| Kiwifruit（Actinidia）is the characteristic fruit in China.The industrial processing of juice will become the main direction of industry with the development of technology of storage for kiwifruit consumed in fresh.However,the deterioration of flavor quality in traditional processing of kiwifruit juice is a key technical bottleneck solved.In view of the complexity and instability of aroma compounds,the selection of aroma-active compounds in freshly squeezed kiwifruit juice and the determination of mechanism about changes of flavor quality in traditional processing are beneficial to the development of technology for the control of flavor quality.Xuxiang（XX）,Cuixiang（CX）and Huayou（HY）,as characteristic kiwifruits in Shaanxi province,were selected as objects studied.The ripeness degree of kiwifruit suitable for processing juice was selected by employing descriptive sensory evaluation combined with physicochemical index analysis,aroma molecular analysis and multivariate statistical methods.The aroma-active compounds were identified by sensory analysis,gas chromatography-mass spectrometry-olfactory detection（GC-MS-O）,aroma activity value（OAV）and other methods.Then gas chromatography-ion migration spectrometry（GC-IMS）was used to compare the composition and aroma properties of different fresh kiwifruit juice,and the one suitable for kiwifruit juice processing was selected.The changes of aroma compounds and sensory properties of kiwifruit juice sterilized by different methods were clarified by gas chromatography-mass spectrometry（GC-MS）,sensory analysis and multivariate analysis（PLSR,OPLS-DA）.The results could provide a theoretical basis for the further analysis of mechanism about aroma alienation in kiwifruit juice processing and the methods of controlling flavor quality.The results were as follows:（1）The volatile compounds in Xuxiang（XX）,cuixiang（CX）and Huayou（HY）during ripening were analyzed by gas chromatography-mass spectrometry（GC-MS）coupled with physical and chemical index analysis,and their flavor qualities were also compared by descriptive sensory evaluation.At the same time,clustering and principal component analysis（PCA）of the volatile compounds were used to determine appropriate ripeness degree for kiwifruit juice processing.The results showed that total soluble solids（TSS）and juice yield increased first and then stabilized with contents of titratable acid decreasing gradually during ripening（30℃,7 to 11 days）.When the contents of titratable acid in XX,CX and HY were 1.14%,1.04%and 1.16%,respectively,their contents of TSS were 16.86%,17.87%and 14.86%,respectively,which their freshly squeezed juice showed typical flavor of kiwifruit.The types and contents of esters increased significantly while the contents of alcohols and aldehydes with the grassy note decreased during ripening,and many volatile compounds were detected.Clustering and PCA analysis indicated that the ones with contents of titratable acid and TSS characterized by XX,CX and HY were suitable indexes for processing juice.The results could provide a reference for the selection of raw materials and the determination of the ripening degree of kiwifruit juice.（2）Aroma compounds were screened in freshly squeezed juice by gas chromatography-mass spectrometry combined with frequency detection method（GC-MS-O/DFA）.Aroma activity value（OAV）analysis was carried out based on quantitative analysis of external standard method.The aroma-active compounds were further verified by aroma recombination and aroma omission tests.It showed that 17,19 and 16 aroma compounds were screened in XX,CX and HY,respectively,and 15,15 and 13 aroma-active compounds were identified by OAV analysis（OAV≥1）in XX,CX and HY,respectively.The recombination model was constructed by adding the aroma-active compounds in the simulation system（containing aqueous solution with the same TSS and titratable acid content as fresh juice）.Descriptive sensory analysis showed that the recombination model fitted well with the flavor profile of the actual sample.The selected aroma-active compounds could form the characteristic flavor of fresh kiwifruit juice.The omission tests of CX sample with strong aroma were conducted.It showed that the omissions of methyl butyrate,ethyl butyrate,hexanal,（E）-2-hexenal,hexanol and（E）-2-hexenol could lead to obvious changes of flavor profile of recombination model,which indicated that these aroma-active compounds were important to flavor profile of CX fresh juice.The omissions of methyl caproate and 3-hexenal could lead to changes of flavor profile of recombination model,which indicated that these aroma-active compounds were influential to flavor profile of CX fresh juice.The omissions of octanal,（Z,E）-2,6-nonadienal and 1-octene-3-ol could lead to less changes of flavor profile of recombination model,which indicated that these aroma-active compounds were less influential to flavor profile of CX fresh juice.The identification of aroma-active compounds above in fresh kiwifruit juice is an important basis for the study of the changes of the overall aroma composition of kiwifruit juice during the sterilization process.（3）The GC-MS and GC-IMS were used to compare the volatile composition and aroma properties of different fresh kiwifruit juice,and the one with high solid acid ratio,juice yield and strong characteristic aroma was selected for processing kiwifruit juice.The fresh juice was processed by pasteurization（PS）,high temperature and short time sterilization（HTST）and ultra-high pressure sterilization（HPP）.The volatile compounds in kiwifruit juice processing were identified by GC-MS,sensory evaluation and multivariate analysis（PLSR,OPLS-DA）.At the same time,the relationships between volatile compounds and aroma properties were analyzed,and the key compounds resulting in the deterioration of flavor quality were found.It indicated that CX freshly squeezed juice with high solid acid ratio and juice yield showed strong fruity and grassy notes.The aroma profile of kiwifruit juice subjected to sterilization changed a lot.The contents of methyl butyrate and ethyl butyrate with the fruity note,C6 aldehydes or C6 alcohols with the grassy note changed differently.But HPP had little effect on the compounds above.The contents of methyl butyrate and hexanal were 477.89 μg/kg and 913.04 μg/kg,respectively,in fresh juice while contents of methyl butyrate were 163.46 μg/kg,154.58 μg/kg and 417.22 μg/kg in PS,HTST and HPP samples,and contents of hexanal were 561.13 μg/kg,634.76 μg/kg and 783.05 μg/kg in PS,HTST and HPP samples.Descriptive sensory evaluation analysis found the fruity and grassy notes of kiwifruit juice subjected to different sterilization changed a lot,and the cooked-flavor was generated after sterilization.Ethyl butyrate and methyl benzoate were associated with the fruity note while ethyl acetate,methyl butyrate and the two compounds were associated with the sweet note.Hexanal,3-hexenal and（E）-2-hexenal are the key compounds to the grassy note.The pungent note was mainly related to 1-pentene-3-one,and the cucumber note was mainly related to（Z,E）-2,6-nonadienal.However,the cooked-flavor was formed by α-terpinol,p-cymene,methyl heptenone and 1-hydroxy-3-pentanone.Then methyl butyrate,ethyl butyrate,hexanal,（E）-2-hexenal,hexanol,（E）-2-hexenol and 1-pentene-3-one were key compounds for the deterioration of flavor quality in heat sterilization samples.The results could provide the theoretical basis and technical support for further analysis of the mechanism about deterioration of flavor quality in processing of kiwifruit juice and the development of technology for the control of flavor quality. |
