Analysis of Nutrition and Major Flavor of Different Muscle Parts of Thunnus thynnus

At present, local and foreign research reports on Thunnus thynnus have focused on fishery biology, fishing, and genetic diversity, among others. There are few reports on nutrition and flavor related to the different muscles of T. thynnus. To scientifically evaluate the nutritional quality and major flavor of different muscle parts of T. thynnus, the basic nutritional components, amino acid composition, fatty acid composition, and volatile flavor substances of dorsal and abdominal muscles were compared. Except for ash, the moisture, crude protein, and fat contents varied significantly between the dorsal and abdominal muscles (P < 0.05). The moisture contents of the dorsal and abdominal muscles were 57.79 g/100 g and 50.16 g/100 g, respectively. The protein content was 24.70 g/100 g and 18.61 g/100 g, respectively. The protein content of dorsal muscles was 24.70 g/100 g, which was 1.33 times that of the abdomen. The crude fat content of the dorsal muscles was 19.34 g/100 g, and in the abdomen was 30.29 g/100 g, which was 1.57 times that in the dorsal muscles. The crude ash content of dorsal and abdominal muscles was 1.01 g/100 g and 1.08 g/100 g, respectively. Seventeen amino acids were detected in different parts of T. thynnus. The total amino acid content differed significantly between the dorsal (22.07±0.74) g/100 gand abdominal muscles (16.57±0.47) g/100 g. The essential amino acid content of dorsal and abdominal muscles was (8.61±0.29) g/100 g and (6.28±0.17) g/100 g, respectively. The content of glutamate was the highest among the amino acids, followed by aspartic acid, and the content of cystine was the lowest. The ratio of essential amino acids to total amino acids of different muscle parts of T. thynnus was 40%, and the ratio of essential amino acids to nonessential amino acids was > 60%, which was close to the ideal protein nutrition evaluation model recommended by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO). Thus, the dorsal and abdominal muscles are high-quality proteins. The umami taste of aquatic products is closely related to the content of umami amino acids. Four types of umami amino acids showed the highest content levels, namely glutamic acid, aspartic acid, alanine, and glycine (in descending order of content level). The proportion of umami amino acids in dorsal and abdominal muscles was 34.93% and 35.91%, respectively, both exceeding 30%, which indicated that the dorsal and abdominal muscles of T. thynnus have a delicious quality. In the dorsal and abdominal muscles, 24 and 29 kinds of fatty acids were detected, including 8 and 12 kinds of saturated fatty acids, 6 kinds of monounsaturated fatty acids, and 10 and 11 kinds of polyunsaturated fatty acids, respectively, and there was a significant difference among the contents of different types of fatty acids. The contents of myristic acid, palmitic acid, and stearic acid were higher than those of other saturated fatty acids. The contents of oleic acid and eicosanoic acid were higher in monounsaturated fatty acids. The total content of eicosapentaenoic acid and docosahexaenoic acid in the dorsal and abdominal muscles accounted for 88.55% and 75.13% of polyunsaturated fatty acids, respectively. Therefore, the dorsal and abdominal muscles have good functional properties. The FAO/WHO recommends that the intake ratio of n-6 to n-3 polyunsaturated fatty acids in the diet should be 4:6, and the ratio of dorsal and abdominal muscles ranged from 0.41 to 0.44, which is much smaller than the standard. These results indicate that T. thynnus is rich in n-3 polyunsaturated fatty acids. A total of 39 volatile substances were detected in the dorsal and abdominal muscles of T. thynnus, including aldehydes, ketones, alcohols, esters, acids, alkenes, and compounds containing nitrogen and sulfur. PCA analysis showed that there was a significant difference between the odors of the dorsal and abdominal muscles of T. thynnus. Gas chromatography–ion mobility spectrometry can effectively be used to distinguish the different volatiles associated with the muscles. The relative contents of 2-methylthiophene, (E)-2-hexen-1-ol, pentanoic acid, alpha-pinene, propanoic acid, butyl butanoate, 2-pentylfuran, pentyl butanoate, dipropyl disulfide, diallyl disulfide, ethylheptanoate, alpha-terpineol, methylisobutylketone, 2-methylbutanal-D, and 2-methylbutanal-M were higher in the abdomen; the relative contents of (E)-2-pentenal, N-nitromethylethylamine, methylpyrazine, 3-methylbutanol, 1-2-dimethylbenzene, triethylamine, E-3-hexene-1-ol, 4-methyl-2-pentanol, ethyl 2-methylpropanoate, 2-butanone, ethyl acetate, 3-butenonitrile, and 2, 4, 5-trimethylthiazole were higher in abdominal muscle. The difference in composition and content caused the dorsal and abdominal muscles to form their own flavor characteristics, which were mainly related to the amino acid and fatty acid composition of the two parts. In general, the dorsal muscles mainly presented a fatty fragrance, whereas the abdominal muscles mainly presented a clear fragrance. The dorsal and abdominal muscles of T. thynnus are rich in nutrients and n-3 polyunsaturated fatty acids. Both muscles have a delicious taste, though they vary in flavor characteristics. This study provides a scientific basis for the deep processing and utilization of T. thynnus.