Aquaculture Nutrition - QIM Eurofish · 2013-02-13 · Cobia, Rachycentron canadum (500 g) cultured in pond cages for a 3-month experiment were fed two moist diets based on raw ﬁsh

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1 Applied and Industrial Biology, Department of Biology, University of Bergen, Bergen, Norway; 2 Aquaculture Faculty, Nha

Trang University, Nha Trang, Khanh Hoa, Vietnam

Cobia, Rachycentron canadum (500 g) cultured in pond

cages for a 3-month experiment were fed two moist diets

based on raw fish with or without added fish silage. No sig-

nificant differences in nutritional composition were

observed between the fillet groups, which were of high

quality with a balance of essential and non-essential amino

acids (EAA/NEAA = 1) and medium levels of omega-3

fatty acid composition (210 g kg�1 total fatty acids). The

total quality index method and quantitative descriptive

analysis from both groups were correlated throughout stor-

age (r2 = 0.83–0.86). After 15 days iced storage, the scores

of most attributes were low compared to maximum

accepted values. The thiobarbituric acid reactive substances

and microbial counts were also below the accepted limits

after the storage trial. It might be concluded that the nutri-

tional composition and the fillet quality were similar for

the groups fed raw fish with or without added fish silage,

and the estimated shelf life for cobia was >15 days.

KEY WORDS: cobia, fish silage, quality index method, quanti-

tative descriptive analysis, sensory evaluation, shelf life

Received 31 January 2012; accepted 22 May 2012

Correspondence: Diep T. N. Mach, Applied and Industrial Biology,

Department of Biology, University of Bergen, PO Box 7800, 5020

Bergen, Norway. E-mail: [email protected]

Cobia, Rachycentron canadum Linnaeus (1766), with excel-

lent characteristics, for example good fillet quality, high

commercial prices and fast growth, are considered to be a

noteworthy candidate species for commercial aquaculture.

As a potential species in aquaculture, cobia research has

been devoted to nutritional demands, diseases and aquacul-

ture conditions, while work on fillet quality and storage of

cobia fillets has not been published.

Sensory evaluation is considered as a rapid, cost-efficient

and accurate method for the assessment of quality, shelf

life and storage conditions of food (Nielsen 1997; Martins-

dottir 2002). The first sensory method was developed by

Torry Research Station (Shewan et al. 1953). The new

method, the Quality Index Method (QIM), is based upon a

scheme that was developed by the Tasmanian Food

Research (Bremner 1985). Now, QIM has been developed

for many species in Europe and Nordic countries (Larsen

et al. 1992; Huss 1995) including: Red fish Sebastes marinus

(Martinsdottir & Arnason 1992), Atlantic mackerel, horse

mackerel and European sardines (Andrade et al. 1997),

brill, dab, haddock, pollock, sole, turbot and shrimp

(Luten 2000), and gilthead sea bream (Huidobro et al.

2000), Atlantic salmon (Sveinsdottir et al. 2002, 2003), her-

ring (Jonsdottir 1992; Nielsen & Hyldig 2004), common

octopus (Barbosa & Vaz-Pires 2004), flounder (Massa et al.

2005), Atlantic halibut Hippoglossus hippoglossus (Guil-

lerm-Regost et al. 2006), cuttlefish and shortfin squid (Vaz-

Pires & Seixas 2006), hybrid striped bass (Nielsen & Green

2007), and Atlantic cod (Jonsdottir 1992; Bonilla 2004;

Bonilla et al. 2007). The most commonly used attributes

for fish are the appearance of eyes, skin and gills, together

with odour and texture. The development of QIM for a

particular seafood or fish species involves the selection of

appropriate and best fitting attributes to observe a linear

increase in the QI during iced storage time.

The maximum storage time and thus the limit of rejec-

tion of fish can be determined by the sensory evaluation of

cooked samples using Quantitative Descriptive Analysis

(QDA) (Stone & Sidel 1993; Huss 1995; Sveinsdottir et al.

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ª 2012 Blackwell Publishing Ltd

2012 doi: 10.1111/j.1365-2095.2012.00969.x. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Aquaculture Nutrition

2002, 2003; Bonilla et al. 2007; Nielsen & Hyldig 2004).

The results from QDA should be used as a reference when

developing QIM for fresh fish.

The aim of this study was to determine whether inclusion

of fish silage in a raw fish diet has an influence on fillet qual-

ity and storage of cobia, through comparing the nutritional

composition of fillets and shelf life of whole gutted fish by

sensory evaluation throughout iced storage. In comparison

with sensory evaluation, microbial growth and lipid oxida-

tion were also investigated in fillets during iced storage.

Twenty-five cobia (500 g) were randomly placed in each of

six cages (2 diets 9 3 replicates) in a pond at the Institute

of Aquaculture Research – Nha Trang University at Cam-

Ranh district, Khanh Hoa province, Vietnam, for

3 months. Temperature (25.9–31.9 °C), salinity (32.2–

37.0 g L�1), pH (7.9–8.5) and dissolved oxygen (4.2–

8.1 mg L�1) of water in the pond were measured twice per

day (6:00 and 14:00) with YSI 556 Multi-parameter. Two

moist diets based on raw lizardfish (Saurida undosquamis)

with or without added lizardfish silage were fed in the pres-

ent experiment (Table 1). Lizardfish was purchased from a

local market in Cam Ranh district, Khanh Hoa province,

Vietnam. Lizardfish was minced and mixed with 25 g kg�1

of formic acid (85%) and 2.2 g kg�1 of potassium sorbate

to prevent the growth of bacteria and fungi, respectively

(Mach & Nortvedt 2009). Because lizardfish is lean fish

(crude lipid <10 g kg�1), antioxidants were not added in

the silage (Mach & Nortvedt 2009). The silage was stored

in 100-L plastic buckets indoors at ambient temperature

(28 ± 3 °C) and stirred daily. After 2 weeks, the silage was

solar-dried to achieve a moisture content of approximately

450 g kg�1. The feed was prepared with silage stored for

1 month. Fresh moist pellets were made every 3 or 4 days

and stored in a refrigerator at 5 °C. All fish were healthy

and survived until the termination of the experiment.

After 3 months, 100 cobia (50 fish of each dietary group)

were randomly sampled after 24-h starvation. Individual

fish was killed by a strong blow to the head and the gills

were cut. After measuring length and weight, the fish were

immersed for a few seconds in water containing

100 mg L�1 chlorine and then gutted. Fish were rinsed

with water containing 50 mg L�1 chlorine before filleting

or packing. Viscera and livers were weighed for the calcula-

tion of a viscera somatic index (VSI) and hepatic somatic

index (HSI). The raw biological data (weight, VSI and

HSI) showed no significant differences within replicated

groups, and sampling was therefore designed based on

pooled dietary groups. Seven cobia of each dietary group

were sampled for chemical and microbiological analyses.

One side of fillets was immediately collected after filleting

and stored at �80 °C for crude protein, total lipid, amino

acid and fatty acid analyses, while the matching fillets from

the other side of the fish were stored in ice (0 °C) and sam-

pled after 5, 10 and 15 storage days for lipid oxidation and

microbiological determination. For shelf life study, 35

cobia of each dietary group were used. The gutted fish were

packed in plastic bags and eventually stored in ice boxes at

0 °C for sensory evaluation of the shelf life after 3, 5, 7, 9,

11, 13 and 15 storage days. The remaining eight fish of

each dietary group were used for training purposes.

Quality assessment schemes for cobia in the study were

developed based on references (EEC 1976; Howgate et al.

1992; Jonsdottir 1992; Larsen et al. 1992; Huss 1995). QIM

was applied to estimate the freshness and quality of the

gutted cobia during storage. Moreover, QDA was also used

Table 1 Formulation and composition of the experimental diets

Diet A Diet B

Ingredient (g kg�1)

Raw fish (moisture 780 g kg�1) 800 600

Fish silage (moisture 440 g kg�1) 200

Fish meal 80 80

Wheat 53 40

Fish and plant oil (1 : 1) 50 50

Premix-Vitamin and mineral 10 10

Sodium alginate 7 20

Proximate composition (g kg�1)

Dry matter 377.1 405.9

Crude protein (dry wt) 483.0 459.4

NPN (g kg�1 total N) 164.7 288.8

Crude lipid (dry wt) 184.3 171.5

Ash (dry wt) 167.8 194.2

pH 7.76 7.06

1 Premix-vitamin and mineral (9100-Vemedin) consisted of

(mg kg�1 wet diets): retinol 4000 IU; cholecalciferol 800 IU;

tocopherol 12; ascorbic acid 60; menadione 2.4; niacin 10; thia-

min 1.6; riboflavin 3; pyridoxol 1; folic acid 0.32; inositol 15; cho-

line chloride 48; calcium pan 4; iron 200; zinc 110; manganese 20;

magnesium 75; copper 100; cobalt 1.2; iodine 0.04; methionin 30;

lysine 25; Supplied by Veterinary Stock Company (Vietnam).

NPN, Non-Protein Nitrogen.

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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd

as a reference for the developed QIM scheme to determine

the maximum storage time of the fish. Flesh from the gut-

ted fish was cooked and consumed by a panel to determine

whether it was acceptable after storage.

Ten trained assessors in a sensory testing panel from the

Quality of Seafood Department, Seafood Processing Tech-

nology Faculty, Nha Trang University participated in the

development and evaluation of the QIM and QDA schemes

(Tables 2 & 3). Thirty-five cobia from each group (five fish

per storage time) were assessed after 3, 5, 7, 9, 11, 13 and

15 storage days. Each cobia was coded with a random

number unrelated to storage time. Fish were randomly and

individually assessed according to the QIM principles.

After QIM assessment, the fish were used for QDA evalua-

tion. After filleting, three pieces (3 9 3 cm, with skin) were

prepared from both fillets of each fish. The six coded pieces

were individually placed in aluminium boxes and cooked in

a steam oven at 100 °C for 7 min; afterwards, the pieces

were randomly served to each panellist for sensory evalua-

tion based on the QDA scheme.

The samples were analysed at different laboratories in Viet-

nam. Total nitrogen (N) was determined by the combustion

method (CHNS-O Analyzer Model FLASH EA 1112 series

made by Thermo Finnigan, Italy) at Nha Trang Oceanog-

raphy Institute, and crude protein was estimated as

N 9 6.25. Non-protein nitrogen (NPN) in diets was

extracted in 20% tricloroacetic acid (Backhoff 1976), and

nitrogen was determined by the same method and labora-

tory as crude protein. Amino acid and fatty acid composi-

tions were determined at the Advanced Laboratory, Can

Tho University. Total amino acids were determined with

the EZ:faast LC/MS kit (Phenomenex, Torrance, CA,

USA) by LC/MS (Finnigan LCQ Advantage Max, Wal-

ham, MA, USA); however, tryptophan was not determined

in this study because of the high costs of these specific

analyses. The lipids from the fillets and diets were extracted

using chloroform/methanol (2 : 1, v/v) and analysed for

fatty acid composition as described by Lie et al. (1986)

with GLC (Carlo Erba Vega GLC, CAE, Redwood

City, CA, USA). Moisture, ash, pH and crude lipid were

Table 3 The quantitative descriptive analysis (QDA) (scheme

developed for cooked cobia

Parameter Description Score

Odour Sweet fresh fish 0

Metallic 1

Oily 2

Off odour 3

Colour White 0

Whitish 1

Yellow 2

Texture Stiffness 0

Little softness 1

Softness 2

Flavour Sweet of fresh fish 0

Metallic 1

Sourness 2

Strong sourness 3

QDA total 0–10

Table 2 The quality index method scheme developed for cobia

Parameter Description Score

Skin

Colour Natural colour (black–silver) 0

Some reduction in lustre and

colour

1

Distinct reduction 2

Mucus Transparent and not clotted 0

Milky and clotted 1

Yellow and clotted 2

Odour Fresh seaweedy 0

Slightly seaweedy, neutral 1

Rancid 2

Rotten 3

Eyes

Pupils Bright and clear 0

Cloudy 1

Matt 2

Shape Convex 0

Flat 1

Sunken 2

Belly

Blood in abdomen Red 0

Light red 1

Brownish colour 2

Odour Fresh sea odour 0

Neutral 1

Slight sourness 2

Strong sour to spoilt odour 3

Gills

Colour Red 0

Light red 1

Grey, green 2

Mucus Transparent 0

Yellow, clotted 1

Brown 2

Odour Seaweedy 0

Neutral 1

Sour 2

Rotten 3

Texture

Elasticity Finger mark returns quickly (<2 s) 0

Finger mark returns slowly (>3 s) 1

Finger leaves mark 2

Quality index total 0–25

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determined at the laboratories of the Institute of Biotech-

nology and Environment, Nha Trang University. Moisture

was determined by oven-drying at 105 °C for 48 h. Ash

content was determined by combustion at 550 °C in a muf-

fle furnace for 24 h. Crude lipid was determined gravimetri-

cally after extraction with ethyl acetate (Losnegard et al.

1979). Lipid oxidation was estimated by the thiobarbituric

acid reactive substances (TBARS) method (Pikul et al.

1989). The pH was determined according to Fagbenro &

Jauncey with a digital pH meter (Omega, Stamford, CT,

USA). Microbiological counts were determined immedi-

ately after sampling by the ‘Aerobic Plate Count at 30 °C:

Surface plate method’ (Health Protection Agency 2004).

The StatisticaTM (version 7.0) software program (StatSoft,

Inc. Tulsa, OK, USA) was used for one-way analysis of var-

iance (ANOVA). Significant differences (P < 0.05) between

means were tested by Duncan’s multiple range test, accord-

ing to Duncan (1955). Multivariate correlations between

objects and variables were revealed by principal component

analyses (PCA) using the SiriusTM (version 7.0) software

program (Pattern Recognition Systems AS, Bergen, Nor-

way), according to Kvalheim & Karstang (1987). The reason

for applying PCA is its ability to reveal complex correlation

patterns between variables (loadings) and samples (scores),

for example found in the present study between the loadings

of the QIM variables and the storage days.

The two diets had similar content of crude protein and

crude lipids (Table 1). Levels of dry matter, NPN and ash

were higher in diet B than in diet A, but pH was lower in

diet B than in diet A (Table 1). The fatty acid and amino

acid compositions of the two diets were mostly the same

(Tables 4 & 5). Fatty acids in the diets consisted mainly of

monounsaturated fatty acids [MUFA, >436 g kg�1 total

fatty acids (TFA)], while polyunsaturated fatty acids

(PUFA) accounted for only 240 g kg�1 TFA, which n-6

PUFA dominated (670–700 g kg�1 total PUFA, Table 5).

No significant differences (P > 0.05) in nutritional quality

of the fillets were observed between the two cobia groups

fed diets with or without added fish silage (Tables 4–6).

Nutritional composition of fish fillets varies greatly from

species to species and individual to individual, which

depends on feed intake, sex, size, reproductive status, geo-

graphic location, seasonal changes and tissue. According to

Stansby (1962) and Love (1970), lipid content in fish fillets

ranges from 2 to 250 g kg�1, while protein levels range

from 160 to 210 g kg�1 total fillets. In the present study,

lipid content of cobia fillet was 31.6–36.1 g kg�1 and pro-

tein was 197–203 g kg�1. Cobia fillets showed a balance of

essential amino acid (EAA), and non-essential amino acid

(NEAA) compositions with ratios of EAA/NEAA were

approximately equal 1. EAA comprised high levels of

lysine (112.1–116.5 g kg�1 protein) and leucine (81.6–87.4).

Amino acid profile of the cobia fillets was quite similar to

that of rainbow trout (EAA/NEAA ratios about 1.1) (Unu-

san 2007), and no significant difference in the fatty acid

composition was found between the two fillet groups. The

three groups of fatty acids – saturated fatty acid (SFA),

MUFA and PUFA in cobia fillet – were divided into quite

similar proportions from 300 to 330 g kg�1 TFA (Table 5).

PUFA accounted for approximately 303 g kg�1 TFA. Con-

trary to the diets, PUFA in the cobia fillet comprised

mainly n-3 PUFA (690 g kg�1 total PUFA), which con-

sisted mainly of docosahexaenoic acid (DHA, 22:6n-3; 460–

490 g kg�1 total PUFA) and eicosapentaenoic acid (EPA,

20:5n-3; 110–120 g kg�1 total PUFA). MUFA shared

Table 4 Amino acid composition in the experimental diets (n = 3)

and in the cobia fillets (n = 7) (g kg�1 protein)

Amino acids

Diets Fillets

A B A B

Arginine 47.7 45.6 39.6 ± 0.4 39.5 ± 0.5

Serine 30.6 26.3 38.1 ± 0.7 39.2 ± 0.5

Hydroxyproline 10.6 9.3 8.7 ± 0.2 8.6 ± 0.2

Glycine 69.3 66.6 50.3 ± 0.5 50.7 ± 0.5

Threonine 17.3 15.8 30.8 ± 0.5 29.8 ± 0.6

Alanine 50.6 53.7 56.5 ± 0.8 61.9 ± 1.9

Proline 73.3 75.1 48.5 ± 0.5 47.7 ± 0.8

Methionine 22.1 20.0 23.4 ± 0.6 27.6 ± 0.7

Aspartic acid1 64.9 67.2 76.3 ± 1.0 78.9 ± 1.3

Valine 54.1 53.4 50.0 ± 0.7 50.2 ± 1.0

Histidine 33.5 39.4 30.6 ± 0.9 32.0 ± 1.2

Lysine 95.0 95.4 116.5 ± 2.6 112.1 ± 2.6

Glutamic acid1 121.3 121.6 118.0 ± 2.2 116.8 ± 1.2

Leucine 80.9 87.4 81.6 ± 1.2 84.2 ± 1.3

Phenylalanine 39.9 39.5 40.6 ± 0.4 40.8 ± 1.1

Isoleucine 54.1 54.3 47.7 ± 0.8 53.9 ± 1.5

Cystine 5.1 5.4 8.1 ± 0.1 8.3 ± 0.1

Tyrosine 29.1 28.5 36.3 ± 0.6 38.3 ± 1.5

1 Aspartic acid included asparagine; Glutamic acid included gluta-

mine; Tryptophan was not analysed in the AA analyses.

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approximately 310 g kg�1 TFA and was composed domi-

nantly of C18:1n-9 (640–650 g kg�1 total MUFA). Simi-

larly, SFA accounted for 328 g kg�1 TFA and comprised

mainly C16:0 (610–620 g kg�1 total SFA).

The present result was consistent with results in commer-

cial cobia (5–6 kg) by Liu et al. (2009). PUFA, MUFA

and SFA consisted mainly of DHA (481–499 g kg�1 total

PUFA), C18:1n-9 (715–722 g kg�1 total MUFA) and

C16:0 (550–595 g kg�1 total SFA), respectively (Liu et al.

2009). PUFA in the present study was higher (303 versus

177 g kg�1 TFA), but n-3 PUFA composition was lower

(690 versus 838 g kg�1 total PUFA) than that in the study

by Liu et al. As a marine fish, cobia can convert n-6 PUFA

from diets into n-3 PUFA and their PUFA consist mainly

n-3 PUFA. Similar results were reported in Atlantic salmon

where they were fed diets with and without added fish

silage (Lie et al. 1988; Heras et al. 1994). Minor differences

in fatty acid composition were observed between the two

salmon fillet groups, and PUFA levels accounted for 245–

291 g kg�1 total TFA, with n-3 PUFA dominant (680–

810 g kg�1 total PUFA). By comparison with Atlantic cod

fillets, PUFA composition in cobia fillets was lower (300

versus 520–610 g kg�1 TFA) (Ackman & Burgher 1964;

Jangaard et al. 1967; Addison et al. 1968; Lie et al. 1986),

but similar to sea bass or sea bream fillets (Testi et al.

2006; Yanar et al. 2007; Yildiz et al. 2008). In cod fillets,

DHA accounted for 560–600 g kg�1 total PUFA and the

ratios of n-3/n-6 ranged between 7.7–15.2, whereas the

ratio for the cobia fillets in the present study was 2.3. The

total fat content (32–36 g kg�1) in the present cobia fillets

was, however, higher than observed in Atlantic cod.

Sensory evaluation for gutted cobia No significant differ-

ences were observed in scores for the attributes and total

QI (sum of all attributes) between two cobia groups

throughout trial (Fig. 1). The correlations (r2 = 0.83–0.84)

between the total QI scores and the storage time in the

present study was higher than that (r2 = 0.74) in the study

by Martinsdottir et al. (2001), but lower (r2 = 0.85–0.99)

compared to the studies by Sveinsdottir et al. (2003), Niel-

sen & Hyldig (2004), Nielsen & Green (2007) and Bonilla

et al. (2007). The values showed that the attributes gradu-

ally and naturally decayed throughout the storage time.

Scores for the attributes increased more sharply in gills

Table 6 Length, weight and biological indices of cobia (n = 3);

and dry matter, lipid, protein and ash content of fillet cobia fed

with the experimental diets (n = 7: seven cobia per pooled dietary

group); Mean ± SEM (g kg�1, wet wt)

Cobia A Cobia B

Final weight (kg) 1.49 ± 0.03 1.56 ± 0.04

Total length (cm) 61.18 ± 0.54 61.32 ± 0.66

Viscera somatic index 69.9 ± 0.3 70.4 ± 1.9

Hepatic somatic index 12.1 ± 0.5 12.0 ± 1.0

Fillet yield 498.5 ± 0.5 476.6 ± 1.7

Dry matter in fillets 246.3 ± 5.1 244.5 ± 2.6

Fat in fillets 36.3 ± 3.1 31.6 ± 2.1

Protein in fillets 203.3 ± 2.4 196.9 ± 2.8

Ash in fillets 13.6 ± 0.3 13.4 ± 0.4

Table 5 Fatty acid composition in the experimental diets (n = 3)

and the cobia fillets (n = 7) (g kg�1 total fatty acids)

Fatty acids

Diets Fillets

A B A B

14:0 14 15 37.2 ± 0.1 42.8 ± 1.9

15:0 3 3 7.7 ± 0.2 7.9 ± 0.2

16:0 227 218 203.4 ± 2.2 203.5 ± 2.6

16:1n-9 3 3 6.5 ± 0.0 6.2 ± 0.1

16:1n-7 18 18 58.3 ± 1.6 61.1 ± 1.2

17:0 4 4 9.0 ± 0.3 9.1 ± 0.3

16:2n-4 2.1 ± 0.1 2.7 ± 0.1

18:0 49 51 65.3 ± 1.4 67.9 ± 0.8

18:1n-9 362 364 204.4 ± 1.0 196.4 ± 5.4

18:1n-7 21 21 30.1 ± 0.0 29.7 ± 0.9

18:2n-6 158 151 36.0 ± 1.4 36.7 ± 1.1

20:0 3 3

18:3n-3 11 12 6.0 ± 0.2 6.9 ± 0.1

20:1n-11 2 2 1.2 ± 0.6 0.7 ± 0.7

20:1n-9 22 23 7.2 ± 0.1 7.0 ± 0.5

20:1n-7 1 2 0.8 ± 0.4 0.9 ± 0.5

18:4n-3 4.3 ± 0.2 4.8 ± 0.1

20:2n-6 2 2 2.6 ± 0.1 2.4 ± 0.1

22:0 2 2

20:3n-6 1.7 ± 0.1 1.8 ± 0.1

20:4n-6 9 10 27.1 ± 0.2 27.9 ± 2.4

22:1n-11 2 2

22:1n-9 2 2

20:4n-3 1 2 2.9 ± 0.2 2.9 ± 0.2

20:5n-3 12 14 32.9 ± 1.6 34.9 ± 1.3

24:0 1.1 ± 0.5 0.9 ± 0.4

24:1n-9 3 3 3.6 ± 0.2 3.5 ± 0.1

22:4n-6 7.2 ± 0.2 6.9 ± 0.1

22:5n-3 6 8 17.5 ± 0.3 17.5 ± 0.1

22:5n-6 15.8 ± 0.4 15.3 ± 0.8

22:6n-3 41 46 147.0 ± 2.9 139.4 ± 10.2

SFA 303 296 323.7 ± 3.2 332.0 ± 3.6

MUFA 436 439 312.2 ± 1.4 305.4 ± 8.0

PUFA 240 243 303.2 ± 4.9 302.1 ± 14.4

sum n-3 71 81 210.7 ± 4.7 208.3 ± 10.8

sum n-6 168 163 90.4 ± 0.8 91.1 ± 3.7

n3/n6 0.4 0.5 2.33 ± 0.06 2.28 ± 0.03

MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty

acids; SFA, saturated fatty acid.

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(colour and mucus), and eyes (shape) than in skin (colour,

mucus and odour), and texture during storage (Fig. 1).

PCA showed clearly the correlation between the parameters

and storage time. All attributes received high scores related

to storage time (gills, belly odour and eye shape), which

were located on the right side of the first principal compo-

nent axis that explained approximately 76.3% of the varia-

tion between the samples, while skin colour, texture, pupil

colour and abdomen blood, which received low scores,

were located on the opposite side (Fig. 2). At the end of

the storage time, the scores of the attributes were quite low

with the exception of those for gill colour, gill odour and

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Gill colour Skin colour Texture Pupil colour

Gill mucus Skin mucus Abdomen blood colour Eye shape

Gill odour Skin odour Belly odourIQ score

yA = 0.78x + 2.24; rA2 = 0.83 yB = 0.71x + 2.79; rB2 = 0.84

Figure 1 Mean ( ± SEM) scores of each quality attribute and sum of all attributes (QI) assessed with QIM scheme for gutted cobia (n = 5)

fed the experimental diets versus storage time; ■ fish fed diet A; * fish fed diet B. QIM, quality index method.

Comp. 1 (76.3%)

Com

p. 2

(11.

1%)

–0.90 –0.30 0.29 0.88 1.48–3.8

–1.0

1.9

4.7

7.6*10–1

Skin color ASkin color B

Skin mucus ASkin mucus B

Skin odour A

Skin odour B

Pupil color A

Pupil color B

Eye shape A

Eye shape B

Abdomen blood A

Abdomen blood BBelly odour ABelly odour B

Gill color A

Gill color B

Gill mucus A

Gill mucus B

Gill odour A

Gill odour 2

Texture A

Texture B

Day 3

Day 5

Day 7

Day 9Day 11Day 13

Day 15

Figure 2 Principal component analyses

(PCA) loading plot of QIM data from

gutted cobia (n = 5) fed the experimen-

tal diets A and B against storage time.

The two-component PCA model

explained 87.4% of the total variation

in the quality attributes. QIM, quality

index method.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


eye shape, which ranged 33–63% of the maximum scores

(2 or 3) given in the QIM scheme. Eventually, the total QI

score values were quite low (13.7–13.8), compared to the

maximum total value given in the scheme (25), (Fig. 1)

after 15 days stored in ice; therefore, QDA played an

important role in determining the shelf life of the fish in

the study.

Similarly, there were insignificant differences in scores

for the attributes and total QDA between the two cobia

groups throughout the trial (Fig. 3). The average scores for

the individual attributes fluctuated, but the total of all the

attributes increased with storage in ice. The correlation

(r2 = 0.86) between the total QDA and the storage time

indicates that the quality of the fillets gradually deterio-

rated with time. The highest scores for odour and flavour

attributes were approximately 40% of maximum values

given in the QDA scheme by the end of the storage, while

the highest values in colour and texture were 60–70% of

maximum (Fig. 3). At the end of the storage trial, the total

QDA score values were 4.6–5.0, compared to 10 scores of a

total QDA. Total QDA scores were significantly different

at the sampling times, but the distinctions were not clear

enough to be used as references for assessing the fish qual-

ity during storage. The fluctuated and low scores of the

attributes of sensory evaluation may be caused by confu-

sion about attributes, individual differences in the use of

the scale, or individual differences in precision (Næs et al.

1994), which are reflected in Fig. 4. The QIM evaluation of

cobia given by the individual panellist was variable. Panel-

lists 1, 2, 4, 5 and 9 gave stable increases in scores during

storage time, while the QI scores of the other panellists

fluctuated. The variation was lowest at day 3 and 15, but

highest at day 7 of storage in ice. This was probably due to

clearer quality attributes at the beginning and at the end of

the storage trial.

Lipid oxidation of cobia fillets Lipid oxidation is consid-

ered to be one of the most important factors responsible

0

1

2

3

0

1

2

0

1

2

0

1

2

3

Days in ice

3 5 7 9 11 13 15 3 5 7 9 11 13 15

1 3 5 7 9 11 13 150

2

4

6

8

10

Odour

Colour Texture

Flavour

QD

A sc

ore

Scor

eyA = 0.26x + 0.63; rA

2 = 0.86yB = 0.25x + 1.28; rB

2 = 0.86

Figure 3 Mean ( ± SEM) scores of

quantitative descriptive analysis (QDA)

of cooked cobia (n = 5) fed the experi-

mental diets against storage time; ■ fish

fed diet A; * fish fed diet B. Maximum

potential QDA score is 10.

3 5 7 9 11 13 15

Days in ice

0

4

8

12

16

20

IQ s

core

Figure 4 Average QI given by each QIM panellist in the shelf life

study of the gutted cobia fed diet A throughout iced storage. ■ 1,

* 2, ▲ 3, ○ 4, □ 5, ♦ 6, △ 7, ● 8, + 9, ♢ 10. QIM, quality index

method.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


for quality deterioration of fish during storage. In the pres-

ent study, no statistically significant differences in lipid oxi-

dation were observed between the two fillet groups during

iced storage (Fig. 5). TBARS values of the both groups

rapidly increased from day 5 to day 10 (7–17 nmol g�1 fil-

let), but slightly decreased at day 15 (16 nmol g�1 fillet).

The fatty acid composition of the two groups was similar,

which might explain the absence of significant differences

in rancid development between them. The reduction in

TBARS values at day 15 was probably due to deficiency of

substrate, for example free fatty acids. It is well known that

the initiation of lipid oxidation probably involves non-

enzymatic and enzymatic reactions. The development of

lipid oxidation depends on several factors, such as storage

period, temperature, presence of inhibitors or catalysts,

availability of oxygen, and degree of unsaturated fatty

acids (Maclean & Castell 1964; Castell et al. 1966; Castell

& Bishop 1969; Aubourg & Medina 1999; Erickson 2002).

Unsaturated fatty acids are known to be more susceptible

to oxidation than SFA because of lowered activation

energy in the initiation of free radical formation for triplet

oxygen auto-oxidation (Holman & Elmer 1947; Lea 1952).

Seafood, particularly fatty fish with highly unsaturated

fatty acid composition, is sensitive to oxidation during stor-

age, especially in iced storage. According to Nunes et al.

(1992), the limit of acceptability of lipid oxidation for fish

stored in ice is 70–110 nmol TBARS g�1 flesh (equal to 5–

8 mg of malondialdehyde kg�1 flesh). The TBARS values

in the present study were low compared to the limitation

during storage.

Microbial counts of cobia fillets No significant differences

were found in total aerobic plate counts (APC) between the

two fillet groups throughout the trial; even the mean APC

of fish fed diet A was lower than that of fish fed diet B at

day 15 (Fig. 5). The International Commission on Microbi-

ological Specifications for Food (ICMSF) recommends that

total APC should not exceed 107 cfu g�1 wet weight during

iced storage (ICMSF 1978). In the present trial, the total

aerobic bacterial counts slowly increased from day 5 to day

10 (0.25 9 104–1.68 9 104 cfu g�1) in both groups. The

values sharply increased at day 15 (9.55 9 104 in fish fed

diet A and 14.47 9 104 for fish fed diet B), but still satis-

fied this recommendation.

Based on the above results from QIM and QDA for the

gutted cobia, and for lipid oxidation and microbial counts

of the fillet, the quality of the cobia was probably accept-

able after 15 days stored in ice.

There were no significant differences in nutritional com-

position between the 2 9 3 pooled replicated cobia fillet

groups after 3 months feeding trial given the diets with

or without added fish silage, and no significant differ-

ences were observed in the shelf life study between the

two cobia groups. With high nutritional composition,

particularly balance of EAA and NEAA and high levels

of n-3 PUFA, cobia fillets demonstrate good quality,

compared to Atlantic cod or Atlantic salmon. The use of

the QIM and QDA schemes developed for the gutted

cobia in the present study showed clear correlations

between the attributes and storage time in ice. However,

the scores for most attributes were low compared to

maximum values given in the schemes by the end of the

trial, which was probably due to the short period of stor-

age. Moreover, the TBARS values and microbial counts

were below acceptable limits in the cobia fillets at the

end of storage. Consequently, the shelf life of the cobia

was estimated to be >15 days; thus, further studies are

needed in the future to estimate more accurately the shelf

life of this species.

Days in ice

0

5

10

15

20

CFU

g–1

5 10 155 10 15Days in ice

4

6

8

10

12

14

16

18

20

22TB

AR

S (n

mol

g–1

fille

t)

Figure 5 Means ( ± SEM) of lipid oxi-

dation (TBARS) and aerobic plate

counts (APC, cfu g�1 fillet) of cobia fil-

lets (n = 7) from the dietary trial versus

days in ice; ■ fish fed diet A; * fish fed

diet B. TBARS, thiobarbituric acid

reactive substances.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


The authors are grateful for financial support from the pro-

ject ‘Improving training research capacity at University of

Fisheries’ funded by NORAD (The Norwegian Agency for

Development Cooperation) (NORAD SRV 2701 project).

We also thank the Institute of Aquaculture Research, the

Institute of Biotechnology and Environment, and the Fac-

ulty of Seafood Processing Technology – Nha Trang Uni-

versity, Nha Trang Oceanography Institute, and the

Advanced Laboratory – Can Tho University in Vietnam

for assistance with facilities and to their colleagues for

advice and collaboration in the execution of the present

study.

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