Study On The Composition And Change Profile Of Flavor Compounds In Fresh Walnut

So far,little research has been conducted on flavour substances of fresh walnuts at home and abroad,which makes the quality evaluation index system of fresh walnuts incomplete.In this study,colorimetry,solid phase microextraction(SPME),gas chromatography-mass spectrometry(GC-MS),and gas-phase ion mobility spectroscopy(GC-IMS)were used to detection of the taste substances and volatile compounds.The main composition and content of fresh walnut flavor substances in different varieties,different development periods of the same variety and different preservation methods were systematically measured and analyzed.And compare the change of flavor substance of 'Qingxiang' walnut before and after drying.It provides a flavor evaluation basis for the selection of the harvest period and preservation method of fresh walnuts in production.It also provides a reference for walnut flavor quality breeding and achieve the following main results.(1)The difference between the two methods of steam distillation(SD)and headspace solid-phase microextraction(HS-SPME)for the extraction of volatile substances from fresh walnut kernels was compared.It is concluded that the types and contents of compounds extracted by HS-SPME are greater than SD,which is more suitable for the extraction and analysis of volatile flavor substances of fresh walnuts.(2)The parameters of the HS-SPME method were optimized,and the results showed that the optimal parameters for the analysis of walnut volatile flavor substances were: extraction head 50 / 30?m PDMS / DVB / DVR,extraction temperature 70 ?,extraction Time 40 min,sample dosage 2.00 g.(3)Result from fresh walnuts of four varieties of 'Qingxiang','Xifu1','Liaohe4' and 'Xiangling ' harvested at the commodity maturity turned out that among the four taste related substances,'Qingxiang' had the highest water content,the lowest soluble sugar,less oil and tannin content,resulting its crisper taste compared with other varieties.GC-MS measurement shows that the volatiles contained in 4 varieties mainly include 6 categories of esters,aldehydes,hydrocarbons,ketones,ethers,and benzene.Among the four walnut varieties number of volatile component was the most(29)in 'Qingxiang',and the least in 'Xiangling'(21).Unique flavor of different varieties ascribe to the differences in the content and compositions of volatile substances.(4)Using "Qingxiang" as the test material,taste related substances were determined.The results showed that water and soluble sugar content in fresh walnut kernel decreased,oil content increased,and the content of glutamic acid and aspartic acid increased,then decreased while fruit developed from shell-harden to mature stage.The detection results of HS-GC-IMS in 5 points during the period found that the volatile components of fresh walnut kernels in different growth and development stages are very different.Hexanal and methyl 3-methylbutyrate also appear as common components of fresh walnuts in the detection results of HS-SPME-GC-MS.During walnut ripening,the contents of 3-methylbutyric acid methyl ester,2-heptanone and 2-butanone increased significantly,and they are considered to be unique volatile components of 'Qingxiang' during walnut ripening.(5)HS-GC-IMS was used to identify the volatile components of the fresh walnuts of the 'Qingxiang' after drying.The results showed that drying made the original 4 kinds of volatile substances disappear and 10 kinds of volatile substances emerged.The new substances included ethyl acetate,2-methylbutyraldehyde,valeraldehyde,hexanal,butyl acetate,etc.lading basis for the flavor difference between dry and fresh walnuts.After 90 days of storage,the contents of 4 volatile substances decreased,7 increased and 6 new components were added.At the end of cold storage,increased and new appeared compounts in dry walnut included propanol,3-Methylbutyraldehyde,2-methylbutyraldehyde,acetal,acetone and hexanal.(6)Compared with the initial stage of storage,four volatile substances disappeared and 11 new ones were found after 90 days' storage under PE50 package(MAP)and food wraping film package(CK).The content of 10 new components in MAP fruit was higher than that in CK,but the content of glyoxal in CK was higher than that in map fruit.The increased or new components of MAP fruit include ethanol,2,3-pentanedione,ethyl acetate,2,3-butanedione and propanol.Compared with map,the new components of CK include propanol,3-Methylbutyraldehyde,2-methylbutyraldehyde,acetal,acetone and hexanal.