Identification Of Key Aroma-active Compounds From Chinese Mitten Crab And Mangrove Crab

Aroma is one of the most important factor to evaluate the quality of food. Currently, Gas Chromatography-Mass Spectrometry (GC-MS) is widely used to identify the composition of volatiles from food. However, it could not offer the imformation about the aroma activity of the volatiles. In the aroma analysis, it is more important to indentify those volatiles which can contribute effectively to the overall aroma of the food. The development of Gas Chromatography-Mass Spectrometry-Olfactometry (GC/MS-O) combined with analytical techniques, offer a good solution for this problem.The objective of this thesis is to first identify the volatile composition for Chinese mitten crab and Mangrove crab. Furtherly find the key aroma-active compounds among them and evaluate the overall aroma of the two crabs by sensory analysis. As Chinese mitten crab and Mangrove crab are both characteristic crabs for China, there`ve been no systemly study about their aroma. What`s more, the results of this study will also be helpful for the cultivation and intensive processing of the two crabs. On the other hand, the methods used and discussed in this study could be useful in the aroma analysis for other seafood crustrceas.Odorants of steamed crabmeat from Chinese mitten crab and Mangrove crab were extracted by simultaneous steam distillation extraction (SDE) and investigated by GC-MS. Compounds identification was confirmed on the basis of matching retention indices (RI)and mass spectra (MS), and further quantified by adding internal standard.There were 81 compounds identified in both of the crabs, with 59 compounds in Chinese mitten crab and 47 one in Mangrove crab. Aldehydes, ketones and alcohols comprised the mainly group of compounds deteced in Chinese mitten crab, while in Mangrove crab they were aldehydes, S-containing and N-containing compounds in turn. Among the volatiles identified in both crabs, the composition of S- and N- containing compounds showed the most differences, and the amount of aldehydes varied a lot. Considering the aroma traits of these groups of compounds, they may contribute a lot to the difference of the two crab`s aroma.The overall aroma of the two crabs was analyzed by flavor profile analysis methods. Results showed that the aroma of steamed Chinese mitten crab can be described as an intensive fishy smell, accompanied by strong meaty and earthy smell, morderate egg yolk-like, metalic and fatty smell and a very little grass and creamy-like smell; while the fishy smell in steamed Mangrove crab was weaker. The two crabs showed most differences in earthy, fatty, grass and meaty smell; but in the item of creamy and metalic, it was close in intensity.Odorants of steamed Mangrove crabmeat extracted by SDE methods were investigated by MS hyphenated to the GC-Olfactometry (GC/MS-O )system with two olfactometric methods: detection frequency (DF)methods and aroma extract dilution analysis (AEDA). Odor activity values (OAVs) were calculated to confirm the results.A total of 38 aroma-active compounds were detected and 31 identified. The results of the two detection methods were close and well correlated, compounds with high detection frequencies also had higher flavor dilution factors. The following 5 were the most important contributors to the aroma of steamed mangrove crab: 2,3-butanedione (creamy, caramel), 2,5-dimethylpyrazine (roasted, nutty), 3-methylbutanal (chocolate), 2-acetyl-1-pyrroline (popcorn, nutty), 2-acetylthiazole (roasted, sulfury).Key aroma-active compounds of steamed Chinese mitten crab extracted by SDE methods were investigated by GC/MS-O system with two judges performed in the AEDA. The results obtained by the two judges in the AEDA showed consistency for the key aroma-active compounds, but with differences in those compounds contributed less to the overall aroma. It`s furtherly confirmed the effectiveness of AEDA in evaluating aroma-active compounds from the complicated samples.A total of 34 aroma-active compounds were detected and 22 identified by both MS and RI. The aroma of cooked Chinese mitten crab could be primarily attributed to 3-methylbutanal (black chocolate), 2-methylpyrazine (popcorn), 2,3-butanedione (creamy), 2,5-dimethylpyrazine (roasted, nutty), (E)-2-?hexenal (fruity), (Z)-4-heptenal (boiled potato) and nonanal (fatty).