A Nutrição de Peixe Tradução

1        Evaluation of Quality in Cultured Fish

HEISUKE NAKAGAWA

Graduate School of Biosphere Science, Hiroshima University, Higashi-hiroshima 739-8528, Japan

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Mass production techniques in fish culture have been well established over  the last two decades with regard to elevating growth and feed efficiency. However, very little work has been done on fish quality,  which  might influence market value. Criteria used to evaluate the quality of cultured fish would include: meat quality, durability of freshness, colour (meat and skin), metabolism, disease resistance, stress response, and physical activity besides growth and feed utilization. Factors concerning growth performance are essential and should not be disregarded in relation to evaluating quality of cultured fish. The fish quality, including carcass quality and physiological condition, should be evaluated in fish with maximum growth performance.

The feeding activity, which is influenced by dietary  composition,  rearing density, rearing environment, stressful conditions, disease, etc., influences growth and feed efficiency. At present, fish culturists may not take improvement of fish quality seriously because in the current system high quality fish does not always equate to high market value. However, internationalization of markets for marine products may demand higher product quality in the near future. This chapter summarizes possible  indices of quality control in cultured fish relative to feed supplements. A lot of feed supplements, which are not essential nutrients, are used in fish culture. Although an adequate use of some may give positive effects on harvested products, excessive use of some supplements may give negative effects. Such effects of various supplements on cultured fish will be covered in Chapters 3 to 12.

1. Evaluation of Carcass Quality and Taste

The criteria for sensory evaluation of fish meat vary according to fish species, nationality, cooking and season. As primary factors dominating

© CAB International 2007. Dietary Supplements for the Health and Quality of

Cultured Fish (eds H. Nakagawa, M. Sato and D.M. Gatlin III)        1

carcass quality, quantitative and qualitative properties of the reserved lipid and protein composition cannot be disregarded. A comparison of them between wild and cultured fish show that cultured fish are generally characterized by high lipid content and tenderness compared to wild fish. Protein composition influences hardness of meat and maintenance of freshness.

Sensory evaluation of the feed supplement Spirulina was determined by free amino acids (Liao et al., 1990; Watanabe et al., 1993) and proximate composition (Nandeesha et al., 1998). In yellowtail fed a diet supplemented with Chlorella-extract, sensory evaluation such as taste and texture improved (Nakagawa et al., 1985).

1. Muscle protein

Muscle protein is an important factor affecting physical characteristics such as carcass quality. Low strength of cultured fish meat is due to a weak Z-  line in muscle fibres, compared to wild fish. The strength of the Z-line may be improved by exercise during the rearing period. Strength of muscle protein can be evaluated by the methods of Lavéty and Love (1972) and Ando et al. (1991, 1992). Ando et al. (1992) pointed out muscle firmness related to density and the arrangement of collagen fibrils in the pericellular connective tissue. While the stromal fraction in meat, which contains collagen, contributes to stiffness in raw fish meat, heating during cooking tenderizes the meat by solubilizing the collagen. Vitamin C is well known   as a cofactor in hydroxylation of proline to hydroxyproline in collagen synthesis. As observed in sensory evaluation, the feeding of Spirulina and tea catechin elevated the stromal fraction, meat firmness and taste of red sea bream Pagrus major (Mustafa et al., 1994). These results agreed with studies of Liao et al. (1990) carried out in striped jack Pseudocaranx dentex. As polyphenols have synergistic effects on vitamin C metabolism (Bai and Gatlin, 1992; Nakagawa et al., 2000), supplementation of polyphenols might improve carcass quality (Tanimoto et al., 1993; Nakagawa et al., 2000).

Protein sythesis activity can be assessed by a combination of

RNA/DNA ratio, as a parameter of protein synthesis, and acid protease activity, as that of protein degradation (Mustafa et al., 1995). Yone et al. (1986) evaluated the effects of algal supplements on dietary protein and carbohydrate, using levels of plasma free amino-N and blood sugar.

1. Lipids and lipid metabolism

Surplus energy is reserved in the muscle, liver, bone and adipose tissue as lipid reserves, mainly triglycerides. The site of lipid deposit is dependent on fish species. Excessive lipid accumulation in non-edible parts does not benefit consumers and is a great loss of feed energy. Lipid content, lipid class composition and fatty acids also may influence taste and tenderness of fish.

Triglycerides are elemental and important factors in the taste of meat, but relationships between meat quality and fatty acid composition is not clear.

Lipid content in the muscle and adipocytes is generally higher in cultured fish than wild fish because of overfeeding and low physical movement. Muscle lipid content highly influences taste, but high lipid or low lipid content is not always good. Accordingly, controlling lipid content should be one important factor in quality control, but depression of lipid levels should not be accompanied by depression of growth and feed efficiency. The lipid content is dependent upon the balance between lipogenesis and lipolysis. The reserved lipids derived from the surplus energy should be preferentially utilized as an energy source. However, reserved lipids accumulated under nutrient imbalance are consequently deposited as celoid and lipofustin, which cannot be used for energy. The utility of the reserved lipids can be assessed by short-term starvation. Constitutional change by starvation reveals lipolysis activity; the reserved lipids should be primarily mobilized to energy prior to conversion of  muscle protein in response to energy requirements. If energy is supplied from muscle protein in place of reserved lipids during food shortage, the consumption of muscle protein causes much body weight loss. Also this reduction of muscle protein can result in depression of physical activity and high mortality. Lipolysis activity of the adipocytes can be evaluated by in vitro analysis (Nematipour et al., 1990). Lipoperoxide has been analysed to evaluate the effect of supplements (Bai and Gatlin, 1992; Nakagawa et al., 1997; Ji et al., 2003b).

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