Nn./ & ~. pp 7-/6
Nutritional evaluation in five species of tuna
and M V.E. Attygalle
Department of Zoology, University of Sri Jayewardenepura,
Nugegoda, Sri Lanka
Received on - 20 - 10 - 2009
Accepted on -
13 - 11 -
Proximate composition was determined in different body parts (skin, white
muscle, red muscle, head muscle and belly flap) of five species of tuna;
Katsuwonus pelamis (skipjack, balaya,) , Thunnus albacares (yellow fin tuna,
kellawalla), Auxis rochei (Bullet, tuna, ragoduwa), Auxis thazard (frigate
tuna, alagoduwa) and Euthynnus affinis (kawakawa, attawalla.) obtained
from the Negombo fish landing site. Fatty acid profiles were also analyzed in
the skin, red and white muscle of the five species. No significant differences
between the tuna species were observed with respect to protein, total fat, and
moisture contents. The ash content in Frigate tuna and Kawakawa were
significantly higher than the other species. The muscle tissue in all the
species was rich in protein (20-25%) and low in fat «2%). The skin of all the
species recorded high protein (27-32%) and lipid (6-8%) levels. The moisture
content was low in the skin compared to the other tissues. All five species of
tuna studied here recorded less SFA (11-33%) and more PUF A (50-74%) in
the muscle tissues. The MUF A content in muscle tissue ranged from 8-25%
and all the species contained both EPA (eicosapentaenoic acid, C20:5n-3) (2-
19.7%) and DHA (docosahexaenoic acid, C22:6n-3) (4-42%) in varying
Key words: Tuna, proximate composition, n-3 fatty acids, n-6 fatty acids,
Nutrition has been cited as one of the primary reasons why consumers are
attracted to seafood (Gall et al, 1983). They also provide a good balance of
proteins, lipids, vitamins and minerals ( Edirisinghe et al, 2000). Edible fish
muscle normally contains about 18% protein, 1-2% ash and the balance 80%
of the wet weight of muscle is made up of lipid and water (Ackman, 1995).
Like most animal foods, seafood proteins have excellent nutritive value. Fish
protein contains all the essential amino acids and it is highly digestible.
(Jhaveri et ai, 1984). In terms of nutritive value, fish protein ranks above
Karunarathna and Aff.vgallc
casein (Haard 1995; Snook, 1984). Fish is the chief source of animal protein
in the cereal-based diet of Sri Lankans (Nathanael et al, 1997).
Lipids have been considered as a key energy source for growth metabolism,
visceral organs and muscle functions. But now, marine lipids are receiving
increasing attention as a source of C20 and C22 carbon omega 3 polyenoil
fatty acids which have profound implications for health and disease (Uauy-
dagach and Valenzuela, 1992).
The unique feature that differentiates lipids of marine species from those of
land animals is the presence of long chain PUF A, namely eicosapentaenoic
acid (EPA C20:5 w-3), docosahexaenoic acid (DHA C22:6 w-3) and to a
lesser extent, docosapentaenoic acid (DPA C22:5 w-3) (Shahidi, 1998),
which are important in the prevention and treatment of cardiovascular
diseases, hypertension, arthritis, other inflammatory and autoimmune
disorders, and cancers (Jones, 2002).
Anatomical position of the flesh sampled is an important factor because
nutrients are not distributed evenly in all the body parts of the fish. Lipid
content has been found to vary from 2% to almost 30% depending on the area
of the body being sampled (Porter, 1992). Red or blood meat has been found
to contain more fat and less protein than white meat (Geiger. and Borgstrom,
1962). The major portion of minerals in the fish body is distributed in skeletal
tissues. Generally most of the bones and other skeletal tissues of fish are
removed prior to consumption.
Tunas are among the largest, most specialized and commercially important of
all fishes. Belonging to the genus Thunnus of the family Scombridae, they
are found in temperate and tropical oceans around the world and account for
a major proportion of the world fishery products (Lee et aI, 2005).
In the present study, moisture, ash, total fat, and protein contents of five
species of tunas namely; Katsuwonus pelamis (skipjack, balaya,) , Thunnus
albacares (yellow fin tuna, kellawalla), Auxis rochei (Bullet, tuna,
ragoduwa), Auxis thazard (frigate tuna, alagoduwa) and Euthynnus affinis
(kawakawa, attawalla,) were determined in different body parts (skin, red
muscle, white muscle, head muscle and belly flap) and the fatty acid profiles
of the skin, red muscle and white muscle of these five species of tuna were
Sample size and the average size (Fork length) of the fish are given in
Materials and Methods
Fresh fish samples purchased from Pitipana, Negombo fish landing site were
packed in ice and transported to the laboratory from July 2006 to April 2008.
Samples of skin, belly flap, red, white and head muscles were separated to
determine moisture, ash, total fat, and protein contents in each species
respectively. Moisture content was determined by oven drying at 105 DCand
the ash content of each sample was determined by using the muffle furnace at
550DC.Total fat and protein contents were determined by Majonnier method
and Kjeldhal method respectively (Kirk and Sawyer, 1991). Then oils were
extracted from the Bligh and Dyer method and the Fatty Acid Methyl Esters
(FAMEs) were prepared by sodium methoxide method. The methyl esters of
each fatty acid were then analyzed by Gas chromatography (Agilent, 4890 D,
Innowax,) (Temperature; injector 270°C and detector 250°C, the oven was
first maintained at 170°C and then programmed to225°C at the rate of
1°C/minute) The chromatographic peaks were then identified by comparison
of the retention time with cod liver oil as standard and GLC 411 as internal
standard. Percentages of each fatty acid was calculated as a percentage of
FAMEs. Statistical differences between species and within species were
determined at 5% level using One-Way variance of analysis (ANOVA)
Minitab version 14.
Results and Discussion
From table 1 it is observed that the highest constituent in all the samples was
moisture. The average moisture content did not vary significantly between
the tuna species. The skin contained significantly (p<0.05) less moisture (58-
60%) compared to the other body parts which contained between 69- 74%.
Edirisinghe et al (2000) had reported higher values (69-80%) for of moisture
content of some marine fish. Jayasinghe (1996) reported values for fresh
water species in the range of 60-84%.
No significant differences in the average protein content were observed
between the species. But the protein content of the skin was significantly
higher (p<0.05) than the other body parts in all the tuna species (Table 1) and
ranged from 27-32%. The protein content in the red muscle, white muscle
and head muscle were 20-25%, 20-23% and 20-25% respectively. There were
no significant differences in the protein contents among red, white and head
muscles. According to Sidwell (1981) the belly flap recorded significantly
low amount of protein (16-17%) than the other body parts. Most fin fish
muscle contains about 18-22% protein and the average for 540 analyses made
was 18.5±3.6%. In the present study protein values recorded for the tuna
species are higher than this average. Suzuki (1981) has reported that within
Karunarathna and Anvgallc
species, white flesh has more protein than dark flesh. In the present study no
significant difference in protein content was evident in the white and red
muscle of tuna.
Kawakawa and Frigate tuna recorded significantly higher (p<0.05) amount of
ash compared to the other species (Table 1). These two species also recorded
significantly much higher contents of ash in their skins than the other tissues
studied. In most fish the average ash content in the edible muscle portion has
been found to range between 0.5 to 1.8% of wet weight (Sidwell, 1981). In
the present study the ash content in muscle and belly flap varied between 0.4
to 1.2%, which is similar to the values reported by Sidwell (1981) for the
edible muscle portion.
There was no significant difference in total fat content among the different
species of tuna (Table 1). The percentage of total fat in the skin of all tunas
was significantly higher (p<0.05) than in the other body parts. In the skin
total fat varied from 6-9% and in the other body parts it was less than 2%.
The total fat content in the red muscle, white muscle, head muscle and belly
flap ranged from 1-1.3%, 0.6-0.9%, 0.7-1.5% and 0.9-1.3% respectively.
There is considerable variation in the distribution of fat in different tissues.
The belly flap of Salmon has been reported to contain 30-50% fat (Ackman,
1995). The Indian oil Sardine (Sadinella longicepsi has been reported to have
27% or more lipid in the skin and only 6% in the muscles (Nair et al,1979).
In the capelin (Mallotus villosus) the highest amount of fat (35%) was found
in the belly flap, followed by the skin (25%). In the mackerel the Skin
contained most fat followed by the white muscle. In all tunas investigated
here the skin recorded the highest amount of fat compared to the other
Nutritional Evaluation of tuna
Table 1. Proximate Composition in the different body parts of five species of
tuna ( n>20, mean
Skipjack Yellow fin Frigate
Fish Type tuna tuna tuna Bullet tuna Kawakawa
Samples 29 26 28 22 27
Fork Length 49.50± 1.8 52.00±2.1 47.20± 1.6 45.00± 1.7 25.50 ± 2.3
Skin 59.60±1.61 58.13±3A3 58.19±1.87 58.65±2.39 58.19±1.87
Red 71.60±1.05 70.83±0.70 71.39± lA5 70.68±1.l5 72.77±lA5
~White 72.05±1.20 72A4±IAl 73.00±1.14 71.06±1.03 73Al±IA2
Head 71.83±1.38 71.93±0.71 73Al±2.80 69.86±2.74 73.73±1.42
~flap 71.97±0.78 70.53±0.76 71.67±IA9 68.60±1.89 71.02±0.77
Skin 28.31± 0.53 27.12±0.5 31.67±0.38 32.67±OA2 30.06±OA2
Red 24.53± 0.24 0.19 0.21 21.95±0.31 21A2±OAO
White 23.79±0.38 21A2±0.25 23.92±OA3 23.37±OAl 20.73±0.29
Head 20A8±0.85 20.98±0.34 24.83±OA1 25.6±0.26 20.7±0.39
flap 16.04±0.24 16.94±0.67 19.74±0.18 17A7±0.23 16.85±OA2
Skin 7A5±1.34 8.87±1.23 8.92±1.03 6.71±l.23 6.31±1.l2
~muscle 0.98±0.23 1.01±0.23 l.08±0.77 0.99±0.23 1.26±OA3
~Muscle 0.77±0.23 0.88±0.25 0.75±0.12 0.68±0.54 0.60±0.14
Head 0.72±0.17 0.98±0.13 1.54±0.13 1.21±0.12 0.67±0.32
flap 1.34±OA2 0.90±0.16 0.99±0.12 0.88±0.37 1.26±0.21
Skin 1.33±0.98 1.03±OA5 3.21±2.27 0.98±0.63 6.06±0.98
muscle 1.09±0.22 0.92±0.21 0.73±0.12 1.19±0.17 0.90±0.36
Muscle 1.02±0.10 1.12±0.15 0.79±0.35 0.71±0AO 1.03±0.04
<Head 0.88±0.07 1.00±0.06 OAO±0.28 0.69±0.70 0.65±OA6
flap 0.58±0.12 0.67±0.38 1.03±0.59 0.78±0.34 0.92±0.59
Katunarathna and AHygallc
In all the tuna species studied here the skin recorded the highest amounts of
protein, total fat and ash (only in 2 species), compared to the other body
parts. Therefore if the skin is not consumed valuable nutrients may be lost
with it. In the smaller species of tuna such as Bullet tuna (Ragoduwa), Frigate
tuna (Alagoduwa) and Kawakawa (Attawalla) the skin is not as tough as in
Yellow fin tuna, and may be consumed with the flesh.
Fatty acid profiles of the skin and muscle for the five species of tuna are
given in table 2. In the present study significant differences (p<0.05) between
the total saturated fatty acids and unsaturated fatty acids were observed
within the species. Total saturated fatty acids (SF A) in the muscle tissue (red
and white muscle) of the tunas were low and varied between 11% (skipjack,
white muscle) to 33% (skipjack, red muscle). Total polyunsaturated fatty
acids (PUF A) in the muscle tissues of all species of tuna were high and
ranged between 50% (frigate tuna, red muscle) to 74% (kawakawa, red
muscle). Total monounsaturated fatty acids in the muscle tissues of tunas
were also low and varied between 8% (skipjack tuna, red muscle) and 25%
(yellow fin tuna, white muscle).
Percentage of saturated fatty acids (SFA), MUF A (Monounsaturated
Fatty acids) and PUFA (Poly unsaturated fatty acids) in five species of tuna.
Fish Type Body part SFA MUFA PUFA
Skin 23A7±0.90 24.57±1.61 51.27±1.40
Skipjack White 10.92±1.44 21.05±lAl 68.05±1.99
Red 33.11±2.57 7.52±1.14 59.36±1.89
Skin 18.18±1.01 36.53±2.53 45.02±1.93
Kawakawa White 26.64±1.62 10.71±1.21 61.92±1.88
Red 13.91±1.98 11.85±0.78 73.94±2.83
Yellow fin Skin 33.77±1.63 11.61±1.01 53.33±2.19
White 12.30±1.52 14.l1±0.93 72.36±2.56
tuna Red 22.74±lA6 21.11±0.94 55.85±1.78
Skin 13.65±l.46 1O.67±1.09 75.69±3.32
Bullet tuna White 17.58±1.63 25A4±1.07 56.01±1.95
Red 21.31±1.35 11AO±0.75 64.06±2.09
Skin 26.35±2.52 34.93± lA3 38.73±1.62
Frigate tuna White 24.92±1.80 22.02±1.07 53.05±2.13
Red 21.33±1.46 20.81±1.15 49.91±1.78
Nutritional Evaluation of tuna
Total SFA, MUFA, and PUFA levels in herring (Clupea harrengus) from
USA were found to be 26%, 47% and 27% respectively (Ackman, 1995). In
fresh water carp (Cyprinus carpio) from Turkey the corresponding values
were 36%, 32% and 32% (Donmez, 2009) respectively. Tunas in this study
were characterized by high levels of PUF A, while in herring higher
concentrations of MUF A were found. Carp was characterized by sightly
higher concentrations of SFA over MUF A and PUF A. In little tuna
(Euthunnus alletteratus) from the Mediterranean similar results to the tunas
in this study were reported, as the major fatty acid class was PUF A followed
by SFA and MUF A (Selmi et aI, 2008)
Table 3. PUF A/SF A ratio for five species of tuna
Fish Type SFA% PUFA% PUF A/SF A/ratio
White Red Skin White Red Skin White Red Skin
Skip jack 10.92 33.11 23.47 68.05 59.36 5l.27 6.23 1.79 2.18
Kawakawa 26.64 13.91 18.18 6l.92 73.94 45.02 2.32 5.32 2.47
Yellow fin tuna 12.3 22.74 33.77 72.36 55.85 53.33 5.88 2.46 l.58
Bullet tuna 17.58 21.31 13.65 56.01 64.06 75.69 3.19 3.01 5.55
Frigate tuna 24.92 21.33 26.35 53.05 49.91 38.72 2.13 2.34 1.46
The total SFA, MUF A and PUF A in the skin of the tunas varied between
13.65 - 33.77%, 10.67 - 36.53% and 38.75 to 75.69% respectively. The ratio
of PUFAlSF A for white and red muscle of the five species of tuna is given in
table 3. A minimum value for PUFAISFA ratio recommended by nutritionists
is 0.45. From table 3 it is seen that for the tuna species investigated here.
PUFAISfA ratio varied between 1.79 and 6.23, which was well above the
recommended minimum value.
Table 4. Ratio of n6/n3 fatty acids for five species of tuna
Species Total n-6 Fatty acids Total n-3 Fatty acids n6/n3 ratio
White Red Skin White Red Skin White Red Skin
Skip jack 15.47 15.31 18.87 57.45 60.51 45.42 0.27 0.25 0.42
Kawaka 2l.27 37.97 26.38 53.20 40.11 31.09 0.40 0.95 0.85
Yellow 20.77 1.30 16.7 61.09 44.77 48.19 0.34 0.03 0.35
Bullet 21.73 41.46 18.31 48.38 35.29 56.51 0.45 l.l7 0.32
Frigate 36.46 26.70 46.51 25.60 40.19 12.08 1.42 0.66 3.85
The n6/n3 fatty acid ratio for white and red muscle of the five species of tuna
is given in table 4. Nutritionists believe that the desirable n6/n3 fatty acid
. ratio should be 5 at a maximum. The
ratios in both white and red
muscle of the five species of tuna investigated here were found to be between
0.03 and 1.42 which was well below the recommended maximum value of 5.
Kawakawa Yellow tin tuna Bullet tuna Ftig
at c tun a
I. EPA and DHA in skin, red and white flesh of five species of
Figure 1 shows the distribution of EPA (eicosapentaenoic acid, C20:5n-3)
and DHA (docosahexaenoic acid, C22:6n-3) which are considered as
beneficial fatty acids in health care, especially coronary heart diseases. The
highest DHA value (42%) and the lowest EPA value (2.4%) were recorded in
the red muscle of yellow fin tuna. The highest EPA and value (19.73%) was
recorded in the red muscle of skipjack tuna. The sum of EPA and DHA in the
muscle tissue of the tunas ranged from 16% (frigate tuna, white muscle) to
44% (yellow fin tuna, red muscle). Therefore these tuna species can be
considered as good source of these fatty acids. Reena
(1994) report that
the Indian fish they evaluated were excellent source of EPA and DHA as the
major constituent of PUF A were these two fatty acids.Although the muscle
tissue of the tuna species contain low levels of fat (2%) they have desirable
fatty acid profiles which can contribute to good health.
Authors acknowledge financial support by University of Sri Jayawardenapura
research grant No. ASP/61R12006/08
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