Nutritional composition and lipid content of skin and muscle of wild giant mottle eels anguilla marmorata in Thua Thien Hue, Vietnam

ent 
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) 
Protein content (mg/g) =
𝑋
1000 × 𝑚
 (3) 
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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) 
𝐶 =
𝑉1 × 𝑥
𝑉2 × 𝑚
 (4) 
The total sugar content in the sample was then calculated according to formula (5) 
𝑋 (mg/g) =
𝑚1 × 𝑉1 × 𝑛
𝑉2 × 𝑚
 (5) 
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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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 1000 g 
Water 72.8 ± 1.56 65.0 ± 1.87 60.3 ± 2.74 55.8 ± 1.18 
Lipid 16.1 ± 0.50 18.7 ± 0.60 21.5 ± 1.15 26.7 ± 1.66 
Protein 13.98 ± 2.22 19.80 ± 1.90 22.13 ± 0.79 25.30 ± 1.21 
Total sugar 1.28 ± 0.23 1.32 ± 0.33 1.58 ±0.09 2.09 ± 0.19 
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 65.51 ± 0.42 57.17 ± 0.98 61.07 ± 0.03 
Lipid (%) 18.2 ± 1.02 21.35 ± 2.48 13.26 ± 0.61 17.23 ± 0.73 
Protein (%) 19.54 ± 4.31 17.17 ± 0.71 16.78 ± 2.8 20.28 ± 0.06 
Total sugar (mg/g) 1.34 ± 0.34 – – – 
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Table 4. Correlation between nutritional components and weight in the flesh of A. marmorata 
Nutritional ingredients Correlation equation R2 R 
Water Y = –3.806 × ln(W) + 84.553 0.88 0.94 
Protein Y = 2.6607 × ln(W) + 5.2529 0.95 0.97 
Lipid Y = 1.8018 × ln(W) + 1.6694 0.82 0.91 
Sugar Y = 0.0798 × ln(W) + 1.0129 0.92 0.96 
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% 
(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. 
Kieu Thi Huyen, Nguyen Quang Linh Vol. 129, No. 3C, 2020 
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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 
<100 
13.5 ± 0.29a 21.0 ± 0.79c 16.1 ± 0.50b 
2 
100–500 
15.1 ± 0.92a 26.1 ± 1.21c 18.7 ± 0.60b 
3 
500–1000 
20.2 ± 2.02a 26.3 ± 2.14b 21.5 ± 1.15b 
4 
>1000 
26.2 ± 0.89a 32.4 ± 1.32b 26.7 ± 1.66a 
5 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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