Chemicalcomposition of fish The main constituents of freshfish are water (65-85 %), protein (15-24 %), fat (0.1-22 %), carbohydrate (1-3%) and inorganic substances (0.8-2 %). The amount of fish meat varies accordingto the species, age, part of body, pre or post-spawning season and the feedingconditions (Suzuki, 1981). Protein is a major composition of fish muscle withthe range of 15-20 % (wet weight).
Protein compositions of fish vary, dependingupon muscle type, feeding period, and spawning, etc. Hashimoto, et al. (1979) determined the proteincompositions of the dark and the white muscle from sardine (Sardinops melanosticta).2.2.
Muscle protein 2.2.1 Myofibrillarprotein Myofibrillar proteins are the majorproteins in fish muscle. Normally, these proteins account for 65-75 % of totalprotein in muscle, compared with 52-56 % in mammals (Mackie, 1994). Theseproteins can be extracted from the muscle tissue with neutral salt solutions ofionic strength above 0.15, usually ranging from 0.30 to 0.
70. The myofibrillarproteins are related with the water hoding capacity and other func tional propertiesof proteins such as gelation etc. (McCormick, 1994). Myofibrillar proteins are highly reactive in their unfolded state.A gel results when reactive protein surfaces form a 3 dimensional network entrapping water. Figure 17illustrates the 3 dimensional protein network formed during gelation of surimi(Morrissey et al.
, 1995). Contractile proteins, whichare different in size and location in the muscle, are listed in Table 1 (Ashieand Simpson, 1997). Myosin Myosinis a large asymmetric molecule that has a long ?-helical coiled-coil tail andtwo globular heads with an approximate weight of 500 kDa (Hodge et al., 2000). The basic body plan of myosin consists of anN-terminal head or motor domain, a light chain-binding neck domain, and a classconserved, C -terminal tail domain and has been categorized into over twentydifferent classes (Mooseker et al.,2008) . The head or motor domain has acore sequence that is highly conserved in all myosin classes, and it containsthe ATPase active site (Holmes, 2008)Myosin is the major muscle protein that isfound in fish and comprises approximately 55-60% of the myofibrillar proteins(Lanier et al., 2005) .
Skeletal myosincan be broken up into six polypeptide chains, two heavy chains and four light chains. Myosin light chains typically range from 17to 25 kDa. These amino acid chains arenon -covalently attached to the myosin head (Lanier et al., 2005) . Myosin canalso be broken into fragments by proteolysis (Szent -Görgyi, 1953). Table1.Contractile protein in food myosystems.
Protein Relative abundance (%) Size (kDa) Location Myosin 50-60 470 Thick filaments Actin 15-30 43-48 Thin filaments Tropomyosin 5 65-70 Thin filaments Troponins 5 Thin filaments Troponin-C 17-18 Troponin-I 20-24 Troponin-T 37-40 C- protein – 140 Thick filaments ? – Actin – 180-206 Z-disc Z-nin – 300-400 Z-disc Connective / Titin 5 700-1000 Gap filaments Nebulin 5 ~ 600 N2-line ActinActin,which comprises of about 20% of the total myofibrillar protein, is the majorcomponent of the thin filament of the myofibril (Suzuki, 1981). Actin exists intwo forms – globular, G-actin and fibrous, F-actin. G-actin is a monomer of 42KD molecular weight proteins and polymerises to F-actin depending on the ionicstrength of the medium of extraction (Shenouda and Pigott, 1975; Stryer, 1995).Fish actin in contrast to that from bovine sources, does not gel on heating inthe presence of sodium chloride, a property characteristic of myosin.It forms acurd instead of a gel and thus, does not contribute to the elasticity of fishgels (Sano et al.
, 1989a). Sano et. aI., (1989b) further stated that theincrease in elasticity of fish protein gel is proportional to the F-actin/myosinratio and F-actin adds the viscous element to the natural actomyosinsuspension.TropomyosinTropomyosin,the third major component of structural proteins (Mannherz and Good, 1976),plays a role in the regulation of calcium dependent interaction of actin and myosin.In skeletal and cardiac muscles it forms an integral part of the thin filamentof sarcomere and involves in the calcium regulatory system for contraction andrelaxation. Situated in the two grooves of the double stranded structure of filamentousactin, tropomyosin forms a long filament by aggregation of individualmolecules.
Throughout its entire length it interacts with seven actin monomerson each of the two strands of F-actin. It also binds one Mole of troponincomplex. As a result of the binding of Ca2+ to troponin C, tropomyosin mayalter its position in the groove of actin filament and permits interaction of myosinheads and actin monorners (Mannherz and Good, 1976). Tropomyosin does not contribute to the gel-forming ability of fish meat.
Alaska Pollock tropomyosin forms a transparent solution of low viscosity in NaCI even at 9% w/w (Sikorski. 1994a). ropomyosin is an a-helical protein that forms a two-stranded coiled-coil(Huang et al., 2004). It is similar to myosin in amino acid content and it accounts for about 10 -12 % of the total myofibrillar proteins. In skeletal and cardiac muscle, it forms an integral part ofthe thin filament of sarcomere and involved in the calciulll regulatory system for contraction and relaxation (Mannterz and Good. 1976).TroponinTroponin(Suzuki, 1981) is a protein highly essential for the action of tropomyosin inmuscle contraction.
The protein isolated from rabbit has a molecular weight ofabout 80,000 and is formed of three subunits namely troponin-T which combineswith tropomyosin, troponin -1 which inhibits the action of ATPase andtroponin-C which combines with calcium. The troponin from fish sources is alsomore or less similar,of course with species difference. Troponins isolatedfrom the skeletal muscles of carp. tilapia. big eye tuna. mackerel.
and rainbowtrout ‘were able to form a functional complex with carp tropomyosin (Seki andHasegawa. 1978). Troponin C and troponin I from lobster and crayfisll muscleswere found to exist in several isoforms (Nishita and Ojima.
1990).ParamyosinParamyosin(Sikorski, 1994) is characteristic of invertebrates (3% in scallop, 14% insquid, 19% in oyster and 38% in smooth muscle of oyster) and is present in 0.1to 10% of myosin. Theparamyosin is characterised by the presence of large concentrationsof amide, acidic amino acid residues like glutamic acid (20-23%), aspartic acid(12%), and basic amino acid residues like arginine (12%), lysine (9%) and smallamounts of proline (Kantha, et al.
, 1990). The paramyosin rods form the thickcore of myofibrilsof invertebrate muscle, which is covered by a layer of myosinand was reported to have a structural function affecting the orientation of themyosin molecule. Functionally paramyosin affects the rheological properties ofgels prepared from invertebrate meat by adding elasticity and cohesiveness tothe gel than that prepared from fish gels. The action of paramyosin is due tothe inhibition of dissociation of myosin from actin (Sano et. al.
, 1989c). Sarcoplasmicprotein Scopes (1970) described Weber and Meyer’s (1933) experiments onextraction of sarcoplasmic proteins. Henoted that “the early work was carried out with water extracts, which were then dialysed to verylow ionic strength, precipitating the so- called globulins, the albuminremaining in solution.” Bate-Smith (1937) subdivided the albumin portion of thesarcoplasmic proteins by electrophoresis into those that were “slow migratory,” referred to a “myogens,”and those that were “fast migratory”,referred to as “myoalbumins”. Baranowski (1939) crystallized a muscle protein, that he called “myogen A.” Jacob(1947) showed that the myogen componentcould be separated into several fractions by electrophoresis The sarcoplasmic proteins usually refer to the proteins of die sarcoplasmaswell as die components of the extracellular fluid and the sarcoplasm.
The sarcoplasmicproteins comprise about 20-35% of the total muscle proteins and are commonlycalledmyogens (Mackie, 1994: Pearsons and young,1989). Despite their diversity,sarcoplasmicproteins share many common physicochemical properties. Most are of relativelylowmolecular weight high isoelectric pH, and globular or rod-shaped structures. Thesarcoplasmic proteins are extracted by homogenizing the muscle tissue withwater orsolutions of neutral salts of ionic strength below 0.15. Among the sarcoplasmicenzymesinfluencing the quality of fish, the enzymes of die glycolytic pathway and thehydrolyticenzymes of die lysosomes are found to be important (Sikorski et al.,1990a).
The content of sarcoplasmic proteinin fish meat varies with fish species, but is generally higher in pelagic fishsuch as sardine and mackerel and lower in demersal fish (Suzuki, 1981).Benjakul et al. (2004a) reported that the sarcoplasmicfraction from bigeye snapper muscle possessed cross-linking activity towardsmyosin heavy chain (MHC). StromaproteinStroma is the protein, which forms connective tissue, representingapproximately 3% of total proteincontent of fish muscle. It cannot be extractedby water, acid, or alkali solution an d neutralsalt solution of 0.01-0.1 M concentration.
The component of stroma is collagen, elastin or both (Suzuki, 1981). Elastin is very resistant to moist heat andcooking, normally it is a reflection of the different structural arrangementsof muscles in fish, compared to mammals (Mackie,1994).Suitablespecies for surimi productionThe technology of surimi processing was first commercialisedin 1960. By 1965 Alaska Pollock (Thengra calcogrammis) surimi was being producedon factory ships (Suzuki, 1981). Surimi Processing According to him Atka mackeral(Pleurogrammus azonus), horse mackeral (Trachus japonicus) and lizard fish(Shurida undosquamis) were also used for production of surimi. Fish like cod,hake, whiting, Atlantic menhaden, croaker, Chilean mackeral, New Zealand hokiare found to be suitable for producing surimi (Young, 1978). Mac Donald et al.
(1990) demonstrated the use of stabilized mince produced from New Zealand hoki(Mhcruronus novaezelandiae) as surimi source. Pacific whiting (flerluccius productus)has been a good source of surimi production (Chang-Lee et al., 1990).Manyresearchers have investigated the use of fatty fish such as herring (Hastingset al., 1990; Gill et al.
, 1992), mackeral (Shimizu, 1976; Katoh et al., 1989)and sardines (Nonaka et al., 1989; Roussel and Cheftel, 1990; Saeki et al.
,1991) in production of surimi. Species such as Alaska pollock, croaker,jackmackeral, threadfin bream, blue whiting, sardine, lizard fish, eel, barracudaand leather jacket, have been recognised to give-good quality surimi (Lee,1984, 1986; Yean, 1993). The gel-forming ability of dark muscle fish meat has been known to be lower than that of ordinary muscle (Chen, 2002; Ochiai et al., 2001).