Atlantic salmon muscle structure

The muscle can be divided into two types, fast muscle or white muscle, and slow-muscle or red muscle. Salmon do not have a distinctive intermediate layer of pink muscle (Altringham, 1999; Johnsen, 2011).The slow muscle have a red color results of high concentration of myoglobin and well developed capillaries and contain high concentration of mitochondria (20-50%),lipid, glycogen, cytochromes (Johnsen, 2011; Johnston, 2001).They function maintaining a sustainable slow swimming (Altringham, 1999). Fast muscle makes up the bulk of the fish, typically 80–100 % of the fish (Johnston, 2001; Lecture).The fast muscle fibres have a rare capillary supply, contain relatively small number of mitochondria and are largely supported by anaerobic metabolic pathways, function fast swimming but is rapidly exhausted (Altringham, 1999; Johnston, 2001). Slow muscle fibers are placed parallel to the longitudinal axis of the fish, and deeper fast muscle take place to the horizontal and median planes(Johnston, 1989; Johnston, 1999). Fish macrostructure is arranged in a series of blocks or myotomes, separated by sheets of collagenous tissue called myocommata (Nielsen, 2001). Individual muscle fibers are relatively short, and they are normally inserted through the short tendon collagen sheets called myosepts, which linked each myotome. Myotomes are tapered and stacked in metametric agreement on both sides of the medium septum(Johnston, 2001).
Flesh texture and factors effecting on

Texture is one of the most important quality parameters of fish for producers, processors, and consumers(Nielsen, 2001).Market research indicates that 75% of buyers of Norwegian salmon rate texture as one of the most important quality parameters (Nielsen, 2001). The main reasons of downgrading of Atlantic salmon is fillet gaping, feebleness of color and softness fillet (Kiessling, Espe, Ruohonen, Mørkøre, 2004).The red color meat of Atlantic salmon is associated with carotenoids deposition(Johnston, 2001; Kiessling, Espe, Ruohonen, Mørkøre, 2004; Nielsen, 2001). Some researchers suggested that color regardless of the content of carotenoids may be affected by the method of slaughter that change color with the slaughter in response to changes in muscle structure, the structural changes associated with reduced post-mortem liquid holding capacitance(Kiessling, Espe, Ruohonen, Mørkøre, 2004).

Distribution of fat, dryness, color, chewiness and flavor are all important quality characteristics(Casas, Martinez, Guillen, Pin, Salmeron, 2006; Kiessling, Espe, Ruohonen, Mørkøre, 2004; Lie, 2001). One of the common method of sensory evaluation texture is ”finger method”(Lie, 2001; Nielsen, 2001). Instrumental measurement of texture is preferable to sensory evaluation, the measurements more accurate, because it eliminated the human factor(Casas, Martinez, Guillen, Pin, Salmeron, 2006).

Many factors affect the texture of fish, they include: age, size, species, nutrition, collagen structure (Lie, 2001; Nielsen, 2001), photoperiod regimes and harvesting season (Hagen, 2007). Postmortem factors also effect on softening muscles, gaping and water-binding capacity, include glycolysis, postmortem pH, rigor mortis and handling stress before slaughter, starvation before slaughtering(Einen, Thomassen, 1998; Kiessling, Espe, Ruohonen, Mørkøre, 2004), the temperature profile during storage(Johnston 1999).

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