Study On The Aroma-giving Mechanism Of The Epidermis In Hot-air-dried Shrimp(Litopenaeus Vannamei)

Shrimp is one of the most important aquatic products in China,both in terms of earning foreign exchange through exports and providing income for fishermen.Dried shrimp and shrimp meal are the main-processed products of shrimp.Dried shrimp has unique aroma characteristics,and the level and purity of aroma are one of the important indicators for evaluating the quality of dried shrimp.The researchers of dried shrimp products were mainly focused on the identification and analysis of aroma components,while there were few reports about the formation parts and mechanism.Preliminary research by the research team found that the epidermis was the key aroma-giving part of hot-air-dried whole shrimp,and lipids were one of the main precursors of aroma-active compounds in dried shrimp.In this study,the key aroma compounds of hot-air-dried whole shrimp and epidermis were identified by aroma recombination and omission tests.The changes of key aroma compounds of whole shrimp and shrimp epidermis were investigated during the drying process.Then,the change rules in the precursors of fresh whole shrimp and different parts and the key aroma compounds of their dried products were analyzed.The effect of astaxanthin esters on the formation of key aroma compounds was analyzed.The types of amino acids and reducing sugars were screened based on their degree of impact on the content of key aroma compounds,and their dose-response relationship was investigated.Finally,the aroma-giving mechanism of hot-air-dried shrimp epidermis was verified through the construction of reaction model.The main research contents and results were as follows:1.The aroma components of hot-air-dried whole shrimp and shrimp epidermis were analyzed by gas chromatography-mass spectrometry(GC-MS)and gas chromatography-olfactometry(GC-O)combined with the aroma extract dilution assay(AEDA).Based on odor activity value(OAV)and flavor dilution factor(FD),the volatile compounds with OAV≥1 or FD≥27 were used as the main aroma components,and compounds with a dilution factor(FD)of ≥27 were accurately quantified.The key aroma compounds of hot-air-dried whole shrimp and shrimp epidermis were identified by recombination and omission tests.The results showed that pyrazines,nitrogen compounds and aldehydes were the three groups of compounds with high volatile compound content,OAV and FD values of hot-air-dried whole shrimp and epidermis.Eleven key aroma compounds were identified as2-ethyl-3,6-dimethylpyrazine,2-ethyl-5-methylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine,3-methylbutyraldehyde,benzaldehyde,valeraldehyde,2-nonanone,1-octen-3-ol,trimethylamine and 2-acetyl-1-pyrroline in hot-air-dried whole shrimp and epidermis.There was a high degree of consistency in the types and contents of volatile compounds,FD values,OAV of key aroma compounds,and types and contents of key aroma compounds in the hot-air-dried whole shrimp and epidermis.2.The changes in aroma characteristics and key aroma compounds of whole shrimp and shrimp epidermis during hot air drying were followed by GC-MS,electronic nose and sensory evaluation.The results showed that the aroma characteristics of whole shrimp and shrimp epidermis as well as the types and contents of key aroma compounds changed significantly during the whole drying process,among which pyrazines,aldehydes and nitrogen-containing compounds showed the most significant changes in the types and contents.The sensory scores,types and contents of volatile compounds and OAV of key aroma compounds of epidermis and whole shrimp increased significantly at the later stage of drying.Among them,the most significant changes were observed in the contents of2-ethyl-3,6-dimethylpyrazine,2-ethyl-5-methylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine,3-methylbutanal,benzaldehyde and trimethylamine.3.The contents of amino acids,monosaccharides,fatty acids and astaxanthin esters in fresh whole shrimp and different parts were determined by the instrument.The aroma characteristics and volatile compounds of hot-air-dried whole shrimps and their different parts were determined by electronic nose,sensory evaluation,gas chromatography-ion mobility spectrometry(GC-IMS)and GC-MS.The main aroma precursors were identified based on the relationship between key aroma compounds and aroma precursors.The results showed that hot-air-dried whole shrimp and epidermis were the most similar in terms of odour profile,odour fingerprint spectra,type and content of key aroma compounds and aroma characteristics,and were significantly different from shrimp head,shell and meat.The composition of amino acids,reducing sugars,fatty acids and astaxanthin esters was most similar between fresh whole shrimp and epidermis.Lipids and astaxanthin esters were the main reasons that make the aroma characteristics of whole shrimp and epidermis different from other parts of shrimp heads,meat and shells.4.The addition test,removal test and in vitro recombination models of astaxanthin ester were constructed.The key aroma compounds and aroma characteristics of those models were analyzed using GC-IMS,GC-MS and sensory evaluation,in order to study the effect of astaxanthin esters on the formation of key aroma compounds in hot-air-dried shrimp.The results show that astaxanthin ester significantly increases the content of key aroma compounds,such as2,5-dimethylpyrazine,2,3,5-trimethylpyrazine and 2-ethyl-5-methylpyrazine,2-Ethyl-3,6-dimethylpyrazine,3-methylbutyraldehyde,benzaldehyde,valeraldehyde,2-nonanone and 1-octen-3-ol.Astaxanthin ester significantly promoted hot-air-dried shrimp protein and fish meat to produce more volatile compounds belonging to hot-air-dried shrimp,changed their aroma characteristics,increased the sensory scores of the unique aroma characteristics of dried shrimps(cooked meat,smoky and roasted aromas),and produced a pure shrimp aroma in hot-air-dried shrimp protein and fish meat.Therefore,astaxanthin ester was a key precursor for producing the characteristic aroma of dried shrimp.5.The amino acid contents of shrimp epidermis during the drying process were measured by the amino acid automatic analyzer.The correlation between amino acids and key aroma compounds was established through the Pearson correlation coefficient,and the amino acids affecting the content of key aroma compounds were screened.The effects of various amino acids and reducing sugars on the content of key aroma compounds were analyzed by the addition test.On this basis,the dosage of precursors was changed to further analyze the dose-effect relationship between them.The results showed that the amino precursors(Lys,Arg,Leu and Phe)and carbonyl precursors(glucose,arabinose and galactose)significantly affected the content of key aroma compounds in hot-air-dried shrimp epidermis.Arg and Lys significantly increased the content of 2-ethyl-5-methylpyrazine,2-ethyl-3,6-dimethylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine and trimethylamine.Leu and Phe significantly increased the levels of 3-methylbutyraldehyde and benzaldehyde.Glucose,arabinose and galactose significantly increased the levels of pyrazine and aldehyde key aroma compounds.The contents of 2-ethyl-5-methylpyrazine,2-ethyl-3,6-dimethylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine,and trimethylamine showed an increasing and then flattening out trend with the addition of Arg and Lys,while the contents of 3-methylbutanal and benzaldehyde decreased.With increasing addition of Leu and Phe addition,the content of 3-methylbutanal and benzaldehyde increased,respectively,but they had no significant effect on the content of 2-ethyl-5-methylpyrazine,2-ethyl-3,6-dimethylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine and trimethylamine.With increasing addition of glucose,arabinose and galactose,2-ethyl-5-methylpyrazine,2-ethyl-3,6-dimethylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine,3-methylbutyraldehyde and benzaldehyde then tended to increase and then level off,but there was no significant effect on the content of trimethylamine.6.Based on the basic composition of shrimp epidermis,Arg,Lys,Phe,Leu,glucose,galactose,arabinose and lipids were selected as aroma precursors to construct the Maillard reaction model,lipid oxidation model and interaction model.Drying was carried out according to the conditions for drying shrimp epidermis.The types and contents of key aroma compounds in each model were determined by GC-MS,and the relationship between aroma precursors and key aroma compounds was visually analyzed.The results showed that Arg,Lys,Phe,Leu,glucose,galactose and arabinose and lipids were important aroma precursors of key aroma compounds in dried shrimp products.However,their contribution to the formation of different key aroma compounds were variable.Lys and Arg are the key amino precursors of2-ethyl-3,6-dimethylpyrazine,2-ethyl-5-methylpyrazine,2,5-dimethylpyrazine,2,3,5-trimethylpyrazine and trimethylamine.Phe and Leu are the amino precursors of benzaldehyde and 3-methylbutyraldehyde,respectively.The reducing sugar that contributes most to the formation of key pyrazine and aldehyde aroma compounds is glucose.Lipids were not only important aroma precursors of valeraldehyde,2-nonanone and 1-octen-3-ol,but also increased the content of pyrazines and aldehydes key aroma compounds.