Research On Characteristic Aroma Of Jujube During Ripening Stages And Processing

Jujube was studied with headspace solid phase micro-extraction/gas chromatography-mass spectrometry(HS-SPME/GC-MS),electronic nose(E-nose)and GC-flash e-nose for analysis of its characteristic aroma.The characteristic of aroma in jujube of different varieties,changes of characteristic aroma and relationship between aroma-related compounds and their precursors during ripeing stages and dried maturity stages,as well as the effects of processing methods on the characteristic flavor of jujube were comprehensively studied in this study.Results showed that alcohols,aldehydes,acids and esters were the main aroma components in fresh jujube;small molecule sugar,fatty acid and amino acid were the important precursors for the aroma formation of jujube,which were closely related to the formation and change of aroma during the ripening and dried ripening of jujube;different drying methods had significant effects on the aroma quality of jujube.The main results and conclusions are as follows:(1)Volatile profile of ten different varieties of fresh jujubes was characterized by HS-SPME/GC–MS and E-nose.GC–MS results showed that a total of 51 aroma compounds were identified in jujubes,hexanoic acid,hexanal,(E)-2-hexenal,(Z)-2-heptenal,benzaldehyde and(E)-2-nonenal were the main aroma components with contributions that over 70%.Differentiation of jujube varieties was conducted by cluster analysis of GC–MS data and principal component analysis & linear discriminant analysis of E-nose data.Both results showed that jujubes could be mainly divided into two groups: group A(JZ,PDDZ,JSXZ and LWZZ)and group B(BZ,YZ,MZ,XZ and DZ).There were significant differences in contents of alcohols,acids and aromatic compounds between group A and B.GC–MS coupled with E-nose could be a fast and accurate method to identify the general flavor difference in different varieties of jujubes.(2)Volatile profiles of jujube fruits at different ripening stages were characterized by GC–MS and E-nose.Changes in the possible precursors of volatile components were also investigated.Four ripening stages were established: GM,green maturity;YM,yellow maturity;HRM,half-red maturity;RM,red maturity.Fruit maturity showed significant effect on the volatile components and their precursors.Alcohols(2-ethyl-1-hexanol,benzyl alcohol),acids(hexanoic acid,octanoic acid and decanoic acid)and aldehydes(hexanal,trans-2-hexenoaldehyde,benzaldehyde and nonanal)were the main aroma components in ripening jujube.(E)-2-hexenal was the most abundant component which showed rising trend from 169.2 ?g/kg(GM)to 733.4 ?g/kg(RM).Glucose and fructose showed a decreasing tendency during ripening.As the important precursors of jujube flavor,seven fatty acids were identified,the predominant ones were C16:0,C16:1n7,C18:1n9c and C18:2n6c;twenty-six free amino acids were quantified,in which L-Asp NH2 and L-Pro were the abundant components.Owing to the highest content of total aldehydes(59.33%)and the strongest responses of E-nose,jujubes at HRM stage showed superior overall aroma quality among all the samples.(3)A total of 27,27,22,20,14 and 18 volatile compounds were identified in jujube dried mature stage of S1 ? S6,respectively.Volatile profile of S5 closed to S6 but different from S1 ? S4.Glucose and fructose presented the similar changes at S1 ? S6 which were reversed to sucrose.Eight fatty acids and twenty-six free amino acids were identified at S1 ? S6,respectively.Considerable differences were found in unsaturated FAs including C14:1n5,C16:1n7 and C18:2n6c(p < 0.05).L-Pro was the most abundant with about 50%.Negative relationship was found between volatile compounds and their potential precursors analyzed by PCA-X model.Red jujube at S6 was the best time to harvest the fruits which presented highest aroma content and stronger responses values of E-nose.(4)GC-MS coupled with MOS E-nose and flash GC E-nose were used to analyze the volatile profiles of red jujubes that dried by hot-air drying(HAD),heat-pump drying(HPD),infrared radiation drying(IRD),vacuum drying(VD),vacuum freeze drying(VFD)and instant controlled pressure drop drying(DIC).Acids were the predominant family that made over 90% contribution to jujube aroma.Acetic acid,propanoic acids,butanoic acid,hexanoic acid,octanoic acid and decanoic acid were most common components presented in all dried jujubes.In terms of volatile contents,VFD was superior to obtain the aroma compounds especially acids(998.5 ?g/kg).However,DIC samples showed the most diversity of volatile compounds(26 species).Besides,samples of HPD,IRD,HAD and VD presented similar volatile profile in the results of GC-MS and flash GC E-nose.Although the results of GC-MS,MOS E-nose and flash GC E-nose showed some difference with each other,they could provide comprehensive volatile analysis of different dried jujubes samples.