Nutritional composition and lipid content of skin and muscle of wild giant mottle eels anguilla marmorata in Thua Thien Hue, Vietnam
Hue University Journal of Science: Agriculture and Rural Development
pISSN: 2588-1191; eISSN: 2615-9708
Vol. 129, No. 3C, 2020, P.5–14; DOI: 10.26459/hueunijard.v129i3C.5848
NUTRITIONAL COMPOSITION AND LIPID CONTENT OF
SKIN AND MUSCLE OF WILD GIANT MOTTLE EELS
ANGUILLA MARMORATA IN THUA THIEN HUE, VIETNAM
Kieu Thi Huyen1*, Nguyen Quang Linh2
1 University of Agriculture and Forestry, Hue University, 102 Phung Hung St., Hue, Vietnam
2 Hue University, 1 Dien Bien Phu St., Hue, Vietnam
Abstract: In Vietnam, the giant mottle eel Anguilla marmorata is the most widely distributed species and
being exploited for seed in aquaculture as well as for human consumption. This study aims to investigate
the basic nutritional components of the fish. The eels were collected from six locations of Thua Thien Hue
province, with weights from 5 to 3200 g. In addition, the content of lipid in skin and tissue was also
examined. The results show that eel flesh has a relatively high nutritional value. The water, protein, lipid,
and total sugar content of the fish meat is 60.4 ± 0.94%, 19.54 ± 4.31%, 18.2 ± 1.02%, and 1.34 ± 0.34 (mg/g),
respectively. The nutritional components of the eel have a good correlation with the weight according to the
equation: Y = a × ln (W) + b (where W is the weight of eels; Y is the content of nutritional components; a is
the correlation coefficient b is a constant) with r > 0.9. The lipid content of the fish skin is higher than that of
muscle and meat.
Keywords: eel, protein, lipid, water, total sugar, weight
1
Introduction
Anguilla marmorata is the second largest of the 16 species and three sub-species of eels of Anguilla
[6, 16–18], with the most extensive geographical distribution in two different oceans (tropical and
subtropical central Pacific and the Indian Ocean) [13]. The habitat covers the east coast of Africa
across the Indian Ocean [6], including India and Sri Lanka, the Indo-Pacific region (Indonesia,
Philippines, and Papua New Guinea), and the island chains in the central South Pacific (the
Marquesas Islands and French Polynesia). Vertically, this species is distributed in South-Western
Japan, Taiwan, South-east China, in the south (Vietnam and Malaysia), and especially the
Southern Cape in South Africa [19].
In Vietnam, Anguilla marmorata is distributed widely in coastal areas, estuaries, lagoons,
lakes, rivers, and freshwater streams from Ha Tinh to Vung Tau, the Highlands, and Phu Quoc
Island, most of which is from Thua Thien Hue to Khanh Hoa [1]. Giant mottle eels are a valuable
and preferred species for human consumption because of their high nutritional value, good flesh
quality, and export value, and thus they are considered as a species with high economic
importance [3]. Huss [9] reported that the nutrition in the fish meat comprises water (26–96%),
Corresponding: [email protected]
Submitted: June 8, 2020; Revised: September 28, 2020; Accepted: September 28, 2020
Kieu Thi Huyen, Nguyen Quang Linh
Vol. 129, No. 3C, 2020
lipid (0.2–25%), protein (6–28%), ash (0.4–1.5%), and carbohydrate (<0.5%). The nutritional
composition of the fish fluctuates, depending on internal and external factors [21], such as species,
age, gender, stage of development, diet, and habitat [6, 21]. Similarly, the nutritional content of
wild eels is expected to vary with the growth stages by the differences in natural foods [10, 5].
The body composition is, therefore, a true reflection of eating habits and the type of food available
[21]. In this study, the variation of nutritional components, namely protein, lipid, total sugars,
and water content, in the meat of the giant mottle eels naturally distributed in Thua Thien Hue is
studied. Besides, this study also looks at the distribution of lipid in different parts of the eels’
flesh. The results will provide the basis for selecting the commercial quantity and food processing
from the eels.
2
Materials and methods
2.1
Sampling
One hundred and twenty-seven giant mottle eels (Anguilla marmorata) sized 17–1080 mm
and weighted 10–3200 g were collected from six locations in Thua Thien Hue (Fig. 1) in July 2018
and August 2019 (Table 1). Eel’s meat was divided into two parts: skin and tissue (Fig. 2). In
particular, the meat (skin and muscle) was used to analyze the protein, lipid, total sugar, and
water. The lipid content was analyzed separately for skin and muscle to see their distribution in
the wild eel’s meat. The samples were divided into four groups according to the weight of fish:
<100 g (glass eels), 100–500 g (elver eels), 500–1000 g (golden eels), and >1000 g (silver eels).
Table 1. Sample characterization
Number of
samples
Total weight
(g)
Total length
(mm)
303–1080
513.00 ± 186.13
215–527
333.8 ± 126.39
170–620
337.1 ± 110.74
205–610
413.2 ± 116.1
150–362
No. Locations
Ratio (%)
20.5
52.3–3200.0
511.5 ± 776.69
24.5–269.5
104,7 ± 103.64
9.9–511.1
107.2 ± 117.54
12.9–493.9
184.3 ± 140.52
5.9–89.0
1
Thao Long Dam
26
23
26
2
Truoi Dam
18.1
3
Nam Dong district
20.5
4
Phong Dien district 28
22.0
Bu Lu river and
24
5
19.0
Lang Co town
27.2 ± 18.91
236.4 ± 53.75
9.9–3200
202.5 ± 444.40
17–1080
365.66 ± 163.75
Total
127
100.0
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Fig. 1. Sample collecting map
Fig. 2. Eel pattern
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Vol. 129, No. 3C, 2020
2.2
Water content
The water content was determined before and after drying in three replicates by drying 1 g of
sample in an oven at 105 °C until constant weight (after the first three times more weight). The
water content is calculated according to formula (1) [4]
푚 − 푚0
(1)
Water content (%) =
× 100
푚0
where m and m0 are the weight of the sample after and before drying.
2.3 Lipid content
Two grams (W0) of eel meat/skin/muscle was wrapped in one layer of cotton and one sheet of filter
paper on the outside. The sample was extracted with n-hexane in Soxhlet apparatus for 6 hours.
The extract is then dried in an oven at 105 °C to remove the solvent. The lipid content was
calculated according to formula (2) [4]
푊
(2)
Lipid (%) =
× 100
푊
0
where W and W0 are the weight of the fat and the sample, respectively.
2.4 Protein content
The protein content of the sample was determined with the Bradford method [11]. The basic
principle of this method is based on the change of the maximum absorption wavelength of
Coomassie dye Brilliant Blue when creating complexes with proteins. In an acidic solution
without protein, the red dye has a maximum absorption wavelength of 465 nm. When combined
with the protein, the color turns blue and maximizes absorption at 595 nm. The absorption at 596
nm is directly related to protein concentration.
To determine the protein in a sample, first, a calibration curve with a known standard
protein solution was constructed. After adding the protein solution to the dye, the color appeared
after two minutes and lasted up to one hour. The absorbance of the solution was measured on a
spectrophotometer (ODX). The absorbance is proportional to the amount of protein in the sample.
A control with HCl (ODO) was carried out. The value ODOD = ODX – ODO was calculated. The
amount of protein in a sample was determined according to the calibration curve from the ØOD
value on the vertical axis, thus deducing the corresponding protein concentration on the
horizontal axis. From the standard equation and the optical density of the sample, the protein
content of the sample is given by formula (3)
푋
(3)
Protein content (mg/g) =
1000 × 푚
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Vol. 129, No. 3C, 2020
where X is the density/concentration of the sample; m is the sample size for analysis.
2.5 Total sugar content
Two grams of eel flesh was used to analyze the total sugar content, following the color reaction
between sugar and dinitrosalicylic acid (DNS), according to the method described by AOAC [4].
The color intensity of the reaction mixture is directly proportional to the strength of the
dinitrosalicylic acid-reducing sugar, which expresses the total sugar of the sample. Eel flesh after
weighing was added to 50 mL of distilled water in a beaker, which was kept in a water bath at 74
°C for 2 hours. Next, 1 mL of 0.5% HCl solution was added to the beaker and kept for another 15
minutes, and the beaker was cooled quickly under running water. The solution was neutralized
with NaOH until the solution turned pink (with phenolphthalein). Ten millilitres of the
neutralized solution was concentrated; then, 2 mL of distilled water was added to make a
standard solution. The absorbance of the resulting complex solution was measured at 530 nm .
From the calibration curve y = 12.20 × x – 0.818 (where x is the absorbance, and y is the content of
glucose), the standard glucose content was calculated according to formula (4)
푉 × 푥
1
(4)
퐶 =
푉 × 푚
2
The total sugar content in the sample was then calculated according to formula (5)
푚1 × 푉 × 푛
1
(5)
푋 (mg/g) =
푉 × 푚
2
where m1 is the concentration of sugar in the standard solution (mg/mL); m is the weight of the
sample (g); n is the dilution factor; V1 is the initial standard volume; V2 is the volume of the
reaction.
The correlation between the nutrient composition and the weight of eel flesh was
determined by using multivariate regression analysis in the SPSS 22.0 software.
3
Results and discussion
3.1
Nutrient content in meat of Anguilla marmorata
Table 2 indicates that water is the highest proportion in the flesh of fresh eels, accounting for 60.4
± 0.94%, followed by protein (19.54%), lipid (18.2%), and sugar (1.34%). The water content of the
present study is lower than that of A. marmorata but higher than that of A. bicolor bicolor (57.17 ±
0.98%) [12].
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Vol. 129, No. 3C, 2020
Table 2. Average value of nutrient content of Anguilla marmorata
Nutritional
ingredients
A. marmorata
A. marmorata [12]
A. bicolor bicolor [12] Salmo salar [8]
Water (%)
60.4 ± 0.94
18.2 ± 1.02
19.54 ± 4.31
1.34 ± 0.34
65.51 ± 0.42
21.35 ± 2.48
17.17 ± 0.71
–
57.17 ± 0.98
13.26 ± 0.61
16.78 ± 2.8
–
61.07 ± 0.03
17.23 ± 0.73
20.28 ± 0.06
–
Lipid (%)
Protein (%)
Total sugar (mg/g)
The lipid content ranges from 17.18 to 19.22%, which is lower than that of A. marmorata
(21.35 ± 2.48%) [12] and A. bicolor (28.29%) [20]. The lipid content of A. marmorata is higher than
that of A. japonica (10.85–19.44%) [15] and A. bicolor bicolor (13.26 ± 0.61%) [12].
Protein plays a vital role in the development of organisms. In this study, the protein content
of wild eel flesh is 19.54 ± 4.31%, higher than that of A. marmorata (17.17 ± 0.71%) and A. bicolor
bicolor (16.78 ± 2.8%) [12]. The high protein content of eel flesh is related to the animal-based diet,
with the preferred food being the crustaceans [10].
The total sugar content of eel flesh is 1.34 ± 0.34 mg/g (Table 2).
The nutritional contents in the flesh of giant mottle eels distributed in Thua Thien Hue are
comparable to that of salmon with the water, lipid, and protein content of 61.07 ± 0.03, 17.23 ±
0.73, and 20.28 ± 0.06%, respectively [8].
It is difficult to distinguish between young eels and underdeveloped eels in different life
stages. In particular, small eels and underdeveloped eels begin to grow in the same phase (glass
eels) and have the same weight [7]. Table 3 shows the fluctuation in the nutritional content in eel
flesh of different size groups. Accordingly, the water content of eel flesh varies from 72.8 to 55.8%
and decreases gradually with increasing fish bodyweight. Meanwhile, the value of protein, lipid,
and sugar content increases. These results are consistent with those reported by Huss [9] and
show a similar trend with those of European eels (A. anguilla) weighted 9–420 g [7].
Table 3. Fluctuation of nutritional content in eel flesh by weight (%)
Weight
Water
<100 g
100–500 g
65.0 ± 1.87
18.7 ± 0.60
19.80 ± 1.90
1.32 ± 0.33
500–1000 g
60.3 ± 2.74
21.5 ± 1.15
22.13 ± 0.79
1.58 ±0.09
>1000 g
55.8 ± 1.18
26.7 ± 1.66
25.30 ± 1.21
2.09 ± 0.19
72.8 ± 1.56
16.1 ± 0.50
13.98 ± 2.22
1.28 ± 0.23
Lipid
Protein
Total sugar
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Table 4. Correlation between nutritional components and weight in the flesh of A. marmorata
Nutritional ingredients
Correlation equation
Y = –3.806 × ln(W) + 84.553
Y = 2.6607 × ln(W) + 5.2529
Y = 1.8018 × ln(W) + 1.6694
Y = 0.0798 × ln(W) + 1.0129
R2
R
Water
Protein
Lipid
0.88
0.95
0.82
0.92
0.94
0.97
0.91
0.96
Sugar
Note: Y is the nutritional value, and W is the weight of the eel
According to the value of the nutritional ingredients in A. marmorata, a good positive
correlation between the nutrition components (protein, lipid, and sugar) and the eel body weight
is observed with R > 0.9 [2] (Table 4). This means that within the permitted limits, the content of
nutrients increases with body weight. In contrast, water content shows a negative correlation.
Premature eels contain less water than the young. These results are consistent with those reported
by Degani et al. [7] when analyzing the relationship between the nutritional composition and the
weight of European eels A. anguilla weighted 9–420 g.
Besides, from the correlation equations, we can see that significant changes of nutritional
components (the decrease of water and increase in other ingredients) are observed when the eels
are in the small stage weight >400 g. In the weight range of 400–1000 g, the nutritional components
of giant mottle eels A. marmorata tend to decrease; when the fish weights are greater than 1000 g,
the variations reach a steady-state (Table 3 and Table 4).
3.2
Distribution of total lipid content in eel meat
The flesh of all wild giant mottle eels A. marmorata in Thua Thien Hue, weighted 7–3200 g,
has a higher content of lipid in the skin (28.56 ± 2.15%) than in the muscle (18.10 ± 1.57%) and
meat (23.33 ± 1.89%). The lipid content increases with the bodyweight groups from 13.5 ± 0.29%
(<100 g) to 26.2 ± 0.89% (>1000 g) in muscles, 21.0 ± 0.79% to 32.4 ± 1.32% in skin, and 16.1 ± 0.50
to 26.7 ± 1.66% in meat (Table 5). On the other hand, a small increase in the lipid content in muscle
is observed for the eels lighter than 500 g (from 13.5 to 15.1%). When the eels reach weight in the
range of 500–1000 g, there is a sudden increase in lipid content in the muscle (15.1 ± 0.92% to 20.2
± 2.02%). It is possibly due to the accumulation of fat during the migration and reproduction of
eels in the wild [14]. The differentiation of the fat content is also due to different parts of the fish’s
body and different seasons throughout the year [14, 21]. In this study, the lipid content in flesh is
the sum of the lipid content in the skin and muscle of the eels.
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Kieu Thi Huyen, Nguyen Quang Linh
Vol. 129, No. 3C, 2020
Table 5. Total lipid content in Anguilla marmorata eel flesh by weight
No.
Weight
(g)
Lipid in muscles
(%)
Lipid in the skin
(%)
Lipid in meat
(%)
1
2
3
4
5
13.5 ± 0.29a
21.0 ± 0.79c
16.1 ± 0.50b
<100
15.1 ± 0.92a
20.2 ± 2.02a
26.2 ± 0.89a
26.1 ± 1.21c
26.3 ± 2.14b
32.4 ± 1.32b
18.7 ± 0.60b
21.5 ± 1.15b
26.7 ± 1.66a
100–500
500–1000
>1000
Average
18.10 ± 1.57
28.56 ± 2.15
23.33 ± 1.89
Note: In the same row, letters a, b, and c show a statistically significant difference with p < 0.05
4
Conclusion
Wild A. marmorata has a high nutritional value, which is dependent on their body weight. The
nutritional components increase with weight, except the water content. They have a close
correlation with the body weight and reach a stable value when the weight of the fish is greater
than 400 g. In the flesh of the giant mottle eels A. marmorata, lipid accounts for a greater proportion
than other nutritional components and is mostly distributed in the skin.
Funding statement
This research was supported by Hue University, Vietnam, under project number DHH 2019-113.
Acknowledgments
To complete this study, the authors would like to sincerely thank the students at the Faculty of
Fishery, University of Agriculture and Forestry, Hue University and the fishermen for the
collection of specimens, and MSc. Dang Thanh Long and Biotechnology Institute, Hue University
for sample analysis.
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