During the COVID-19 epidemic, quarantine personnel at the Fang Cabin Hospital posted a recipe specially equipped with turtles. Turtle, also known as Wang Ba, TuanYu, etc., scientific name Chinese turtle (Trionyx sinensis), is a famous aquatic product in China, its nutritional value is high, and the flavor is unique, since ancient times has been regarded as an excellent tonic ingredient that helps to enhance human immunity and is well received by consumers.
With the decline of wild Chinese turtle resources in China, various breeding modes (such as greenhouse culture, imitation ecological culture, etc.) have gradually emerged, which has promoted the vigorous development of the turtle market. Among them, greenhouse culture is an important turtle culture model in recent decades, but this mode usually requires a large amount of artificial feed, water quality pollution is serious, and the breeding process often uses a large number of artificial drugs to treat and prevent diseases, and there are certain drug residues in turtle meat. In addition, compared with wild turtles, artificially farmed turtles (especially greenhouse turtles) are usually rougher in meat and have decreased nutritional quality, especially the taste and flavor caused by consumer criticism.
In recent years, the development of comprehensive rice and fishery breeding in China is particularly rapid, in addition to the large scale of rice and shrimp, rice field turtle farming has also become an ecological breeding model worth promoting. "Rice turtle model" and "rice shrimp model", is a three-dimensional ecological agricultural production mode that organically combines rice planting and aquaculture, that is, both rice and turtle are planted in the same field, which reasonably improves the growth and development conditions of turtles, and the metabolites of turtles can promote the growth of rice and achieve the goal of double harvest of rice turtles. At the same time, rice field turtles basically do not put or put less artificial feed, its disease is less, drug residues are low, due to its good growth conditions (close to the wild environment), high product quality, increase the commodity value of Chinese turtles, greatly improving the additional output value of rice field agriculture.
Comprehensive rice and fishery breeding is an important breeding model promoted by the Ministry of Agriculture and Rural Affairs of China, and it is also an important starting point for poverty alleviation in the national industry and an effective means to get rid of poverty and get rich. In recent years, the national rice and fishery comprehensive breeding mainly implements the "rice shrimp model" in a large area, and with the rapid development of the rice and shrimp industry, the crayfish consumption market is gradually saturated. By cultivating rice turtles, rice shrimp can be partially replaced, which can not only alleviate the oversaturated crayfish market in the future, but also help farmers further enhance the added value of rice and fishery products. But compared to greenhouse turtles and wild turtles, what is the nutritional and flavor quality of rice field turtles? Is it worth further investment and promotion by farmers? In order to answer these questions, Lu Jianfeng, a national modern agricultural industry technology system processing post scientist, an expert in the Anhui aquatic industry technology system, and a professor at the School of Food and Biological Engineering of Hefei University of Technology, led a scientific and technological agricultural support team to compare and analyze the nutritional quality of Chinese turtle muscles and the differences in volatile flavor substances in three different environments with rice field breeding, greenhouse culture and wild Chinese turtle as materials, providing a very meaningful reference for improving the breeding quality of Chinese turtles in the future.
The greenhouse turtles and rice field turtles used in the experimental testing process were taken from Anhui Jiehexiang Aquaculture Company in Feixi County, Hefei City, and the wild turtles were taken from the natural waters of Chaohu Lake around the company's farm, weighing more than 600 g and similar in size.
Studies have shown that in addition to fatty acids, the nutritional quality and volatile flavor of rice turtles and wild turtles are mostly better than those of greenhouse turtles, and the nutritional quality and volatile flavor of rice turtles are closer to wild turtles (or even better), and the main research data and conclusions are as follows.
1. Basic nutritional composition and comparison of Chinese turtle muscle
From Table 1, it can be seen that the water content in the muscles of rice turtles, wild turtles and greenhouse turtles is 76.79%, 74.70%, 78.38%, and there is no significant difference between the muscle water content of greenhouse turtles and rice turtles (P>0.05), but it is significantly higher than that of wild turtles (P<0.05); the content of crude protein in muscles is wild turtle (18.13%), > rice turtle (18.02%), > greenhouse turtle (16.41%). The crude protein content in rice field turtle and wild turtle muscle was significantly higher than that of greenhouse turtle (P<0.05); the crude fat content of wild turtle (2.37%) > greenhouse turtle (2.16%) > rice field turtle (2.04%), wild turtle was significantly higher than that of rice field turtle and greenhouse turtle (P<0.05); ash content was wild turtle (1.38%) > greenhouse turtle (1.28%), > rice field turtle (1.14%), and wild turtle was slightly higher than greenhouse turtle, However, it was significantly higher than that of rice field turtle (P<0.05). Through comprehensive comparison, it is found that wild turtles have low muscle moisture content and the highest crude protein, ash and crude fat, which may be one of the main reasons for the better quality of wild turtles.
Table 1 Content of basic nutrients in Chinese turtle muscle under three different environments (%)
Note: Different letters in the same column indicate significant differences, P<0.05.
2. Amino acid composition and comparison in Chinese turtle muscle
Proteins constitute the body's tissues and maintain the growth, renewal and repair of the tissues, participate in the body's important physiological functions and oxidation functions, are extremely important nutrients in the body, and are also the extremely important material basis for various life activities. Protein is composed of different amino acids, and the proportion and variety of amino acids in muscle protein determine the nutritional value of turtle muscle. As can be seen from Table 2, a total of 17 amino acids were detected in the Chinese turtle muscles in three different environments. Among them, the total content of dry sample amino acids was rice turtle (77.83 g/100g) > greenhouse turtle (75.81 g/100g) > wild turtle (72.68 g/100g), but there was no significant difference (P>0.05); the essential amino acid content was paddy field turtle (30.27 g/100g) > greenhouse turtle (30.87 g/100g) > wild turtle (28.71 g/100g), and there was no significant difference (P> 0.05）。 In terms of individual amino acid content, Glu content was the highest, and paddy field turtle (13.09 g/100 g) and greenhouse turtle (12.87 g/100 g) were slightly higher than wild turtle (12.36 g/100 g), while Cys content was the lowest (about 0.80 g/100 g).
The delicious taste of animal protein is related to its umami taste and sweet amino acids. From Table 2, it can be seen that in the fresh sample, the total umami and sweet amino acids of rice field turtles, wild turtles and greenhouse turtles are similar in umami and sweet amino acids in rice field turtles (8.48 g/100 g) > wild turtles (8.47 g/100 g) > greenhouse turtles (7.38 g/100 g), and the umami and sweet amino acid content of rice field turtles and wild turtles is similar, and both are significantly higher than that of greenhouse turtles (P<0.05), indicating that wild turtles and rice turtles may be sweeter, that is, the taste is relatively better.
Medicinal amino acids refer to amino acids that can play a certain medicinal role in the body's biochemical reactions and disease treatment, and the types and content of amino acids used in food and traditional Chinese medicine determine the medicinal effects and health care effects of food. From Table 2, it can be seen that a total of 10 kinds of medicinal amino acids were detected in the chinese turtle muscles in three different environments, accounting for about 65% of the total amino acids. In the fresh samples, the medicinal amino acid content of rice field turtle and wild turtle muscle was also similar, and both were slightly higher than that of greenhouse turtle, but there was no significant difference (P>0.05), and the total content was wild turtle (12.36 g/100 g) > field turtle (12.17 g/100 g) > greenhouse turtle (11.05 g/100 g), indicating that wild turtle and paddy field turtle may be slightly better than greenhouse turtle in terms of medicinal and health care.
In addition, according to the ideal model of FAO/WHO, the ratio of essential amino acids to the total amount of amino acids (EAA/TAA) in the amino acids that make up proteins is about 0.4, and the ratio of essential amino acids to non-essential amino acids (EAA/NEAA) is above 0.6, so that the protein composition mode is considered to be a better quality protein. From Table 2, it can be seen that the EAA/TAA in the muscles of rice field turtles, wild turtles and greenhouse turtles is close to 0.39, 0.40 and 0.41, respectively; EAA/NEAA is also relatively close, 0.77, 0.79 and 0.83, respectively, and all meet the FAO/WHO recommended standards. It can be seen that the types of amino acids in the Chinese turtle muscle are complete, and the proportion between amino acids is appropriate, which is a high-quality protein source.
Table 2 Content of muscle hydrolyzed amino acids in Chinese turtle under three different environments (g/100g)
Note: # Medicinal amino acids;* Umami amino acids. Different letters indicate significant differences. The same applies below.
3. Composition and comparison of muscle mineral content of Chinese turtle
Minerals are essential for maintaining the body's normal metabolism and body tissues, and are also important nutrients for the human body. Minerals, especially some trace elements, play an extremely important role in the life activities of the human body, but minerals cannot be synthesized by themselves in the body and must be ingested from the outside world, so it becomes particularly important for people to consume minerals in their daily diet. From Table 3, it can be seen that the muscle mineral content of Chinese turtles in three different environments is different. Among the constant elements (dry sample), there was no significant difference in the content of K and P (P>0.05), the content of Na was that of paddy turtle (323.58 mg/100 g >), wild turtle (247.29 mg/100 g) > greenhouse turtle (208.07 mg/100 g), paddy field turtle was significantly higher than that of greenhouse turtle and wild turtle (P< 0.05), and the content of Mg was greenhouse turtle (95.46 mg/100 g) > rice field turtle (89.93). the content > of rice turtles was significantly lower than that of greenhouse turtles(P<0.05), and the content of Ca was significantly lower than that of wild turtles (P<0.05), and the content of Ca was significantly lower than that of rice turtles (43.7 mg/100 g) > wild turtles (40.12 mg/100 g) > greenhouse turtles (13.8 mg/100 g), and greenhouse turtles were significantly lower than those of rice field turtles and wild turtles (P<0.05). Among the trace elements (dry sample), cu content was rice field turtle (0.33 mg/100 g) > wild turtle (0.14 mg/100 g) > greenhouse turtle (0.08 mg/100 g), rice field turtle was significantly higher than wild turtle (P<0.05), and wild turtle was significantly higher than greenhouse turtle (P<0.05); Zn element played an important role in the development of the human brain and reproductive system, immune regulation, etc., Zn content was rice field turtle (9.06 mg/ 100 g) > wild turtle (7.14 mg/100 g) > greenhouse turtle (4.41 mg/100 g), paddy turtle and wild turtle were significantly higher than greenhouse turtle (P<0.05); Fe content was significantly higher than that of wild turtle (21.11 mg/100 g) > wild turtle (5.13 mg/100 g) > greenhouse turtle (3.19 mg/100 g), rice field turtle was significantly higher than wild turtle and greenhouse turtle (P<0.05) Mn is related to the normal physiological metabolism, bone growth and reproductive system development of the human body, only a trace amount of Mn (0.27 mg/100 g) was detected in the muscle of rice field turtles; Se plays a role in anti-cancer, anti-cancer, anti-aging and other effects in humans, se content is wild turtle (0.11 mg/100 g) > rice field turtle (0.10 mg/100 g) > greenhouse turtle (0.02 mg/100 g), wild turtle and rice field turtle are significantly higher than greenhouse turtle (P< 0.05）。 Comparing the difference in trace element content, it can be seen that the trace elements of paddy field turtle and wild turtle are significantly higher than those of greenhouse turtle overall, and the total mineral content of greenhouse turtle (dry sample) is also significantly lower (P<0.05), but the mineral content of all three is relatively rich, which can be considered to be a relatively excellent mineral source.
Table 3 Mineral composition and content of Chinese turtle muscle in three different environments
Note: * is a trace element.
4. Composition and comparison of muscle fatty acid content of Chinese turtle
Fat is an indispensable substance for food heating to produce aroma components, and the high content of saturated fatty acids (SFAs) in muscles is low in aroma, tenderness, juiciness and overall acceptability, while the content of polyunsaturated fatty acids (PUFA) is high, and its acceptance is high. As shown in Table 4, a total of 16 fatty acids were detected in rice field turtle muscle fat, a total of 16 fatty acids were detected in wild turtle muscle fat, and a total of 15 fatty acids were detected in greenhouse turtle muscle fat.
In a comprehensive comparison of fatty acids in muscle fat of Chinese turtles in three different environments, the SFA content was that of wild turtle (44.83 g/100g) > rice field turtle (40.15 g/100 g) > greenhouse turtle (29.31 g/100 g), and wild turtles were significantly higher than rice field turtles, and rice field turtles were significantly higher than greenhouse turtles (P<0.05). The UFA content was greenhouse turtle (69.21 g/100 g) > paddy field turtle (51.49 g/100 g) > wild turtle (50.78 g/100 g), and greenhouse turtles were significantly higher than rice field turtles and wild turtles (P<0.05), while rice field turtles and wild turtles were close. This may be due to the fact that most UFA are essential fatty acids of turtles, which cannot be synthesized through their own lipid metabolism pathways and must be ingested by the outside world, while the breeding method of greenhouse turtles is mainly based on feeding a large amount of artificial compound feed (fishmeal and fish oil are abundant), which provides a large amount of UFA for its growth process, but rice field turtles and wild turtles mainly rely on wild (rice fields, rivers or lakes) to supplement unsaturated fatty acids in the body, so the UFA content of greenhouse turtles is higher than that of wild turtles and rice turtles. Usually, the diet is rich in MUFA, which can effectively reduce the mortality rate of cardiovascular disease and the incidence of coronary heart disease, and the CONTENT of MUFA is greenhouse turtle (53.97 g/100g) > rice field turtle (43.58 g/100g) > wild turtle (36.82 g/100g), which are relatively rich. PUFA is a class of active substances with special physiological functions, can ester cholesterol, reduce the content of cholesterol and triglycerides in the blood, thereby helping to prevent hypertension, atherosclerosis, cardiovascular disease, etc., PUFA content is greenhouse turtle (15.23 g / 100g) > wild turtle (13.96 g / 100g) > rice field turtle (7.91 g / 100g), greenhouse turtle PUFA content is about 2 times that of rice turtle, Moreover, the PUFA content of wild turtles is also significantly higher than that of rice field turtles (P<0.05), which may be due to the fact that the predatory sources of wild turtles are mainly from wild fish and shrimp in rivers or lakes, which may be more abundant than the food sources in rice field ecosystems. Therefore, in order to improve and enhance the UFA composition and proportion of rice field turtles, it is recommended that in the future, artificial compound feed or a small amount of wild trash fish should be appropriately assisted in their breeding process.
Table 4 Muscle fatty acid content of Chinese turtle under three different environments (g/100g)
5. Composition and comparison of volatile flavor substances in Chinese turtle muscles
HS-SPME-GC-MS method was used to analyze the composition and content of volatile flavor compounds in chinese turtle muscle in three different environments: greenhouse, rice field and wild. As shown in Table 5, a total of 99 volatile flavor compounds were detected from the chinese turtle muscles in three different environments. The main analysis is as follows:
Hydrocarbon compounds have a higher aromatic threshold and cause olfactory reactions only at high concentrations, and are generally considered to contribute less to the overall flavor of the food. The hydrocarbon volatile content in the muscles of Chinese turtles in three different environments was rice field turtle (13.41%)> wild turtle (12.45%) > greenhouse turtle (11.02%). The hydrocarbons contained in the three are not exactly the same, among which 10 hydrocarbon compounds were detected in rice field turtle muscle, 19 hydrocarbon compounds were detected in wild turtle muscle, and 18 hydrocarbon compounds were detected in greenhouse turtle muscle. Although more hydrocarbon compounds were detected, they contributed less to the overall flavor of chinese turtle muscles.
Aldehyde compounds are produced by the oxidation of unsaturated fatty acids and the degradation of amino acid Strecker, which is a low threshold odorous substance with odor superposition effect and plays an important role in food flavor characteristics. Aldehyde compounds are one of the main odorous substances in turtle muscles, a total of 15 kinds of aldehyde substances were detected in rice field turtle muscles, 16 kinds of wild turtle muscles were detected, 16 kinds were detected in greenhouse turtle muscles, tetradecanal, heptarinal, caprylyral, decanoaldehyde, pentadecanal, hexanal are the aldehydes common to the three. The content of aldehyde compounds in the muscles of Chinese turtles is that of rice field turtle (75.00%)>, wild turtle (66.42%) > greenhouse turtle (54.07%), which may be an important reason why the flavor of rice field turtle and wild turtle is significantly better than that of greenhouse turtle. The four aldehydes with the highest content in muscle are nonaldehyde, hexanal, heptanal and octanal, hexanaldehyde is an important indicator for evaluating the flavor of meat products, in the turtle muscle in three different environments, the hexaldehyde content is rice field turtle (29.24%) > wild turtle (15.61%) > greenhouse turtle (9.91%), rice field turtle and wild turtle are much higher than greenhouse turtle, which may be one of the main reasons why the flavor of rice field turtle and wild turtle is better than that of greenhouse turtle. Caprylyl, heptanolic, and nonaldehyde have been shown to have a fishy flavor[28], with heptaldehyde containing greenhouse turtle (14.03%) > wild turtle (10.54%) > rice field turtle (8.41%), caprylylaldehyde containing wild turtle (4.73%) > paddy turtle (4.22%) > greenhouse turtle (3.89%); nonaldehyde content is wild turtle (26.05%) > rice field turtle (22.7%), > greenhouse turtle (19.85%), This is one of the main reasons why all three have a fishy smell. The unique aldehydes in the muscles contribute to the special flavor of the Chinese turtle muscles, there are 3 kinds of aldehydes unique to the rice field turtle and greenhouse turtle muscles, and there are 7 kinds of aldehydes unique to the wild turtle muscles, which may be one of the reasons why the muscle flavor of wild turtles is more unique.
The content of ketones is lower, 0.81% of wild turtles, 0.65% of greenhouse turtles, and rice field turtles are not detected, and due to the higher sensory threshold of ketones, the contribution to the muscle flavor of Chinese turtles may be small.
Esters and aromatic compounds play an important role in the flavor of turtle muscles, esters will give food a sweet fruity flavor, in general, except for lactones and thioesters are higher ester thresholds, less impact on the flavor of meat. A total of 4 ester compounds were detected in the muscles of Chinese turtles in three different environments, of which 2 species were detected in greenhouse turtles and 2 species in wild turtles, and 1 species was detected in rice field turtles, and their content was relatively low, which had little impact on the overall flavor formation of Chinese turtle muscles. But aromatic compounds are important flavor substances in meat, especially benzaldehyde, which has a pleasant almond, nutty and fruity aroma. The content of benzaldehyde was wild turtle (4.33%)> paddy field turtle (4.05%) > greenhouse turtle (3.16%), and the overall content of aromatic substances was also wild turtle (34.17%)> paddy field turtle (26.70%) > greenhouse turtle (25.08%). This may reflect the superiority of wild turtle and rice field turtle muscle flavor over greenhouse turtle overall.
Alcohol compounds may be formed by the decomposition of secondary hydroperoxides of fatty acids, the action of lipid oxidase on fatty acids, or from the reduction of carbonyl compounds. The saturated alcohol threshold is higher and contributes less to flavor, and the lower unsaturated alcohol threshold contributes more to flavor. The alcohol content is wild turtle (2.10%) > paddy field turtle (1.91%) > greenhouse turtle (0.70%). Alcohols, in particular 1-octen-3-ol, are thought to be associated with fragrance substances in fresh freshwater fish and are ubiquitous in volatile fragrance substances in freshwater and marine fish. In the chinese turtle muscle in three different environments, the content of 1-octen-3-alcohol in the three environments was that of wild turtle (1.13%) > rice field turtle (0.54%) > greenhouse turtle (0.12%), which may be one of the reasons why the muscle flavor of wild turtle and rice field turtle is superior to that of greenhouse turtle.
Furan compounds were detected in smaller amounts, but all three contained 2-pentylfurans. The threshold of 2-pentylfuran is relatively low, has a grilled meat flavor, and its role as an indicator of lipid oxidation of meat may have a huge effect on the overall flavor of meat. In the chinese turtle muscle in three different environments, the content of 2-pentyl-furan is that of paddy field turtle (0.8%) > wild turtle (0.42%) > greenhouse turtle (0.3%), which may also be one of the reasons why the muscle flavor of wild turtle and rice field turtle is superior to that of greenhouse turtle.
Table 5 Volatile flavor substances of Chinese turtle muscles in three different environments (%)
Note: "-" indicates that the substance is not or has not been detected.
6. Conclusion
Comprehensive rice and fishery breeding has become a beautiful business card for poverty alleviation in the national fishery industry. By comparing and analyzing the differences in meat nutritional quality and volatile flavor substances of rice field farming, greenhouse culture and wild Chinese turtle, the team of Professor Lu Jianfeng of the School of Food and Biological Engineering of Hefei University of Technology showed that the moisture content of wild turtle was significantly low, but the crude fat and ash content was significantly higher (P<0.05), and the crude protein content of rice turtle was close to that of wild turtle and significantly higher than that of greenhouse turtle (P<0.05), but the EAA/TAA and EAA/EAA/ of the three were EAA/TAA and EAA/EAA/. NeAA's ratio is relatively close, all in line with fao/WHO recommended standards; among the 17 amino acids detected, wild turtles and rice turtles have similar content of umami and sweet amino acids and medicinal amino acids in their muscles, all of which are higher than those of greenhouse turtles; compared with greenhouse turtles, wild turtles and rice turtles not only have good volatile flavor (overall better than greenhouse turtles), but also have rich mineral content, Na, Ca, Cu, Zn, Fe, se content, and only rice field turtle muscles contain trace amounts of Mn However, the content of unsaturated fatty acids (UFA) in the muscle fat of greenhouse turtles was significantly higher than that of rice field turtles and wild turtles (P<0.05). In summary, in addition to fatty acids, the nutritional quality and flavor of paddy turtles and wild turtles are mostly better than those of greenhouse turtles, and the nutritional quality and flavor of rice turtles are closer to wild turtles, and in the case of rapid decay of wild turtle resources, paddy turtles have good development prospects.
For details, please refer to the literature: Wang Futian, Lai Nianyue, Cheng Huafeng, Wu Haoran, Liang Feng, Ye Tao, Lin Lin, Jiang Shaotong, Lu Jianfeng*. The nutritional qualities and volatile flavor substances of Chinese turtle in three different environments were compared and analyzed. Food and Fermentation Industry, 2019, 45(22): 253-261.