Quality Analysis and Evaluation of Scylla paramamosain from Three Different Areas

Aixia HUANG Feng LIN Lihui SUN Qian LI Haiqi ZHANG Rongxiang JIANG

Abstract [Objectives] The quality of Scylla paramammosain produced in Fujian， Sanmen and Shanghai were analyzed and evaluated.

[Methods] Adult Scylla paramammosain from Fujian， Sanmen and Shanghai were separated for their muscles and gonads， which were analyzed for nutritional components.

[Results] The muscle moisture content of S. paramammosain from Sanmen was significantly higher than those in S. paramammosain from Fujian and Shanghai. The muscle protein contents of S. paramammosain from the three regions ranked as Shanghai>Fujian>Sanmen （P<0.05）. The muscles of S. paramammosain from Shanghai had the highest crude fat and crude ash contents. The moisture contents of the gonads of the S. paramammosain from the three regions had no significant differences. The protein contents of the gonads of S. paramammosain from the three regions ranked as Shanghai>Sanmen>Fujian （P<0.05）. The S. paramammosain from Sanmen had the highest crude fat and crude ash contents in gonads. The contents of 17 amino acids detected in the muscles and gonads of S. paramammosain from the three different regions were different. The total amounts of total amino acids （TAA）， essential amino acids （EAA） and flavor amino acids （FAA） in the muscles of S. paramammosain from Sanmen were significantly lower than those produced in Fujian and Shanghai （P<0.05）， but the proportions of essential amino acids （EAA） and flavor amino acids were not significantly different. The proportion of EAA and FAA in the gonads of S. paramammosain produced in the three areas were significantly different （P<0.05）. There were also differences in the fatty acid composition of the muscles and gonads of S. paramammosain from the three different regions. The proportion of EPA in the muscles of S. paramammosain from Fujian was higher than those of other two groups， but the differences were not significant; the proportion of DHA was significantly higher than those of other two groups， and the differences were significant （P<0.05）; and the proportions of n3 PUFA in muscles of S. paramammosain produced in Fujian and Sanmen were significantly higher than that of S. paramammosain produced in Shanghai （P<0.05）. The proportions of EPA， DHA and n6 PUFA in the gonads of S. paramammosain produced in Sanmen were significantly higher than those produced in Fujian and Shanghai （P<0.05）， while the n3 PUFA was the highest in S. paramammosain produced in Fujian. The proportions of n6/n3 PUFA in S. paramammosain produced in the three different regions were significantly different （P<0.05）. In summary， it could be seen that the nutritional composition of S. paramammosain produced in different regions has its own characteristics.

[Conclusions] This study provides basic data for the establishment of quality evaluation standards for S. paramammosain， and provides a new perspective for the evaluation of breeding models.

Key words Scylla paramammosain; Muscle; Gonad; Nutrient composition; Amino acid; Fatty acid

Scylla paramammosain， commonly known as "Xun"， belongs to Scylla in Portunidae of Decapoda of Crustacea. Its meat is delicious， rich in nutrients， and has nourishing and strengthening effects. It is known as "Sea Ginseng" and is widely distributed in the temperate， subtropical and tropical mangrove areas and estuary areas of the Pacific and Indian Oceans. It is an important breeding economic crab in the southeast coastal provinces such as Zhejiang[1-2]. S. paramammosain breeding is one of the four leading aquaculture industries in Zhejiang Province. In 2014， the breeding area of S. paramammosain in Zhejiang Province reached 10 533 hm2， with an output of 27 300 t， accounting for about 20% of the national output， with an output value exceeding 2.5 billion yuan. There may be significant differences in the nutritional quality and taste of crabs in different ecological environments[3]. There has been no systematic study on the quality of S. paramammosain. In this study， the basic nutritional composition， amino acid content， and fatty acid composition of the muscle and gonads of S. paramammosain from different regions were analyzed and compared， aiming to fully understand the quality of S. paramammosain. This study provides basic data for the establishment of quality evaluation standards for S. paramammosain， provides a new perspective for the evaluation of breeding models， and accumulates basic data for bait development.

Materials and Methods

Experimental materials

The S. paramammosain produced in Sanmen were provided by the Sanmen Luyang Special Aquaculture Cooperative， and the S. paramammosain produced in Fujian and Shanghai were purchased from markets in the two areas. The crabs used were male crabs weighing （183.98±14.25） g， and the collection time was mid March 2020.

Experimental methods

The muscles and gonads of S. paramammosain were separated， and freeze-dried for the analysis of amino acids and fatty acids and the analysis of basic nutrients.

The determination of moisture， crude protein， crude fat and ash content referred to the AOAC method[4]. The Kjeldahl method was used for determination of crude protein. The Soxhlet extraction method was used for determination of crude fat. The ash content was determined by muffle furnace 550 ℃ incineration method.

Amino acid content determination： The acid hydrolysis method was adopted. The material was pretreated with 6 mol/l concentrated hydrochloric acid at 110 ℃ for 22 h， and determined by Hitachi L8900 amino acid analyzer.

Fatty acid composition： A certain amount of the vacuum freeze-dried crushed sample （100 mg） was weighed， put it into a 10 ml graduated test tube with stopper， and added with 3 ml of 1 mol/L KOH-methanol solution. The test tube was heated in a water bath at 75-80 ℃ for 20 min， and cooled. A certain amount of n-hexane （1 ml） was added， and extracted with shaking. The reaction system was stood for layering. A certain amount of supernatant （1 μl） was loaded and determined on a GC-MS combination instrument （Agilent technologies 7890B-5977A， USA）.

Evaluation of amino acid nutritional quality

According to the amino acid scoring standard model and the amino acid model of whole egg protein recommended by the Food and Agriculture Organization of the United Nations/World Health Organization （WHO/FAO） in 1973， the amino acid score （ASS）， chemical score （CS） and essential amino acid index （EAAI） were analyzed according to following formulas[5-7]：

AAS=Amino acid content of the sample （mg/g pro）/Requirement of identical amino acid for adults by WHO （mg/g pro）

CS=Amino acid content of the sample （mg/g pro）/Identical amino acid content in whole egg protein （mg/g pro）

EAAI=n100AAE×100BEE×100CCE×…×100JJE

In the formula： n is the number of essential amino acids to be compared; A， B， C… J are the contents of essential amino acids in muscle protein （mg/g pro） of S. paramammosain; and AE， BE， CE… JE are the contents of the same amino acids in whole egg protein （mg/g pro）.

Amino acid content （mg/g pro）= Content of certain amino acid in the sample （mg/100g wet sample）/Crude protein content of the sample （mg/100g wet sample）

Data processing and statistical analysis

The experimental data was expressed as mean±standard deviation. The SPSS16.0 for Windows analysis software （One-way ANOVA） was used to perform one-way analysis of variance on the obtained experimental data. If there are significant differences， SNK multiple comparisons should be performed to determine the differences between groups. The significance level P was 0.05. If P<0.05， the difference is significant.

Results and Analysis

Analysis of basic nutritional components in the muscles and gonads of S. paramammosain

From the results shown in Table 1 and Table 2， it could be seen that the muscle moisture content of S. paramammosain from Sanmen was significantly higher than those in S. paramammosain from Fujian and Shanghai. The muscle protein contents of S. paramammosain from the three regions ranked as Shanghai>Fujian>Sanmen （P<0.05）. The muscles of S. paramammosain from Shanghai had the highest crude fat and crude ash contents. The moisture contents of the gonads of the S. paramammosain from the three regions had no significant differences. The protein contents of the gonads of S. paramammosain from the three regions ranked as Shanghai>Sanmen>Fujian （P<0.05）. The S. paramammosain from Sanmen had the highest crude fat and crude ash contents.

Analysis of amino acid contents in muscles and gonads of S. paramammosain

The contents of 17 amino acids （tryptophan not tested） in the muscles and gonads of S. paramammosain from the three different regions are shown in Table 3 and Table 4. The contents of 17 amino acids detected in the muscles and gonads of S. paramammosain from the three different regions were different. The total amounts of total amino acids （TAA）， essential amino acids （EAA） and flavor amino acids （FAA） in the muscles of S. paramammosain from Sanmen were significantly lower than those produced in Fujian and Shanghai （P<0.05）， but the proportions of essential amino acids （EAA） and flavor amino acids were not significantly different. The proportion of EAA and FAA in the gonads of S. paramammosain produced in the three areas were significantly different （P<0.05）.

Composition and evaluation of essential amino acids in muscles and gonads of S. paramammosain

According to the essential amino acid pattern and the amino acid pattern of egg protein recommended by FAO/WHO， the amino acid score （AAS）， chemical score （CS） and essential amino acid index （EAAI） were calculated for the muscles and gonads of S. paramammosain from the three different regions， so as to evaluate the protein quality of their muscles and gonads， as shown in Table 5 and 6. The AAS values of the muscles of S. paramammosain from the three different regions were all greater than 0.58， and the CS values were all greater than 0.38. The EAAI values of the muscles of S. paramammosain from Fujian， Sanmen and Shanghai were 78.92， 72.08 and 76.36， respectively. According to the ASS and CS scores， the first limiting amino acid in the muscles of S. paramammosain produced in three different regions was methionine + cystine， and the second limiting amino acid was threonine. The AAS values of the gonads of S. paramammosain from the three different regions were greater than 0.93， and the CS values were all greater than 0.6. The EAAI values of the gonads of S. paramammosain from Fujian， Sanmen and Shanghai were 87.95， 81.74 and 91.04， respectively. According to the ASS score， the first limiting amino acid of the gonads of S. paramammosain from Fujian and Sanmen was valine， and the second limiting amino acid was methionine+cystine; and the first limiting amino acid of the gonads of S. paramammosain from Shanghai was methionine+ cystine， and the second limiting amino acid was valine. According to the CS score， the first limiting amino acid of the gonads of S. paramammosain produced in the three different regions was methionine + cystine， and the second limiting amino acid was valine.

Fatty acid composition in muscles and gonads of S. paramammosain

The results of determination of fatty acid composition in muscles and gonads of S. paramammosain produced in the three differ-ent regions are shown in Table 7 and Table 8. It could be seen from the table that there were also differences in the fatty acid composition of the muscles and gonads of S. paramammosain from the three different regions. The proportion of EPA in the muscles of S. paramammosain from Fujian was higher than those of other two groups， but the differences were not significant; the proportion of DHA was significantly higher than those of other two groups， and the differences were significant （P<0.05）; and the proportions of n3 PUFA in muscles of S. paramammosain produced in Fujian and Sanmen were significantly higher than that of S. paramammosain produced in Shanghai （P<0.05）. The proportions of EPA， DHA and n6 PUFA in the gonads of S. paramammosain produced in Sanmen were significantly higher than those produced in Fujian and Shanghai （P<0.05）， while the n3 PUFA was the highest in S. paramammosain produced in Fujian. The proportions of n6/n3 PUFA in S. paramammosain produced in the three different regions were significantly different （P<0.05）.

Discussion

The nutritional value of S. paramammosain is mainly reflected in the edible ratio， basic nutrients， amino acid composition， fatty acid composition， and mineral elements[8-9]. The proportions of high protein and highly unsaturated fatty acids （UFA） are the basic characteristics of aquatic products as a high-quality food source. Therefore， the nutritional value evaluation of crabs is mainly based on protein content and quality and PUFA， SFA， MUFA composition analysis.

The crude protein content in the muscles of S. paramammosain is higher than 6% of ordinary fish and 13%-16% of pork （wet weight）， and the crude fat content is lower than 5% of ordinary fish. The protein and fat contents of the edible parts of S. paramammosain from different regions are in line with the aquatic food characteristics of high protein and low fat aquatic food[10]. By comparing the analysis results of the conventional muscle composition of different species of aquatic products， it is found that the conventional muscle composition has a large change. It is speculated that these differences may be caused by the species， living environment， and baits ingested by organisms. Meanwhile， the different nutritional components of protein and fat in different diets are the reasons for these differences[11-14].

Since the human body cannot synthesize amino acids or the synthesis speed cannot meet the needs of the body， they need to be obtained from external foods. Therefore， the nutritional value of food protein mainly depends on the type， quantity and composition ratio of essential amino acids contained in it[15-16]. Amino acid composition， especially EAA composition， is a good indicator for evaluating the nutritional value of amino acid protein[17-19]. Flavor amino acids， as one of the important sources of fish flavor， also affect the quality of crab meat and gonads[20]. The muscle TAA and EAA of S. paramammosain produced in Fujian and Shanghai were significantly higher than those produced in Sanmen. The total amounts of FAA in the muscles of S. paramammosain produced in Fujian and Shanghai were significantly higher than that produced in Sanmen， but there were no significant differences in the proportions of essential amino acids and flavor amino acids between the three kinds of S. paramammosain. The proportion of EAA and FAA in the gonads of S. paramammosain in the three areas were significantly different （P<0.05）. The contents of FAA in the muscles and gonads of S. paramammosain were higher， slightly higher than that of Portunus trituberculatus and Eriocheir sinensis[21-22].

These differences may be due to the differences in the biological composition of the seas in the three regions， leading to differences in the food intake of S. paramammosain， resulting in greater differences in the amino acid composition[23-24]. Comparing the ASS and CS values of the muscles and gonads of S. paramammosain produced in the three different regions， it was found that the lysine contents of the muscles and gonads of S. paramammosain produced in the three regions exceeded the FAO/WHO model， and S. paramammosain could be used to supplement lysine. The ASS and CS scores were used to determine the limiting amino acids of S. paramammosain， which provided a basis for optimizing the compound feed formula of S. paramammosain. The EAAI index is a method to evaluate protein nutrition， and the higher the ratio， the closer the protein to the egg white standard， and the higher the nutritional value. The EAAI indexes of this study were all higher than 72.08， higher than many marine fishes[25-27]， so S. paramammosain has a high edible value.

The fatty acids that make up fat are roughly equally divided into saturated fatty acids （SFA）， monounsaturated fatty acids （MUFA） and polyunsaturated fatty acids （PUFA）. SFA is the main component of cell energy supply， but too much intake will increase cholesterol， and UFA can reduce blood lipids to protect cardiovascular function. Therefore， both saturated fats and unsaturated fatty acids are essential nutritional components. The fatty acid composition of crab meat can be used to judge the nutritional value of its fat， because crab meat is an important source of 15 PUFAs of the n3 series in the human body， and is of great significance for improving human nutrition and health[28]. EPA and DHA are also essential fatty acids for human growth and development， which can promote brain development and have obvious effects on the prevention of heart disease and cerebrovascular disease. The proportion of EPA in the muscles of S. paramammosain from Fujian was higher than those of other two groups， but the differences was not significant; and the proportion of DHA was significantly higher than the other two groups， and the differences were significant （P<0.05）. The proportions of n3 polyunsaturated fatty acids （PUFA） in the muscles of S. paramammosain from Fujian and Sanmen were significantly higher than that in Shanghai （P<0.05）. The proportions of EPA， DHA and n6 PUFA in the gonads of S. paramammosain produced in Sanmen were significantly higher than those in Fujian and Shanghai （P<0.05）， while n3 PUFA was the highest from in S. paramammosain from Fujian; and the proportions of n6/n3 PUFA in S. paramammosain from the three sources were significantly different （P<0.05）. The n6/n3 values of muscles and gonads of S. paramammosain from the three different sources were all lower than the upper limit of the n6/n3 value of healthy food recommended by the British Ministry of Health （4.0）[29]， which is consistent with the results of the study by Li et al. in 2019[30]. Therefore， the fat of S. paramammosain is one of the high-quality healthy foods for human beings.

Conclusions

To sum up， due to the differences in the environment and diet composition in different producing areas， the nutritional composition of the edible parts of S. paramammosain from Fujian， Sanmen and Shanghai have their own characteristics. S. paramammosain has the characteristics of high protein， low fat， high protein quality， rich variety of fatty acids required by the human body， and good taste， and thus has high nutritional value and health care effects.

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