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flesh quality The role of nutrition

Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
Flesh quality ± the role of nutrition
é Lie
Institute of Nutrition, Directorate of Fisheries, P.O. Box 185 Sentrum N-5804 Bergen, Norway
Correspondence: éyvind Lie, Institute of Nutrition, Directorate of Fisheries, P.Box 185 Sentrum N-5804 Bergen, Norway. E-mail:
[email protected]®
Product quality includes a variety of aspects
(Fig. 1), with both biological and nonbiological
causes and parameters. Fish nutrition has an
important impact on several parameters directly
in¯uencing the quality of the ®sh, such as colour
and appearance, smell and taste, texture, nutritional quality, shelf life, and level of contaminants. Further, consumers (market) are becoming
more concerned about how the ®sh are produced,
which type of feed ingredients are used and other
issues. Authorities in different countries have an
increased focus on food safety and traceability of
the production from egg to plate (for ®sh
farming). The need for improved knowledge of
®sh nutrition is therefore of great importance.
The present paper presents an overview of the
role of ®sh nutrition on ¯esh quality.
Keywords: ®sh nutrition, product quality
Colour and appearance
Product freshness and appearance are important
quality parameters (Fig. 1); however, ®sh nutrition
plays a minor role and therefore these characteristics will not be discussed in this paper. The
characteristic red or pink pigmentation of the ®llet
of salmonids is an important quality parameter and
the supplementation of dietary carotenoids to
Atlantic salmon (Salmo salar) represents a relative
high proportion of the feed costs (Torrissen,
Christiansen, Struksnñs & Estermann 1995).
Dietary astaxanthin is poorly absorbed and retained
in the muscle of salmonids; only 10%±15% of
ã 2001 Blackwell Science Ltd
dietary intake is retained in the ®llet (Torrissen,
Hardy & Shearer 1989; Storebakken & no. 1992).
Several factors in¯uence astaxanthin deposition
in the ®llet: dietary source (Foss, Storebakken,
Austreng & Liaaen-Jensen 1987; Storebakken,
Foss, Schiedt, Austreng, Liaaen-Jensen & Manz
1987) and dietary level of astaxanthin (Torrissen
1985; Storebakken et al. 1987; Torrissen et al.
1989; Choubert & Storebakken 1989). In a study
with Atlantic salmon where the dietary level of
astaxanthin ranged from 0 to 200 mg kg±1,
Torrissen et al. (1995) reported that no signi®cant
effects on ®llet deposition were achieved by increasing the astaxanthin level above 60 mg kg dry feed±
. The authors further claimed that to obtain
maximal astaxanthin level in the ®llet, Atlantic
salmon should have a dietary supply of astaxanthin
during the whole sea water stage, rather than just
at the ®nal phase of production.
The absorption of astaxanthin is positively
affected by high dietary lipid level (Torrissen,
Hardy, Shearer, Scott & Stone 1990; Bjerkeng,
Refstie, Fjalestad, Storebakken, Rùdbotten & Roem
1997a), whereas in rainbow trout (Oncorhynchus
mykiss) the presence of the z-isomers of astaxanthin
has been reported to hamper the relative bioavailability of astaxanthin (Bjerkeng, Fùlling, Lagocki,
Storebakken, Olli & Alsted 1997b; ésterlie, Bjerkeng
& Liaan-Jensen 1999). Further, dietary a-tocopherol is claimed to enhance the deposition of
canthaxanthin in rainbow trout (Pozo, Lavety &
Love 1988), although Jensen, Birk, Jokumsen,
Skibsted & Bertelsen (1998) were not able to
demonstrate a similar effect with dietarya-tocopheryl acetate. Recently. Bjerkeng, Hatlen &
Wathne (1999a) reported from an experiment
Flesh quality ± the role of nutrition é Lie
Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
Figure 1 Schematic overview of
parameters important for ¯esh
with Atlantic salmon a bene®cial effect of dietary atocopherol on ®llet deposition of astaxanthin. The
dietary levels of a-tocopherol used were 200, 400
and 800 mg kg±1 for each level of astaxanthin (30
and 50 mg kg±1 added); however, the authors
pointed out that the effect was rather small.
Texture is one of the main quality parameters for
fresh salmon (Sigurgisladottir, Torrissen, Lie,
Thomassen & Hafsteinsson 1997). Texture of ®sh
is de®ned by its dryness, chewiness and juiciness
and is traditionally determined in conjunction with
¯avour characteristics. It is commonly tested in the
industry by the `®nger method', which to a large
extent depends on the subjective evaluation of the
person who does the test. However, instruments for
the determination of texture are becoming more
commonly used (Sigurgisladottir et al. 1997) and
different methods of analysis have recently been
evaluated (Sigurgisladottir, Hafsteinsson, Jonsson,
Lie, Nortvedt, Thomassen & Torrissen 1999). Lipid
content and distribution have consequences for
textural properties. Faergemand, Ronsholdt, Alsted
& Borresen (1995) found in an experiment with
rainbow trout that the ®llet level of lipids could be
increased by 20% without any affect on the texture
measurement. The instrumental measurements of
texture (Instron Universal Testing Machine) were
veri®ed by sensory analyses. According to
Andersen, Thomassen & Rora (1997), ®llets from
rainbow trout fed high-lipid diets were evaluated as
softer than ®llets from ®sh fed a low-lipid diet. These
results were based on instrumental measurements
Figure 2 Routes of contaminants of farmed ®sh.
(Instron, Mass., USA). According to Sigurgisladottir,
Parrish, Lall & Ackman (1994a), dietary levels of
pigment and tocopherol did not affect ®llet texture of
Atlantic salmon. Nevertheless, there is little information about the role of ®sh nutrition on the
texture of ®sh ®llets.
In the last few years, gaping has been a quality
problem with farmed salmon (Fig. 2). After smoking, gaping has caused dif®culties with slicing of the
®llet. Many reasons for gaping have been discussed,
such as high-energy diets, fast growth and changes
in muscle collagen structure during chilled storage;
however, the mechanism(s) causing these problems
are still unknown. Several studies have focused on
the role of collagen on texture (Sato, Yoshinaka,
Sato & Shimizu 1986; Hatae, Tobimatsu,
Takeyama, & Matsumoto 1986). Fish are often
stored for several days on ice before being used;
studies have shown that ®sh ®llets soften after 1 day
of chilled storage (Montero & Borderias 1990; Ando,
Toyohara, Shimizu & Sakaguchi 1991a; Sato,
Ohashi, Ohtsuki & Kawabata 1991). Furthermore,
ã 2001 Blackwell Science Ltd, Aquaculture Research, 32 (Suppl. 1), 341±348
Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
it has been demonstrated by histological studies that
rapid softening of ®sh ®llets is caused by disintegration of thin collagen ®brils (Hallet & Bremner 1988;
Ando, Toyohara, Shimizu & Sakaguchi 1991b;
Ando, Toyohara & Sakaguchi 1992). Type I and V
collagen has been identi®ed in connective tissue
from several ®sh species (Sato, Yoshinaka, Sato, Itoh
& Shimizu 1988; Sato, Yoshinaka, Itoh & Sato
1989a; Sato, Yoshinaka, Sato, & Tomita 1989b;
Sato et al. 1991; Sato, Sakuma, Ohtsuki &
Kamabata 1994a; Sato, Koike, Yoshinaka, Sato, &
Shimizu 1994b; Aidos, Lie & Espe 1999). A study by
Sato et al. (1991) indicates that type V collagen is
involved in the rapid softening of ®sh muscle.
Collagen ®bres of Atlantic salmon have a high
solubility in acid and salt and contain few crosslinks, and some of these links seem to be cleaved
during storage on ice (Eckhoff, Aidos, Hemre & Lie
1998). In a recent study of collagen in Atlantic
salmon no signi®cant differences in solubility of
types I and V during storage were found (Aidos et al.
1999). However, seasonal differences in solubilities
of collagen were found (acid soluble, pepsin soluble
and insoluble collagen) in connective tissues from
Atlantic salmon (Espe, personal communication,
2001). Further studies are necessary to elucidate
the mechanisms behind the problem of gaping.
Dietary lipid level
Wild ®sh tend to be leaner than farmed ®sh
(Haard 1992); however, large variations due to
size, nutritional status and season do occur.
Dietary lipids and retention of lipids, particularly
in the ®llet, are often parameters discussed in
connection with quality. For farmed ®sh the
relationship between dietary lipids and deposition
of fat in the ®llet has been studied for several
species. In a study with white sturgeon (Acipenser
transmontanus) Hung, Storebakken, Cui, Tian &
Einen (1997) reported that dietary lipid levels
between 26% and 36% gave good growth without major effects on body composition, whereas
40% dietary lipid lowered speci®c growth rate
and increased levels of liver lipids. However, they
found no differences in lipid content of the whole
body of ®sh fed the different dietary lipid levels.
Montero, Izquierdo & Aksnes (1999) reported a
study with gilthead sea bream (Sparus aurata) fed
three different lipid levels (15%, 22% and 28%)
combined with two qualities of ®sh meal, a
Flesh quality ± the role of nutrition é Lie
signi®cant increase in growth with increasing
dietary lipids. A signi®cant increase in total body
lipid content was only seen in the ®sh fed the
high-quality ®sh meal and 28% lipid. It has been
suggested that the dietary lipid level for Atlantic
halibut should be optimised at a level near 40%
of the diet sometime before slaughtering
(Nortvedt & Tuene 1998).
Ê sgaÊrd (1998)
Hillestad, Johnsen, Austreng & A
reported from two experiments with Atlantic
salmon, where the ®sh were fed two dietary
lipid levels (22% and 30%) in isocaloric diets at
two feeding rates for the whole sea rearing period
(0.2±0.3 kg to 3±4 kg). No signi®cant differences
in growth between the two diets at the same
feeding range were found. The lipid content in
cutlets and dressed carcasses were signi®cantly
affected by feeding rate but not by dietary lipid
In an recent experiment with Atlantic salmon
fed 31%, 38% and 47% dietary lipid where the
®sh grew from 1.2 kg to 2.2±2.7 kg, a signi®cantly improved growth was found in ®sh fed
either the 38% or 47% lipid diet compared with
the 31% lipid diet (Hemre & Sandnes 1999).
There was an increase of ®llet fat (wet weight)
level from 7.7% to 12% and 16%, respectively,
with an increase in the dietary lipid level.
Correlated for weight there was a positive
correlation between dietary lipid and lipid content
of the ®llet.
Although there are several studies showing
that the deposition on fat is dependent on the
level of dietary lipid, the optimum (maximum)
dietary level of lipids in diets for the different
farmed ®sh species regarding ®llet quality has not
yet been clearly de®ned. Furthermore, lipids are
not homogenously distributed throughout the
®llet in ®sh species having high lipid levels in
the ®llet-like salmon (Lie & Huse 1992; Nortvedt
& Tuene 1998). There is a need for a more basic
understanding of the lipid metabolism of ®sh.
According to Frùyland, Madsen, Eckhoff, Lie &
Berge (1998), Atlantic salmon seem to possess a
high capacity to utilise dietary lipid as an energy
source based on carnitine palmitoyltransferase II
(CPT-II) activity and palmitoyl-L-carnitine oxidation. Furthermore, results suggest that an
increased dietary level of lipids increase the
mitochondrial b-oxidation of Atlantic salmon
(Frùyland et al., unpublished).
ã 2001 Blackwell Science Ltd, Aquaculture Research, 32 (Suppl. 1), 341±348
Flesh quality ± the role of nutrition é Lie
Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
Factors other than dietary lipid level
in¯uence lipid deposition in ®sh.
Garcia-Gallego & Akharbach (1998) found that the
dietary lipid level in¯uenced the level of lipids in the
muscle of European eel (Anguilla anguilla) and that
there was a direct relationship between ®sh size and
fat content of the eel. Earlier Lie, Hemre &
Lambertsen (1990) reported that ®llets of small
wild-caught European eel contained 12.4% lipid,
®llets of market-size wild eel contained 21.7% lipid,
whereas ®llet of fed marked-size eel contained 28%
Ê sgaÊrd, & Berglund
lipid. Johansson, Kiessling, A
(1995) reported from an experiment with rainbow
trout that the ®llet lipid content related strongly to
the ration level, and ®nally Aksnes (1995) found
that increased dietary level of carbohydrate (®xed
lipid level) gave increased storage of lipids in the
®llet and in the abdominal fat of Atlantic salmon.
Nutritional quality
Nutritional quality of food is related to its content of
nutrients and their bioavailability. The fact that
there is a link between diet and human health has
increased consumers' interests in healthy food and
thereby the nutritional quality of food. Fish is an
excellent source of protein and ®sh lipid has received
much attention for its content of n-3 fatty acids.
Furthermore, high-lipid ®sh are the only natural
source of vitamin D and ®sh in general provide a
number of B vitamins as well as a number of
minerals and trace elements. In the farming of ®sh it
is important to achieve the same nutritional quality
as wild ®sh and further improve this quality aspect.
acid composition of the ®llets of several ®sh species
(Steffens 1997; Brodtkorb, Rosenlund & Lie 1997;
Dosanjh, Higgs, McKenzie, Randall, Eales,
Rowshandeli, Rowshandeli & Deacon 1998;
Bjerkeng, Hatlen & Wathne 1999b). For several
species it is possible to produce a ®sh ®llet with a
more or less de®ned level of n-3 fatty acids.
However, how changes in the fatty acid composition
of the main phospholipids (PC and PE) affect quality
aspects connected to freezing, thawing and leaching
of ®llets remains to be elucidated.
Vitamins, minerals and trace elements
The deposition of lipid-soluble vitamins seems to be
dependent on dietary level. This has been demonstrated for vitamin D (Mattila, Piironen, Haapala,
Hirvi & UusiRauva 1997; Horvli, Aksnes & Lie
1998) and vitamin E (Bai & Gatlin 1993;
Sigurgisladottir, Parrish, Ackman & Lall 1994b;
Hamre & Lie 1997), whereas information regarding
vitamins A and K and water-soluble vitamin
deposition seems to be scarce.
There is some information regarding dietary
levels of mineral and trace elements. Lorentzen,
Maage & Julshamn (1994) reported that dietary
selenite was poorly deposited in the ®llet, whereas
selenomethionine was deposited. Furthermore, dietary zinc and iron (added as sulphates) seem not to
be deposited in the ®sh ®llet (Maage & Julshamn
1993; Bjùrnevik & Maage 1993). However, knowledge regarding nutritional quality, except for fatty
acids, is limited and more work is needed to utilize
the possibilities to design different nutritional
qualities of ®sh ®llet to match consumer expectations.
Dietary fatty acids
The fatty acid composition of tissue lipids is the
momentary net result of complex dynamic interrelationships of a number of factors, the details of
which are not fully understood. The major factors
are: dietary fatty acids intakes; rates of oxidative
catabolism of the fatty acids; kinetics of desaturation
and elongation reactions; and competitive incorporation and retroconversions among fatty acids.
Fish is unique as a food item in that it provides longchain n-3 polyunsaturated fatty acids. In farmed
®sh the fatty acid composition as well as other ®llet
lipids may be altered by feeding. There are
numerous studies that have demonstrated the
in¯uence of the dietary fatty acids on the fatty
Oxidative stability
Oxidation of ®sh lipids is a major quality problem;
the primary decrease in quality observed during
oxidation is due to the production of off-¯avour
compounds. The oxidation process can also lower
nutritional quality and modify texture and colour.
In ®sh tissue, there a number of naturally present
components that can serve as pro-oxidants or
antioxidants. The main function of tocopherols is
to protect unsaturated lipids in living tissues against
free radical-mediated oxidation. Fish ®llets enriched
with vitamin E (increased dietary level) have in
some studies showed higher oxidative stability
during fresh and frozen storage (Boggio, Hardy,
ã 2001 Blackwell Science Ltd, Aquaculture Research, 32 (Suppl. 1), 341±348
Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
Flesh quality ± the role of nutrition é Lie
Figure 3 Example of gaping in the
®llet of Atlantic salmon.
Babbitt & Brannon 1985; Frigg, Prabucki & Ruhdel
1990; Waagbù, Sandnes, Torrissen, Sandvin & Lie
1993; Bai & Gatlin 1993; Hamre & Lie 1997;
Santana & Mancini 2000). There has been some
discussion about which of the tocopherols are most
effective antioxidants: a-tocopherol is the most
frequently used; however, g- and d-tocopherols are
claimed to be better antioxidants (Frankel 1996). A
recent study with Atlantic salmon concluded that
a-tocopherol is preferred for protection of the ®llet
against lipid oxidation due to faster excretion of gand d-tocopherols from the ®sh body (Hamre, Berge
& Lie 1998).
Food safety
Food safety is a topic receiving increasing focus in
many countries, often in connection with ®sh
farming. Dietary composition as well as environmental factors are important parameters regarding
contaminants in farmed ®sh (Fig. 3). Contaminants
are general highly stable, bioaccumulated, acutely
toxic at high concentrations and chronically toxic
at low concentrations. Examples of contaminants
are heavy metals and other inorganic elements (Hg,
Cd, As, Cu, etc.), organic contaminants (Dioxin,
PCB, etc.) and radioactive isotopes (cesium, technesium, etc.). There are maximum allowable limits in
food and feed for some contaminants, and there are
ongoing processes for setting limits for a range of
Fillets are in general the last target organ for
dietary metal deposition. Dietary metals will initially
be deposited in the intestine, kidney and liver.
Contamination of ®llets is therefore unlikely to
occur. However, only a life-cycle experiment can
give real information on the exact ®llet dietary
Table 1 Dioxin content in ®sh ®llet
ng WHO-TEQ/kg wet.wt.
Atlantic salmon
metal loading. The nonessential metal cadmium
(Cd) has a low bioavailability. Recent experiments
with Atlantic salmon have demonstrated that only
2%±3% of dietary Cd is retained in the carcasses of
®sh fed elevated dietary levels and it takes a long
time before dietary Cd starts to accumulate in the
®llet (Berntssen, Lundebye & Maage 1999). The
uptake of dietary copper (an essential metal) is well
down-regulated at the intestinal level, and the
retention of excess dietary Cu is therefore low
(Berntssen et al. 1999).
Dioxins have been a focus in Europe the last year
due to the ®ndings of very high levels in chicken
feed. Dioxins (PCDD, polychlorinated dibenzo-pdioxins and PCDF, polychlorinated-di benzofurans)
are a large group of compounds (at least 210) with
variable stability and toxicity. They are extremely
acutely toxic to certain mammals, although large
species differences exist. LD50 for the guinea pig is
reported to be 0.6±2 mg g body weight±1, whereas
LD50 is reported to be 1150±5050 mg g body
weight±1 for hamsters. The information regarding
dioxin levels in ®sh and particular farmed ®sh are
scarce (some examples are given in Table 1).
Furthermore, retention studies of dietary dioxins in
farmed ®sh are not yet published.
ã 2001 Blackwell Science Ltd, Aquaculture Research, 32 (Suppl. 1), 341±348
Flesh quality ± the role of nutrition é Lie
Aquaculture Research, 2001, 32 (Suppl. 1), 341±348
To summarize, ®sh nutrition has an important
role in regulating ¯esh quality of farmed ®sh, and it
will even be more important in the future, particular
within tailor-made products and food safety.
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