Article pp.365-377 du Vol.28 n°4-5 (2008)

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FOCUS : JSMTV

Biological basis of fish quality

F. Lefevre, J. Bugeon

RÉSUMÉ

Déterminisme biologique de la qualité des poissons

Le terme de « qualité », qu’il s’agisse de poisson ou de tout autre produit alimen- taire, intègre de nombreuses caractéristiques, dont l’étude relève de domaines de compétences très variés. L’objectif de cette synthèse est de passer en revue les déterminismes, d’origine biologique, des qualités technologiques, nutrition- nelles et organoleptiques des poissons. La qualité technologique est principale- ment reliée aux premières étapes de transformation (éviscération, filetage, parage) et dépend de la morphologie des poissons, et de la distribution et de la composition des tissus musculaires et adipeux. La qualité nutritionnelle dépend directement de la composition de la chair en macro- et micro-nutriments. Les qualités organoleptiques recouvrent l’ensemble des perceptions sensorielles du produit incluant l’apparence, la couleur, la flaveur et la texture de la chair. Dans un objectif de maîtrise de ces qualités par le contrôle des pratiques piscicoles, nous avons axé cette revue sur les différentes pratiques d’élevage (génétique, nutrition ou environnement d’élevage) qui peuvent modifier les caractéristiques des animaux et affecter ainsi la qualité des poissons et de leur chair.

Mots clés

Poisson, qualité technologique, qualité nutritionnelle, qualités organoleptiques, conditions d'élevage.

SUMMARY

The concept of “quality”, for fish as for other food products, involves many different aspects requiring various scientific competences to be studied. The biological basis of the technological, nutritional and organoleptic fish quality was reviewed. Technological quality is related to primary processing (gutting, filleting, trimming) and depends on fish morphology, distribution and composition of muscle and adipose tissues. Nutritional quality is directly related to flesh macro- and micro-nutriments composition. The organoleptic qualities gather the sensorial perception of the products including appearance, colour, flavour and texture.

In order to manipulate adequately these qualities in fish farms, we focused on the different rearing practices, genetics, nutrition and rearing environment which can modify the fish characteristics and thereby the fish and flesh quality.

Keywords

fish, technological quality, nutritional quality, organoleptic quality, rearing conditions.

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1 – INTRODUCTION

The concept of “quality”, for fish as for other food products, involves many different aspects requiring various scientific competences to be studied. Moreover, the determinism of fish quality depends on all the stages from capture of fish or rearing, to the distribution to consumers, including the potential processing steps. “Quality”

includes sanitary, technological, nutritional and organoleptic criteria. Sanitary quality depends on the absence of parasites, pollutants, toxic molecules or pathogenic micro-organisms for human consumption. It’s the fundamental quality for all food products to be safe for consumption. Sanitary quality relies entirely on fish environment, feed quality and the safety of the processing and marketing. Technological quality depends on fish morphology, distribution and composition of tissues. Nutri- tional quality is directly related to flesh macro- and micro-nutriments composition.

Finally, the organoleptic qualities gather the sensorial perception of the products including appearance, colour, flavour and texture. The determinism of these sensorial criteria may or may not be of biological origin.

The main goal of this review is to describe the biological basis of the technological, nutritional and organoleptic fish quality. We focused also on the different rearing practices which can modify the fish characteristics and thereby the fish and flesh quality, in order to adequately manipulate theses qualities in fish farms.

2 – CARCASS QUALITY

Most of the fish species are sold after primary processing like gutting, filleting, trimming, skinning and sometimes cutting cutlets. From an economical point of view, it’s important to minimize the processing loss, because for example the value of the fillet is three times that of the whole fish. The knowledge of the biological basis of these processing yields is necessary in order to improve it.

Schematic representation of round fish main tissues is presented in figure 1.

Epaxial muscle

Hypaxial muscle Horizontal myosepta

Vertical myosepta White muscle

Red muscle Fin muscle + dorsal adipose tissue

Abdominal cavity:

Gut+adipose tissue Vertebral axis

Ventral adipose tissue Figure 1

Schematic representation of a trout cutlet showing macroscopic organization of muscle, connective and adipose tissues.

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2.1 Gutting

The first stage in the primary processing (except for little fish sold as whole fish) is gutting and consists in discarding all the internal organs (gut, liver, spleen, heart, …) and the mesenteric fat tissue. Gutting is a requisite for good shelf-life of the products, because of possible muscle damage by digestive enzymes and flesh contamination by the digestive flora. The gutting yield or slaughter yield (gutted body weight/live body weight) presents a large variability between fish species and is negatively correlated to the viscera yield (viscera weight/live body weight). The viscera can represent more than 20% of the body weight for cod or rainbow trout or less than 10% for Atlantic salmon or halibut. These discrepancies are related to the adipose tissue deposited by the fish. For example, cod presents a high fat content in the liver (with, as a consequence, a liver weight representing up to 12% of the live weight) whereas rainbow trout stores lipid in the mesenteric tissue. Finally both species present a rather low gutted yield. On the other hand, Atlantic salmon presents a high muscle fat content but less fat in the liver and mesenteric tissue and so a better gutted yield.

For a species like rainbow trout the viscera yield can greatly differ between indi- viduals (variation coefficient 15%) (Kause et al., 2007). This variability is related to the development of the mesenteric fat tissue as shown in sea bass (Haffray et al., 2007). The mesenteric fat tissue presents a positive allometry for immature rainbow trout (Weatherley and Gill 1983). This tissue is an energetic store that fish can use during long periods of starvation or for sexual maturation (Einen et al., 1998; Bugeon et al., 2004). The viscera yield presents good heritability (h²= 0.58 for rainbow trout) (Kause et al., 2007), and so it can be improved by selective breeding.

2.2 Filleting

Following gutting, the filleting consists in setting apart the main skeletal muscle from the carcass (head and skeleton). The filleting yield differs greatly from one species to another (from 35% up to 62% of the live body weight). The filleting yield is not directly related to the overall morphology of the species, for example two round fishes, like cod and Atlantic salmon, present rather low (35-40%) and high (55-60%) filleting yields, respectively (Jobling et al., 1994). On an other hand, fish with very different body morphology like Atlantic salmon and halibut (flat fish) can present similar and high filleting yields (around 60%). For a species like rainbow trout or European catfish the coefficient of variation (CV) of filleting yield is rather low (3.85 and 5%, respectively) compared to the CV for live body weight (23% and 18%, respectively) (Haffray et al., 1998; Kause et al., 2007). The skeletal muscle tissue presents a positive allometry (1.05 for the carcass without head in carp) (Goolish and Adelman 1988, cited by Fauconneau et al., 1995). The rapid increase in the muscle mass compared to body weight leads to an increase in the condition factor (body weight/

length³) during growth. The different filleting yields can be related, within a species, to both body morphology and muscle width, especially around the abdominal cavity.

For Atlantic salmon, the filleting yield depends both on body size and body morphology, the biggest and widest fish present the highest fillet yield. However, “optimal”

body morphology seems necessary and for example above a value of 1.5 for condition factor the filleting yield decreases (Rora et al., 2001). Relationships between body morphology and filleting yield have been demonstrated in catfish (Dunham et al., 1985; Bosworth et al., 2001), carp (Fauconneau et al., 1997; Cibert et al., 1999), hybrid bass (Bosworth et al., 1998) and tilapia (Rutten et al., 2004). For catfish the combination of the estimation by ultrasound of muscle thickness and the external

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measure of body morphology gives the best prediction of the filleting yield compared to the analysis of body morphology alone (Bosworth et al., 2001). Depending on the studies, the filleting yield presents low or intermediate values of heritability. A direct and indirect selection using an index including the gutting yield, the head vol- ume and the filleting yield seems to be a good way to obtain an increase in fillet yield (Kause et al., 2007). The body morphology presents also a great environmental plas- ticity and for example Atlantic salmon in wild or rearing conditions present different head and caudal conformation (Cramon-Taubadel et al., 2005).

2.3 Trimming/skinning

Finally the last step to obtain directly consumable fish fillet consists in discarding the bone, the subcutaneous adipose tissue (dorsal and ventral) and the skin. The trimming yield is related to the differential growth of these tissues compared to skeletal muscle tissue. In order to obtain a fillet with a standard shape, some muscle tissue is discarded from the fillet, this can explain the rather low correlation between area of adipose tissue and trimming yield. Finally the best estimation of the trimming yield is obtained with the use of the condition factor. Thus this yield is also related to body morphology (Rora et al., 1998). A more precise quantification of fat tissue using MRI (Magnetic Resonance Imaging) and body morphology would allow improving the estimation of the trimming yield from measurement on whole fish body.

2.4 Processing

The processes like cooking, salting or smoking present also yields that can be related to the properties of the raw products. The salting yield is negatively correlated to the fillet fat content (Rora et al., 1998; Morkore et al., 2001), because of a lower water content of the fattiest fillet. The shape of the fillet is also important, the deepest fillet present the highest processing yield because of a better mass/area ratio (Morkore et al., 2001).

The shape of the fillet is also important for all the mechanical cut like slicing. In order to obtain slices with a similar weight, the slicing machine adjusts the width of the slice, for too high fillet, the slice can be too thin and difficult to cut. Such prob- lems are currently stated by the processing industry.

The cooking yield depends also on the fillet composition and the functional properties of muscle proteins that determine the water holding capacity. Such characteristics are very different from one fish species to another.

3 – FLESH NUTRITIONAL QUALITY

From the nutritional quality point of view, fish flesh can be considered as a meat product. This quality was already thoroughly described previously (Medale, 2004), and so will not be developed here. Fish flesh contains 70-80% water, 16-22% protein, a variable amount of lipid and a low quantity of glycogen (Medale et al., 2003).

The protein content is quite constant between fish species, it increases during growth and stabilizes around 20% and is not modified by the diet composition. Long

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periods of starvation can decrease the protein content of the muscle, for example during sexual maturation, especially before spawning. The amino acid composition is well-balanced and the content in essential amino acid is similar to that observed in meat from land species.

The lipid content varies greatly amongst fish species because of specific storage of fat by different tissue (mainly muscle, liver or peri-visceral fat tissues). Above all, fish flesh fatty acid composition is very different from meat, and very plastic in quantity and quality. The triglycerides are the main storage of energy and contain a great amount of n-3 polyunsaturated fatty acid (PUFA) (Medale et al., 2003). The flesh lipid content depends both on nutritional (mainly feed fat content) and genetic factors (Medale et al., 2003; Quillet et al., 2005). The fatty acid composition of the flesh is greatly dependent on the diet, ie the origin of the oil added in the feed or the nature of prey for wild fish.

The micronutriments (carotenoid, vitamin, minerals and trace element) are highly variable between fish species, depending on their environment and diet but can be especially interesting for human consumption, as for example phosphorus content (review of Medale et al., 2003; Medale, 2004).

4 – ORGANOLEPTIC FLESH QUALITY

The sensorial flesh quality of the fresh muscle is difficult to evaluate because of its subtle flavour, rather soft texture and low jutosity compared to meat (Fauconneau and Laroche, 1996).

4.1 Flesh colour and appearance

Many commercial fish species present a white flesh. The typical pink-red colour of salmonids (salmon, trout) flesh is an important quality criterion for this species.

This colour results from the fixation in the muscle of carotenoid pigments, astaxanthin or cantaxanthin, precursors of vitamin A. Such a colour is essential in the choice of the consumer to buy or not a salmonids product. The colour must be sufficient and homogenous all along the fillet. In Atlantic salmon bad colour may represent up to 39% of the flesh quality downgrading in a processing plant (Michie, 2001).

Fish can not synthesize carotenoid pigments and thus depend on the feeding supply (Choubert, 1992). In fish farm the diet composition contains pigment, whose quantity and quality are adjusted to obtain the required flesh colour for consumer acceptance. Astaxanthin is presently the main pigment added in the feed. The deposition of the pigment in the flesh increases with the concentration in the diet, up to a limit (1 mg/100 g) above which the fish muscle can not deposit more pigment. The deposition of carotenoids depends on endogenous factor like pigment digestibility, intestinal absorption, lipoprotein blood transport, its metabolism and attachment to the muscle fibre (Nickell and Springate, 2001). During sexual maturation carotenoid pigments are caught up by the roe for the female and by the skin for the male, leading to a discolouration of the flesh. The carotenoid deposition is also dependent on exogenous factors like the origin of the carotenoid of the diet, lipid content, or dura- tion of the feeding with a diet supplemented with carotenoid (Choubert, 2001). How- ever, carotenoid deposition also presents a genetic determinism with heritability

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values (h² values of 0.58, 0.46, and 0.36, for lightness, chroma, and hue angle, respectively) good enough to achieve a selective breeding (Kause et al., 2008).

Astaxanthin links non specific hydrophobic sites of muscle protein like actomy- osin (Henmi et al., 1987), but also of some other protein (Saha et al., 2006). The abil- ity of the fish to pigment deposition is thus related to the transport of the pigment from the blood to the muscle (Saha et al., 2006).

The flesh colour is not only related to carotenoid content. The redness value (a*) of the flesh is only partially correlated to the pigment concentration (Ronsholdt, 2005), suggesting that other parameters may affect flesh colour. Muscle structure, mainly muscle fibre density, can account for 27 to 44% of the discrepancy observed in flesh colour, independently of pigment concentration (Johnston et al., 2000). Fish with a high muscle fibre density present a more colourful flesh, due to different light scattering properties.

Another important quality attribute is the appearance of the fillet. The adipose tissue does not fix carotenoid pigment, or at a low level compared to muscle fibre, leading to a colour contrast between the muscle (pink-red) and the adipose (pink- white) tissues. For example the myosepta (sheet of connective tissue) contain many adipose cells (Zhou et al., 1995) and appear as white stria in the fillet, contributing to a fatty appearance of the fillet. Adipose cells are also present between the muscle fibres in the perymisium and contribute to the marbling of the fillet. Besides, the lightness of the flesh is positively correlated to the area of the myosepta (Marty- Mahe et al., 2004).

The gaping phenomenon corresponds to a failure between the muscle sheets in the myosepta or the myotendinous junction leading to the appearance of gaps along the length of the fillet, especially in the dorsal part. Such a defect concerns species like cod and salmonids and, for Atlantic salmon, may account for up to 38% of the downgrading of fillets because such fillets are difficult to process and give a bad appearance not acceptable for the consumers. The comprehension of the origin of gaping is still incomplete, with probably a multifactorial determinism (biological and technological). The appearance of this default is often seasonal, with an increase in spring and summer (Morkore and Rorvik 2001). The components of the extracellular matrix, like glycosaminoglycan, differ between fish presenting gaping or not. For example, the muscle of the spotted wolffish contains more chondroitin sulphate and does not present gaping compared to that of cod which contains more heparan sulphate and presents more gaping. A higher content of insoluble collagen is linked to a lower extent of gaping (Bjornevik et al, 2004; Espe et al., 2004). The muscle fibre density is also negatively related to level of gaping (Johnston et al., 2002; Bjornevik et al., 2004). The copper content of the blood (cofactor of the lysyl oxydase) is nega- tively correlated to the level of gaping (Morkore and Austreng 2004). Finally, triploid salmon are more prone to gape than diploid ones (Johnston et al., 2007).

4.2 Flavour

Fish flesh presents a rather neutral flavour. The free amino acid, peptide, organic acid, ammoniacal quaternary bases and minerals are the main components involved in fish flesh flavour (Haard 1992). These components are often more concentrated in wild than in reared fish muscle and thus wild fish usually present a more intense flavour. The oxidation of PUFA can explain the specific odour of the fish. Fish from freshwater and saltwater also present different volatile compounds (Haard, 1992).

The main determinism of flesh flavour is the living environment of the fish, for example the off-flavours (earthy/musty odour) perceived in freshwater fish are due to

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significant amount of geosmin or 2-methyl-iso-borneol in fish flesh. These compounds are produced by microorganism (cyanobacteria and actinomycetes) present in the water (Fauconneau and Laroche, 1996; Robin et al., 2006). Flavour can also be related to the lipid content of the flesh and the nature of the fatty acid in relation with their origin.

4.3 Texture

Texture is a quality parameter quite complex to describe, implying flesh firmness, jutosity or fibrosity. The determinism of flesh texture is related to

1) muscle composition, content and nature of the proteic compounds of the myofibrillar and connective tissues and the content of flesh fat;

2) the three-dimensional organization of the contractile and connective structu- res.

Comparison of different fish species shows a positive correlation between mechanical resistance of the raw flesh and collagen content (main connective tissue protein) (Hatae et al., 1986; Sato et al., 1986). After cooking, a negative relationship is observed because of thermal denaturation of fish collagen. Nevertheless this relationship is not systematically observed when studying only one fish species. A positive relationship between the cross-link content of the collagen and the mechanical resistance of the flesh is demonstrated in raw flesh from Atlantic salmon (Li et al., 2005). The mechanical resistance of the connective tissue is also related to the three-dimensional organization of the collagen fibrils (Ando et al., 1992).

Because of the low collagen content and its low thermal stability we can hypoth- esize that flesh texture, especially after cooking, is more related to muscle fibre and myofibrillar protein. Unlike in mammals and birds, both hyperplasia and hypertrophy contribute to the muscle growth all along the life in fish, and consequences on muscle structure is that muscle tissue appears histologically as a mosaic with the coex- istence of fibre with a large variety of size and with big and small fibres in the same muscle area. Comparing different fish species a negative correlation is observed between flesh firmness and muscle fibre area (Hatae et al., 1990; Hurling et al., 1996), fish with thinner muscle fibres present a firmer flesh compared to fish with bigger muscle fibres. A similar relationship is observed within the same species on smoked flesh from Atlantic salmon, and raw flesh from brown trout and rainbow trout (Johnston et al., 2000; Bugeon et al., 2003; Lefevre et al., 2008a). However, in other studies no relationship is observed between muscle fibre size and texture on Atlantic salmon and cod (Sigurgisladottir et al., 2000; Bjornevik et al., 2003). Such a relationship is also observed on cooked flesh, comparing wild and reared sea bass (Periago et al., 2005) but in an other study no relationship is observed for sea bass reared with different incubation temperatures (Lopez-Albors et al., 2008). Thus the relationship between flesh firmness and muscle fibre area for the same fish species can depend on the experiment.

The lipid content of the flesh clearly contributes to the sensorial properties of hydration. For example, the flesh of the fattiest fish is perceived as more juicy (Robb et al., 2002; Lefevre et al., 2006). There is however no systematic relationship between flesh firmness and fillet fat content. An inverse relationships can however be observed, with a softer flesh measured for the fattiest fish (figure 2).

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The biological basis of flesh texture are in a large extent described comparing different fish species or comparing fish within the same species submitted to different environmental, nutritional or genetic factors. Another variability observed for flesh texture is an antero-posterior gradient of fish texture, with a firmer flesh in the caudal part than near the head. The dorsal part of the fillet is also firmer than the ventral one. This difference can be related to the increase in red muscle and connective tissue content from the head to the posterior zone, and to thinner muscle fibre and a lower fat content of the caudal part of the fillet compared to the anterior part. The lower firmness of the ventral part can be attributed to the higher content of fat tissue, due to the vicinity of abdominal cavity.

5 – WAYS OF CONTROL

5.1 Genetics

The genetic origin of the fish is one of the main biological determinants for all the quality parameters. Indeed genes determine, at least partly, body morphology, body composition, and the organization and repartition of tissues. In farmed fish, the comparison of genetically distinct families, essentially for salmonid species, clearly shows that some morphological criteria, body fat content, flesh colour or white muscle fibre density are heritable characteristics. However genetic selection programs rarely include, at the moment, quality criteria other than yields. Muscle lipid content was shown to be possibly selected in rainbow trout, independently of total body fat content. This could allow controlling a part of flesh nutritional quality without impair- ing overall body compostion (Quillet et al., 2005). In many works, quality is analysed as a correlated response to genetic selection based on other criteria. The compari-

5 7 9 11 13 15 17 19 21

25 26 27 28 29 30 31 32 33

Muscle dry matter content (%)

Specific resistance of fillet (N/g)

F 2n L 2n F 3n L 3n

Figure 2

An illustration of the effects of genetic origin (lines and ploidy) and fat content on the texture. Correlation between muscle dry matter content (correlated with lipid content) and mechanical resistance of the raw flesh from pan-size rainbow trout selected

from four generations on muscle fat content. F: Fat line. L: Lean line. 2n: diploid fish.

3n: triploid fish. n = 15.

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son of distinct genotypes often reveals differences on quality parameters as a result of some modification of muscle characteristics. For example, divergent selection based on plasma cortisol response following an acute confinement stress, leads to fish lines for which most of characteristics (growth, morphology) are distinct, and, as a consequence, the quality parameters measured for those fish are drastically different (Lefevre et al., 2008b). However, genetic selection only based on quality criteria is not realistic, as it won’t give information on real genetic selection, as performed by fish breeders, who need to preserve in their program many quality criteria patiently obtained for their strains, including growth performance, body morphology, yields, fat content…

Amongst the influence of genetic background on quality, a particularity in fish is the production, in fish farming, at the industrial level of triploid animal to avoid any deleterious effect of sexual maturity on flesh quality. Compared to their diploid counterparts, triploid fish have distinct characteristics concerning their morphology, fat content, and muscle organization, with bigger muscle fibres leading to a softer flesh.

5.2 Nutrition

Feeding level has an effect on fish quality as a determinant of overall and muscle growth, and thus affects muscle structural characteristics, and fish adiposity. Ration level was shown to also affect flesh flavour in rainbow trout (Johansson et al., 2000).

Long-term starvation increase carcass yield by lowering overall fish adiposity. How- ever, fasting affects organoleptic quality especially texture with a firmer but less juicy flesh (Regost et al., 2001; Bugeon et al., 2004).

Food quality for fish is one of the main determinants of the product nutritional quality. The food composition in particular (energetic and fat contents) determines the overall carcass fat content (and yields as a consequence) and flesh lipid content (and nutritional quality by the presence of PUFA). As previously explained, food fatty acid composition determines muscle fatty acid composition and so the nutritional value of the flesh. The replacement of meal and oil from marine origin by other ingre- dients, and in particular from plant origin, may affect organoleptic quality of the flesh, especially its colour (Liu et al., 2004, Menoyo et al., 2004), sometimes its flavour (Luzzana et al., 2003), and can or not affect its texture depending on the studies (Regost et al., 2003; Menoyo et al., 2004).

Salmonids flesh colour is firstly determined by the control of the quantity and the nature of carotenoids pigments in food (Choubert, 1992).

5.3 Living environment

The influence of environmental parameters on fish quality is sometimes difficult to demonstrate as it often needs long-term experiments, in which all breeding parameters should be rigorously controlled upon a long period. Moreover, some of the observed effects may be the result of a differential growth rate due to the treatment. However, some environmental factors were shown to affect fish quality.

Different levels of exercise, provoked in fish by increasing water flow and thus swimming speed, have an effect of fish morphology, yields and body composition.

At the level of the muscle, increasing swimming speed leads to muscle fibre hypertrophy and changes some characteristics of collagen, and can thus affect some texture parameters (Bugeon et al., 2003).

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As poïkilotherm animals, the biological functions of the fish depend on living temperature. In particular, growth rate and as a consequence muscle characteristics are dependent on temperature. Moreover, living temperature can affect, at the same size, contractile and metabolic properties of muscle and thus flesh quality. These effects of living temperature may be as important as conditioning gene expression, as for example the expression of thermal isoforms of myosin heavy chain in carp.

This thermal dependent expression of structural proteins may influence the thermal behaviour of flesh during cooking process. More generally, the comparison of fish species living in various thermal environment shows that the thermal stability of myofibrilar proteins and collagen is positively correlated to the living temperature of the species (Sikorski et al., 1984; Poulter et al., 1985). Surprisingly very few studies report effects of adaptation temperature on flesh quality, within a species, which would be independent of any effect on growth rate (Lopez-Albors et al., 2008). How- ever, an effect of acclimation temperature on the proportion of red muscle, and thus on the carcass quality, was demonstrated in the scup (Zhang et al., 1996), and on the colour and texture of fillets from Arctic charr (Gines et al., 2004).

Living oxygen level is of great importance for fish, as severe hypoxia affects all biological functions until lethal consequence. Nevertheless, even oxygen levels close to normoxia may affect muscle tissue. Indeed, the increase in water dissolved oxygen, form 76% to 117% saturation level, increase muscle mass, estimated by fillet yield, and increase the proportion of red muscle in rainbow trout (Lefevre et al., 2007). Consequences on flesh quality are not so drastic, but a lower mechanical resistance of the fillet at low deformation is however observed for fish reared at 76%

oxygen saturation (Lefevre et al., 2008a) (table 1).

Table 1

Example of the effects of external factors on muscle characteristics and flesh quality.

Effects of oxygen level on fillet yield, relative width of red muscle, and mechanical resistance of the flesh in rainbow trout at pan size (400g). NS: not significant (p > 0,05),

* : p < 0,05, **: p < 0,01, ***: p < 0,001, n = 24 per treatment.

Values with the same letter in the same row are not significantly different (p > 0.05).

The photoperiod may affect overall and muscle growth in some fish species. As a consequence of these effects on growth, some effects on body composition and flesh quality are usually observed (Nordgarden et al., 2003; Johnston et al., 2004).

One of the limits of these studies, in the controlled environment of the experi- mental conditions, is that they only allow studying each factor (genetic origin, food, etc.) one by one, whereas different factors may act in synergy and/or interaction to affect the characteristics of muscle and flesh quality. One way to overpass this risk is to build multifactorial experiments which allow to grade the influence of several factors and to study simultaneously their potential interactions (Gardeur et al., 2007).

Oxygen level 76% 98% 117%

Effect of oxygen

level

Fillet yield (%) 48,7 ± 4,2 b 50,7 ± 3,4 a 51,3 ± 2,4 a *

Relative width of red muscle (%) 41,4 ± 8,2 b 41,3 ± 7,3 b 49,8 ± 11,0 a ***

Low deformation mechanical resistance (N/g) 1,15 ± 0,17 b 1,31 ± 021 a 1,34 ± 0,18 a **

High deformation mechanical resistance (N/g) 15,3 ± 3,1 16,6 ± 2,9 16,3 ± 3,1 NS

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6 – CONCLUSIONS

From the quality point of view, fish get some specificity concerning both their flesh nutritional quality due to their composition, and their sensorial quality due to the particular structural organization and some original properties of muscle components. Qualitative characteristics of the fish, and of their flesh, have a major biological determinism, which is, for example, particularly underlined by the importance of animal genetic origin on quality. However, fish and fish-products are very sensitive products. Indeed, the control of all peri-mortem steps, pre-slaughter and slaughter conditions, and the post-mortem storage and processing conditions, have to be totally controlled to keep the initial qualitative characteristics of the products, as the result of all the conditions of production.

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