Linseed oil inclusion in sea bream diets: effect on muscle quality and shelf life

Linseed oil inclusion in sea bream diets: effect on

muscle quality and shelf life

Pedro L Castro1, Mar�ıa J Caballero1, Rafael Gin�es1, Juan C Penedo2, Daniel Montero1,

Mar�ıa T Lastilla3 & Marisol Izquierdo1

1Grupo de Investigaci�on en Acuicultura (GIA), Universidad de Las Palmas de Gran Canaria & Instituto Canario de

Ciencias Marinas, Las Palmas de Gran Canaria, Canary Islands, Spain2Bromatology and Food Technology Section, Veterinary Faculty, University of Las Palmas de Gran Canaria, Las Palmas

de Gran Canaria, Canary Island, Spain3Animal Production Department, Universit�a degli Studi di Bari, Bari, Italy

Correspondence: P C Alonso, Veterinary Faculty, Trasmonta~na s/n 35413 Arucas, Las Palmas. Spain. E-mail: pcastro@dmor.

ulpgc.es

Abstract

Dietary lipid source in aquaculture has become a

central topic in research whilst natural resources

availability diminishes. Hence, to weigh up and

forecast consumers impressions, the impact of par-

tial (70%) and complete (100%) dietary replace-

ment of fish oil (FO) by linseed oil (LO) on sensory

and quality attributes was studied during the

edible shelf life of gilthead sea bream (Sparus aurata).

Physico-chemical parameters (pH, torrymeter, total

volatile basic nitrogen, thiobarbituric acid reactive

substances and texture), and sensory analysis,

both in cooked and raw fish were carried out

during 17 days of ice storage. Throughout ice

storage, feeding with LO diets, TBARS values

remained lower on muscle than those found when

feeding FO control diet. On freshly caught fish

(day 0 of ice storage), statistically significant die-

tary texture variations were recorded on cooked

fillet fed FO diet. No sensory differences on Quality

Index Method, sensory profile or Torry scheme

were found with partial or total LO replacement

diets.

Keywords: linseed oil, sea bream, quality, sen-

sory assessment, texture, shelf life

Introduction

The increase in global production volume in aqua-

culture by 8–10% a year (Tacon, Hasan & Suba-

singhe 2006) has resulted in increasing prices and

limited availability of some ingredients. It is

expected that the demand for fish oil (FO) by aqua-

culture will probably exceed available resources

over the next decade (Bostock, McAndrew, Rich-

ards, Jauncey, Telfer, Lorenzen, Little, Ross, Handi-

syde, Gatward & Corner 2010). Some vegetable

oils are considered alternatives to FO, such as

soybean oil, canola oil and linseed oil (LO), and

have been tested as alternative lipid source in

different marine fish species and particularly in

gilthead sea bream (Sparus aurata; Menoyo,

Izquierdo, Robaina, Gin�es, L�opez-Bote & Bautista

2004; Izquierdo, Montero, Robaina, Caballero,

Rosenlund & Gines 2005) one of the most impor-

tant marine fish for Mediterranean aquaculture.

Feed with different proportions of LO, a vegetable

oil rich on n-3 polyunsaturated fatty acids (PUFA)

and specifically on linolenic acid (18:3n-3), as

alternative lipid source, seems to produce a more

similar fatty acid profile to FO than other vegetable

oils in gilthead sea bream, without compromising

fish growth or feed utilization (Izquierdo, Obach,

Arantzamendi, Montero & Robaina 2003; Menoyo

et al. 2004; Izquierdo et al. 2005). Increases in

fillet n-3 fatty acids lead to a more balanced n-3:n-6

ratio which may improve the commercial and

health value of cultured fish as western diet con-

tains high levels of n-6 and low levels of 18n-3

PUFAs (€Ozogul, €Ozogul & Alagoz 2007) which is

considered to be an unbalanced diet.

In this sense, fish as a healthy foodstuff and fish

quality has attracted research attention due to

increasing consumer’s health concern. To assess

fish fillet quality, it should be analyzed by means

© 2013 Blackwell Publishing Ltd 1

Aquaculture Research, 2013, 1–11 doi:10.1111/are.12161

of organoleptic characteristics, freshness and nutri-

tional value. Among them, freshness is fundamen-

tal from consumer’s point of view, as fish as food,

is highly perishable. Freshness state can be

described by a variety of properties that can be

assessed by several indicators, such as sensory

assessment or physico-chemical tests. Thus,

parameters as total basic volatile nitrogen (TVBN),

thiobarbituric acid reactive substances (TBARS),

Torrymeter (TMRs) or pH have been widely used

to revise freshness and shelf life in multiple species,

and specifically on sea bream (Kyrana, Lougovois

& Valsamis 1997; Grigorakis 2007). Regarded sen-

sory determinations, Huidobro, Pastor and Tejada

(2000), developed a Quality Index Method (QIM)

for sea bream. Carbonell, Duran, Izquierdo and

Costell (2003) assessed fresh and frozen texture in

commercial and cultured sea bream, whilst Ayala,

Abdela, Santaella, Mart�ınez, Periago, Gil, Blanco

and L�opez Albors (2010) revised texture evolution

throughout shelf life. On cooked fish, Carbonell,

Izquierdo and Costell (2002) assessed fresh and

frozen sea bream with a sensory panel.

Studies on dietary FO replacement for gilthead

sea bream have been focused mainly on growth,

feed utilization (Izquierdo et al. 2005), welfare and

stress resistance (Montero, Kalinowski, Obach,

Robaina, Tort, Caballero & Izquierdo 2003; Mon-

tero, Grasso, Izquierdo, Ganga, Real, Tort, Cabal-

lero & Acosta 2008). Besides, the effect on the

fatty acids profile, tissues modifications and their

relations to overall quality (Izquierdo et al. 2003,

2005; Menoyo et al. 2004; Castro, Cansino,

Mill�an, Gin�es, Montero & Izquierdo 2010) have

received special attention. Regarding quality, stud-

ies conducted in gilthead sea bream fed partial

vegetable oil substitution diets analyzed on the

harvest day, showed some effects on fillet flavour

and texture with soybean oil. Studies on fillet

hardness with gilthead sea bream showed slight

differences at 40% of LO substitution level com-

pared with FO diet (Izquierdo et al. 2003) or no

significant difference at 60% and 80% of LO sub-

stitution (Menoyo et al. 2004; Izquierdo et al.

2005). However, no studies have been conducted

to determine the effects of FO replacement by vege-

table oils on the shelf life, in spite of its commer-

cial significance.

Consequently, considering new research

demands, it seems valuable to farming industries,

retailers and consumers to investigate dietary FO

substitution effect on quality changes of cultured

gilthead sea bream occurring during the ice

storage period. Thus, the objective of this study

was to determine whether fillet quality, fillet sen-

sory profile and fillet texture are affected in

gilthead sea bream fed LO oil-based diets (partial

and complete FO substitution) both freshly caught

and throughout the shelf life period.

Material and methods

Experimental fish

Gilthead sea bream juveniles of 40 � 9 g initial

body weight, supplied from a local farm (ADSA

S.A. Gran Canaria, Spain), were randomly distrib-

uted into six fibreglass tanks of 1000 L (50 fish/

tank) at Instituto Canario de Ciencias Marinas

(Canary Islands, Spain, Lat 26°99′N, Long 15°36′W). All tanks were supplied with continuously

running seawater, constant aeration and natural

photoperiod (12 h:12 h L:D). Along the experi-

mental period, water temperature and dissolved

oxygen ranged between 21.8–22.4°C and

5.5–7.2 mg/L respectively. Fish were fed to satia-

tion three times a day, 6 days a week. The diets

were fed to duplicate groups of fish.

Experimental diets

Two isonitrogenous (450 g/kg protein content)

and isoenergetic diets, with a lipid content of

220 g/kg, were formulated and provided by Bio-

Mar Iberia (Due~nas, Spain) to partial (70LO diet)

and complete substitution of FO (Peruvian

anchovy) by LO (100LO diet; Table 1). A diet con-

taining 100% FO was used as a control diet (FO

diet; Table 1). All tested diets were provided as

extruded 3 and 4.5-mm pellet.

Sample preparation

After 330 days of experimental feeding, 254

gilthead sea bream (42 fish tank/84 fish per diet)

with weight and body length of 506.9 � 12.8 g

and 30.2 � 0.2 cm (FO), 496.0 � 11.2 g y

29.8 � 0.3 cm (70LO) and 501.1 � 12.5 g and

30.1 � 0.3 cm (100LO) were slaughtered by

immersion in ice cold water (fish: ice ratio 2:1) and

packed as whole ungutted fish with flaked ice into

polystyrene boxes with holes for drainage. Fish were

stored in a refrigerator at 4°C for 17 days post

harvest (dph). During storage, 12 fish per diet,

© 2013 Blackwell Publishing Ltd, Aquaculture Research, 1–112

Linseed oil in sea bream diets PL Castro et al. Aquaculture Research, 2013, 1–11

randomly chosen, were obtained at 0, 2, 4, 7, 10,

14 and 17 dph. This storage period is close to the

end of the shelf life of whole ungutted gilthead sea

bream stored in ice (Kyrana et al. 1997; Alasalvar,

Taylor, €Oks€uz, Garthwaite, Alexis & Grigorakis

2001). Fish were removed from ice, evaluated for

QIM, pH and Torrymeter measures and afterwards,

filleted and subjected individually to analytical

determinations conducted in triplicate. Six fish were

fully devoted to texture and sensory assessment and

another six for chemical tests.

Physico-chemical analysis

The pH of the fillet was determined using a Crison

penetration electrode (accurate to 0.01 pH unit,

model 507; Crison Instruments S.A., Barcelona,

Spain) after carrying out an incision on the skin,

in the tail area. Electric conductivity was deter-

mined using a Torry fish freshness meter

(G.R. Torrymeter, Distell, UK). Measurements were

performed on the skin, in the central portion of

the lateral line of the fish. The TVBN content of

samples was determined according to the method

of Antonacopoulos (1968) expressed as mg TVBN

per 100 g fish muscle with a 2100 Kjeltec distilla-

tion unit (Foss Tecator AB, Hillerød, Denmark).

Reactive substances to 2-thiobarbituric acid (TBA)

were determined according to the extract method

proposed by Shahidi and Hong (1991). Chemical

products were analytical grade (Panreac, Barce-

lona, Spain; Sigma-Aldrich, St. Louis, MO, USA).

Texture measurement

Texture measurements were carried out on both

raw and cooked fillet of the gilthead sea bream fed

experimental diets. On raw fish, texture was stud-

ied on 0, 2, 4, 7, 10, 14 and 17 dph, whereas on

cooked fish texture was studied on day 0 dph. All

texture measurements were performed with a

Stable Micro Systems texture analyser (TA.XT2,

Surrey, UK). Texture measurement was conducted

according to Gines, Afonso, Zamorano and Lopez

(2004) and carried out at refrigeration tempera-

ture, keeping the fillet cooled with ice. Skin was

removed and three-square pieces (cranial, central

and caudal pieces of 2.5 9 2.5 cm side) were

collected from each left fillet above the lateral line.

The raw flesh samples were stored on ice until

analysed. The force-deformation curve was ana-

lyzed to determine eight texture parameters

(fracturability, hardness, springiness, cohesiveness,

gumminess, chewiness, adhesiveness and resil-

ience). The cooked flesh samples were prepared in

air-heated oven (Compact; Eurofred, Barcelona,

Spain) for 10 min at 115°C, packed in aluminium

boxes labelled with a three digits code. On cooked

fillet, fracturability was not determined.

Sensory assessment

On the 0, 2, 4, 7, 10, 14 and 17 dph six labora-

tory-trained panellists evaluated sensory attributes

of whole gilthead sea bream depending on fish

freshness: appearance of eyes, skin and gills,

together with odour and texture applying the

quality index method (QIM) for this fish species

(Huidobro et al. 2000).

Fillets from six fish per diet were studied on

0 dph for the cooked sensory profile. A team of

nine trained panel members were selected for their

interest, availability on the experimental days and

sensory capacities, stimuli or discriminating inten-

sities. Tests were conducted in an isolated,

air-conditioned room designed for sensory analysis

divided into six individual boxes with standardized

light and equipped with a computerized system.

Judges were randomly offered the fillets (3–4 cm),

cooked as stated above. Odour (marine, oily and

atypical), appearance (whiteness, brightness,

compactness and liquid expelled), texture

Table 1 Ingredients of the experimental diets used

% of dry weight

Oils (Fish oil†/linseed) 16.32

South-American fish meal† 47.26

Wheat 7.00

Soybean meal 47%‡ 25.00

Sunflower meal 3.67

Vitamins premix§ 0.27

Minerals premix¶ 0.48

†South-American, anchovy oil.

‡Soybean meal with 47% as a dry protein, ‘no GMO’.

§Vitamin premix contained: A – Retinol 11200 IU/kg,

D3 – Cholecalciferol 112 IU/kg,E – Tocopherol 280 mg/kg,

C – Ascorbic acid 336 mg/kg, B1 – Thiamin 9 mg/kg,

B2 – Riboflavin 15.7 mg/kg, B3 – Nicotinic acid/Niacin

179.2 mg/kg, B5 – Panthothenic acid, 31.4 mg/kg, B6 –

Pyridoxin 13.4 mg/kg, B8 – Biotin 0.5 mg/kg, B9 – Folic acid

4.5 mg/kg, B12 – Cyanocobalamin 0.036 mg/kg, K –

Menadion 6.7 mg/kg, Inositol 44.8 mg/kg.

¶Mineral premix: 14.5 mg/kg, Zn 44.8 mg/kg, Fe 67.2 mg/kg,

Cu 3.6 mg/kg, Mn 14.6 mg/kg, Mg 136.1 mg/kg, Co 0.2 mg/

kg and Se 0.06 mg/kg.

© 2013 Blackwell Publishing Ltd, Aquaculture Research, 1–11 3

Aquaculture Research, 2013, 1–11 Linseed oil in sea bream diets PL Castro et al.

(cohesiveness, juiciness, hardness and adhesive-

ness), flavour (marine, oily and atypical), flavour

persistence or residual taste (persistent, oily) and

acceptability were assessed. The panel scored the

sensory attributes on a continuous intensity scale

going from 0 (low intensity) to 100 (high inten-

sity), and the obtained data were subjected to a

principal component analysis (PCA).

The measurements of acceptability of cooked fish

(odour, flavour and texture) were assessed accord-

ing to a modified Torry scheme (Torry advisory

note No. 91) modified for cultured gilthead sea

bream (Alasalvar et al. 2001). Flavour, odour,

texture and acceptance (an overall sensory score

of the separate characteristics) were assessed by

means of a hedonic scale from 10 to �3, 10

being absolutely fresh and �3 completely putrid

or spoiled. Fish fillets were cooked as stated above

then served to the nine panellists the following

dph: 0, 2, 4, 7, 10, 14 and 17.

Statistical analysis

Data were submitted to a two-way ANOVA. Scheffe’s

multiple range test at p < 0.01 was used when ANO-

VA main effect were significantly different using a

SPSS Statistical Software System 17.0 (SPSS Inc.,

Chicago, IL, USA). In addition, the multivariable

structure of the sensory assessment was described

using a PCA (Unscrambler� version 9.8 program;

Camo A/S, Oslo, Norway 2010).

Results and discussion

Partial and total substitution of FO by LO signifi-

cantly (p < 0.05) reduced gilthead sea bream

growth at the end of the experimental period

when compared with fish fed FO based diet. Simi-

larly, fatty acid composition of the fillet was modi-

fied feeding LO experimental diets. Fish oil

replacement by different proportions of LO reduced

the dietary content of saturated and increased n-3

and n-6 polyunsaturated (Table 2) in the gilthead

sea bream fillet, although the level of n-3 HUFA

was reduced. The quantity of 18:2n-6 in the fish

fillet was related to its dietary concentration. Thus,

18:2n-6 was consistently higher (p < 0.05) in the

fish fed LO diets relative to those fish fed FO. Simi-

lar results were found in 18:3n-3, which was the

most increased fatty acid with the LO inclusion.

Furthermore, the levels of 18:3n-3 in fish tissue

were below those observed in LO diets (Table 2).

LO substitution produced a lower fillet n-3:n-6

proportion (3:1 and 2:1, 70LO and 100LO respec-

tively) than that determined in fillet with the FO

commercial diet (4:1) (Table 2).

Table 2 Proximate composition (% dry weight) and selected fatty acid (% total identified fatty acids) of the experimen-

tal diets and gilthead sea bream fillet

Proximate

composition (g/kg)

FO 70LO 100LO

Diet Fillet Diet Fillet Diet Fillet

Protein 456.4 � .5.3 219.6 � 1.4 454.7 � 10.9 224.7 � 10.4 456.4 � 3.9 220.5 � 8.1

Lipid 202.3 � 11.5 43.2 � 17.0 216.9 � 13.7 44.2 � 22.0 219.2 � 19.2 39.4 � 7.5

Moisture 67.0 � 4.7 728.8 � 18.1 62.8 � 4.2 727.7 � 21.2 72.8 � 9.0 734.9 � 0.2

Ash 99.1 � 12.3 17.9 � 2.3 98.6 � 5.6 17.9 � 1.4 98.6 � 6.4 17.6 � 1.0

Main Fatty acid (%)

18:1n-9 9.17 12.28a 14.13 15.99b 18.41 18.25c

18:2n-6 3.00 3.87a 11.87 10.86b 16.31 13.95c

18:3n-3 1.48 1.41a 31.51 20.50b 34.60 26.52c

20:4n-6 0.92 1.14b 0.41 0.70b 0.13 0.28a

20:5n-3 7.58 6.67c 3.52 3.11b 1.01 1.98a

22:6n-3 5.32 10.52b 3.57 6.70a 1.20 5.25a

Saturated 43.91 35.69c 23.71 25.59b 21.21 21.48a

n-3 PUFA 18.54 24.49a 40.11 33.04b 37.31 35.42b

n-6 PUFA 5.12 6.66a 12.53 12.11b 16.65 14.99c

n-3 HUFA 14.91 20.84b 7.68 11.45a 2.34 8.38a

n-3/n-6 3.62 3.78b 3.22 2.86a 2.24 2.35a

FO, fish oil; LO, linseed oil; PUFA, polyunsaturated fatty acids. Different letters (a, b) in a column denote statistically significant

differences in respect with diets (p < 0.05). Data expressed as mean values � SD.



Physico-chemical analysis

pH values measured on gilthead sea bream fed

experimental diets on 0 dph (Fig. 1) ranged

around 6.5. The inclusion of LO in gilthead sea

bream diets did not produce significant changes in

muscle pH values on 0 dph, as was reported for

60% and 80% of LO substitution (Menoyo et al.

2004). The values found in the fillet of the fish fed

FO diet were higher than those found feeding LO

diets, mainly during the first 7 dph, although

these values were found to be statistically not

significantly different. The evolution of the pH val-

ues throughout the time in ice matches those

reported by Kyrana et al. (1997) for gilthead sea

bream fed FO diet: a gradual decrease in the val-

ues during the first days on ice of the edible shelf

life due to the post-mortem anaerobic metabolism

with lactic acid production. On 4th dph, a subse-

quent increase in the pH values was recorded

related with bacterial basic compounds production,

connected with fish muscle spoilage.

Torrymeter values on the gilthead sea bream fed

experimental diets on 0 dph ranged from 11.3 (FO)

to 11.5 (100LO; Fig. 2). Factors that are known to

have an effect in fish dielectric characteristics and

cause more variable TMRs values are fat content

and integrity losses of the skin and/or the muscle

(Pivarnik, Kazantzis, Karakoltsidis, Constantinides,

Jhaveri & Rand 1990). In that sense, LO diets

meant changes in fatty acids proportions compared

with those fish feeding conventional FO diet

(Table 2). Nevertheless, these changes did not affect

total fat content (Table 2) which did not register

significant differences and accordingly Torrymeter

measurements (Fig. 2). The pattern developed by

the average data from the Torrymeter during shelf

life registered a decreasing tendency with ageing.

For gilthead sea bream, some authors have

described values over 12–5 units during 18 days of

ice storage (Lougovois, Kyranas & Kyrana 2003) or

inferior when the fish is considered unacceptable by

sensory assessment (Tejada, Huidobro & Fouad Mo-

hamed 2006). The initial values of around 11.3

were reduced to final values of around 7.2 on day

17.

TVBN values usually oscillate between 5 and

20 mg/100 g muscle in freshly caught fish (Con-

nell 1995). TVBN values were, on 0 dph, around

20.0 (FO) and 21.5 mg/100 g (100LO; Fig. 3).

Experimental diets did not produce significant

TVBN changes in the fillet of gilthead sea bream.

Throughout storage time, this parameter did not

follow significant increase. Similar results were

previously described related to water temperature

and spoilage flora (Tejada & Huidobro 2002;

Castro, Penedo, Cansino, SanJuan & Mill�an 2006).

No significant differences were determined when

comparing diets throughout the storage (Fig. 3).

012345678

0 2 4 7 10 14 17

pH v

alue

s

Days in ice

pH values

FO

70LO

100LO

Figure 1 Changes in pH readings in muscle of

gilthead sea bream fed experimental diets during ice

storage. Data expressed as means values � SD. n = 6.

0246810121416

0 2 4 7 10 14 17

Tor

rim

eter

rea

ding

s

Days in ice

Torrimeter values FO70LO100LO

Figure 2 Changes in Torrymeter readings (TMRs) in

muscle of gilthead sea bream fed experimental diets

during ice storage. Data expressed as means

values � SD. n = 12.

0

5

10

15

20

25

0 2 4 7 10 14 17

mg

nvb

t 100

g–1

mus

cle

Days in ice

TVBN

FO

70LO

100LO

Figure 3 Changes in total basic volatile nitrogen

(TVBN) in muscle of gilthead sea bream fed experimen-

tal diets during ice storage. Data expressed as means

values � SD. n = 6.



Thiobarbituric acid reactive substances values

ranged from 0.12 (100LO) to 0.16 (FO; Fig. 4)

on the fillet of the gilthead sea bream fed experi-

mental diets (0 dph). Even though FO fillet values

were the highest, no diets effect on TBARS values

was determined on the day 0. During ice storage,

TBARS values progressed to final values of

0.20/0.27 (Fig. 4). 100LO diet was found to be

significantly (p < 0.05) different from FO diet on

the 2nd, 4th, 7th, 10th and 17th dph (Fig. 4).

These results are in agreement with those referred

by Menoyo, Lopez-Bote, Obach and Bautista

(2005) where lipid peroxidation after the replace-

ment of dietary FO by vegetable oil on salmon

(Salmo salar) decreased. As a remarkable aspect,

when the LO inclusion level increases in gilthead

sea bream diets, the lipid oxidation susceptibility

and the TBA values were lower. Lipid peroxida-

tion has importance in terms of fish-flesh quality,

because of its negative impact on flavour, colour

and nutritional characteristics (Monahan 2000).

In chilled lean fish, the lipid peroxidation process

has high significance only regarding the last days

of ice storage when microbial spoilage changes

are the main cause of the reduction in edible shelf

life. However, this lower susceptibility to lipid per-

oxidation might be valuable for freezing storage

and technologic process where lipid peroxidation

could be a main and restrictive factor.

Texture studies on 0 dph (Table 3), showed the

highest hardness values on the fillet of the fish fed

FO diet (6.23 N), gumminess (1.48 N), adhesiveness

(�44.69 N*sg) and chewiness (0.76 N). Springiness

(0.55), cohesiveness (0.26) and resilience (0.16)

were determined higher on fish fed 70LO diet. In

spite of these values, dietary treatment did not

produce significant differences. Izquierdo et al.

(2003) found a lower (but not significant) hardness

on the fish feed vegetable oil-based diets than that

found with FO diet. Similar results were reported

with 60% and 80% of LO inclusion level on flesh

compression (whole fish), flesh puncture and fillet

compression (Menoyo et al. 2004; Izquierdo et al.

2005).

As it was determined on 0 dph, diets did not pro-

duce differences on tested attributes during shelf life

(Table 3). The most remarkable changes in texture

occurred with all tested diets during the first

4 days. During this period there was a strong

decrease in all parameters, as was previously

described by Veland and Torrisen (1999). Alasal-

var, Taylor and Shahidi (2002) studied gilthead sea

bream fillet feed FO commercial diet. The authors

referred an initial hardness of 7.4 N on the day 1 of

ice storage and a gradual decrease during ice stor-

age being on day 17 slightly up to 4 N. In this

study the initial values varied from around 6 N

(0 day) to around 3.4–3.7 N (17 day, Table 3).

Texture studies on cooked fish (0 dph) showed

significant (p < 0.05) differences related to diets in

nearly all measured texture parameters (Table 4).

These differences were found in a representative

parameter, hardness. The highest values were

found with FO diet (3.73 N) decreasing according

to the per cent of FO substitution. Thus, the lowest

values were found when fish was fed total substi-

tution diet (2.85 N, 100LO). In those parameters

related to the resistance to the maximum compres-

sion as gumminess and chewiness, the maximum

value was found with FO diet (1.40 N and 0.67 N

respectively) and statistically (p < 0.05) different

from 100LO (0.98 N, gumminess and 0.50 N,

chewiness). However springiness, also related to

the resistance to the maximum compression,

followed an opposite pattern getting maximum

values with LO diets (0.53, 70LO; 0.51 100LO;

0.48 FO). Cohesiveness was significantly

(p < 0.05) higher with 70LO (0.38) than with

100LO diet (0.35) although not different from the

FO diet (0.37). Another significant parameter on

cooked fillet was resilience. The smallest value was

found with FO diet (0.12) and it was significantly

(p < 0.05) different, comparing with LO diets

(around 0.13). Adhesiveness did not show signifi-

cant differences. Fracturability was not determined

in cooked fillets because it depends on the recovery

of the structure by sliding myomeres within the

elastic network of collagen. Collagen content has

00.050.10.150.20.250.30.350.4

0 2 4 7 10 14 17

mg

MA

per

100

g m

uscl

e

Days in ice

TBARS values

b

b

b

bb

abab

abab ab

a

aa

aa FO

70LO

100LO

Figure 4 Changes in Tiobarbituric acid index (TBARS)

values in muscle of gilthead sea bream fed experimental

diets during ice storage. Different letters (a, b) denote

statistically significant differences in respect with diets

(p < 0.05). Data expressed as means values � SD.

n = 6. TBARS, thiobarbituric acid reactive substances.



been correlated with firmness in raw fish (Hatae,

Tobimatsu, Takeyama & Matsumoto 1986). How-

ever, in cooked fish the firmness of the fillet did

not depend on the connective tissue proteins since

after cooking the collagen, responsible for main-

taining the structure of the fillet, was gelatinized

by thermal action. In addition, this fact could be

related to the less noticeable differences in texture

parameters found in raw fillet.

Sensory assessment

On 0 dph when the fish is considered with maxi-

mum freshness, no effect of experimental diets was

found assessing whole gilthead sea bream with the

QIM (Fig. 5). Quality Index Method advantages

take place along with ice storage. Accordingly,

authors that have used QIM to compare different

treatments did not report differences on the 0 dph

(Huidobro et al. 2000; Huidobro, Mendes & Nunes

2001). Similarly, comparing wild and cultured

gilthead sea bream sensory attributes with the

TFRU system (Tasmanian Food Research Unit), a

QIM-like method, no differences were found on the

0 dph (Alasalvar et al. 2002). The values granted

by the panel, increased along with storage time.

However, the tested diets did not produce signifi-

cant differences on the appearance of whole

gilthead sea bream. In markets and research, QIM

has proved to have excellent correlation with ice

Table 3 Fillet texture changes of gilthead sea bream fillet fed experimental diets during 17 days of ice storage

Days

in ice

FO 70LO 100LO FO 70LO 100LO

Hardness (N) Springiness (ratio)

0 6.23 � 1.47 5.93 � 1.64 6.01 � 1.93 0.52 � 0.01 0.55 � 0.079 0.50 � 0.11

2 6.13 � 4.58 5.11 � 6.60 4.91 � 7.81 0.41 � 0.11 0.37 � 0.018 0.39 � 0.04

4 4.83 � 1.14 5.00 � 1.35 4.56 � 1.33 0.38 � 0.04 0.36 � 0.025 0.37 � 0.04

7 4.60 � 9.80 4.68 � 2.49 4.46 � 1.54 0.36 � 0.09 0.34 � 0.066 0.32 � 0.02

10 4.04 � 5.54 4.29 � 4.76 4.40 � 4.01 0.34 � 0.08 0.34 � 0.043 0.31 � 0.01

14 4.02 � 3.47 3.77 � 1.03 4.40 � 5.31 0.33 � 0.02 0.33 � 0.035 0.30 � 0.08

17 3.51 � 6.09 3.41 � 7.36 3.72 � 3.38 0.33 � 0.01 0.30 � 0.060 0.29 � 0.03

Fracturability (N) Cohesiveness (ratio)

0 nd nd nd 0.24 � 0.04 0.26 � 0.04 0.22 � 0.03

2 1.94 � 0.21 2.39 � 0.53 2.22 � 0.83 0.17 � 0.02 0.15 � 0.03 0.29 � 0.19

4 1.88 � 0.15 1.92 � 0.41 2.01 � 0.93 0.17 � 0.02 0.15 � 0.02 0.18 � 0.02

7 1.83 � 0.75 1.61 � 0.47 1.43 � 0.26 0.17 � 0.02 0.17 � 0.02 0.18 � 0.02

10 1.75 � 0.48 1.28 � 0.37 1.38 � 0.29 0.16 � 0.03 0.17 � 0.02 0.17 � 0.03

14 1.26 � 0.17 1.15 � 0.14 1.27 � 0.12 0.16 � 0.02 0.16 � 0.02 0.14 � 0.03

17 1.23 � 0.12 0.98 � 0.08 1.13 � 0.05 0.13 � 0.02 0.15 � 0.02 0.13 � 0.22

Gumminess (N) Adhesiveness (N*sg)

0 1.48 � 0.50 1.25 � 0.32 0.97 � 0.40 �44.69 � 26.15 �36.67 � 9.31 �33.98 � 7.50

2 0.94 � 0.18 0.93 � 0.40 0.92 � 0.93 �51.78 � 12.78 �51.82 � 11.03 �50.55 � 24.08

4 0.80 � 0.20 0.78 � 0.15 0.82 � 0.29 �57.06 � 5.20 �64.09 � 17.99 �61.43 � 11.91

7 0.67 � 0.11 0.66 � 0.20 0.80 � 0.14 �56.47 � 28.62 �54.80 � 14.03 �50.78 � 7.65

10 0.57 � 0.21 0.57 � 0.17 0.72 � 0.23 �54.66 � 18.32 �50.62 � 7.56 �49.04 � 12.10

14 0.42 � 0.12 0.51 � 0.18 0.51 � 0.12 �53.50 � 14.30 �49.62 � 19.52 �47.82 � 11.23

17 0.43 � 0.25 0.50 � 0.15 0.43 � 0.56 �46.69 � 38.59 �47.92 � 27.58 �42.06 � 19.07

Chewiness (N) Resilience (ratio)

0 0.76 � 0.26 0.68 � 0.19 0.49 � 0.25 0.14 � 0.02 0.16 � 0.02 0.14 � 0.03

2 0.30 � 0.05 0.31 � 0.13 0.30 � 0.24 0.09 � 0.02 0.08 � 0.01 0.09 � 0.00

4 0.29 � 0.04 0.27 � 0.05 0.30 � 0.11 0.08 � 0.01 0.08 � 0.01 0.09 � 0.01

7 0.28 � 0.09 0.24 � 0.81 0.21 � 0.08 0.08 � 0.01 0.08 � 0.01 0.08 � 0.02

10 0.20 � 0.09 0.20 � 0.49 0.24 � 0.08 0.08 � 0.01 0.08 � 0.01 0.08 � 0.06

14 0.20 � 0.02 0.20 � 0.07 0.16 � 0.05 0.07 � 0.01 0.07 � 0.01 0.08 � 0.00

17 0.14 � 0.07 0.10 � 0.06 0.18 � 0.07 0.07 � 0.00 0.07 � 0.00 0.07 � 0.01

FO, fish oil; LO, linseed oil. Data expressed as mean values � SD. n = 6. nd, no determined.



storage time and on the basis of the QIM it is pos-

sible to develop a calibration curve of the total

number of demerit points assigned to the fish

(quality index) against time of storage in ice. Thus,

a regression coefficient of 0.954 (p < 0.01) was

determined for the experimental diets. Alasalvar

et al. (2002) compared cultured and wild gilthead

sea bream during 23 days of ice storage by using

TFRU system. The authors reported differences on

raw sensory attributes only from 16 day on ice

on, and considered as feasible causes capture

stress, initial microbial flora and fat content. In

this study, capture stress and initial microbial flora

are supposed to be similar in both sampling due to

the controlled growing conditions. Fatty acids vari-

ations (Table 2) did not cause outward appearance

changes during the 17 dph.

Sensory assessment of the cooked fillet was not

significantly (p < 0.05) affected by LO substitution

in gilthead sea bream diets. These results, analyzed

using PCA (Fig. 6), showed that the fillet of the

gilthead sea bream fed LO diets had the highest

cohesive texture and a whiter and more compact

appearance after cooking. 100LO fillets were juic-

ier and more adhesive to the teeth. On the other

hand, those fillets of the gilthead sea bream fed FO

diet maintained the residual aftertaste for longer.

Table 4 Fillet cooked fillet texture of gilthead sea bream fed experimental diets on the slaughter day (0 days of ice

storage)

Hardness (N) Springiness (ratio) Cohesiveness (ratio) Gumminess (N)

FO 3.73 � 1.05b 0.48 � 0.05a 0.37 � 0.02ab 1.40 � 0.42b

70LO 3.17 � 1.31ab 0.53 � 0.07b 0.38 � 0.03b 1.21 � 0.56ab

100LO 2.85 � 0.9a 0.51 � 0.06ab 0.35 � 0.06a 0.98 � 0.33a

Adhesiveness (N*sg) Chewiness (N) Resilience (ratio)

FO �14.08 � 23.60 0.67 � 0.19b 0.12 � 0.01a

70LO �7.84 � 9.77 0.63 � 0.27ab 0.13 � 0.02b

100LO �6.73 � 8.54 0.50 � 0.18a 0.13 � 0.02b

FO, fish oil; LO, linseed oil. Different letters (a, b) in a column denote statistically significant differences in respect with diets

(p < 0.05). Data expressed as means values � SD. n = 6.

Figure 6 Biplot for the first two

principal components (PC1 and

PC2) of the principal component

analysis (PCA) analysis of the sen-

sorial assessment fillet values of

whole sea bream fed experimental

diets during ice storage. PC1 and

PC2 explained 69% and 31% of

the variations in the data set,

respectively. Main sensorial codes

where A, Aspect; T, Texture; AT,

after taste.

0246810121416

0 2 4 7 10 14 17

Qua

lity

inde

x

Days in ice

Quality index method (QIM)

FO

70LO

100LO

Figure 5 Changes of Quality Index Method (QIM) val-

ues of whole sea bream fed experimental diets during

ice storage. Data expressed as means values � SD.

n = 12.



Within odour parameters, oily values were slightly

higher on 70LO diets than FO or 100LO. Similarly,

flavour and appearance obtained higher scores on

fish fillet fed 70LO diet on all tested attributes.

Flavour should confirm the assessment based on

odour. Thus, ‘oily’ and ‘atypical’, both associated

with odour and flavour, yielded similar results to

those found when fed FO diet. Linseed oil as FO

substitute, even with a 100% substitution level did

not change the sensory perception determined

with traditional FO diet. In this sense, when par-

tial LO substitution was tested on gilthead sea

bream diets, flavour was considered not signifi-

cantly different from fish fed FO (Izquierdo et al.

2003, 2005). In this study, differences were not

found on residual taste and general acceptance on

the fillet of fish fed experimental diets.

Hardness, which was considered different with

FO and 100LO diets using cooked texturometer

analysis (Table 3), was not statistically different

with cooked sensory assessment. Izquierdo et al.

(2003) reported a slightly, non significantly, differ-

ent hardness acceptance on the fillet of gilthead sea

bream fed 60% LO diet than with FO control diet

with cooked sensory assessment. Also within sen-

sory texture attributes, juiciness has been related to

fatty acids profile. The sensory perception of fatness

could improve with diets containing 60% or 80% of

LO, based on the lower content in SFA and higher

in PUFA comparing with FO diet (Izquierdo et al.

2005; Castro et al. 2010). In this study, juiciness

differences were not statistically significant, in spite

of reaching higher scores on fillet fed 100LO diet

that those found with FO and 70LO diets.

Acceptance sensory score (Table 5) on 0 dph,

similarly to sensory cooked assessment, did not

produce statistically significant differences on the

gilthead sea bream fillet fed experimental diets. On

the 7th and 10th dph with FO diet, a significant

highest value of flavour was record (Table 5).

Those values affected ‘Acceptance’ which was

found higher with FO diet on the 7th dph. Unlike

the sensory assessment, acceptance assessment is

not expected to detect slight variations in the

tested attributes. The main objective is to deter-

mine when the fish becomes unacceptable and

especially, in this case, if LO inclusion on gilthead

sea bream diets modifies shelf life reducing post-

cooking fish acceptance. In this scale, 10 showed

absolutely fresh fish and �3 showed completely

putrid or spoiled (Alasalvar et al. 2001). A score of

around five was granted in all tested attributes on

day 17, not being considered unacceptable by the

members of the panel, regardless of the diet.

Table 5 Sensory scores for flavour, odour, texture and acceptance for sea bream fed experimental diets during ice

storage

0 2 4 7 10 14 17

Flavour days in ice

FO 9.9 � 0.3 9.5 � 0.5 7.9 � 1.3 8.1 � 0.9b 7.1 � 0.9b 6.1 � 1.0 5.5 � 0.9

70LO 10.0 � 0.0 9.3 � 0.6 8.3 � 1.2 7.4 � 0.8ab 6.1 � 1.3a 5.6 � 0.9 5.1 � 0.8

100LO 9.9 � 0.3 9.1 � 0.8 8.0 � 1.5 7.1 � 1.2a 6.4 � 0.8ab 5.9 � 1.1 5.2 � 0.9

Odour days in ice

FO 10.0 � 0.0 9.3 � 0.5 8.4 � 0.6 8.3 � 0.9 7.5 � 1.2 6.9 � 1.1 6.5 � 0.9

70LO 10.0 � 0.0 9.5 � 0.7 7.9 � 0.7 7.5 � 1.4 6.9 � 1.0 6.6 � 1.0 5.9 � 1.1

100LO 10.0 � 0.0 8.9 � 0.7 7.9 � 1.0 7.5 � 0.8 6.7 � 1.2 6.7 � 1.4 5.9 � 0.9

Texture days in ice

FO 9.9 � 0.3 9.3 � 0.7 8.2 � 1.3 7.9 � 1.2 6.9 � 1.0 6.2 � 1.2 5.5 � 1.0

70LO 10.0 � 0.0 9.1 � 0.6 7.8 � 0.9 7.3 � 0.9 6.5 � 1.4 6.1 � 1.2 5.3 � 0.9

100LO 9.9 � 0.3 8.7 � 1.1 7.8 � 1.3 7.3 � 0.7 6.6 � 1.2 6.0 � 1.4 5.1 � 0.7

Acceptance days in ice

FO 10.0 � 0.2 9.4 � 0.4 8.2 � 0.9 8.1 � 0.9b 7.2 � 0.8 6.4 � 1.0 5.9 � 0.7

70LO 10.0 � 0.0 9.3 � 0.4 8.0 � 0.8 7.4 � 0.8ab 6.5 � 1.0 6.1 � 0.9 5.5 � 0.5

100LO 10.0 � 0.2 8.9 � 0.7 7.9 � 1.1 7.3 � 0.7a 6.6 � 0.8 6.2 � 1.2 5.4 � 0.6

FO, fish oil; LO, linseed oil. Different letters (a-b) in a column denote statistically significant differences in respect with diets

(p < 0.05). Data expressed as means values � SD. n = 6.



Comparing gilthead sea bream slaughtering meth-

ods (Huidobro et al. 2001) or comparing wild and

cultured gilthead sea bream (Alasalvar et al.

2001) reported no significant differences through-

out chilled shelf life. When FO substitution is

tested, the different fatty acid composition

produced by diet substitution (Table 1) could lead

to different formation of volatile compounds, by

specific spoilage organisms. However, cooking may

mask changes otherwise noticeable and remove

the odour changes, being assessed as acceptable or

considered without differences.

Conclusions

The result of this study indicates that partial and

complete LO substitution does not substantially

modify gilthead sea bream quality from a physico-

chemical point of view. As a favourable factor, a

decrease in the lipid oxidation susceptibility mea-

sured as TBA values was found when the fish is

fed LO diets. Quality, evaluated as raw and cooked

texture and sensory assessment, were affected by

texture parameter only on cooked fillet of gilthead

sea bream fed LO diets.

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Linseed oil inclusion in sea bream diets: effect on muscle quality and shelf life

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