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Application of the SAFES (systematic approach of food engineering systems) methodology to salting, drying and desalting of cod
Journal of Food Engineering 83 (2007) 267–276 www.elsevier.com/locate/jfoodeng
Application of the SAFES (systematic approach of food engineering systems) methodology to salting, drying and desalting of cod A. Heredia *, A. Andre´s, N. Betoret, P. Fito Institute of Food Engineering for Development-Department of Food Technology, Polytechnic University of Valencia, Camino de Vera s/n, 46022 Valencia, Spain Available online 23 February 2007
Abstract SAFES methodology was applied to cod process where three matrix of changes were defined corresponding to the three main stages of the process. Due to insufficient experimental data to completely build the matrix, some related hypotheses were necessary. Specifically, this process consisted of: osmotic dehydration in saturated brine, followed by a drying at 15 °C and desalting by immersion in tap water of cod fillets. During all of which, mass transfer (basically salt, water and protein) occurs by diffusion. A migration of water from extra-intracellular liquid to solid matrix and to external fluid took place in the salting and drying stages. On the other hand, an intake of salt occurred during the process and an amount of it precipitated. However, it was observed that there was a tendency for the opposite to occur in the desalting operation. Moreover, important changes related to protein component were taken into account. During salting, some proteins from raw cod (60%) were denatured (made soluble) due to the high ionic forces in the media. Soluble proteins above mentioned suffered a further precipitation during the drying process and a re-solubilization. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: SAFES; Cod; Salting; Drying; Desalting
1. Introduction Salt-cured cod (Gadus morhua L.) is a highly appreciated and traditional product due to its excellent storage stability, sensorial properties (lightness value (L*), texture), and nutritional value (protein content). It is widely consumed in Spain, Portugal and Latin America, although the largest producers are the North Atlantic countries, such as Norway and Iceland (Bjornsson, 2000). These two countries exported over 40,000 tonnes of salted cod to the largest cod consuming countries in 1999, which shows the importance of the product in economic terms (Gallart-Jornet, Rodrı´guez-Barona, Barat, Andre´s, & Fito, 2003).
*
Corresponding author. Tel.: +34 96 387 36 51; fax: +34 96 387 73 69. E-mail address: [email protected] (A. Heredia).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.02.025
The traditional description of food processing is prone to simplification with possible information loss as a consequence. Food is usually considered as a homogenous fluid composed of one or two phases and of two or three components, where the classic phenomena of thermodynamic and kinetic transport are validated. The obtaining of salted, dried and desalted cod has been approached as a process composed of three unitary operations (salting, drying and desalting) where two major components flow through the cod structure: salt and water. In this typical description, no distinction was made between the apportionment of water (free and bonded water or the compartmentalization between liquid phase in cells) in relation to sensorial attributes (juiciness, firmness, colour) and stability of the product (composition of solid matrix and extracellular and intra-cellular liquid phase), the possible denaturation of proteins has not been studied enough (new con-
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figuration and losses) and the aggregation state of salt was not evaluated during the different stages of the process. The new methodologies attempt to describe the real complexity without leaving out information even if some simplifications are necessary in order to model the system. In this way, SAFES (Systematic Approach of Food Engineering Systems) is a useful methodology which attempts to clarify the complex system using the simplest models applicable without loss of relevant information. The main objective of this study was to apply the SAFES methodology as an approach to cod processing with the purpose of explaining the changes in the product’s properties through complex unitary operations with minimum simplifications. 2. Materials and methods 2.1. Materials The manufacturing of cod is an important industrial process based on three main stages (Fig. 1a): osmotic dehydration (OD) carried out using a saturated brine solution at 5 °C during 15 days, followed by air drying (AD) in an chamber at 15 °C with air velocity of 1.2 m/s and an air relative humidity ranging from 50% to 65% until the moisture content of the dried cod reaches 47%. Finally, desalting (DS) using a ratio of cod: water 1:9 (w/w) at 5 °C during 24 h. Salting is one of the oldest treatments used in food preservation and it consists of transporting salt into food structures while water flows out of them. There are several
advantages to brining: higher weight yield (Beraquet, Iaderoza, Jardim, & Lindo, 1983; Bogason, 1987) caused by uptake of water; protection against oxidative rancidity (Wheaton & Lawson, 1985); and faster salting due to a higher rate of salt penetration in the fish muscle (Akse, Gundersen, Lauritzen, & Ofstad, 1993). Nevertheless, this process is not possible to obtain a stable product and a further drying treatment is required, the objective of which is to drastically reduce water. Traditional method consists of solar drying but it is slow and extremely dependent on climatic conditions making development of industrial methods necessary (Ismail & Wootton, 1982). Although in this case aw, it is the same before and after drying, the latter is a very important process from the preservation point of view as the excess of solid salt can be considered as a reservoir in the case of water gaining during storage at an atmosphere of relative humidity higher than 75% (Andre´s, Rodrı´guez-Barona, Barat, & Fito, 2005). The process ends with the direct consumption of either salted cod or previously desalted cod (under tap water). On one hand, unitary operations during the processing of cod have been studied in general terms, taking into account one or two major components: mainly salt and water (Andre´s et al., 2005; Collignan & Raoult-Wack, 1994). On the other hand, literature revealed numerous specific studies in which one component and its interaction with external media was investigated but neither its aggregation state nor its migration in food systems between phases were investigated. The investigation of proteins has been based on the effect of brine composition and pH (Martı´nez-Alva´rez, 2003), changes in myofibrillar proteins during processing of salted cod (Thorarinsdottir, Arason, Bogason, & Kristbergsson, 2002), or on the effects of the hydration process on water–soluble proteins (Luccia et al., 2005). Studies on product quality have emphasized the influence of additives (Esaiassen et al., 2005 and), freshness (Barat et al., 2006) catching methods (Esaiassen et al., 2004) or state of rigor and freezing (Lauritzsen et al., 2004) and on sensory quality and consumer preference. 2.2. Methods
Fig. 1. Diagrams of the salting, drying and desalting of cod: (a) Traditional flux diagram (N = 3) and (b) Diagram of whole process including SAFES nomenclature for each stage (MC = matrix-changes; M = matrix’s product) (N = 3).
SAFES methodology (Fito, LeMaguer, Betoret, & Fito, 2007) allows the analysis of the migrations of components between phases and of their aggregation state (Table 1)
Table 1 Phases, components and states of aggregation in cod system PHASES
j
COMPONENTS
i
STATES OF AGGREGATION
k
Solid Matrix Liquid extracellular Liquid intracellular Soluble solids (solid) Solid fat Liquid fat Whole Food External fluid
1 2 3 4 5 6 0 7
Water Non soluble solids (native) Native soluble solids NaCl (added soluble solids) Fat Whole food
1 2 3 4 5 0
Liquid Adsorbed Rubber Vitreous Crystal
1 2 3 4 5
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into food matrix in order to achieve a better understanding of the internal processes which could be related to the product’s final quality. This procedure integrates the knowledge about the structure, the chemical reactivity, physic-chemical interactions between chemical species present in the food, phase transitions and specially, the relationships between phases and components. It requires the appropriate definition of the components, phases and aggregation state in the food system (Table 1) with the aim of building descriptive-matrices (Mn1.n1) in relation to intermediate products and matrices of changes (MCn.n1) corresponding to each stage of the process. In general, dotted cells in matrices represent the combination of the component, phase and aggregation state which are not possible in a thermodynamic sense. In descriptive-matrices, the total amount of all mass fractions of each component in each phase ðxki;j Þ must equal 1. Matrix of changes (MCn.n1) is the difference between two consecutives descriptive-matrices, the second one called transformed descriptive-matrix (Mn1.n) referred to the same basis of calculus used in the first one (Mn1.n1). In this case, the sum of all elements is equal to the basis of calculus. Mass transfer phenomena, chemical reactions and/or phase transitions taking part at each stage of change were noted with values different from zero in matrix of changes, as zero value in any of the cells of the matrix of changes. As ruled by the IUPAC notation, positive values implied a gain and negative values implied a loss of any component. The phenomena take place during the different stages of the process are specified with arrows in the matrices of changes. A dotted arrow indicates a second order thermodynamic transition whereas a non-dotted one indi-
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cates a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon. 3. Results and discussion Fig. 1b shows the SAFES diagram which consists of three main stages (N = 3), the raw cod (M0.0) was salted (MC1.0) producing an intermediate flow of salted product (M1.1). This product underwent further drying (MC2.1) with the result of dried salted cod flow (M2.2). Alternative desalting was done (MC3.2) giving as a result desalted cod (M3.3). A final matrix of changes of the global process (MC3.0) was obtained to obtain information about the yield of the process. In the cod food-system, phases, components and aggregation states were: solid matrix (SM) which constituted of bonded water and insoluble proteins (native non-soluble solids), liquid phase (free water and soluble components) (such as salt or soluble proteins) divided into: extra-cellular liquid (LE) and intra-cellular liquid (LI) phase, soluble solids phase (SS) composed basically of soluble proteins and soluble solids added (salt) which are precipitated, liquid fat phase (F), whole food (WF) such as the sum of all components and phases and external fluid (EF) which depends on the media where each step takes place. The states of aggregation are liquid (L), adsorbed (A), rubber (R), vitreous (V) and crystalline (k). The process of salting cod was analyzed using experimental data for cod making process obtained from the Rodriguez-Barona PHD Thesis (Rodriguez-Barona, 2003). Nevertheless, due the lack of experimental data to build the matrix completely, some related hypothesis and calculations were made. Table 2 summarizes the mass frac-
Table 2 Composition of the product at each step (g of component i/g total) Components
Raw cod
Salted cod
Dried cod
Desalted cod
Water bond free total Non soluble proteins (rubber)
0.0118b 0.761b 0.809a 0.188a
0.374b 0.218b 0.592a 0.090b
0.481b 0.000b 0.481a 0.115b
0.326b 0.427b 0.754a 0.078b
Soluble proteins adsorbed liquid rubber total
0.000b 0.000b 0.000b 0.000b
0.000b 0.113b 0.000b 0.113b
0.000b 0.000b 0.145b 0.145b
0.000b 0.076b 0.000b 0.076b
Added soluble solids (salt) adsorbed liquid crystal total Fat content aw
0.000a 0.000a 0.000a 0.000a 0.003a 0.98a
0.000?b 0.073b 0.125b 0.198a 0.004b 0.75a
0.000b 0.000b 0.254b 0.254a 0.005b 0.75a
0.000b 0.089b 0.000b 0.089a 0.003b 0.97a
a b
Experimental data from Rodriguez-Barona PHD Thesis (Rodriguez-Barona, 2003). Data calculated from the combination of hypothesis and equations.
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tion of each component in each intermediate product, identifying those values obtained from experimental data or from hypothesis and equations. 3.1. Hypothesis 3.1.1. Raw cod (M0.0) Hypothesis 1: All proteins are insoluble. Hypothesis 2: The entire liquid phase takes place within the cells. Extra-cellular liquid phase is not considered. Hypothesis 3: The amount of minority soluble solids (vitamins, minerals, etc.) is not taken into account in the liquid phase.
3.1.2. Salted cod (M1.1) Hypothesis 1: Solubilization of all proteins takes place; with a loss of soluble proteins as a consequence of this phenomenon. 3.1.3. Dried cod (M2.2) and desalted cod (M3.3) Hypothesis 1: Insoluble proteins remained unaltered. Hypothesis 2: Migration of water only occurs between external fluid and liquid phase. Solids matrix is not modified. Calculus: Fig. 2 recapitulates the equations required to complete the matrices.
Fig. 2. Equations guide for the obtaining of the needed components to build the matrices. where: x22;1 is the mass fraction of proteins adsorbed in SM (g/g); (% NaCl) is g salt/100 g of protein; X 21;1 is bonded water content (g water/g dried matter); X 11;2=3 is free water content (g water/g dried matter); X1,0 is total o water content (g water/g dried matter); aw is water activity; X W 0 is the monolayer value (g water/g solids); C is the sorption energy constant; DM t is net p w NaCl i mass changes; DM t is net proteins mass changes; DM t is net water mass changes; DM t is net salt mass changes; DM t is variation of mass component i; M ot and M o0 is cod weight at the sampling time t and 0, xit and xi0 the mass fraction of component i expressed by g of component i/total g at the sampling time t and 0, respectively; qi is the density of each component (kg/L); xi,j is the mass fraction of each component in each phase (g/g); Vj is the volume of each phase (L/kg).
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3.2. The apportionment of water content Water bonded by proteins and NaCl was calculated applying the model published by Fito et al. (2001), and after, applied for meat-salted products where Eqs. (2) and (3) relates the monolayer value (g water/g solids) and sorption energy constant with the ‘salt content of the food, respectively and Eq. (4) is the BET model. In raw cod, the complementary model of progressive hydration for dried proteins proposed by Chou and Morr (1979) was necessary in order to quantify the monolayer of water (Eq. (1)). The adsorption of water molecules on polar zones stabilizes the native protein structure. Water (6 mg/g of protein), well bonded and difficult to eliminate, constitutes a non-frozen monolayer which cannot produce chemical reactions. Nevertheless, the adsorption of water can form multiple layers which can take part in chemical reactions. Free water was calculated as the difference between whole water content (determined gravimetrically by drying until constant weight was reached at 105 °C (Boeri, Davidovich, & Lupin, 1978) and bond water (5). 3.3. The distribution of salt NaCl present in the product would be located in the liquid phase at a saturation concentration, z14; 2=3 P 0:25. Since the saturated brine concentration is around 25% (w/w), the evaporation of three units of water implies the formation of one unit of NaCl crystals. Salting involves salt intake with a saturation of the liquid phase. During the
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drying process, the saturation of the cod liquid phase would be maintained by the precipitation of the NaCl, thus forming salt crystals, mainly on the cod surface. Eqs. (6) and (7) were used to determine the distribution of salt during the cod process. The salt does not exist in the liquid phase, it can be absorbed by solid matrix and/or crystallized in the soluble solid phase, but this apportionment was not found at bibliography and the amount absorbed by solid matrix was considered zero. 3.4. Total protein amount To calculate the amount of proteins which become soluble it is necessary to establish the mass balance during the salting process. It was assumed that two main fluxes took place: water loss ðDM wt Þ and salt gain ðDM NaCl Þ. But t the global mass balance ðDM ot Þ, revealed the migration of a third component: variation of proteins ðDM ot Þ which can be calculated as a difference following the sequences of Eqs. (8)–(10). All components of equations are known from Rodriguez-Barona PhD Thesis (2003) except for the net change of proteins ðDM pt Þ and mass fraction of proteins after salting or desalting ðxpt Þ, which can be easily obtained. 3.5. Solubility of proteins The gradual increase in salt concentration in cod muscle results in changes in functional properties. The presence of high concentrations of salt in muscle gradually increases
Fig. 3. Descriptive matrix of raw cod (M0.0).
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Fig. 4. Descriptive matrix of salted cod (M1.1).
the water-holding capacity (WHC), obtaining a maximum at an ionic strength of 1 M (5.8% salt) (Offer & Knight, 1988). At high ionic strengths, water-holding capacity decreases, apparently by a salting-out effect due to water binding due to the salt and concurrent dehydration of the protein. Furthermore, the effect of salt concentration is dependent, in turn, on the pH of the medium. At pH values higher than the isoelectric point of the muscle proteins (pH 5), the protein net charge is increased and the muscle swells. The reason is repulsion between protein groups with the same charge and, in this way, the space between the peptide chains is enlarged and therefore more water can penetrate. Together, salt and pH can alter the net charge of the protein molecule, affecting protein functionality to a greater or lesser extent and reducing protein–water and protein–protein interactions (Stefansson & Hultin, 1994). Martı´nez-Alva´rez (2003) PhD Thesis reported a solubilization of 60% of proteins when brine solution at pH of 4.5 is used in OD. 3.6. Volume of each phase (L/Kg) The volume of each phase was not found in the references, but in the phases in which more than one component interacte was calculated as shown Eq. (11). Composition of product at each step was summarized in Table 1 from the hypothesis and experimental data in order to build the descriptive-matrix (Mn1.n1). Matrices of changes (MCn.n1) were obtained as the difference between Mn1.n1 and Mn.n1 which are both referred to the same basis of calculus. Mass balance of whole food and components are necessary with the aim of obtaining this basis of
calculus. In this case, basis of calculus was obtained from experimental data of Rodriguez-Barona PHD Thesis (2003). In salting, basis of calculus was = 0.8659 g salted cod/g of raw cod ðM ot Þ. In drying, it was 0.78 g of dried cod per gram of salted cod, and for desalting it was 1, 47 g of desalted cod per gram of dried cod. The descriptive-matrix of raw cod, Fig. 3 shows the distribution of cod components, mainly water and proteins, in the different phases and their aggregation states in the food system. Most of the water is located in the liquid phase but a small quantity of water interacts with insoluble proteins in solid matrix. After salting, the flow of salted cod (Fig. 4) presents more water bonded in solid matrix as a consequence of salt intake and more quantity of soluble proteins than insoluble ones. Salt is distributed in liquid phase and in soluble solid phase depending of saturation of liquid phase. Dried cod composition can be looked up in Fig. 5. This product has a total loss of water in liquid phase producing the second order phase transition: from soluble proteins in liquid place to rubber proteins in solid phase. Raw (Fig. 3) and final processed cod (already desalted) (Fig. 6) present several differences. Desalted cod presents more absorbed water in solid matrix than raw cod due to the salt content that remains after desalting. Moreover, the nature of the proteins has changed, while raw cod only contains insoluble ones, final product has soluble and insoluble proteins in the same proportion. In the salting process, (Fig. 7), a transfer of water from extra-intracellular liquid to solid matrix and to external fluid by pseudodiffusinal mechanisms (PSD) due to salt gain occurs producing a reduction of weight of 16.2%. Salt
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Fig. 5. Descriptive matrix of dried cod (M2.2).
Fig. 6. Descriptive matrix of final product (M3.3).
remained in solution until reaching saturation and the excess of salt is distributed in SS and SM but their apportionment is not known. Some proteins from raw cod (60%) were solubilized due to the high ionic forces in that environment. At the beginning of the process, the migration of solubilized proteins to
brine solution takes place as consequence of exuded liquids of raw cod. The solubilized proteins are basically myofibrillar proteins which become soluble during the first period when salt concentration is less than 7% (Thorarinsdottir, Arason, Bogason, & Kristbergsson, 2001). This effect involved changes in proteic matrix properties, high NaCl
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Fig. 7. Matrix of changes of salting stage (MC1.0 = M0.1M0.0). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
Fig. 8. Matrix of changes of drying stage (MC2.1 = M1.2M1.1). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
concentration produces the denaturation of proteins (Duerr & Dyer, 1952) affecting their holding-capacity, their isoelectric point and their functionality. The deformation-relaxation phenomena took place during all stages of the process, being the major loss or gain of
volume, in salting with a reduction of 27% and desalting with the regain of 37% of volume. During the drying process (Fig. 8) the greatest loss of weight took place with 21.8%. Moreover, a precipitation of soluble proteins and a loss of water by PSD from the liquid
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Fig. 9. Matrix of changes of desalting stage (MC3.2 = M2.3M2.2). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
Fig. 10. Matrix of changes of Global process (MC3.0 = M0.3M0.0). The dotted arrows indicate a second order thermodynamic transition whereas the non-dotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
phase occurred, producing the saturation of this phase, and therefore forming salt crystals, mainly on the cod surface. In general, in the desalting stage (Fig. 9) the inverse tendency was observed with a 47% increase of cod weight, a 37% increase in volume as well as a re-solubilization of salt and proteins. Finally, it must be noted that the matrix of changes of global process (Fig. 10) which gives only information about
the efficiency, but it masks the sequence of changes that occur during the process. 4. Conclusions After reviewing the references, a lack of information was detected in terms of denaturation and migration of the proteins mainly in salting process, compartmentalisation of the
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liquid phase and composition of solid matrix in the sense of the interaction among components. The SAFES methodology allowed for a better knowledge of internal changes which take place in the process of manufacturing of cod. Quantification of changes taking into account all phases, components and aggregation state was done starting from experimental data. Some suitable hypothesis had to be made and planning of mass balances implemented when information was not available in the references. Acknowledgement The authors of this paper thank the MEC for their financial support. References Akse, L., Gundersen, B., Lauritzen, K., Ofstad, R., & Solberg T., (1993). Saltfisk: saltmodning utproving av analysemetoder misfarget saltfisk, Fiskeriforskning, Tromso¨, Norway, pp. 1–61. Andre´s, A., Rodrı´guez-Barona, S., Barat, J., & Fito, P. (2005). Salted cod manufacturing: influence of salting procedure on process yield and product characteristics. Journal of Food Engineering, 69, 467–471. Barat, J. M., Gallart-Jornet, L., Andre´s, A., Akse, L., Carleho¨g, M., & Skjerdal, O. T. (2006). Influence of cod freshness on the salting, drying and desalting stages. Journal of Food Engineering, 73(1), 9–19. Beraquet, N. J., Iaderoza, M., Jardim, D. C. P., & Lindo, M. K. K. (1983). Salting of mackerel (Scomber japonicus) II. Comparison between brining and mixed salting in relation to quality and salt uptake. Coletaneado Instituto de Tecnologı´a de Alimentos, 13, 175–198. Bjornsson, A., (2000). Morgunbladid, 13. Groundfish Forum. Madrid. Spain. Boeri, R. L., Davidovich, L. A., & Lupin, H. M. (1978). Comparacio´n de me´todos para la determinacio´n de humedad en productos pesqueros. La Alimentacio´n Latinoamericana, 111, 44. Bogason, S. G. (1987). So¨ltun orskafla (in Icelandic). Fiskvinnslan, 4, 39–44. Chou, D. H., & Morr, C. V. (1979). Protein water interactions and functional properties. Journal of the American Oil Chemists Society, 56(1), 53A–53B. Collignan, A., & Raoult-Wack, A. L. (1994). Dewatering and salting of cod by inmersion in concentrated sugar/salt solutions. Lebensm.-Wiss. u. Technology, 27, 259–264. Duerr, J. D., & Dyer, W. J. (1952). Proteins in fish muscle. IV. Denaturation by salt. Journal of the Fisheries Research Board of Canada, 8(5), 325–331. Esaiassen, M., Nilsen, H., Joensen, S., Skjerdal, T., Carleho¨g, M., Eilertsen, G., et al. (2004). Effects of catching methods on quality
changes during storage of cod (Gadus morhua). Lebensmittel-Wissenschaft und-Technologie, 37(6), 643–648. Esaiassen, M., Østli, J., Joensen, S., Prytz, K., Olsen, V., Carleho¨g, M., et al. (2005). Brining of cod fillets: effects of phosphate, salt, glucose, ascorbate and starch on yield, sensory quality and consumers liking. LWT – Food Science and Technology, 38(6), 641–649. Fito, P., Chiralt, A., Sanz, J., Fito, P. J., Betoret, N., & Argu¨elles, A., (2001). Ana´lisis termodina´mico del equilibrio en alimentos estructurados salados. Influencia sobre la estabilidad y la calidad. Congreso Iberoamericano de Ingenierı´a de Alimentos, Valencia (Espan˜a). Fito, P., LeMaguer, M., Betoret, N. & Fito, P.J. (2007). Advanced food process engineering to design real foods and processes: the safes methodology. Journal Food Engineering, in press, doi:10.1016/ j.jfoodeng.2007.02.017. Gallart-Jornet, L., Rodrı´guez-Barona, S., Barat, J. M., Andre´s, A., & Fito, P. (2003). Elaboracio´n y comercializacio´n del bacalao salado. Alimentaria, 348, 87–94. Ismail, N., & Wootton, M. (1982). Fish salting and drying: a review. ASEAN Food Journal, 7(4), 175–183. Lauritzsen, K., Akse, L., Johansen, A., Joensen, S., Sørensen, N. K., & Olsen, R. L. (2004). Physical and quality attributes of salted cod (Gadus morhua L.) as affected by the state of rigor and freezing prior to salting. Food Research International, 37(7), 677–688. Luccia, A., Alviti, G., Lamacchia, C., Faccia, M., Gambacorta, G., Liuzzi, M., et al. (2005). Effects of the hydration process on water– soluble proteins of preserved cod products. Food Chemistry, 93(3), 385–393. Martı´nez-Alva´rez, O., (2003). Influencia del procesado en la composicio´n y funcionalidad de proteica del bacalao seco salado (Gadus morhua). Estudio del producto desalado. PHD Thesis, Instituto del Frı´o-CSIC, Madrid (Espan˜a). Offer, G., & Knight, P. (1988). The structural basis of water holding in meat. In R. Lawrie (Ed.), Developments in meat science (pp. 63–171). London: Elsevier. Rodrı´guez-Barona, S., (2003). Estudio de proceso de salado y desalado de bacalao (Gadus morhua). Evaluacio´n de alternativas en los procesos tradicionales. PHD Thesis. UPV, Spain. Stefansson, G., & Hultin, H. O. (1994). On the solubility of cod muscle proteins in water. Journal of Agricultural and Food Chemistry, 42, 2656–2664. Thorarinsdottir, K. A., Arason, S., Bogason, S. G., & Kristbergsson, K. (2001). Effects of phosphate on yield, quality, and water-holding capacity in the processing of salted cod (Gadus morhua). Journal of Food Science, 66(6), 821–826. Thorarinsdottir, K. A., Arason, S., Bogason, S. G., & Kristbergsson, K. (2002). Changes in myofibrillar proteins during processing of salted cod (Gadus morhua) as determined by electrophoresis and differential scanning calorimetry. Food Chemistry, 77(3), 377–385. Wheaton, F. W., & Lawson, T. B. (1985). Processing aquatic food products. New York: John Wiley & Sons.
Application of the SAFES (systematic approach of food engineering systems) methodology to salting, drying and desalting of cod A. Heredia *, A. Andre´s, N. Betoret, P. Fito Institute of Food Engineering for Development-Department of Food Technology, Polytechnic University of Valencia, Camino de Vera s/n, 46022 Valencia, Spain Available online 23 February 2007
Abstract SAFES methodology was applied to cod process where three matrix of changes were defined corresponding to the three main stages of the process. Due to insufficient experimental data to completely build the matrix, some related hypotheses were necessary. Specifically, this process consisted of: osmotic dehydration in saturated brine, followed by a drying at 15 °C and desalting by immersion in tap water of cod fillets. During all of which, mass transfer (basically salt, water and protein) occurs by diffusion. A migration of water from extra-intracellular liquid to solid matrix and to external fluid took place in the salting and drying stages. On the other hand, an intake of salt occurred during the process and an amount of it precipitated. However, it was observed that there was a tendency for the opposite to occur in the desalting operation. Moreover, important changes related to protein component were taken into account. During salting, some proteins from raw cod (60%) were denatured (made soluble) due to the high ionic forces in the media. Soluble proteins above mentioned suffered a further precipitation during the drying process and a re-solubilization. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: SAFES; Cod; Salting; Drying; Desalting
1. Introduction Salt-cured cod (Gadus morhua L.) is a highly appreciated and traditional product due to its excellent storage stability, sensorial properties (lightness value (L*), texture), and nutritional value (protein content). It is widely consumed in Spain, Portugal and Latin America, although the largest producers are the North Atlantic countries, such as Norway and Iceland (Bjornsson, 2000). These two countries exported over 40,000 tonnes of salted cod to the largest cod consuming countries in 1999, which shows the importance of the product in economic terms (Gallart-Jornet, Rodrı´guez-Barona, Barat, Andre´s, & Fito, 2003).
*
Corresponding author. Tel.: +34 96 387 36 51; fax: +34 96 387 73 69. E-mail address: [email protected] (A. Heredia).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.02.025
The traditional description of food processing is prone to simplification with possible information loss as a consequence. Food is usually considered as a homogenous fluid composed of one or two phases and of two or three components, where the classic phenomena of thermodynamic and kinetic transport are validated. The obtaining of salted, dried and desalted cod has been approached as a process composed of three unitary operations (salting, drying and desalting) where two major components flow through the cod structure: salt and water. In this typical description, no distinction was made between the apportionment of water (free and bonded water or the compartmentalization between liquid phase in cells) in relation to sensorial attributes (juiciness, firmness, colour) and stability of the product (composition of solid matrix and extracellular and intra-cellular liquid phase), the possible denaturation of proteins has not been studied enough (new con-
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figuration and losses) and the aggregation state of salt was not evaluated during the different stages of the process. The new methodologies attempt to describe the real complexity without leaving out information even if some simplifications are necessary in order to model the system. In this way, SAFES (Systematic Approach of Food Engineering Systems) is a useful methodology which attempts to clarify the complex system using the simplest models applicable without loss of relevant information. The main objective of this study was to apply the SAFES methodology as an approach to cod processing with the purpose of explaining the changes in the product’s properties through complex unitary operations with minimum simplifications. 2. Materials and methods 2.1. Materials The manufacturing of cod is an important industrial process based on three main stages (Fig. 1a): osmotic dehydration (OD) carried out using a saturated brine solution at 5 °C during 15 days, followed by air drying (AD) in an chamber at 15 °C with air velocity of 1.2 m/s and an air relative humidity ranging from 50% to 65% until the moisture content of the dried cod reaches 47%. Finally, desalting (DS) using a ratio of cod: water 1:9 (w/w) at 5 °C during 24 h. Salting is one of the oldest treatments used in food preservation and it consists of transporting salt into food structures while water flows out of them. There are several
advantages to brining: higher weight yield (Beraquet, Iaderoza, Jardim, & Lindo, 1983; Bogason, 1987) caused by uptake of water; protection against oxidative rancidity (Wheaton & Lawson, 1985); and faster salting due to a higher rate of salt penetration in the fish muscle (Akse, Gundersen, Lauritzen, & Ofstad, 1993). Nevertheless, this process is not possible to obtain a stable product and a further drying treatment is required, the objective of which is to drastically reduce water. Traditional method consists of solar drying but it is slow and extremely dependent on climatic conditions making development of industrial methods necessary (Ismail & Wootton, 1982). Although in this case aw, it is the same before and after drying, the latter is a very important process from the preservation point of view as the excess of solid salt can be considered as a reservoir in the case of water gaining during storage at an atmosphere of relative humidity higher than 75% (Andre´s, Rodrı´guez-Barona, Barat, & Fito, 2005). The process ends with the direct consumption of either salted cod or previously desalted cod (under tap water). On one hand, unitary operations during the processing of cod have been studied in general terms, taking into account one or two major components: mainly salt and water (Andre´s et al., 2005; Collignan & Raoult-Wack, 1994). On the other hand, literature revealed numerous specific studies in which one component and its interaction with external media was investigated but neither its aggregation state nor its migration in food systems between phases were investigated. The investigation of proteins has been based on the effect of brine composition and pH (Martı´nez-Alva´rez, 2003), changes in myofibrillar proteins during processing of salted cod (Thorarinsdottir, Arason, Bogason, & Kristbergsson, 2002), or on the effects of the hydration process on water–soluble proteins (Luccia et al., 2005). Studies on product quality have emphasized the influence of additives (Esaiassen et al., 2005 and), freshness (Barat et al., 2006) catching methods (Esaiassen et al., 2004) or state of rigor and freezing (Lauritzsen et al., 2004) and on sensory quality and consumer preference. 2.2. Methods
Fig. 1. Diagrams of the salting, drying and desalting of cod: (a) Traditional flux diagram (N = 3) and (b) Diagram of whole process including SAFES nomenclature for each stage (MC = matrix-changes; M = matrix’s product) (N = 3).
SAFES methodology (Fito, LeMaguer, Betoret, & Fito, 2007) allows the analysis of the migrations of components between phases and of their aggregation state (Table 1)
Table 1 Phases, components and states of aggregation in cod system PHASES
j
COMPONENTS
i
STATES OF AGGREGATION
k
Solid Matrix Liquid extracellular Liquid intracellular Soluble solids (solid) Solid fat Liquid fat Whole Food External fluid
1 2 3 4 5 6 0 7
Water Non soluble solids (native) Native soluble solids NaCl (added soluble solids) Fat Whole food
1 2 3 4 5 0
Liquid Adsorbed Rubber Vitreous Crystal
1 2 3 4 5
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into food matrix in order to achieve a better understanding of the internal processes which could be related to the product’s final quality. This procedure integrates the knowledge about the structure, the chemical reactivity, physic-chemical interactions between chemical species present in the food, phase transitions and specially, the relationships between phases and components. It requires the appropriate definition of the components, phases and aggregation state in the food system (Table 1) with the aim of building descriptive-matrices (Mn1.n1) in relation to intermediate products and matrices of changes (MCn.n1) corresponding to each stage of the process. In general, dotted cells in matrices represent the combination of the component, phase and aggregation state which are not possible in a thermodynamic sense. In descriptive-matrices, the total amount of all mass fractions of each component in each phase ðxki;j Þ must equal 1. Matrix of changes (MCn.n1) is the difference between two consecutives descriptive-matrices, the second one called transformed descriptive-matrix (Mn1.n) referred to the same basis of calculus used in the first one (Mn1.n1). In this case, the sum of all elements is equal to the basis of calculus. Mass transfer phenomena, chemical reactions and/or phase transitions taking part at each stage of change were noted with values different from zero in matrix of changes, as zero value in any of the cells of the matrix of changes. As ruled by the IUPAC notation, positive values implied a gain and negative values implied a loss of any component. The phenomena take place during the different stages of the process are specified with arrows in the matrices of changes. A dotted arrow indicates a second order thermodynamic transition whereas a non-dotted one indi-
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cates a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon. 3. Results and discussion Fig. 1b shows the SAFES diagram which consists of three main stages (N = 3), the raw cod (M0.0) was salted (MC1.0) producing an intermediate flow of salted product (M1.1). This product underwent further drying (MC2.1) with the result of dried salted cod flow (M2.2). Alternative desalting was done (MC3.2) giving as a result desalted cod (M3.3). A final matrix of changes of the global process (MC3.0) was obtained to obtain information about the yield of the process. In the cod food-system, phases, components and aggregation states were: solid matrix (SM) which constituted of bonded water and insoluble proteins (native non-soluble solids), liquid phase (free water and soluble components) (such as salt or soluble proteins) divided into: extra-cellular liquid (LE) and intra-cellular liquid (LI) phase, soluble solids phase (SS) composed basically of soluble proteins and soluble solids added (salt) which are precipitated, liquid fat phase (F), whole food (WF) such as the sum of all components and phases and external fluid (EF) which depends on the media where each step takes place. The states of aggregation are liquid (L), adsorbed (A), rubber (R), vitreous (V) and crystalline (k). The process of salting cod was analyzed using experimental data for cod making process obtained from the Rodriguez-Barona PHD Thesis (Rodriguez-Barona, 2003). Nevertheless, due the lack of experimental data to build the matrix completely, some related hypothesis and calculations were made. Table 2 summarizes the mass frac-
Table 2 Composition of the product at each step (g of component i/g total) Components
Raw cod
Salted cod
Dried cod
Desalted cod
Water bond free total Non soluble proteins (rubber)
0.0118b 0.761b 0.809a 0.188a
0.374b 0.218b 0.592a 0.090b
0.481b 0.000b 0.481a 0.115b
0.326b 0.427b 0.754a 0.078b
Soluble proteins adsorbed liquid rubber total
0.000b 0.000b 0.000b 0.000b
0.000b 0.113b 0.000b 0.113b
0.000b 0.000b 0.145b 0.145b
0.000b 0.076b 0.000b 0.076b
Added soluble solids (salt) adsorbed liquid crystal total Fat content aw
0.000a 0.000a 0.000a 0.000a 0.003a 0.98a
0.000?b 0.073b 0.125b 0.198a 0.004b 0.75a
0.000b 0.000b 0.254b 0.254a 0.005b 0.75a
0.000b 0.089b 0.000b 0.089a 0.003b 0.97a
a b
Experimental data from Rodriguez-Barona PHD Thesis (Rodriguez-Barona, 2003). Data calculated from the combination of hypothesis and equations.
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tion of each component in each intermediate product, identifying those values obtained from experimental data or from hypothesis and equations. 3.1. Hypothesis 3.1.1. Raw cod (M0.0) Hypothesis 1: All proteins are insoluble. Hypothesis 2: The entire liquid phase takes place within the cells. Extra-cellular liquid phase is not considered. Hypothesis 3: The amount of minority soluble solids (vitamins, minerals, etc.) is not taken into account in the liquid phase.
3.1.2. Salted cod (M1.1) Hypothesis 1: Solubilization of all proteins takes place; with a loss of soluble proteins as a consequence of this phenomenon. 3.1.3. Dried cod (M2.2) and desalted cod (M3.3) Hypothesis 1: Insoluble proteins remained unaltered. Hypothesis 2: Migration of water only occurs between external fluid and liquid phase. Solids matrix is not modified. Calculus: Fig. 2 recapitulates the equations required to complete the matrices.
Fig. 2. Equations guide for the obtaining of the needed components to build the matrices. where: x22;1 is the mass fraction of proteins adsorbed in SM (g/g); (% NaCl) is g salt/100 g of protein; X 21;1 is bonded water content (g water/g dried matter); X 11;2=3 is free water content (g water/g dried matter); X1,0 is total o water content (g water/g dried matter); aw is water activity; X W 0 is the monolayer value (g water/g solids); C is the sorption energy constant; DM t is net p w NaCl i mass changes; DM t is net proteins mass changes; DM t is net water mass changes; DM t is net salt mass changes; DM t is variation of mass component i; M ot and M o0 is cod weight at the sampling time t and 0, xit and xi0 the mass fraction of component i expressed by g of component i/total g at the sampling time t and 0, respectively; qi is the density of each component (kg/L); xi,j is the mass fraction of each component in each phase (g/g); Vj is the volume of each phase (L/kg).
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3.2. The apportionment of water content Water bonded by proteins and NaCl was calculated applying the model published by Fito et al. (2001), and after, applied for meat-salted products where Eqs. (2) and (3) relates the monolayer value (g water/g solids) and sorption energy constant with the ‘salt content of the food, respectively and Eq. (4) is the BET model. In raw cod, the complementary model of progressive hydration for dried proteins proposed by Chou and Morr (1979) was necessary in order to quantify the monolayer of water (Eq. (1)). The adsorption of water molecules on polar zones stabilizes the native protein structure. Water (6 mg/g of protein), well bonded and difficult to eliminate, constitutes a non-frozen monolayer which cannot produce chemical reactions. Nevertheless, the adsorption of water can form multiple layers which can take part in chemical reactions. Free water was calculated as the difference between whole water content (determined gravimetrically by drying until constant weight was reached at 105 °C (Boeri, Davidovich, & Lupin, 1978) and bond water (5). 3.3. The distribution of salt NaCl present in the product would be located in the liquid phase at a saturation concentration, z14; 2=3 P 0:25. Since the saturated brine concentration is around 25% (w/w), the evaporation of three units of water implies the formation of one unit of NaCl crystals. Salting involves salt intake with a saturation of the liquid phase. During the
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drying process, the saturation of the cod liquid phase would be maintained by the precipitation of the NaCl, thus forming salt crystals, mainly on the cod surface. Eqs. (6) and (7) were used to determine the distribution of salt during the cod process. The salt does not exist in the liquid phase, it can be absorbed by solid matrix and/or crystallized in the soluble solid phase, but this apportionment was not found at bibliography and the amount absorbed by solid matrix was considered zero. 3.4. Total protein amount To calculate the amount of proteins which become soluble it is necessary to establish the mass balance during the salting process. It was assumed that two main fluxes took place: water loss ðDM wt Þ and salt gain ðDM NaCl Þ. But t the global mass balance ðDM ot Þ, revealed the migration of a third component: variation of proteins ðDM ot Þ which can be calculated as a difference following the sequences of Eqs. (8)–(10). All components of equations are known from Rodriguez-Barona PhD Thesis (2003) except for the net change of proteins ðDM pt Þ and mass fraction of proteins after salting or desalting ðxpt Þ, which can be easily obtained. 3.5. Solubility of proteins The gradual increase in salt concentration in cod muscle results in changes in functional properties. The presence of high concentrations of salt in muscle gradually increases
Fig. 3. Descriptive matrix of raw cod (M0.0).
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Fig. 4. Descriptive matrix of salted cod (M1.1).
the water-holding capacity (WHC), obtaining a maximum at an ionic strength of 1 M (5.8% salt) (Offer & Knight, 1988). At high ionic strengths, water-holding capacity decreases, apparently by a salting-out effect due to water binding due to the salt and concurrent dehydration of the protein. Furthermore, the effect of salt concentration is dependent, in turn, on the pH of the medium. At pH values higher than the isoelectric point of the muscle proteins (pH 5), the protein net charge is increased and the muscle swells. The reason is repulsion between protein groups with the same charge and, in this way, the space between the peptide chains is enlarged and therefore more water can penetrate. Together, salt and pH can alter the net charge of the protein molecule, affecting protein functionality to a greater or lesser extent and reducing protein–water and protein–protein interactions (Stefansson & Hultin, 1994). Martı´nez-Alva´rez (2003) PhD Thesis reported a solubilization of 60% of proteins when brine solution at pH of 4.5 is used in OD. 3.6. Volume of each phase (L/Kg) The volume of each phase was not found in the references, but in the phases in which more than one component interacte was calculated as shown Eq. (11). Composition of product at each step was summarized in Table 1 from the hypothesis and experimental data in order to build the descriptive-matrix (Mn1.n1). Matrices of changes (MCn.n1) were obtained as the difference between Mn1.n1 and Mn.n1 which are both referred to the same basis of calculus. Mass balance of whole food and components are necessary with the aim of obtaining this basis of
calculus. In this case, basis of calculus was obtained from experimental data of Rodriguez-Barona PHD Thesis (2003). In salting, basis of calculus was = 0.8659 g salted cod/g of raw cod ðM ot Þ. In drying, it was 0.78 g of dried cod per gram of salted cod, and for desalting it was 1, 47 g of desalted cod per gram of dried cod. The descriptive-matrix of raw cod, Fig. 3 shows the distribution of cod components, mainly water and proteins, in the different phases and their aggregation states in the food system. Most of the water is located in the liquid phase but a small quantity of water interacts with insoluble proteins in solid matrix. After salting, the flow of salted cod (Fig. 4) presents more water bonded in solid matrix as a consequence of salt intake and more quantity of soluble proteins than insoluble ones. Salt is distributed in liquid phase and in soluble solid phase depending of saturation of liquid phase. Dried cod composition can be looked up in Fig. 5. This product has a total loss of water in liquid phase producing the second order phase transition: from soluble proteins in liquid place to rubber proteins in solid phase. Raw (Fig. 3) and final processed cod (already desalted) (Fig. 6) present several differences. Desalted cod presents more absorbed water in solid matrix than raw cod due to the salt content that remains after desalting. Moreover, the nature of the proteins has changed, while raw cod only contains insoluble ones, final product has soluble and insoluble proteins in the same proportion. In the salting process, (Fig. 7), a transfer of water from extra-intracellular liquid to solid matrix and to external fluid by pseudodiffusinal mechanisms (PSD) due to salt gain occurs producing a reduction of weight of 16.2%. Salt
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Fig. 5. Descriptive matrix of dried cod (M2.2).
Fig. 6. Descriptive matrix of final product (M3.3).
remained in solution until reaching saturation and the excess of salt is distributed in SS and SM but their apportionment is not known. Some proteins from raw cod (60%) were solubilized due to the high ionic forces in that environment. At the beginning of the process, the migration of solubilized proteins to
brine solution takes place as consequence of exuded liquids of raw cod. The solubilized proteins are basically myofibrillar proteins which become soluble during the first period when salt concentration is less than 7% (Thorarinsdottir, Arason, Bogason, & Kristbergsson, 2001). This effect involved changes in proteic matrix properties, high NaCl
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Fig. 7. Matrix of changes of salting stage (MC1.0 = M0.1M0.0). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
Fig. 8. Matrix of changes of drying stage (MC2.1 = M1.2M1.1). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
concentration produces the denaturation of proteins (Duerr & Dyer, 1952) affecting their holding-capacity, their isoelectric point and their functionality. The deformation-relaxation phenomena took place during all stages of the process, being the major loss or gain of
volume, in salting with a reduction of 27% and desalting with the regain of 37% of volume. During the drying process (Fig. 8) the greatest loss of weight took place with 21.8%. Moreover, a precipitation of soluble proteins and a loss of water by PSD from the liquid
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Fig. 9. Matrix of changes of desalting stage (MC3.2 = M2.3M2.2). The dotted arrows indicate a second order thermodynamic transition whereas the nondotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
Fig. 10. Matrix of changes of Global process (MC3.0 = M0.3M0.0). The dotted arrows indicate a second order thermodynamic transition whereas the non-dotted arrows indicate a first order thermodynamic transition; the dotted circle means the deformation-relaxation phenomenon (DRP).
phase occurred, producing the saturation of this phase, and therefore forming salt crystals, mainly on the cod surface. In general, in the desalting stage (Fig. 9) the inverse tendency was observed with a 47% increase of cod weight, a 37% increase in volume as well as a re-solubilization of salt and proteins. Finally, it must be noted that the matrix of changes of global process (Fig. 10) which gives only information about
the efficiency, but it masks the sequence of changes that occur during the process. 4. Conclusions After reviewing the references, a lack of information was detected in terms of denaturation and migration of the proteins mainly in salting process, compartmentalisation of the
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liquid phase and composition of solid matrix in the sense of the interaction among components. The SAFES methodology allowed for a better knowledge of internal changes which take place in the process of manufacturing of cod. Quantification of changes taking into account all phases, components and aggregation state was done starting from experimental data. Some suitable hypothesis had to be made and planning of mass balances implemented when information was not available in the references. Acknowledgement The authors of this paper thank the MEC for their financial support. References Akse, L., Gundersen, B., Lauritzen, K., Ofstad, R., & Solberg T., (1993). Saltfisk: saltmodning utproving av analysemetoder misfarget saltfisk, Fiskeriforskning, Tromso¨, Norway, pp. 1–61. Andre´s, A., Rodrı´guez-Barona, S., Barat, J., & Fito, P. (2005). Salted cod manufacturing: influence of salting procedure on process yield and product characteristics. Journal of Food Engineering, 69, 467–471. Barat, J. M., Gallart-Jornet, L., Andre´s, A., Akse, L., Carleho¨g, M., & Skjerdal, O. T. (2006). Influence of cod freshness on the salting, drying and desalting stages. Journal of Food Engineering, 73(1), 9–19. Beraquet, N. J., Iaderoza, M., Jardim, D. C. P., & Lindo, M. K. K. (1983). Salting of mackerel (Scomber japonicus) II. Comparison between brining and mixed salting in relation to quality and salt uptake. Coletaneado Instituto de Tecnologı´a de Alimentos, 13, 175–198. Bjornsson, A., (2000). Morgunbladid, 13. Groundfish Forum. Madrid. Spain. Boeri, R. L., Davidovich, L. A., & Lupin, H. M. (1978). Comparacio´n de me´todos para la determinacio´n de humedad en productos pesqueros. La Alimentacio´n Latinoamericana, 111, 44. Bogason, S. G. (1987). So¨ltun orskafla (in Icelandic). Fiskvinnslan, 4, 39–44. Chou, D. H., & Morr, C. V. (1979). Protein water interactions and functional properties. Journal of the American Oil Chemists Society, 56(1), 53A–53B. Collignan, A., & Raoult-Wack, A. L. (1994). Dewatering and salting of cod by inmersion in concentrated sugar/salt solutions. Lebensm.-Wiss. u. Technology, 27, 259–264. Duerr, J. D., & Dyer, W. J. (1952). Proteins in fish muscle. IV. Denaturation by salt. Journal of the Fisheries Research Board of Canada, 8(5), 325–331. Esaiassen, M., Nilsen, H., Joensen, S., Skjerdal, T., Carleho¨g, M., Eilertsen, G., et al. (2004). Effects of catching methods on quality
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