Nutritional compositions in different parts of muscle in the longfin batfish, Platax teira (Forsskål, 1775)

ABSTRACT

Longfin batfish (Platax teira) is a coastal marine fish life. The aims of this study are to investigate the compositions and content of amino acid and fatty acid in three parts of P. teira. The results showed that P. teira muscle protein content was as high as 21.00%. For amino acid composition, seventeen kinds of amino acids were detected in three parts. The essential amino acid score (EAAS) and essential amino acids (EAA) / total amino acids (TAA) ratio of P. teira were both above 70 and 0.4. EAAS in the upper part of the abdominal muscle showed higher than those two parts. The content of saturated fatty acids (SFA) and the polyunsaturated fatty acids (PUFA) in the upper part of the abdominal muscle was higher than that in other groups. No significant differences in monounsaturated fatty acids (MUFA) and n-3/n- 6 were detected among three parts. Overall, the upper part of the abdominal muscle was thought to have a higher nutritious value. These results showed P. teira is a high quality marine fish with high protein content and low fat, abundant amino acids with proper proportion, however, it was regarded as an excellent breeding varieties.

KEYWORDS:

• Platax teira
• fatty acid
• amino acid

Introduction

With the improvement of people's living standards, people need more nutrients and food quality than adequate food supplies. The same requirements were imposed on the nutrition and quality of fishery products (He et al. 2017). People pay more and more attention to the quality and safety of aquatic products and the quality of fish. Fish is popular with consumers for its high protein, low fat, rich in EPA and DHA, as well as many essential amino acids in the human body. Fish muscle nutrient is one of the most important indicators for consumers. There are also significant differences in nutrient composition of fish muscles due to different culture conditions and cultured species. The evaluation content of fish muscle quality includes nutrition, sensory characteristics, and texture characteristics (Huss 1995), which are usually measured from the three aspects of fish safety, human health and food satisfaction (Listrat et al. 2016). The main nutrients of fish in muscle are protein, amino acid, fat and fatty acid (Sargent et al. 1995). Muscle nutrients are closely related to muscle quality, the fat and protein content of fish muscles affects the sensory characteristics of muscles (Lie 2002).

Recently, many studies have already reported the diversity of nutritional quality in aquatic animals, including Catla catla, Labeo rohita and Cirrhinus mrigala (Hussain et al. 2018), Ctenopharyngodon idella (Xu et al. 2018), Dicentrarchus labrax (Orban et al. 2002), Ictalurus punctatus (Refaey et al. 2018), Macrobrachium rosenbergii (Asaikkutti et al. 2016), Pagellus bogaraveo (Castro et al. 2018), Perca fluviatilis (Mairesse et al. 2006), Portunus trituberculatus (He et al. 2017). These studies not only provide more basic references for aquatic nutrition, food development and fish processing, but also provide more support to customers on how to choose their favourite food (He et al. 2017). Therefore, the diversity of nutritional quality of aquatic products has become a major concern.

Longfin batfish, Platax teira is a tropical and temperate regions of coastal marine fish life (Li et al. 2016; Leu et al. 2018). P. teira is a targeted ornamental species because they have very long dorsal fins, anal and pelvic fins. It is a potential fine marine aquaculture species. Currently, the research on P. teira mainly focused on the mitochondrial genome, embryonic development and growth (Leu et al. 2018). However, the difference in muscle quality between different parts of P. teira has not been reported. Therefore, it is important practical value to systematically study on the basic characteristics of P. teira in different parts, fully understanding the composition characteristics of the raw materials, providing basic data and theoretical support for the processing of P. teira, further promoting the promotion and sustainable development of P. teira.

Materials and methods

Sample collection

Three experimental fish were obtained from the Tropical Fisheries Research and Development Centre, South China Sea Fisheries Research Institute, Chinese Academy of Fishery Science, Lingshui (Hainan, China). The mean body weight of the experimental fish was 450.07 ± 20.13 g. Fish were fasted for 24 h before sampling, and were anesthetized with 100 mg/L Eugenol (Shanghai Medical Instruments Co., Ltd., Shanghai, China). After this, they were anatomy and their relevant parts (the dorsal muscle, the upper part of the abdominal muscle; the bottom part of the abdominal muscle) were totally taken for weighing. The extracted tissues were immediately stored in liquid nitrogen immediately until used. All experiments in this study were approved by the Animal Care and Use Committee of South China Sea fisheries Research Institute, Chinese Academy of fishery Sciences (no.SCSFRI96-253) and performed according to the regulations and guidelines established by this committee.

Biochemical analysis

Proximate chemical composition

The moisture content was determined by drying the sample at 105°C until constant weight. Protein was determined by Kjeldahl method and crude lipid was determined using a Soxhlet extraction method and estimated by multiplying nitrogen by 6.25. Ash content was determined using a muffle furnace at 550°C for 8 h.

Amino acid composition

Total amino acids were determined by referring to the method of Chen et al. (2007). Details about the extraction of total amino acids follow below. A hundred of sample was placed in 10 mL ampules with 3 mL of 6 mol/L HCl. The amplues were then vacuum-sealed, and these samples were hydrolysed at 110°C for 24 h. Hydrolysates were dissolved to 50 mL using distilled water. After centrifuging, the supernatants were filtered through 0.22 mm pore size membrane. Then, 1 mL of the liquid was placed at 50°C to remove HCl. If necessary, the above step can be repeated two times. After this, 2–5 mL of 0.02 mol/L HCl was supplemented for solution. Finally, 1 mL of sample was analysed by the amino acid auto-analyser S-433D, Sykam Ltd., Germany. For analysis of tryptophane, samples were first hydrolysed with 100 g/L potassium hydroxide and then colorated using 4-dimethylaminobenzaldehyde before being analysed by spectrophotometer at 590 nm (Matheson 1974). The contents of cysteine and methionine were estimated according tothe method of formic acid oxidation and hydrolysation (Spindler et al. 1984). The essential amino acid score (EAAS) was calculated based on the FAO/WHO/UNU (1985) method:$\begin{array}{rl}& \mathrm{E}\mathrm{A}\mathrm{A}\mathrm{S}=100\times \text{the content of essential amino acid}\\ & insample/thecontentofessentialaminoacidin\\ & \text{FAO/WHO referred protein}.\end{array}$

Fatty acid composition

The fatty acid profile of fish tissues were analysed as described by (Zuo et al. 2013) with few modifications. The freeze-dried samples (∼120 mg muscle sample) were added into a 20 ml volumetric screwed tube with cover. Then 3 ml potassium hydroxide methanol (1 N) was added and heated in a water bath at 72℃ for 20 min. After cooling, 3 ml HCL–methanol (2 N) was added and the mixture was heated at 72℃ in a water bath for another 20 min. Previous tests were conducted to make sure that all fatty acids can be esterified following the procedures above. Finally, 1 ml hexane was added to the mixture above, shaken vigorously for 1 min, and then allowed to separate into two layers. Fatty acid methyl esters were separated, and measured by GC-MS (Agilent technologies 7890B -5977A, USA).

Statistical analysis

Data are expressed as the means ± standard error of mean (SE). The mean values were compared using one-way analysis of variance (ANOVA), followed by Tukey’s test. All statistical analyses were performed using SPSS 22.0 (SPSS Inc., Chicago, USA). The significance level adopted was 95% (p < .05).

Results

Proximate chemical composition

The proximate chemical composition of P. teira and other commercial fishes was shown (Table 1). The crude fat content of P. teira was 3.70%. The crude protein content of P. teira was 21.00%. The proximate chemical composition of Trachinotus ovatus was obtained by this subject (unpublished data). The proximate chemical composition of other fish was based on the results of previous studies.

Table 1. Nutritional components in the muscle of Platax teira and other commercial fishes (%).

Species	Moisture	Ash	Crude fat	Crude protein	Reference
Platax teira	72.65	1.55	3.70	21.00
T. ovatus	75.64	1.30	3.17	21.23
Rainbow trout	73.60	1.78	3.34	20.50	Song et al. (1996)
Epinephelus septemfasciatus	74.10	1.70	2.70	19.60	Cheng et al. (2009)
Pampus argenteus	73.11	1.21	4.90	20.16	Xu et al. (2012)
Pampus cinereus	74.08	0.85	6.12	18.45
Pampus sinensis	77.24	1.15	2.31	18.71
Dicentrarchus labrax	68.28	1.21	10.57	19.75	Orban et al. (2002)

Amino acid composition and nutritional quality evaluation

In muscle of three parts, seventeen kinds of amino acids were detected including nine essential amino acids (Table 2). There were some differences in the amino acids detected in the three muscle parts of P. teira. The contents of 11 amino acids of upper part of the abdominal muscle were significantly higher than the dorsal muscle, especially Ile, Leu, Lys, Phe, Thr, Val, Trp, Asp, Ser, Glu and His (P < .05). The amino acid content in the bottom part of the abdominal muscle were higher than in the abdominal muscles, but there were no difference (P > .05).

Table 2. Comparison of amino acids in the different parts of muscle in Platax teira (g/100 g).

Amino acids	Dorsal muscle	Upper part of the abdominal muscle	Bottom part of the abdominal muscle
Ile	0.80 ± 0.02^b	1.00 ± 0.02^a	0.91 ± 0.04^ab
Leu	1.40 ± 0.06^b	1.75 ± 0.05^a	1.59 ± 0.08^ab
Lys	1.58 ± 0.04^b	1.96 ± 0.05^a	1.79 ± 0.08^ab
Met	0.54 ± 0.03^a	0.67 ± 0.03^a	0.60 ± 0.05^a
Phe	0.71 ± 0.04^b	0.91 ± 0.03^a	0.82 ± 0.04^ab
Tyr	0.63 ± 0.06	0.80 ± 0.05	0.73 ± 0.05^a
Thr	0.79 ± 0.02^b	0.98 ± 0.02^a	0.90 ± 0.05^ab
Val	0.90 ± 0.03^b	1.13 ± 0.03^a	1.02 ± 0.05^ab
Trp	0.15 ± 0.01^b	0.20 ± 0.01^a	0.17 ± 0.01^ab
EAA	7.50 ± 0.30^b	9.40 ± 0.23^a	8.52 ± 0.44^ab
Asp	1.71 ± 0.04^b	2.13 ± 0.07^a	1.95 ± 0.11^ab
Ser	0.68 ± 0.01^b	0.84 ± 0.02^a	0.77 ± 0.05^ab
Glu	2.51 ± 0.06^b	3.13 ± 0.08^a	2.87 ± 0.17^ab
Pro	0.58 ± 0.05^a	0.60 ± 0.02^a	0.56 ± 0.06^a
Gly	1.05 ± 0.05^a	1.07 ± 0.06^a	1.06 ± 0.09^a
Ala	1.06 ± 0.01^a	1.24 ± 0.03^a	1.16 ± 0.07^a
His	0.39 ± 0.02^b	0.51 ± 0.02^a	0.46 ± 0.03^ab
Arg	1.15 ± 0.02^a	1.35 ± 0.04^a	1.26 ± 0.07^a
NEAA	9.13 ± 0.05^a	10.86 ± 0.31^a	10.08 ± 0.63^a
TAA	16.63 ± 0.34^b	20.26 ± 0.54^a	18.60 ± 1.06^ab
EAA/TAA	0.45 ± 0.01^a	0.46 ± 0.00^a	0.46 ± 0.00^a

Notes: EAA (essential amino acids), consisted of Thr, Val, Met, Ile, Leu, Phe, Tyr, Lys and Trp amino acid; NEAA (non-essential amino acid), consisted of Asp, Ser, Glu, Gly, Ala and Pro amino acid; TAA: total amino acids. In the same row, values without superscript mean no significiant difference (P > .05), while values with different superscripts mean statistically significiant difference (P < .05)

According to the FAO/WHO/UNU standards, EAAS in three parts of muscle was evaluated (Table 3). As a result, the EAAS in these three parts of P. teira were above 70. Meanwhile, the EAAS in the upper part of the abdominal muscle showed higher than those two parts (P < .05). EAAS in the bottom part of the abdominal muscle were higher than in the dorsal muscle.

Table 3. Comparison of EAAS for amino acids in the different parts of muscle in Platax teira

EAA	EAAS
	Dorsal muscle	Upper part of the abdominal muscle	Bottom part of the abdominal muscle	Averages
Ile	96.33 ± 2.91^b	120.01 ± 5.24^a	108.47 ± 2.91^ab	108
Leu	95.33 ± 2.84^b	118.00 ± 3.21^a	107.67 ± 5.44^ab	107
Lys	138.00 ± 3.78^b	171.67 ± 4.37^a	156.33 ± 7.26^ab	155
Thr	94.00 ± 2.65^b	116.67 ± 2.40^a	106.67 ± 6.39^ab	106
Trp	73.22 ± 6.01^b	100.00 ± 5.00^a	87.02 ± 4.11^ab	86.67
Val	86.33 ± 2.73^b	108.33 ± 2.33^a	97.67 ± 5.24^ab	97.33
Met + Cys	74.03 ± .71^a	90.00 ± 3.79^a	81.00 ± 6.02^a	81.67
Phe + Tyr	105.33 ± 7.22^b	134.00 ± 4.04^a	121.33 ± 6.84^ab	120

Fatty acid composition

The fatty acid composition of P. teira was shown (Table 4). In three parts, 27 kinds of fatty acids were detected in their muscle, including 11 saturated fatty acids (SFA), 6 monounsaturated fatty acids (MUFA) and 10 polyunsaturated fatty acids (PUFA). Among them, SFA was dominated by C16:0 and C18:0, MUFA dominated by C16:1n7 and C18:1n9c, and PUFA by C18:2n6c and C22:6n3. Moreover, the dorsal muscle had four types of fatty acids (C18:2n9c, C20:1, C20:3n3, C20:2) that were no significantly differences than those other groups (P > .05). The upper part of the abdominal muscle had ten types of fatty acids (C18:0, C22:0, C24:0, C24:1n9, C22:1n9, C20:3n6, C20:4n6, C20:5n3, C22:2n6, C22:6n3) and the bottom part of the abdominal muscle had seven types of fatty acids (C14:0, C16:0, C20:0, C16:1n7, C18:2n6c, C18:3n3, C18:3n6) that were no differences (P > .05). Generally, the content of SFA and PUFA in the upper part of the abdominal muscle was higher than that in other groups, but there was no difference (P > .05). No significant differences in MUFA and n-3/n-6 were detected among three parts.

Table 4. Comparison of fatty acid in the different parts of muscle in Platax teira (g/100 g).

Fatty acid	Dorsal muscle	Upper part of the abdominal muscle	Bottom part of the abdominal muscle	Averages
C12:0	0.05 ± 0.00^a	0.05 ± 0.00^a	0.05 ± 0.00^a	0.05
C13:0	0.02 ± 0.00^a	0.01 ± 0.00^a	0.02 ± 0.00^a	0.017
C14:0	2.05 ± 0.11^a	2.06 ± 0.08^a	2.13 ± 0.07^a	2.08
C15:0	0.36 ± 0.02^a	0.35 ± 0.01^a	0.36 ± 0.01^a	0.36
C16:0	22.37 ± 0.62^a	22.47 ± 0.42^a	22.50 ± 0.67^a	22.45
C17:0	0.41 ± 0.01^a	0.39 ± 0.01^a	0.41 ± 0.00^a	0.40
C18:0	7.07 ± 0.33^a	7.11 ± 0.14^a	6.63 ± 0.25^a	6.94
C20:0	0.36 ± 0.01^a	0.35 ± 0.03^a	0.37 ± 0.00^a	0.36
C22:0	0.20 ± 0.01^a	0.23 ± 0.01^a	0.21 ± 0.01^a	0.21
C23:0	0.09 ± 0.05^a	0.12 ± 0.00^a	0.12 ± 0.02^a	0.11
C24:0	1.03 ± 0.02^a	1.09 ± 0.06^a	0.99 ± 0.02^a	1.04
∑SFA	33.63 ± 1.11^a	33.78 ± 0.71^a	33.48 ± 1.04^a	33.63
C14:1n5	0.03 ± 0.00^a	0.03 ± 0.00^a	0.03 ± 0.00^a	0.03
C16:1n7	3.03 ± 0.11^a	3.00 ± 0.07^a	3.09 ± 0.06^a	3.04
C18:1n9c	22.70 ± 0.46^a	21.63 ± 0.44^a	22.50 ± 0.46^a	22.28
C20:1	0.91 ± 0.06^a	0.80 ± 0.03^a	0.86 ± 0.05^a	0.86
C22:1n9	0.28 ± 0.11^a	0.65 ± 0.22^a	0.21 ± 0.05^a	0.38
C24:1n9	0.23 ± 0.05^a	0.31 ± 0.02^a	0.21 ± 0.01^a	0.25
∑MUFA	27.18 ± 0.45^a	26.43 ± 0.32^a	26.89 ± 0.39^a	26.83
C18:2n6c	26.47 ± 0.74^a	26.23 ± 0.27^a	26.87 ± 0.64^a	26.52
C18:3n3	2.73 ± 0.13^a	2.57 ± 0.06^a	2.75 ± 0.11^a	2.68
C18:3n6	1.57 ± 0.14^a	1.51 ± 0.13^a	1.59 ± 0.13^a	1.56
C20:2	0.82 ± 0.04^a	0.80 ± 0.02^a	0.81 ± 0.04^a	0.81
C20:3n6	1.74 ± 0.23^a	1.98 ± 0.25^a	1.72 ± 0.15^a	1.81
C20:4n6(ARA)	0.54 ± 0.12^a	0.75 ± 0.12^a	0.50 ± 0.06^a	0.60
C20:3n3	0.17 ± 0.00^a	0.11 ± 0.06^a	0.16 ± 0.00^a	0.15
C20:5n3(EPA)	1.65 ± 0.06^a	1.72 ± 0.13^a	1.56 ± 0.11^a	1.64
C22:2n6	0.06 ± 0.01^a	0.11 ± 0.00^a	0.07 ± 0.01^a	0.08
C22:6n3(DHA)	3.40 ± 0.19^a	4.06 ± 0.51^a	3.62 ± 0.76^a	3.69
∑PUFA	39.13 ± 1.12^a	39.77 ± 0.96^a	39.65 ± 1.41^a	39.52
∑n-3PUFA	7.96 ± 0.38^a	8.46 ± 0.66^a	8.09 ± 0.98^a	8.17
∑n-6PUFA	28.57 ± 0.77^a	28.49 ± 0.41^a	28.96 ± 0.66^a	28.67
n-3/n-6	0.28 ± 0.01^a	0.30 ± 0.02^a	0.28 ± 0.03^a	0.29
∑HUFA	5.59 ± 0.28^a	6.53 ± 0.73^a	5.68 ± 0.89^a	5.93
EPA/DHA	0.49 ± 0.01^a	0.43 ± 0.03^a	0.45 ± 0.06^a	0.47

Notes: SFA: saturated fatty acids, PUFA: polyunsaturated fatty acids, MUFA: monounsaturated fatty acids, HUFA: highly unsaturated fatty acids acids (carbon chain length ≥ C20 with ≥ 3 double bonds), DHA: docosahexaenoic acid, EPA: eicosapentaenoic acid. In the same row, values without superscript mean no significiant difference (P > .05), while values with different superscripts mean statistically significiant difference (P < .05).

Discussion

As an important marine fish in China, P. teira is a good source of protein in the national diet. This study found that the content of crude protein in the muscle of P. teira was 21.00%, which is higher than Rainbow trout (Song et al. 1996), E. septemfasciatus (Cheng et al. 2009), D. labrax (Orban et al. 2002), P. argenteus, P. cinereus and P. sinensis (Xu et al. 2012), except for T. ovatus; the content of crude fat in the muscle of P. teira was 3.70%, which is lower than D. labrax (Orban et al. 2002), P. argenteus and P. cinereus (Xu et al. 2012), however, which is higher than Rainbow trout (Song et al. 1996), E. septemfasciatus (Cheng et al. 2009), P. sinensis (Xu et al. 2012) and T. ovatus. It could be seen that P. teira was a kind of high-quality fish with high protein and low fat, which could satisfy consumers’ demand for high-protein and low-fat food.

Whether the amino acid composition is comprehensive and the content of essential amino acids determines the edible value of the protein. EAAS and ratio of EAA/TAA are two key factors to evaluate the nutritional quality of amino acid for fishery products (Wu et al. 2014). Some of EAAS’ amino acids over 100 indicate that the amino acid is not limited and the ideal EAA/TAA ratio is about 0.4 (FAO/WHO/UNU 1985). In this study, EAA/TAA ratios in three parts of the muscles of P. teira were all above 0.4, which indicated muscle had high nutritional quality of amino acid. In three parts of the muscles, no significant differences in composition of amino acid were observed. However, differences in composition and content amino acid and EAAS of the upper part of the abdominal muscle were higher that of the dorsal muscle indicating nutritional quality of amino acid of the upper part of the abdominal muscle was higher. Overall, the dorsal muscle had a lower total amino acid content, EAA/TAA ratio and EAAS compared to the upper part of the abdominal muscle and the bottom part of the abdominal muscle, indicating nutritional quality of amino acid of the upper part of the abdominal muscle and the bottom part of the abdominal muscle were higher.

Generally, types of fatty acid in three parts were similar, among SFA, MUFA and PUFA. This indicated the conservation and stability of fatty acids in three parts of P. teira. However, the content of fatty acid showed differences between parts. For the content of SFA and PUFA, the upper part of the abdominal muscle was higher, followed by the bottom part of the abdominal muscle and the dorsal muscle. However, for the content of MUFA, the dorsal muscle was higher, followed by the bottom part of the abdominal muscle and the upper part of the abdominal muscle. In three parts, the content of FUFA was higher, followed by SFA and MUFA. This not indicated the distribution of fatty acid in P. teira has certain specificity to parts but showed contents in three parts was very rich.

Recently, the importance of high-unsaturated fatty acid (HUFA) has been well agreed by the public (He et al. 2017). In PUFA, C22:6n3 (DHA), C20:5n3 (EPA) and C20:4n6 (ARA) are beneficial not only for the growth and reproduction, but also beneficial to the health of humans (Chang et al. 2008). Therefore, the composition and content of HUFA is a key factor to estimate the nutritional value of fatty acid (Muskiet et al. 2006). The upper part of the abdominal muscle has the highest HUFA content, but the dorsal muscle was the lowest. Therefore, fatty acids in the upper part of the abdominal muscle had the highest value of nutrition compared to those in the dorsal muscle and the bottom part of the abdominal muscle.

In our daily life, the intake of n-3 PUFA is much lower than that of n-6 PUFA. Therefore, the ratio of n-3/n-6 is a good indicator of nutritional value and a higher ratio indicates higher nutritional quality. At the same time, the appropriate ratio of n-3/n-6 for human is 0.1–0.2 and the higher is perfect (FAO/WHO, 1994). In this study, this ratio in three parts was above 0.2. Generally speaking, three different parts of the muscle have high nutritional value of fatty acid. Specifically, the upper part of the abdominal muscle contained more DHA, EPA and PUFA, and had a higher ratio of n-3/n-6 PUFAs compared to the dorsal muscle and the bottom part of the abdominal muscle. So the total nutritional quality of the upper part of the abdominal muscle is higher than those of the dorsal muscle and the bottom part of the abdominal muscle in terms of fatty acid.

Conclusions

The nutritional components of the dorsal muscle, the upper part of the abdominal muscle and the bottom part of the abdominal muscle were determined, and their nutritional value was synthetically evaluated. Results showed that there were the muscle protein content was as high as 21.00%. Overall, the upper part of the abdominal muscle nutrition levels was the best, followed by the bottom part of the abdominal muscle. Different parts of P. teira had different nutritional composition, but the overall difference was not large. P. teira was a high quality marine fish with high protein content and low fat, abundant amino acids with proper proportion. It was a fine food protein source, however, P. teira was regarded as an excellent breeding varieties.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Key R&D Program of China (2018YFD0901204), the Guangdong Provincial Science and Technology Project (2019B030316030), the China-ASEAN Maritime Cooperation Fund and the National Infrastructure of Fishery Germplasm Resources Project (2019DKA30407).