Studies On The Key Aroma Compounds And Formation Characteristics In Hot-air-dried Shrimp(Penaeus Vannamei)

Shrimp is the main aquatic product in the market at home and abroad.Dried products are one of the main processed products of shrimps,and deeply favored by consumers because of unique flavor,convenient storage and transportation.The aroma is an important index for the quality of dried shrimp products and the key aroma components are irreplaceable as the quality precision control index.The aroma is mainly evaluated by sensory methods,and currently fell short of the quantitative evaluation index.The researches on dried shrimp at home and abroad mainly focus on the identification and analysis of aroma components,and aroma characteristic changes in the process of processing,key aroma components and aroma sources in dried shrimp products have been not rarely reported.In this study,shrimps（Penaeus vannamei）were used as a raw material,the key aroma compounds in hot-air-dried shrimps were determined by flavoromic technology,and the effect of pyrazine isomers on the overall aroma was analyzed.The similarities between hot-air-dried shrimp and its parts were analyzed by comparing the aroma characteristics in order to reveal the main aroma sources of dried shrimps.The change of key aroma compounds during the hot-air-drying process was studied,and the effect of lipids on the aroma characteristic was analyzed.The similarities between hot-air-dried shrimp and its parts were analyzed by comparing the aroma characteristics in order to reveal the main aroma sources of dried shrimps.The results of this study will provide scientific supports for the objective quality evaluation,quality control and quality improvement of hot-air-dried shrimps.The main research contents and results are as follows.1.Penaeus vannamei was used as a raw material.Aroma compounds in the hot-air-dried shrimp were extracted by solid-phase micro-extraction（SPME）and simultaneous distillation extraction（SDE）,and were analyzed by gas chromatography-mass spectrometry（GC-MS）and gas chromatography-olfactometry（GC-O）combined with aroma extract dilution analysis（AEDA）.Based on odor activity value（OAV）and flavor dilution factor（FD）,the volatile compounds with OAV≥1 or log₃FD≥2 were used as the main aroma components.The standard curve was established by external standard method to quantify the compounds accurately,and the key aroma compounds which had important effect on the overall aroma were determined by aroma recombination and omission experiment.The results showed that there were significant differences in the composition and contents of key aroma compounds determined by SPME and SDE methods.Among them,the SPME aroma recombination model was similar to that of the control group while the SDE aroma recombination model was very different from that of control group.By SPME-GC-MS/O and aroma recombination test,18 key aroma compounds were identified in dried shrimp hot air products,namely 3-ethyl-2,5-dimethylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine,2-methyl-3,5-diethylpyrazine,2,3-dimethylpyrazine,2,3,5,6-tetramethylpyrazine,2-octylketone,3-hydroxy-4,5-dimethyl-2（5H）furanone,trimethylamine,1-octene-3-ol,benzaldehyde,3-methylbutyraldehyde,3-（methylthio）propanal,2-methylhexanal,2-pentylfuran,pyridine,2-acetyl-1-pyrrolidine,dimethyl disulfide.The results of aroma omission experiment show that pyrazines,amines,aldehydes and heterocycles had important effects on the aroma characteristics of dried shrimp.3-Ethyl-2,5-dimethylpyrazine,2,5-dimethylpyrazine,trimethylamine and 3-（methylthio）propanal contributed to roasted,meat-like and fishy odors.2.Volatile compounds in the dried shrimp were separated and identified by comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry（GC×GC-TOFMS）.The effects of pyrazine isomers on aroma characteristics were analyzed by aroma recombination experiment combined with triangle test and sensory evaluation.A total of 20 aroma active compounds were determined,and pyrazines were the main aroma compounds,accounting for 62.36%of the total OAV.There were 10 pyrazines in aroma active compounds,including three isomers:（1）Dimethylpyrazine（C₆H₈N₂）:2,3-dimethylpyrazine,2,5-dimethylpyrazine and 2,6-dimethylpyrazine;（2）Trimethylpyrazine(C₇H₁₀N₂):2,3,5-trimethylpyrazine,2-ethyl-5-methylpyrazine,2-ethyl-6-methylpyrazine and2-ethyl-3-methylpyrazine;（3）Tetramethylpyrazine(C₈H₁₂N₂):3-ethyl-2,5-dimethylpyrazine,5-ethyl-2,3-dimethylpyrazine and 2-ethyl-3,5-dimethylpyrazine.Except for 2-ethyl-6-methylpyrazine and 5-ethyl-2,3-dimethylpyrazine,Aromas recombination models with most single isomer showed no significant difference between the control groups（p>0.05）,and their aroma characteristics are similar.This indicates that most of single isomers can represent the aroma characteristic of compounds with similar chemical structure,and played a similar role in the overall.3.The volatile components in hot-air-dried shrimp and its parts（head,shell,meat,epidermis,removed-epidermis head,removed-epidermis shell,removed-epidermis meat and viscera）were extracted by HS-SPME method.The similarity between whole shrimp and its part was analyzed by sensory evaluation,gas chromatography-ion mobility spectroscopy（GC-IMS）and GC-MS.The main source of aroma was determined by comparing fingerprint similarity and cluster analysis.The results showed that the similarity between the samples containing shrimp epidermis and the whole was generally higher than that of the samples removing shrimp epidermis.There were 14 common aroma active compounds in dried epidermis and dried shrimp,and the composition and contents of these aroma active compounds were very similar.After removing the shrimp epidermis,the total OAV of aroma active compounds in shrimp head,shell and meat decreased by 47.51%,97.55%and 52.04%,respectively.This indicated that the shrimp epidermis was the main aroma source and made an important contribution to the overall aroma formation of dried shrimp.4.Eighteen key aroma compounds were used as a reference,and aroma compounds during the hot-air-drying process of shrimps were investigated using GC-MS,electronic nose（E-nose）and sensory analysis.Partial least square regression（PLSR）model was used to analyze the correlation between volatile compounds,corresponding signals of E-nose and sensory properties.The concentration and quantity of key aroma compounds in shrimps changed significantly during hot air drying.The odor in the fresh shrimp was very light,and the key aroma compounds consisted of trimethylamine and 3 aldehydes.In the incipient stage of the process（0–2h,high water activity）,the content of aldehydes in shrimps was decreased while the content of trimethylamine,esters and pyrazine compounds increased slightly.During the middle hot-air drying（2–7 h,medium water activity）,the aroma intensity in shrimp increased rapidly while the aroma characteristics including mainly roasted and meat-like odors had gradually come into being.At the stage,the number of AACs increased to 17,and four AACs made important contributions,namely,3-ethyl-2,5-dimethylpyrazine,2,5-dimethylpyrazine,trimethylamineand3-（methylthio）propionaldehyde.During the late hot-air drying（7–9 h,low water activity）,the concentration and quantity of of key aroma compounds change little,and the content of benzaldehyde and dimethyl disulfide with smoky/burnt odor increased slightly.By comparing the aroma attributes of shrimps at different stages,the aroma quality of shrimp was the best when the hot air was dried at the hot air drying for 7 h.The PLSR result showed a good correlation between most variables of volatile compounds,E-nose data and sensory attributes.5.Triglycerides or phospholipids in the shrimp were removed by organic reagents,and aroma attributes of different defatted dried shrimps were analyzed by GC-MS and sensory evaluation.The results showed that after removing triglyceride from shrimp,the aroma intensity of dried shrimp products decreased,and sensory scores of roasted,meat-like and fishy odors decreased slightly.The main aroma active compounds（OAV>5）were detected,and the OAV values of pyrazines,heterocycles,amines and aldehydes decreased to some extent.After removing phospholipids from shrimp,the aroma characteristic of dried shrimp changed significantly,and sensory scores of roasted,meat-like,fishy,caramel and sweet odors decreased substantially.Only eight aroma active compounds were detected,two key aroma compounds（2-pentylfuran and 3-（methylthio）propanal）were not found,and OAV value of pyrazines,heterocycles,amines and aldehydes decreased more than that of triglycerides.This indicated that lipid had a great effect on the overall aroma of dried shrimp,while phospholipid had a much greater effect on aroma characteristic than triglyceride.