Abstract

The purpose of this study was to assess the nutritional composition of clam Callista erycina from the South China Sea, and determine its edible value and prospects for aquaculture development. The proximate composition, amino acid composition, fatty acid content and mineral elemental constituents of the soft tissues of clam C. erycina were analyzed. The results showed that the percentage edibility (PE) and condition index (CI) of clam C. erycina were 22.45±3.00 and 95.51±13.34, respectively. The contents of protein, lipid, ash and moisture in the soft tissues (fresh sample) of the clam C. erycina were 13.84±0.27 g, 0.53±0.03 g, 2.56±0.11 g and 77.86±0.74 g per 100g, respectively. A total of 16 common amino acids were detected, including 9 essential amino acids and 4 umami taste amino acids, which accounted for 45.372 % and 42.881 % of the total amino acids. The amino acid score (AAS) of total essential amino acids were 134, and the first limiting amino acid was valine with 93 as AAS, which suggesting that the composition ratio of essential amino acids in soft tissues of clam C. erycina is in line with the FAO/WHO model. The clam C. erycina were rich in polyunsaturated fatty acids (33.08±0.44 % of total fatty acid) with high levels of DHA (13.55±0.55 % of total fatty acid) and EPA (7.58±0.19 % of total fatty acid). Among the mineral elements, sodium (Na) and potassium (K) has the highest mass fraction, followed by phosphorus(P), magnesium (Mg), calcium (Ca), iron (Fe) and zinc (Zn). The results show that clam C. erycina are marine economic shellfish with high nutritional value and edible value, suggesting that the consumption increment and aquaculture activities of clam C. erycina should be encouraged.

1. Introduction

Shelled molluscs, especially mussels, oysters and clams, belong to the commercially important benthic group of organisms and are food resources of nutrients such as proteins, lipids, and minerals. Knowledge about the biochemical composition of edible bivalve molluscs is very necessary for consumers, because the nutritional value is reflected in its biochemical content ¹. Clams are products with great commercial value and are regarded as a traditional food in many countries, because they are high-quality nutritious foods and are considered gourmet foods. The nutritional quality of clams depends not only on the relatively low lipid content and high proportion of polyunsaturated fatty acid (PUFA), but also on its high quality of the protein ^{2, 3}. In recent years, in order to meet the growing consumption of seawater shellfish, China's large-scale fishing and aquaculture of bivalve molluscs has developed rapidly, especially along the coast of the South China Sea.

Callista erycina belongs to the family Veneridae, and is widely distributed in South China Sea ⁴, Indian Ocean ⁵, Arabian Sea ⁶, etc. It mainly inhabits the sandy seabed within 20 m from the intertidal zone to the shallow sea. As a kind of economical shellfish with good taste, the clam C. erycina is getting more popular among consumers in China, and it is not sustainable by mainly be produced through harvesting natural resources. There is very little research on clams C. erycina, only Wei et al. studied the correlation and path analysis of the quantitative traits of the clam C. erycina ⁴. In addition, Palma et al. determined the calcium content in the shells of clams C. erycina and found that there was no significant difference comparing with the Crassostea echinate, Perna viridis, and Placuna placeta ⁷. Zou et al. reported that there were no detectable diarrhetic shellfish poisoning (DSP) toxins in clams C. erycina from Xincun Bay, Hainan Island, the South China Sea ⁸. There is no research report on the comprehensive analysis of the nutrient composition of the clam C. erycina, only Lao et al. reported the fatty acid content and composition of the clam C. erycina collected from seafood market in Guangzhou, China ⁹.

In order to understand the nutritional characteristics of clams C. erycina and provide theoretical guidance for their artificial breeding. This article conducted a comprehensive analysis of the nutritional composition of the soft tissues of the clams C. erycina from South China Sea, including the proximate composition, amino acid content, fatty acid composition and mineral elements.

2. Materials and Methods

July 2020, a total of 124 clams Callista erycina with a similar size (body weight 76.48±14.38 g, shell length 71.09±4.86 mm, shell height 53.77±3.18 mm, shell width 33.46±2.39 mm) were collected from offshore in southern Hainan, China. After 7 days restoration at the floating raft of Tropical Aquaculture Research and Development Center, South China Sea Fisheries Research Institute in Xincun Port, Lingshui, Hainan (18° 40' N, 109° 96' E), clams C. erycina were transferred to the laboratory for a 12 h starvation treatment in a cement pool with 5000 L filtered sea water. The clams C. erycina were randomly divided into four groups (31 clams for each group). The total clam, shell and soft tissues from each C. erycina were weighted, and mixed the soft tissues in one group then stored at -80 °C until use for biochemical analysis. Percentage edibility (PE) and condition index (CI) of clams C. erycina were calculated following the Eqs. (1) and (2) described by Mohite et al. ¹⁰ and Orban et al. ¹¹.

(1)

(2)

The proximate composition of soft tissues from each group was analyzed based on AOAC ¹². Determine moisture by drying the sample in the oven (DHG-9245A, Shanghai Yiheng Scientific Instrument Co., Ltd. China) at 105 °C overnight until reaching constant weight. Protein content was determined by an Automatic Kjeldahl nitrogen analyzer (SKD-5000, Shanghai Peiou Analytical Instrument Co., Ltd. China). The crude lipid was extracted with petroleum ether, and the ash content was determined in the muffle furnace (JC-MF12-16, Qingdao Juchuang Environmental Protection Group Co., Ltd. China) at 550 °C overnight. The contents of moisture, protein, lipid and ash were expressed on g/100g wet soft tissues weight.

In a sealed glass tube filled with nitrogen, the frozen portions of the sample were hydrolyzed with 6 M HCl at 110±1°C for 22 hours. After hydrolysis, 1 mL of the hydrolyzate was taken out and evaporated to dryness under vacuum at 50 °C. The hydrolysate was filtered with a 0.22 μm membrane after dissolved in 1 mL of sodium citrate buffer (pH 2.2). Amino acids were assessed by comparison with the standards (Sigma) by an Automatic Amino Acid Analyzer (SYKAM, S-433D). All measurements of the sample are repeated three times, and the contents of amino acids were expressed as mg/g wet soft tissues weight.

The amino acid score (AAS) was calculated using Eqs. (3) as reported by Yang et al. ¹³.

(13)

For fatty acid analysis, the fatty acids in the samples were extracted with a 2:1 (v/v) chloroform/methanol mixture ¹⁴, and the fatty acid methyl esters (FAMEs) were prepared by esterification using 2% sodium hydroxide methanol. FAMEs were separated and quantified by means of a gas chromatography (Agilent 7890A) equipped with a Supelco SPTM-2560 capillary column (100 m long, 0.25-mm internal diameter, and 0.20 mm film thickness) and flame ionization detector (GCFID). Helium was used as carrier gas at a flow rate of 1.0 mL/min, while injector and detector with temperatures of 250 and 260 °C respectively were used. The temperature program was: initial temperature 60 °C, increasing up to 160 °C at 4 °C per minute, then increasing up to 240 °C at 2 °C per minute. Each of the specific FAME peaks was identified by the retention time with reference to the known standard (Anpel Laboratory Technologies Inc., Shanghai, China). All samples were performed in triplicate, and the fatty acid content was measured using the peak area ratio (% total fatty acids).

Sodium (Na), potassium (K), Calcium (Ca), magnesium (Mg), zinc (Zn), iron (Fe), copper (Cu) and selenium (Se) contents of frozen-dried portions were determined by iCE 3500 atomic absorption spectrometer (THERMO FISHER, USA), and the Phosphorus (P) was determined by UV-2550 UV-Visible Spectrophotometer (Shimadzu, Japan) according to the method of AOAC ¹⁵. All samples were performed in triplicate.

3. Results and Discussion

Soft tissues of C. erycina is a high protein and moisture, and low-fat food based on its proximate composition (Table 1). Such composition is typical of other bivalve molluscs ^{16, 17}. Similar to the Meretrix lusoria ¹⁸, it was established that the moisture content of the clams C. erycina soft tissues is 77.86±0.74 g/100 g wet soft tissues weight. Since the percentage of edibility and condition index reflect the individual’s status of gametogenesis and nutrient reserve storage consumption, they are of great economic significance because they can detect the commercial quality of clam species exploited. ^{19, 20}. PE and CI, together with proximate composition of clams may be the simplest and most useful standards used in international trade ²¹. In this study, the PE of C. erycina in July was 22.45±3.00, which is lower than that of the clam Ruditapes decussatus cultivated in the Ria Formosa (South Portugal) from the same period, while the CI of C. erycina was 95.51±13.34, in agreement to clams R. decussatus from the same period reported by Aníbal et al. ²¹.

The protein content of C. erycina in this work was 13.84±0.27 g/100g (Table 1), this value is similar to that of Mactra chinensis reported by Tabakaeva et al. ²², and slightly higher than that of M. lusoria and Mercenaria Mercenaria reported by Karnjanapratum et al. ¹⁸ and Ogidi et al. ¹⁷ respectively, but significantly lower than that of Egeria radiata reported by Ivon et al. ²³. Apart from its rich protein content, the typical characteristic of C. erycina is its high ash content, which can reach to 2.56±0.11 g/100g. The ash content of C. erycina was higher than that of M. chinensis and Anadara broughtonii from Amur Bay, Sea of Japan ²², Meretrix meretrix from the coast of Andaman Sea ¹⁸ and Haliotis discus hannai Ino from the Nanri Island, Putian, China ²⁴. And the ash content of C. erycina was significantly lower than that of Flexopecten glaber from the Central Mediterranean Sea ²⁵, Egeria radiata from the Calabar river, Cross River State, ²³ and M. Mercenaria from the Ekowe community ¹⁷. The lipid content of C. erycina in this work was 0.53±0.03 g/100g, which was close to the level of M. chinensis ²², but significantly lower than that of Anadara broughtonii ²², M. lusoria ¹⁸, F. glaber ²⁵ and E. radiata ²³.

The ratio of major amino acids varies in clams ²². In this work, the content of 16 amino acids (9 kinds essential amino acids and 7 kinds non-essential amino acids) in C. erycina were detected, and the amino acid profile was provided in Table 2. The soft tissues of the clam C. erycina showed high content of amino acids, the major amino acids in soft tissues of C. erycina were glutamic acid and aspartic acid, followed by arginine, leucine, lysine, glycine and alanine. Which was different from the others marine bivalve mollusks, such as Perna viridis L. ²⁶, Paphia malabarica and Villorita cyprinoids ²⁷ indicated arginine, leucine, and lysine as dominating fundamental amino acids. The purpose of protein intake is to obtain various amino acids suitable for human needs. Essential amino acids (EAA) are the nutritional needs of humans under all conditions, and they play a central role in the quality of food protein ²⁸. Therefore, EAA level is the main factor affecting the nutritional value of protein. In this study, 9 essential amino acids (including arginine and histidine) of the brown-banded fairy clam were tested, and the ratio of essential amino acids to total amino acids (TAA) is 45.372 %. Which is slightly below than that of Japanese abalone (H. discus hannai Ino) (47.37 %) ²⁴, but higher than that of M. chinensis and A. broughtonii (32.1 %-41.8 %) ²². The highest values of EAA in soft tissues of C. erycina was observed for arginine (9.555±0.108 mg/g wet soft tissues weight), followed by lysine and leucine (8.830±0.035 and 8.752±0.277 mg/g wet soft tissues weight respectively). The similar results were also found at the foot of M. lusoria reported by Karnjanapratum et al. ¹⁸. According to reports, arginine can regulate the synthesis of nitric oxide in endothelial cells, which is related to reducing the risk of cardiovascular disease ²⁹. Intake of arginine in protein can also have a beneficial effect on nutritional and metabolic status by regulating the utilization and metabolism of amino acids in the small intestinal microflora ³⁰. The nutritional importance of lysine is a possible limiting amino acid in grains (especially wheat), so its nutritional importance has attracted much attention ³¹, and leucine supplementation in the diet has been widely used to treat obesity and metabolic dysfunction caused by obesity ³².

There are 4 kinds of umami taste amino acids (UAA) in C. erycina, account for 42.881 % of the total amino acids (Table 2), which is higher than that of M. lusoria (35.907 %) ¹⁸. Glycine and valine are characteristic amino acids with sweet taste, the contents were 8.700±0.334 and 8.545±0.144 mg/g wet soft tissues weight, respectively. The umami taste of clam C. erycina is sufficient, as glutamic acid and aspartic acid are characteristic amino acids with umami taste, and their contents were 20.010±0.124 and 13.147±0.235 mg/g wet soft tissues weight, respectively. Which were higher than that of H. discus hannai Ino ²⁴, M. lusoria ¹⁸, M. chinensis and A. broughtonii ²². In addition, there are 7 kinds non-essential amino acids (NEAA), account for 54.628 % of the total amino acids in C. erycina (Table 2). The highest NEAA content of C. erycina was determined to be glutamic acid (20.010±0.124 mg/g wet soft tissues weight), followed by aspartic acid. Glutamic acid has been known to be one of the most abundant intracellular amino acids in mammals. The pool size of free glutamic acid in most brain regions is quite high ³³. The high concentration of glutamic acid may be because glutamate not only acts as a neurotransmitter, but also acts as a key component of intermediate metabolism in brain ³⁴. Generally, the protein with larger EAA/NEAA ratio is considered to be more beneficial for well-being. Similar to the EAA/NEAA ratio for Ensis siliqua (~80 %) ³⁵, the ratio for C. erycina was 83.056 % in this study. Which was higher than that of H. discus hannai Ino (24 %~67 %) ²⁴, but less than those found in P. malabarica and V. cyprinoids (107 %~113 %) ²⁷.

The results of the evaluation of amino acid scores (AAS) for the soft tissues of C. erycina are presented in Table 3. Amino acid scores and the first limiting amino acid are established methods of assessing the quality of proteins, which are widely used in studies on the nutritive value of proteins ^{31, 36}. In this study, the total essential amino acids content in the soft tissues of C. erycina (347.4 mg/g protein) was higher than the recommended daily intake (260 mg/g protein) ³¹. Comparing our results with the amino acid composition of the reference protein recommended by FAO/WHO/UNU, it was found that almost all the main indicators of the amino acids in the soft tissues of clams C. erycina were higher than 100 %, with the exception of valine (93 %). Therefore, C. erycina had only one limiting amino acid (valine), which is less than the M. chinensis and A. broughtonii ²². The first limiting amino acid score in the soft tissues of clams C. erycina was close to 100 %, indicating that clams C. erycina possesses good protein nutrients. By contrast, the AAS of histidine in soft tissues of clams C. erycina was highest as 249 %, followed by threonine (177 %), which was approximately twice as high as the reference standard concentration. Histidine supplementation is beneficial to various several health benefits, such as antioxidant, anti-inflammatory, body weight management, glucoregulatory and cognitive function ³⁷, the lack of threonine is known to impair intestinal paracellular permeability and is associated with fewer goblet cells and mucus synthesis ^{38, 39}, and the clams C. erycina happens to be a rich source of histidine and threonine. All in all, the amino acid pattern of the C. erycina is basically in line with the ideal model of FAO/WHO, indicating that the C. erycina has high nutritional value and it’s an ideal high-quality protein source for humans.

In this study, the contents of 9 kinds saturated fatty acids (SFA), 5 kinds monounsaturated fatty acids (MUFA) and 7 kinds polyunsaturated fatty acids (PUFA) in soft tissues of C. erycina is determined, and the content of the other 15 kinds fatty acids is too low to be detected (Table 4). The highest values of SFA in soft tissues of C. erycina was observed for palmitic acid (C16:0, 28.15±0.87 %), followed by stearic acid (C18:0, 12.02±0.36 %). The same results were also found at the Flexopecten Glaber ²⁴, Paphia malabarica and V. cyprinoids ²⁷. Same as Asian hard clam M. lusoria ¹⁸, Palmitoleic acid (C16:1n7, 8.16±0.11 %) and Cis-9-octadecenoic acid (C18:1n9c, 7.45±0.06 %) were also the major MUFAs in clams C. erycina. But in the bivalves Arca noae and F. glaber ⁴⁰, Cis-9-hexadecenoic acid (C16:1) and C18:1n9c are the major MUFAs. Among PUFAs, Cis-4,7,10,13,16, 19-docosahexaenoic acid (C22:6n3, DHA, 13.55±0.55 %), Cis-5, 8, 11, 14, 17-eicosapentaenoic acid (C20:5n3, EPA, 7.58±0.19 %) and Cis-5, 8, 11, 14-eicosatetraenoic acid (C20:4n6, 6.25±0.23 %) were dominant in clams C. erycina, which was similar to the results found in Crassostrea gigas, but different from the results in Limaria tuberculate and Mimachlamys varia ⁴⁰.

SFA (48.86±0.44 % of total fatty acids (FA)) was the dominant group in clams C. erycina, followed by PUFA (33.08±0.44 % of FA) and MUFA (18.43±0.32 % of FA, Table 4). The high intake of SFA could increase the low-density lipoprotein in the blood ⁴¹, and increase the risk of cardiovascular disease or coronary heart disease ⁴². In contrast, MUFA and PUFA are considered good fats, because they help in reducing risk of chronic metabolic diseases, different cancers, asthma, rheumatoid arthritis, osteoporosis, psoriasis and inflammatory bowel disease ^{43, 44, 45}. Most healthy diet recommendations focus on reducing SFA intake, but increasing MUFA and PUFA intake. Σn3, Σn6 and Σn3/Σn6, ΣPUFA/ΣSFA, ΣUNS/ΣSFA ratios are widely used to assess the nutritional value of lipids ⁴⁶. It is well known that of n3 FA have beneficial effects on human health ⁴⁵, and seafood products are the important source of high level n3 FA in the diet ⁴⁷. In this study, the n3 PUFA (23.08±0.47 % of FA) with DHA and EPA identified as the main PUFAs was the dominant family (Table 4). The DHA/FA and EPA/FA ratio in C. erycina were much higher than that in Ruditapes philippinarum (3.09 % and 1.92 % of FA, respectively), but little lower than that in M. varia (15.16 % and 9.71 % of FA, respectively) ⁴⁶. According to the reports by UK Department of Health ⁴⁸, Simopoulos ⁴⁹ and Chow ⁵⁰, high ratio of Σn3/Σn6 is favorable to human health, and the recommended ratio for health benefits is 1.0 at minimum, but of 4.0 at maximum. In this study, the ratio of Σn3/Σn6 in C. erycina was 2.95, suggesting that clams C. erycina could be categorized as ideal to human health consumption. And which was similar to the F. Glaber (2.7-3.1) ²⁵ but higher than that in Asian hard clam M. lusoria (0.99-1.58) ¹⁸, and lower than that in C. gigas (6.35-8.11) ⁵¹. According to the current nutritional recommendations ⁴⁸, the recommended ratio of PUFA/SFA is above 0.45. In our study, the value (0.68) in clams C. erycina was well above of established limits.

Figure 1 shows the level of elements in the soft tissues of C. erycina. Sodium (Na, 4292.500 ± 187.506 mg/kg soft tissues wet weight) and potassium (K, 2794.925±202.484 mg/kg soft tissues wet weight) were the main macro elements in clams C. erycina. followed by magnesium (Mg, 887.350±34.596 mg/kg soft tissues wet weight), calcium (Ca, 637.350±15.186 mg/kg soft tissues wet weight). Similar results were also found in P. malabarica ²⁷. Na and K are the minerals involved in maintenance of muscular irritability, acid-base balance osmotic equilibrium, and normal water balance. The suitable of Na/K ratio in the human body play an important role in preventing hypertension and arteriosclerosis, and it is recommended that the Na/K ratio be less than one ⁵². In our study, the Na/K ratio in clams C. erycina was 1.54, much higher than that in H. discus hannai Ino (0.69), suggesting that the soft tissues of clams C. erycina was not as suitable as abalone muscle for humans with high blood pressure ²⁴. Ca is important to the human body health for the effects on the bone health and play an important role in the contraction and relaxation of muscle ⁵³, and Mg is a coenzyme involved in many important biochemical reactions in the body ¹⁸. Dietary Reference Intakes (DRI) of Ca and Mg for adult human are not more than 1000 and 400 mg per day, respectively ⁵⁴. The Ca and Mg contents in clams C. erycina were higher than that in P. malabarica (342.5 and 367.5 mg/kg wet soft tissues weight respectively) and V. cyprinoids (285.2 and 258.6 mg/kg wet soft tissues weight respectively) reported by Joy et al. ²⁷. Phosphorus(P) is an important component of human bones, teeth and nerve tissue. The P content in C. erycina was (1451.750±53.631 mg/kg wet soft tissues weight), which was lower than that in Crassostrea madrasensis L. (3197-5302 mg/kg wet soft tissues weight) ⁵⁵, but higher than that in H. discus hannai Ino (1001.21 mg/kg wet soft tissues weight) ²⁴.

The essential trace minerals, such as iron (Fe), zinc (Zn), copper (Cu) and selenium (Se) play a key role in keeping good condition of physiological fluid ⁵⁶. In addition, iron is participating in a wide variety of metabolic processes in human body, including electron transport, oxygen transport and DNA synthesis ⁵⁷, and Zn deficiency could lead children to increase in infectious diseases, retarded growth and impaired cognitive function. Cu is an important component of certain proteins that have biological functions required for development and growth ⁵⁸, and Se as an antioxidant, plays a pivotal role in proper organ function and development ⁵⁹. In this study, the Fe and Zn contents of clam C. erycina (31.775±1.654, 0.515±0.019 mg/kg wet soft tissues weight respectively) were lower than the P. malabarica(76.4±1.4 and 30.1±1.3 mg/kg wet soft tissues weight respectively) and V. cyprinoids (56.5±0.4 and 32.6±0.7 mg/kg wet soft tissues weight respectively) ²⁷, while the Cu and Se contents of clam C. erycina ( 4.6±0.6 and 0.302±0.001 mg/kg wet soft tissues weight respectively) were higher than the P. malabarica (3.0±0.2 and 0.273±0.000 mg/kg wet soft tissues weight respectively) and V. cyprinoids (56.5±0.4 and 32.6±0.7 mg/kg wet soft tissues weight respectively) ²⁷. Overall, soft tissues of clam C. erycina could be a good source of nutritionally important elements for consumers.

4. Conclusion

The major amino acids in soft tissues of C. erycina were glutamic acid and aspartic acid, followed by arginine, leucine, lysine, glycine and alanine. C16:0, C18:0, C22:6n3, C16:1n7, C18:1n9c, C20:5n3, and C20:4n6 were the major fatty acids in clams C. erycina. Among the mineral elements, Na and K has the highest mass fraction, followed by P, Mg, Ca, Fe and Zn. Based on the results of this study, the soft tissues of clam C. erycina contained high amount of water, whereas the dry mass was rich in nutritionally important amino acids, fatty acid and mineral elements. Suggesting that clam C. erycina are suitable for human consumption. Therefore, seed breeding and aquaculture activities related to clams C. erycina should be encouraged to reduce the dependence on wild stock.

Acknowledgements

This study was supported by the grants from the Science and Technology Cooperation Projects of Sanya Scientific Research Institutions(2019YD05), Central Public-interest Scientific Institution Basal Research Fund, South China Sea Fisheries Research Institute, CAFS (2019TS07), Financial Fund of Ministry of Agriculture and Rural affairs of China (NHYYSWZZZYKZX2020), Special Funds for the Construction of Modern Agricultural Industry Technology System (CARS-49).

Conflict of Interest

The authors declare that there are no conflicts of interest.

References