
Only brining (2 d); of dry-salted and rehydrated cod. injection of only S. (2011b)
Pickling (3 d); Protein denaturation was lower in brine injected fillets (during salting and desalting) 
 (all followed by dry-salting (22 to  and higher in the pickled and kench-salted fillets. 
  23 d)); Brine injection followed by brining, with low salt concentrations, was the recommended 
Kench-salting (26 d) (reference group).  presalting method in the production of dry-salted cod. 

Drying

In 2005, Denmark and Norway were the major producers and exporters of dried preserved cod (Di Luccia and others 2005). The fish were headed, gutted, and left to dry for about 3 months during winter or spring, and then the product was stored under cool, dry conditions (Di Luccia and others 2005). However, the preparation of dried salted cod is a different process, and usually the salted cod is dried in factories located in the importer countries (Rodrigues and others 2003, 2005).

The dried products are suitable for transport and storage because they are light and do not take much space (Di Luccia and others 2005). The dried salted cod is sold mainly at retail as unpacked split fish, without storage and/or desalting recommendations, and is generally consumed cooked after soaking, and in some cases raw, after being desalted or not (Rodrigues and others 2003; Pedro and others 2004).

Dried salted cod processing includes the following steps: salting, washing, ‘‘water-horsing’’ (predrying where the green salted cod is kept for several days in piles outside the drying chambers), drying, grading, and packaging (Rodrigues and others 2003).

The salted cod storage period is increased by dehydrating the fish in a hot-air tunnel or using atmospheric agents (Di Luccia and others 2005). This is a time-consuming process, highly hand labor-intensive manipulative, and with great potential for environmental transfer of microorganisms to the product (Rodrigues and others 2003).

Drying of cod is done with temperatures around 20 °C and relative humidities below 70% (Barat and others 2006; Brás and Costa 2010). The water present on the salted cod surface evaporates and is transported by convection into the dried air. The decrease of the water content at the surface establishes a concentration difference with the inner zone, which constitutes the driving force for water migration from the inner zones to the outer surface. Jason and Peters (1973) state that drying of cod, including that of salted cod, occurs in 3 stages: the first of constant water loss rate and the second and third of decreasing water loss rate. The first stage is controlled by convection of dry air and by heat transfer and the second and third stages are controlled by Fickian diffusion of water inside the fish. The second stage of water loss involves a diffusion coefficient of water greater than in the third stage (Jason (1958), as pointed out by Del Valle and Nickerson (1968)). Both diffusion coefficients can be considered isotropic and depend on the fat content of the fish. Jason (1958) also stated that both diffusion coefficients follow an Arrhenius relationship with temperature.

The diffusion of water is also related to other factors. Del Valle and Nickerson (1968) proposed a diffusivity dependence on the concentration of water, in the fish, which is lowered proportionally with a decrease of the water content. This is explained by the decrease in water content itself. With the decrease of water content, the water left in the fish is more strongly bounded to proteins and salt ions, which limits the mobility resulting in a decrease in the rate of water loss. However, the consequent reduction in volume of fish due to water loss shortens the way for water diffusion which contributes to a smaller decrease in the rate of water loss (Del Valle and Nickerson 1968). Jason and Peters (1973) presented a more elaborate explanation, proposing that the second stage would be controlled by diffusion of water molecules and the third would be controlled by diffusion of water molecules aggregated to sodium ions.

In the drying of salted cod, there is formation of an impermeable layer of salt and protein at the surface of the fish (Barat and others 2004b). Linton and Wood (1945) suggested that this effect is caused by the high driving forces used for drying. Del Valle and Nickerson (1968) found that the formation of such layer is the result of syneresis of the salt solution in the salted fish. They suggested that shrinkage of the muscle was the cause of syneresis and that, in turn, the shrinkage is due to water loss and denaturation of proteins because of high salt concentrations. Others attributed the phenomenon to capillary action (Barat and others 2004b). Jason (1958) also observed it and suggested alternating placement of cod during drying in a pile to break this layer (Del Valle and Nickerson 1968).

In addition to water loss, as a result of drying salted cod, there is also a slight loss of salt. Barat and others (2006) found losses of 2% to 4% salt from the surface resulting from the separation of crystals of sodium chloride due to the handling of the cod.

Quality changes during drying have been identified as color, texture, chemical, and microbiological changes. Drying causes a yellowing of the salted cod. Lauritzsen and others (2004a) reported that reduction of water content is sufficient to cause changes in the color of fish. Stien and others (2005) indicated that loss of water reduces light scattering and can cause loss of transparency, thus increasing the luminosity L* (in the colorimetric system L*a*b*). Brás and Costa (2010) measured an increase in lightness (L*) and yellowness (b*) with drying cod, noting a trend inversely proportional to water content. The color of the salted fish and probably of the dried salted fish is also influenced by other factors such as the presence of ions of calcium or magnesium in the salt, contributing to a whiter tone (Horner 1992; Lauritzsen and others 2004a; Martínez-Alvarez and Gómez-Guillén 2005). This effect may be a consequence of increased water retention, which is a common result with the use of these ions in the preparation of gels in various other food products. Other causes have been suggested for color change in salted fish and can be the reasons of this change during drying: protein denaturation due to low pH (Lauritzsen and others 2004a; Stien and others 2005), oxidation of phospholipids (Anon 1967; Stien and others 2005), and reactions resulting from the presence of ions of iron or copper (Anon 1967).

Cod hardens with drying due to protein denaturation and reduction of hydration of the proteins (Brás and Costa 2010). However, the hardness evidenced after drying depends on the method of salting used. Brás and Costa (2010) measured a lower firmness when cod was salted in brines where a lower salt content was used, obtaining an inversely proportional relationship between firmness and water content beneath the surface of the dried fish.

The firmness of the dried fish can also be due to a lower pH of the muscle (Love 1988; Lauritzsen and others 2004a) or the presence of ions of calcium or magnesium in the salt (Horner 1992; Lauritzsen and others 2004a).

In addition to the aforementioned chemical composition changes, other changes occur, such as the conformation of molecules and degradation (like denaturation of proteins and/or proteolysis). The changes in proteins were analyzed by Thorarinsdóttir and others (2002) who reported that proteins denature by a complicated process, due to the extreme changes in water and salt contents of the muscle. Lipid oxidation is a type of reaction expected during drying, but the low lipid content of raw cod (less than 1%) suggests little importance of this reaction in the production of dried salted cod. However, despite the lipid content in the muscle of cod being low and limited to mainly phospholipids, a recent study found flavors most likely derived from lipid molecules (unpublished results). Moreover, yield of dried salted cod depends on the composition of the raw cod. Brás and Costa (2010) studied the salting and drying of cod species and observed that a higher concentration of water (low protein concentration) in raw cod led to lower yields of dried salted cod.

The low water content and high salt content of dried salt cod result in water activities of less than 0.75, a value not favorable for the growth of halophilic bacteria. Beyond the development of these bacteria, other bacteria are halotolerant, particularly Gram-positive cocci which can survive salting and drying, and when facing optimum conditions, like in soaking, may grow and deteriorate the product (Rodrigues and others 2003).

During drying, the water activity sees a slight diminution from 0.73 to 0.75 (unpublished values) to 0.70 (Rodrigues and others 2003) when compared to the water content value which then decreases from close to 60% (m/m; total basis) to less than 50% (m/m; total basis). The relationship of water activity to water content of salted cod during drying and storage is given by isohalic isotherms (Doe and others 1982).

The thickness of the split fish is a limiting parameter in the mass transfer rate when the transfer depends on the internal mechanisms like during drying of salted cod. Del Valle and Nickerson (1968) reported that a greater mass of fish takes longer to dry because it has a greater thickness. Brás and Costa (2010) measured a linear increase in drying time with the weight of the fish, up to 20 h per extra kg of fish above fishes of 0.5 kg.

The end point of drying in the Portuguese industry is usually determined by the time at which the dried salted cod stays in a horizontal position when held with one hand on the loin (Brás and Costa 2010).

The optimal conditions of drying of salted cod were proposed by Linton and Wood (1945) with the air velocity being 1 to 1.25 m/s, the relative humidity 45% to 55%, and the temperature at 26 °C (Jason 1965). Del Valle and Nickerson (1968) cite other authors with similar values. Linton and Wood (1945) proposed, for lightly salted cod, an air velocity of 1.5 to 2.0 m/s, 50% to 55% relative humidity, and a temperature of 30 °C. Industrial practice currently is variable, and in Portugal, the used temperatures is below 25 °C, with air velocity and humidity constant throughout the drying process, but studies in Norway revealed varying conditions during the drying process, with higher temperature (20 to 26 °C) and air velocity (1.9 to 2.4 m/s) and lower relative humidity (50% to 35%) (Barat and others 2006).

Storage

Precise control of temperature and humidity is essential with regard to the growth of halophilic bacteria and the stability of quality parameters, as some microorganisms and enzymes are still active despite the high salt content and chilled storage conditions (Rodrigues and others 2003; Pedro and others 2004). Moreover, the packaging method may be useful for logistic solutions where cold storage is not possible because it has been reported that storage in modified atmosphere packaging inhibited halophilic bacteria (Aas and others 2010). Thus, a growth inhibiting effect in the oxygen-reduced packing atmosphere, would be expected as red halophilic bacteria are strictly aerobic (Elazari-Volcani (1957), as pointed out by Aas and others (2010)).

Furthermore, it is not advisable to store heavily salted cod at −4 °C or lower (Nguyen and others 2011). Nguyen and others (2011) showed that the storage of heavily salted cod at superchilling (−4°C) and lower storage temperatures (freezing temperatures) had a detrimental influence on the color of the product which is the main quality criterion for salted cod.

To minimize weight changes during storage, it is also important to maintain equilibrium between the water activity (aw) in the salted fish and the relative humidity (RH) in the air where the fish is stored. When the RH is higher than aw in fully salted muscle (0.75), the salted fillets absorb water and gain weight, whereas the opposite has been observed when RH is lower than the aw of the salted cod (Doe and others 1982).

Although dried salted cod is salted and dried, different problems (such as less interesting sensory characteristics among others) can arise due to deficient or improper preparation, packing, drying, or salting. The major preparation defects considered in dried salted cod are slits, clots, patches of liver, and treacle. The presence of foreign bodies and the presence of parasites or parasitic infections detectable by the naked eye are also considered preparation defects and presentation defects. The conservation defects considered in dried salted cod may be presence of reddening (pink) (Huss and Valdimarsson 1990; Abel and Consiglieri 1998), dun or brown spots (Huss and Valdimarsson 1990), unpleasant smell, patches of abnormal color, and sour cod (cod looks cooked on the ventral surface) (IPCP-2 1991).

The high salt and low moisture contents characteristic of salted and dried salted products have given rise to the fact that the extreme halophiles are traditionally the bacteria mostly widely studied (Hess 1942b,c; Hess and Gibbons 1942; Freixo and Botelho 1947; Venkataraman and Screenivasam 1954; Tropa and Galamba 1955; Vilhelmsson and others 1996). Thus, reddening (pink) caused by extremely halophilic Archaea (Bjarnason 1986) at too high storage temperatures and brown spots (dun) caused by extremely xerophilic molds (Beatty and Fougère 1957) have been considered the only microbes growing in salt-cured and dried salt-cured cod (Bjørkevoll and others 2003). Since 1950s, a few studies have been carried out on the survival of moderately halophilic and halotolerant bacteria of green salted and dried salted cod and their soaked cod products (Dussault 1962; Ishida and others 1976; Fujii and others 1977; Vilhelmsson and others 1996, 1997; Rodrigues and others 2003; Aas and others 2010). Several Gram-positive bacteria and moderate halophilic Gram-negative rods, in relatively high numbers, have also been isolated from cod in the early stages of salt-curing and from salt-cured and dried salt-cured cod (Vilhelmsson and others 1996, 1997). Vilhelmsson and others (1996) characterized a great number of different (128) moderately halophilic bacteria although they did not identify them. However, Staphylococcus arlettae/xylosus has been identified as the phenotype found in higher numbers in dried salted cod by Vilhelmsson and others (1997). Moreover, Doe and Heruwati (1988) detected Staphylococcus xylosus in spoiled dried salted cod.

Additionally, the increased tolerance to salt and moisture limits, combined with other production abuses, may contribute to an extension of the survival period of halotolerant and moderately halophilic bacteria present in fully cured cod products (Huss and Valdimarsson 1990); some strains may even show growth. Furthermore, some of the industrial abuses may lead to a condition named sliming, characterized by the appearance of a semi-greasy, sticky, glistening layer of yellow-gray or beige color and a sour pungent smell not caused by extreme halophiles (van Klaveren and Legendre 1965). Freixo (1947) studied green salted cod with a low salt content and found many yellow-orange cocci classified as S. pyogenes aureus. The increasing bacterial survival in salted green and dried salted cod product could lead to a greater diversity of microorganisms in the desalted product. This happens because, after rehydration, the conditions for bacterial growth become very favorable, due to high water content (approximately 70% w/w) and low salt concentration (2% to 4% w/w NaCl) in the product (Bjørkevoll and others 2003). As a consequence and during desalting, pathogenic bacteria could grow and threaten the public's health, mainly because these foods can be eaten after a fast cooking or without cooking (Rodrigues and others 2003). So, some new desalted cod products found in the market present microbiological quality problems that are probably related to the desalting method (Pedro and others 2004). For this reason, some work has been carried out on the microbiota of desalted products (Vilhelmsson and others 1996; Pedro and others 2002a; Bjørkevoll and others 2003; Rodrigues and others 2003; Pedro and others 2004; Lorentzen and others 2010a,b, 2011).

Desalting

Due to the unpalatable high salt concentration in the fish muscle (approximately 16% to 20% w/w, Table 3), salt-cured and dried salt-cured cod must be desalted before consumption (Bjørkevoll and others 2003, 2004; Barat and others 2006; Fernández-Segovia and others 2006, 2007; Magnússon and others 2006; Muñoz-Guerrero and others 2010); and therefore, the appearance of the final product (salt content of 2% to 3%) (Pedro and others 2002a; Lorentzen and others 2010a) is not directly related to that of the salted fish (Barat and others 2002). The desalting process is largely traditional and is a time-consuming and tedious process that is usually carried out at home by the final consumer (Barat and others 2004b,c; Bjørkevoll and others 2004; Muñoz-Guerrero and others 2010). Very often the fish is cut into serving size pieces, and then the product is soaked in tap water for at least 24 h at room temperature or under refrigeration (Barat and others 2004c; Andrés and others 2005a) with several changes of water (Martínez-Alvarez 2002; Escriche and others 2003; Barat and others 2004c; Martínez-Alvarez and others 2005b). The process generally takes about 2 d although it depends on the thickness of the fish pieces (Andrés and others 2005a; Martínez-Alvarez and others 2005b).

The process of desalting cod at home needs to be planned and takes time. This fact, together with changes in lifestyle and nutrition habits in society, has increased the demand for “easy or ready to use” products (Barat and others 2004c; Muñoz-Guerrero and others 2010). Nowadays, consumers tend to spend less time on food preparation and prefer more convenient products like ready-to-eat and ready-to-heat foods (Shiu and others 2004). Thus, there is a growing popularity of salt-cured or dried salt-cured rehydrated products due to the fact that these presentations are commercialized almost boneless, with no major parts to be discarded (Bjørkevoll and others 2004), and also ready-to-cook.

So, the increasing consumer demand for easy or ready-to-use products, the need for desalting at home, the alarming drop in cod catches, the attendant rise in retail prices, and the current trend in low sodium diets have decreased the demand for heavy-salted cod (Kurlansky 1999; Skjerdal and others 2002; Gallart-Jornet and others 2003; Barat and others 2004b; Andrés and others 2005a; Martínez-Alvarez and others 2005b).

In order to adapt the cod industry to the new market requirements, it is nowadays appropriate to include the desalting step among the industrial operations (Barat and others 2004b,c; Muñoz-Guerrero and others 2010). Consequently, large-scale industrial production of rehydrated products has become more interesting (Bjørkevoll and others 2004) and at the end of the 20th century, new ready-to-use desalted cod products, with reduced preparation times, have been launched on the market (Martínez-Alvarez and others 2005b). This has been done in several factories, mainly to produce frozen desalted cod (Barat and others 2004b,c). Thus, the products are sold both as frozen, with a relatively long shelf life (several months) (Fernández-Segovia and others 2006), or chilled. However, the shelf life of chilled products is very short, between 1 and 5 d (Pedro and others 2002a) due to the favorable conditions for bacterial growth in this product (Bjørkevoll and others 2003; Fernández-Segovia and others 2003a), as well as to sensory spoilage (Akse and Joensen 1996; Fernández-Segovia and others 2000). This implies the development of undesirable odors and tastes. Therefore, to develop a product of desalted cod suitable to be commercialized under refrigeration for up to 1 month, the use of an additional preservation method is needed (Fernández-Segovia and others 2006).

The procedure of desalting in industrial operations is similar to that performed by consumers at home (Barat and others 2004b; Martínez-Alvarez and others 2005b). Nevertheless, traditional rehydration large scale, using water containers, has shown problems with process efficiency and product quality. The process of desalting is long and handling the large open tubs with water is problematic. Furthermore, there is a high risk of microbial contamination due to the large areas of floor space the tubs occupy (Bjørkevoll and others 2004). In addition, commercial rehydration and distribution often lead to a longer storage period between completed rehydration and consumption than traditional rehydration at home (Lorentzen and others 2010a).

The changes experienced by the cod during the desalting operation are well known; the protein/cod matrix is rehydrated resulting in an improvement of the cod texture (Barat and others 2004a) by decreasing firmness (Martínez-Alvarez and others 2005b) of the muscle obtained by salting, which strongly affects the yield (defined as weight of final desalted cod divided by the weight of initial salted cod), and consequently, increases the industrial economic benefits (Barat and others 2004b,c). The NaCl content decreases to concentrations suitable for human consumption (Hall 1997; Martínez-Alvarez 2002), implying the proteins absorb water and increase WHC, thus contributing to the total cod weight increase (Barat and others 2004a, c). For these reasons, samples become more spongy and softer during desalting (Barat and others 2004c). Thus, the cod desalting process is considered as simultaneous leaching and hydrating processes (Barat and others 2004b). The same authors showed that the mass transfer fluxes that occur during the desalting process are due not only to the diffusion mechanism, but also to the hydrodynamic processes promoted by pressure gradients that occur during the cod matrix leaching and rehydration. Furthermore, the fish rehydration technique could also be considered, in a certain sense, as the opposite process to salting (Barat and others 2004c; Heredia and others 2007).

So, desalting is a solid–liquid extraction operation in which several components are transferred from salted cod to the desalting water (Escriche and others 2003; Barat and others 2004b; Muñoz-Guerrero and others 2010). An analysis of the mass transfer phenomena in the desalting process indicates that the main components transferred were water and NaCl (Na+ and Cl ions) and, in smaller proportion, protein, resulting in protein-containing residual brine. This residual brine is an effluent with dissolved and suspended solids that must be treated before being discharged to the municipal sewage system (Muñoz-Guerrero and others 2010). This effluent has high levels of salt which make it difficult to degrade the organic matter present (Pedro and others 2002b).

The small loss of protein during the rehydration process is due to protein precipitation and aggregation and the short soaking time (Thorarinsdóttir and others 2002; Di Luccia and others 2005). At a high salt concentration, protein solubility decreases because of the solvation competition between salt ions and proteins. The higher solvation capacity of ions reduces the hydrodynamic radius of proteins, and protein–protein interactions become stronger than protein–water interaction. Thus, polar and hydrophobic interaction of proteins increases, favoring their hydrophobicity, aggregation, and precipitation (Duerr and Dyer 1952; Tanford 1970; Cheftel and others 1989; Caflisch and Karplus 1994; Di Luccia and others 2005).

The optimization of cod desalting on an industrial scale involves the analysis of many process variables, such as stirring level, process temperature, the quality of raw material (cod origin/history, feeding, maturity, freshness (Barat and others 2006)), sample size, the fish muscle zone since the thickness and the number of bones can affect the characteristics of the desalted product, the additives used in the desalting water, cod/water ratio, contact time, and water management (Barat and others 2004c). Water hardness can greatly vary depending on the geographical region and may also affect cod desalting, and the different methods used for cod salting can also affect desalting and consequently the final quality (Andrés and others 2005a).

Temperature control during desalting and storage is extremely important, among other reasons, due to the rapidity of microbial growth in cod once it is desalted (Pedro and others 2002a,b). High temperatures increase the speed of desalting, but the product is highly unstable and, as a consequence, temperatures of 0 to 4 °C are preferable (Martínez-Alvarez and others 2005b).

Before desalting salted cod is usually cut into pieces of different size and shape that should be commercially acceptable. It is common to find in the market cod pieces corresponding to loin, fins, tail, and muscle with important mass differences depending on the fish part. This is a vital practical aspect that has to be taken into consideration by the cod industry because the time needed for cod desalting will depend largely on the muscle zone (Andrés and others 2005a). Cod skin does not present any resistance to mass transfer mainly due to its low fat content (0.3%) (Burgess and others 1987; Andrés and others 2005a).

Some experiments (Barat and others 2004b,c, 2006; Andrés and others 2005a) showed that a cod : water mass ratio of 1 : 9 can be considered adequate to carry out the process. However, in the study by Muñoz-Guerrero and others (2010), the residual brine used came from desalting of salted cod with a 1:7 cod:water ratio (w/w). This cod:water mass ratio was also considered adequate to carry out the process (Muñoz-Guerrero and others 2010). The process of cod desalting without water change was studied by Barat and others (2004c). They found that the water weight changes increased significantly from 24 to 130 h of processing, while the NaCl losses were almost constant. The obtained results supported the idea that the cod desalting process without water changes would be the best one from an industrial point of view, since it was the one that gave a commercial product with higher process yields and lower water wastes than the traditional process (which involves several water changes). So, the desalting process without water changes was considered as the simplest way of desalting with obvious economic (most economic) and environmental advantages (Barat and others 2004c).

A possible explanation for the higher weight increase in the desalting process without water changes could be the lesser manipulation of the sample and the high NaCl contents at the end of the desalting experiment, which could be influencing cod WHC and thus the final weight gain (Offer and Trinick 1983; Barat and others 2004c).

Barat and others (2006) considered the influence of raw material of different freshness levels (0, 7, and 12 d stored in ice) on the sensory quality of desalted cod. They found that the major sensory differences were in texture where the freshest raw material tended to lead to a harder and less flaky desalted product, while the less fresh raw material (close to the limits of acceptable freshness quality) resulted in a flakier product (as referred to above), probably due to higher proteolytic degradation of the muscle prior to salting. However, this study did not describe which kind of texture the consumers preferred. Texture and taste have been considered the main parameters for sensory ranking of cooked samples of desalted cod among consumers familiar with traditionally desalted cod (Barat and others 2006).

The use of additives in the desalting water may improve the characteristics of the final product and could be of interest for its preservation (Andrés and others 2005a; Fernández-Segovia and others 2006). Some studies report the use of oxygenated water on desalted cod (Martínez-Alvarez 2002; Martínez-Alvarez and others 2005b, 2008) obtaining an increase of the product's shelf life because hydrogen peroxide added to the desalting solution in small concentrations considerably reduced or delayed the microbial development on the end product. Thus, the use of hydrogen peroxide has been shown to improve microbiological quality (Stout and Carter 1983; McNeillie and Bieser 1993; Juven and Pierson 1996; Himonides and others 1999; Martínez-Alvarez and others 2005b). An increase in firmness and lightness/whitening without affecting either yield or protein functionality to any great extent was also observed with the addition of hydrogen peroxide on desalted cod (Young and others 1980; Martínez-Alvarez and others 2005b). However, and depending on the application conditions of oxygenated water, some disadvantages such as unpleasant changes in the appearance and texture of the muscle tissue or abnormal coloring of the skin (between grayish to brown) have been reported. These problems and the lack of legislation concerning oxygenated water make it hardly an advisable alternative (Fernández-Segovia and others 2006).

The incorporation of additives, authorized by most food regulations for potassium sorbate, sorbic acid, citric acid, and so on, has also been studied for the preservation of cod (Osthold and Leinstner 1983; Shaw and others 1983; Ampola and Keller 1985; Licciardello and others 1986; Fernández-Segovia and others 2003a, 2006, 2007). The use of various concentrations of citric acid and potassium sorbate on the microbial growth on chilled desalted cod has shown that these additives could be a suitable choice for the preservation of this product from a microbiological point of view (Fernández-Segovia and others 2003a, 2006, 2007).

Moreover, the addition of certain salts of weak acids like phosphates or citrates to the desalting water has been found to be an effective aid to the proper binding of water by protein (Hamm 1961; Chang and Regenstein 1997; Thorarinsdóttir and others 2001), which is extremely important in desalting. In the case of cod desalting, the effect of direct addition of salts of weak acids to the soaking water was studied by Martínez-Alvarez and others (2005b) who observed that the use of an alkaline pH (pH 9.5) carbonate/bicarbonate buffer solution during the early hours of desalting can be extremely useful for industrial purposes in that it positively affects yield and juiciness of the product and enhances the functional quality of the muscle protein. However, the final product has only a limited shelf life due to microbial growth.

NaCl affects the WHC of proteins, and for this reason, cod desalted with a NaCl solution will improve process yield and will help to control the final salt content in cod pieces (Andrés and others 2005a).

Measurement of the NaCl concentration in the desalting water has been proposed as a possible way to estimate the NaCl concentration in the cod liquid phase during desalting and at equilibrium (Barat and others 2006).

Recently, Aliňo and others (2011) tried to obtain ready-to-eat low-sodium desalted cod with improved quality and shelf life. They proposed an alternative methodology for sodium replacement in the cod that consists in substituting NaCl by KCl not during salting but during the desalting process. The obtained results showed that it is possible to obtain low-sodium desalted cod, safe under refrigerated storage conditions, up to 100% and to 75% NaCl substitution by KCl, in the raw and cooked product, respectively, without adversely affecting the sensory quality of the product. However, the presence of potassium decreased process yield (Aliňo and others 2011).

Vacuum tumbling is considered to be a suitable method when adding additives to cod and similar fish fillets (Esaiassen and others 2004, 2005). It has been reported that this technique results in a more even distribution, better WHC, and less cooking loss (Esaiassen and others 2004). Mechanical tumbling is considered to be sufficient to incorporate ingredients in small-size products. When large pieces of muscle are to be treated, injection is often preferred (Xiong and Kupski 1999).

Mild thermal treatments (microwave and water and steam blanching) have also been applied in studies on desalted cod (Escriche and others 2001; Fernández-Segovia and others 2000, 2003c, 2006, 2007). The results showed an important reduction in microbial growth (Fernández-Segovia and others 2000, 2003b). However, it has been reported that it was necessary to combine blanching with vacuum or modified atmosphere packaging to obtain a chilled product of appropriate microbiological quality (Fernández-Segovia and others 2007). Moreover, the heating process induces alterations in the original texture (softening), color, and other important characteristics, as compared to untreated desalted cod (Fernández-Segovia and others 2003c). Nevertheless, a sensory study made by Escriche and others (2001) revealed that after cooking, there were no differences in the organoleptic characteristics between the thermally treated product and the untreated samples. Thus, mild thermal treatments could be used for extending the shelf life of desalted cod as a means of offering consumers precooked ready-to-use desalted cod. In this way, the thermal treatments would not only preserve the fish, but would also contribute to add value to the desalted cod (Fernández-Segovia and others 2007).

It has been shown that long-term exposure to very high salt concentrations does not eliminate Listeria spp. and that Listeria present in the fish prior to salt-curing can recover and grow in rehydrated salt-cured cod during chilled storage (Lorentzen and others 2010a). Also, it has been demonstrated that Listeria monocytogenes grows well in rehydrated salt-cured cod when it is introduced to the desalting water; the bacteria may reach high levels within a few days (Skjerdal and others 2002; Fernández-Segovia and others 2003b). So, numerous species found in the desalted products were also isolated from the salted and dried salted cod. These bacteria were able to survive in the low water activity conditions of the products and some of these had the ability to produce H2S and/or displayed decarboxylase activity (Rodrigues and others 2003). Desalting could either introduce or recover off-odor and/or ornithine-producing organisms (Pedro and others 2002a). Thus, it is probable that the growth of bacteria during soaking might contribute to the spoilage of desalted cod (Rodrigues and others 2003). Psychrobacter spp. are considered typical spoilage organisms in rehydrated salt-cured cod (Bjørkevoll and others 2003).

Conclusion

Despite the lower availability of cod that has been observed in recent years and a corresponding reduction in the number of landings, cod will never lose its place of prominence and will always be a product with considerable market space. However, in the society in which we live today, it is necessary to make it a more convenient product.

Cod and its products are a very good example of a working area in which industry is the main inducer of research, and where research is mainly driven by commerce. Despite this undeniable fact, some of the research found for this review seems to be focused on very small details, parts of which the fish industry does not seem to be directly interested in. The cod industry is a very strong and financially important one, but, probably due to tradition and instinct of keeping industrial secrets (the best way of inducing added value), it seems difficult to establish a direct relationship between the cod industry and cod research, as it is so evident in the pharmaceutical industry with its research on human health.

Due to the complexity and variability of cod processing techniques, it is difficult to work with cod as the raw material. Working with samples directly obtained from the industry is difficult due to the many variations that are sometimes impossible to control or to avoid. And in laboratories, the real conditions used in industry are difficult to imitate or simulate.

This situation seems to have induced 2 main different kinds of publications: institutional researchers publish articles on processing, quality, or marketing details, and industry investigators study the more interesting and useful processing components, but do not always publish them, as industrial activities and finding incur high costs and cannot be shared with competitors.

One must not forget that strong financial interests in successful countries like Norway and Iceland are the major cod industry movers, so a little action by them can make all the difference in this competitive market to shape and a well-defined strategy for cod product development.

The cod industry is obviously following the modern tendencies of the society which demands acceptable products that are easy to choose, buy, keep, manipulate, cook, eat, and digest with benefits. New products will be, as usual, the result of industry developments and research discoveries, but it seems that better coordination of overall efforts would increase efficacy in this process and be beneficial to everyone.

Acknowledgments

The authors thank the Portuguese Foundation for Science and Technology (FCT) for the funding provided to H. Oliveira through a Ph.D. grant (SFRH/BD/33394/2008).

Ancillary