Effects Of Extracellular Enzymes From Aspergillus Niger On Volatile Components And Flavor Of Green Tea

Green tea is a kind of unfermented tea,which contains relatively few kinds and contents of volatile substances.It can be improved by using exogenous enzymes.Aspergillus niger is internationally recognized as a safe strain.So,it is significance to study the effect of extracellular enzymes secreted by Aspergillus niger on the flavor and the interaction between volatiles of green tea to improve the flavor.In this paper,the effects of Aspergillus niger extracellular enzymes on the volatile components and flavor profile of green tea were investigated by gas chromatography-mass spectrometer(GC-MS),sensory evaluation combined with gas chromatography-olfactometry(GC-O).On this basis,the interaction mechanism and mode of main volatile aroma components in green tea treated by extracellular enzymes of Aspergillus niger were preliminarily explored.(1)Thirty-two volatiles were identified by GC-MS in green tea,including 9 aldehydes,2 ketones,9 alcohols,7 alkanes,and 5 other substances.Among the 32 compounds,20 and 24 were identified in the green tea control sample and green tea sample treated with crude enzyme,respectively.Quantitative results showed that the contents of 2-methyl-butanal,benzaldehyde,benzeneacetaldehyde,decanal,3-octanone,β-ionone,linalool,1-octen-3-ol,trans-linalool oxide,ethyl-hexanol,cis-3-hexenol,2-ethyl-1-hexanol,cis-linalool oxide,toluene and n-hexadecanoic acid increased significantly in green tea control sample that was treated with crude enzyme,while the contents of 3-methyl-butanal,pentanal,hexanal,dehydro-cis-linalool oxide,ethyl acetate,dibutyl phthalate and most of the alkanes were decreased significantly.(2)The sensory evaluation results indicated that after treated with crude enzyme,the green tea of grass and flower notes increased markly,while sweet,woody and caramel notes decreased significantly.The results of-GC-O showed that dehydro-trans-linalool oxide and limonene were the main aroma contributors in the green tea control sample,while 3-octanone,ethyl-hexanol,traps-linalool oxide,linalool,nonanal,safranal and β-ionone were the main aroma contributors in green tea sample treated with crude enzyme.In addition,the flavor verification test further confirmed that cis-3-hexenol,linalool oxide and β-ionone were the key compounds to change the flavor profile of green tea after treatment with crude enzyme.(3)In green tea,the interaction between volatiles showed that the floral and sweet notes contributed by β-ionone and linalool oxides and the grass note contributed by cis-3-hexenol inhibited each other.When the concentration of β-ionone was 0-0.6 mg/L,the floral note contributed could inhibit the grass note contributed by cis-3-hexenol.Moreover,as the content was greater,the inhibition effect was more obvious;when the concentration of cis-3-hexenol was 0-0.3 mg/L,the grass note could also inhibit the floral note contributed by β-ionone,and the content was greater,the inhibition effect was more obvious.Besides,molecular docking results indicated that both the ligands cis-3-hexenol and β-ionone bind to the bottom of the hydrophobic cavity of 3D structure of the olfactory receptor OR51E2.The binding energies were-5 Kcal/moL and-6.8 Kcal/moL,respectively.Therefore,the latter was more easily bind to the olfactory receptor OR51E2.In summary,the results of this study indicated that Aspergillus niger extracellular enzyme had a significant impact on the volatile components and flavor profile of green tea.In addition,the material basis of the change of the flavor profile of green tea was explored from the molecular level for the first time,which provided a new idea for the flavor research of tea and related deep-processed products.