Comparison of fillet composition and initial estimation of shelf life of

Aquaculture Nutrition
2012
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1,2
1
doi: 10.1111/j.1365-2095.2012.00969.x
1
Applied and Industrial Biology, Department of Biology, University of Bergen, Bergen, Norway;
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 significant 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 storage (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 nutritional 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, quantitative 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: diep.mach@bio.uib.no
Cobia, Rachycentron canadum Linnaeus (1766), with excellent characteristics, for example good fillet quality, high
commercial prices and fast growth, are considered to be a
noteworthy candidate species for commercial aquaculture.
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ª 2012 Blackwell Publishing Ltd
2
Aquaculture Faculty, Nha
As a potential species in aquaculture, cobia research has
been devoted to nutritional demands, diseases and aquaculture 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; Martinsdottir 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), herring (Jonsdottir 1992; Nielsen & Hyldig 2004), common
octopus (Barbosa & Vaz-Pires 2004), flounder (Massa et al.
2005), Atlantic halibut Hippoglossus hippoglossus (Guillerm-Regost et al. 2006), cuttlefish and shortfin squid (VazPires & 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 rejection 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.
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 quality 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 oxidation 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 CamRanh 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 present experiment (Table 1). Lizardfish was purchased from a
local market in Cam Ranh district, Khanh Hoa province,
Table 1 Formulation and composition of the experimental diets
Ingredient (g kg 1)
Raw fish (moisture 780 g kg 1)
Fish silage (moisture 440 g kg 1)
Fish meal
Wheat
Fish and plant oil (1 : 1)
Premix-Vitamin and mineral
Sodium alginate
Proximate composition (g kg 1)
Dry matter
Crude protein (dry wt)
NPN (g kg 1 total N)
Crude lipid (dry wt)
Ash (dry wt)
pH
Diet A
Diet B
800
600
200
80
40
50
10
20
80
53
50
10
7
377.1
483.0
164.7
184.3
167.8
7.76
405.9
459.4
288.8
171.5
194.2
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; thiamin 1.6; riboflavin 3; pyridoxol 1; folic acid 0.32; inositol 15; choline 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.
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 calculation 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 sampled 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
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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 gutted 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 Technology 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
Table 3 The quantitative descriptive analysis (QDA) (scheme
developed for cooked cobia
Parameter
Description
Score
Odour
Sweet fresh fish
Metallic
Oily
Off odour
White
Whitish
Yellow
Stiffness
Little softness
Softness
Sweet of fresh fish
Metallic
Sourness
Strong sourness
0
1
2
3
0
1
2
0
1
2
0
1
2
3
0–10
Colour
Texture
Flavour
Table 2 The quality index method scheme developed for cobia
Parameter
Skin
Colour
Mucus
Odour
Eyes
Pupils
Shape
Belly
Blood in abdomen
Odour
Gills
Colour
Mucus
Odour
Texture
Elasticity
Description
Score
Natural colour (black–silver)
Some reduction in lustre and
colour
Distinct reduction
Transparent and not clotted
Milky and clotted
Yellow and clotted
Fresh seaweedy
Slightly seaweedy, neutral
Rancid
Rotten
0
1
Bright and clear
Cloudy
Matt
Convex
Flat
Sunken
0
1
2
0
1
2
Red
Light red
Brownish colour
Fresh sea odour
Neutral
Slight sourness
Strong sour to spoilt odour
0
1
2
0
1
2
3
Red
Light red
Grey, green
Transparent
Yellow, clotted
Brown
Seaweedy
Neutral
Sour
Rotten
0
1
2
0
1
2
0
1
2
3
Finger mark returns quickly (<2 s)
Finger mark returns slowly (>3 s)
Finger leaves mark
Quality index total
2
0
1
2
0
1
2
3
0
1
2
0–25
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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
QDA total
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 evaluation. 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 evaluation based on the QDA scheme.
The samples were analysed at different laboratories in Vietnam. 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 Oceanography 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 laboratory as crude protein. Amino acid and fatty acid compositions 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, Walham, 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
determined at the laboratories of the Institute of Biotechnology 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 muffle furnace for 24 h. Crude lipid was determined gravimetrically 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 immediately 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 variance (ANOVA). Significant differences (P < 0.05) between
means were tested by Duncan’s multiple range test, according 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, Norway), 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
Table 4 Amino acid composition in the experimental diets (n = 3)
and in the cobia fillets (n = 7) (g kg 1 protein)
Diets
Fillets
Amino acids
A
B
A
Arginine
Serine
Hydroxyproline
Glycine
Threonine
Alanine
Proline
Methionine
Aspartic acid1
Valine
Histidine
Lysine
Glutamic acid1
Leucine
Phenylalanine
Isoleucine
Cystine
Tyrosine
47.7
30.6
10.6
69.3
17.3
50.6
73.3
22.1
64.9
54.1
33.5
95.0
121.3
80.9
39.9
54.1
5.1
29.1
45.6
26.3
9.3
66.6
15.8
53.7
75.1
20.0
67.2
53.4
39.4
95.4
121.6
87.4
39.5
54.3
5.4
28.5
39.6
38.1
8.7
50.3
30.8
56.5
48.5
23.4
76.3
50.0
30.6
116.5
118.0
81.6
40.6
47.7
8.1
36.3
B
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.4
0.7
0.2
0.5
0.5
0.8
0.5
0.6
1.0
0.7
0.9
2.6
2.2
1.2
0.4
0.8
0.1
0.6
39.5
39.2
8.6
50.7
29.8
61.9
47.7
27.6
78.9
50.2
32.0
112.1
116.8
84.2
40.8
53.9
8.3
38.3
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.5
0.5
0.2
0.5
0.6
1.9
0.8
0.7
1.3
1.0
1.2
2.6
1.2
1.3
1.1
1.5
0.1
1.5
1
Aspartic acid included asparagine; Glutamic acid included glutamine; Tryptophan was not analysed in the AA analyses.
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, geographic 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 protein 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) (Unusan 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. Contrary to the diets, PUFA in the cobia fillet comprised
mainly n-3 PUFA (690 g kg 1 total PUFA), which consisted 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
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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
Table 5 Fatty acid composition in the experimental diets (n = 3)
and the cobia fillets (n = 7) (g kg 1 total fatty acids)
Diets
Fillets
Fatty acids
A
B
A
14:0
15:0
16:0
16:1n-9
16:1n-7
17:0
16:2n-4
18:0
18:1n-9
18:1n-7
18:2n-6
20:0
18:3n-3
20:1n-11
20:1n-9
20:1n-7
18:4n-3
20:2n-6
22:0
20:3n-6
20:4n-6
22:1n-11
22:1n-9
20:4n-3
20:5n-3
24:0
24:1n-9
22:4n-6
22:5n-3
22:5n-6
22:6n-3
SFA
MUFA
PUFA
sum n-3
sum n-6
n3/n6
14
3
227
3
18
4
15
3
218
3
18
4
49
362
21
158
3
11
2
22
1
51
364
21
151
3
12
2
23
2
37.2
7.7
203.4
6.5
58.3
9.0
2.1
65.3
204.4
30.1
36.0
±
±
±
±
±
±
±
±
±
±
±
0.1
0.2
2.2
0.0
1.6
0.3
0.1
1.4
1.0
0.0
1.4
42.8
7.9
203.5
6.2
61.1
9.1
2.7
67.9
196.4
29.7
36.7
±
±
±
±
±
±
±
±
±
±
±
1.9
0.2
2.6
0.1
1.2
0.3
0.1
0.8
5.4
0.9
1.1
2
2
2
2
6.0
1.2
7.2
0.8
4.3
2.6
±
±
±
±
±
±
0.2
0.6
0.1
0.4
0.2
0.1
6.9
0.7
7.0
0.9
4.8
2.4
±
±
±
±
±
±
0.1
0.7
0.5
0.5
0.1
0.1
9
2
2
1
12
10
2
2
2
14
3
3
6
8
41
303
436
240
71
168
0.4
46
296
439
243
81
163
0.5
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)
B
1.7 ± 0.1
27.1 ± 0.2
2.9
32.9
1.1
3.6
7.2
17.5
15.8
147.0
323.7
312.2
303.2
210.7
90.4
2.33
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.2
1.6
0.5
0.2
0.2
0.3
0.4
2.9
3.2
1.4
4.9
4.7
0.8
0.06
Cobia A
1.8 ± 0.1
27.9 ± 2.4
2.9
34.9
0.9
3.5
6.9
17.5
15.3
139.4
332.0
305.4
302.1
208.3
91.1
2.28
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.2
1.3
0.4
0.1
0.1
0.1
0.8
10.2
3.6
8.0
14.4
10.8
3.7
0.03
Final weight (kg)
Total length (cm)
Viscera somatic index
Hepatic somatic index
Fillet yield
Dry matter in fillets
Fat in fillets
Protein in fillets
Ash in fillets
1.49
61.18
69.9
12.1
498.5
246.3
36.3
203.3
13.6
±
±
±
±
±
±
±
±
±
Cobia B
0.03
0.54
0.3
0.5
0.5
5.1
3.1
2.4
0.3
1.56
61.32
70.4
12.0
476.6
244.5
31.6
196.9
13.4
±
±
±
±
±
±
±
±
±
0.04
0.66
1.9
1.0
1.7
2.6
2.1
2.8
0.4
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.
MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty
acids; SFA, saturated fatty acid.
approximately 310 g kg 1 TFA and was composed dominantly of C18:1n-9 (640–650 g kg 1 total MUFA). Similarly, 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 commercial 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
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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
Sensory evaluation for gutted cobia No significant differences 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), Nielsen & Hyldig (2004), Nielsen & Green (2007) and Bonilla
et al. (2007). The values showed that the attributes gradually and naturally decayed throughout the storage time.
Scores for the attributes increased more sharply in gills
2
2
Gill colour
2
Skin colour
2
Texture
1
1
1
1
0
0
0
0
2
2
2
Skin mucus
Gill mucus
2
Abdomen blood colour
Eye shape
1
1
1
1
0
0
0
0
3
3
3
Gill odour
Skin odour
2
2
2
1
1
1
0
0
0
Pupil colour
IQ score
24
y A = 0.78x + 2.24; rA2 = 0.83
20
y B = 0.71x + 2.79; rB 2 = 0.84
Belly odour
16
12
8
4
3
5
7
9
11 13 15
3
5
7
9
11 13 15
3
5
7
9
11 13 15
0
3
5
7
9
11 13 15
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.
(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-
7.6
*10–1
Day 3
4.7
Comp. 2 (11.1%)
nent axis that explained approximately 76.3% of the variation 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
Skin mucus A Day 5
Skin mucus B
Skin odour B
Eye shape A
1.9
Gill mucus B
Day 15
Day 7
Belly odour B
Belly odour A
Abdomen blood B
Pupil color B Skin odour A
–1.0
Pupil color A
B
Skin color Texture
B
Skin color A
Abdomen blood A
Gill
mucus
Gill
odourA2
Eye shape B
Day 9
Gill odour
Day
A 11
Day 13
Texture A
Gill color A
–3.8
–0.90
Gill color B
–0.30
0.29
Comp. 1 (76.3%)
0.88
1.48
Figure 2 Principal component analyses
(PCA) loading plot of QIM data from
gutted cobia (n = 5) fed the experimental 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.
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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
3
3
Score
Odour
2
2
1
1
0
0
2
2
Colour
3
5
7
9
11
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 deteriorated 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 quality during storage. The fluctuated and low scores of the
attributes of sensory evaluation may be caused by confusion 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-
..............................................................................................
13
0
15
3
5
7
11
13
9
11
15
13
y A = 0.26x + 0.63; r A2 = 0.86
8
y B = 0.25x + 1.28; r B2 = 0.86
6
4
2
0
1
3
5
7
9
15
Days in ice
20
16
IQ score
Figure 3 Mean ( ± SEM) scores of
quantitative descriptive analysis (QDA)
of cooked cobia (n = 5) fed the experimental diets against storage time; ■ fish
fed diet A; * fish fed diet B. Maximum
potential QDA score is 10.
QDA score
10
Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
Texture
1
1
0
Flavour
12
8
4
0
3
5
7
9
11
13
15
Days in ice
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.
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 considered to be one of the most important factors responsible
22
20
18
15
16
CFU g–1
TBARS (nmol g–1 fillet)
20
14
12
10
10
5
8
6
4
5
10
Days in ice
15
0
5
10
15
Days in ice
for quality deterioration of fish during storage. In the present study, no statistically significant differences in lipid oxidation 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 fillet), 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 nonenzymatic 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 storage, 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-
Figure 5 Means ( ± SEM) of lipid oxidation (TBARS) and aerobic plate
counts (APC, cfu g 1 fillet) of cobia fillets (n = 7) from the dietary trial versus
days in ice; ■ fish fed diet A; * fish fed
diet B. TBARS, thiobarbituric acid
reactive substances.
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 satisfied 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 acceptable after 15 days stored in ice.
There were no significant differences in nutritional composition 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 differences 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 storage. 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.
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Aquaculture Nutrition ª 2012 Blackwell Publishing Ltd
The authors are grateful for financial support from the project ‘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 Faculty of Seafood Processing Technology – Nha Trang University, 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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