| During the post-harvest storage process,wheat is very vulnerable to destructive pests.The Sitophilus zeamais is known as the number one stored grain pest in my country.These unpleasant odors are thought to be produced by the hydrolysis and oxidation of lipids.In this study,the wheat uninfected by Sitophilus zeamais was used as the control,based on the changes of related enzyme activities in the process of lipid hydrolysis and oxidation,and the combination of solid-phase microextraction-gas chromatography-mass spectrometry(SPME-GC-MS)was used.Multivariate statistical analysis to study the effects of the development of offspring to different insect states after Sitophilus zeamais adult infestation on the composition and relative content of volatile compounds in wheat grains,whole wheat flour and whole wheat flour steamed bread,In order to clarify the relationship between the changes of volatile compounds and lipids in insect-eaten wheat,and to provide a theoretical basis for the monitoring of insect-eating pests in wheat.Below are key research findings:(1)With the growth and development of the offspring inside the wheat grains after the adult Sitophilus zeamais infestation,the crude fat content and peroxidase activity of wheat both showed a downward trend,and both decreased to the minimum when the Sitophilus zeamais developed to the adult stage;fatty acid value,malondialdehyde content,lipase,lipoxygenase and alcohol dehydrogenase activities increased with the growth and development of Sitophilus zeamais.The relative content of linoleic acid in wheat was the largest,and there were significant differences in wheat samples at different stages.The relative content of palmitic acid and stearic acid both increased to the maximum at the adult stage.With the growth and development of Sitophilus zeamais,the changes of oleic acid,linolenic acid and eicosenoic acid were not significant.The results showed that the growth and development of the offspring in wheat grains after the infestation of Sitophilus zeamais would accelerate the hydrolysis and oxidation of wheat lipids.(2)A total of 115 volatile compounds were detected in the wheat grains of the original samples,eggs,larvae,pupa,and adults and their corresponding control samples.The relative contents of hydrocarbons,alcohols,aldehydes,esters and ketones all increased with the growth and development of Sitophilus zeamais,and the relative contents of hydrocarbons in the larval wheat samples reached the maximum value of 59.44%,which was higher than the the corresponding control group increased by 23.29%;alcohols,aldehydes,esters and ketones all increased to the maximum values of 38.2%,46.53%,8.56% and 9.68% in the adult samples,acidic compounds were only detected in the adult stage and its control samples.The characteristic volatile compounds of wheat after different types of Sitophilus zeamais are peach aldehyde,1-octen-3-ol,1-dodecene,nonanal,octanol,trans-2-nonenal,dodecanol,octanal,diisobutyl phthalate,nonadecane,5-(2-methylpropyl)nonane.When it is detected that these substances start to change,it can be considered that Sitophilus zeamais appears in the grain pile,and early prevention and control is required.The results of cluster analysis indicated that the volatile compounds began to change greatly in the later period of Sitophilus zeamais infestation.(3)The wheat samples corresponding to different insect states after Sitophilus zeamais were crushed to make whole wheat flour,and then the volatile compounds were detected and analyzed.A total of 89 volatile compounds were detected in 9 groups of whole wheat flour samples.Compared with the wheat grain samples,62 volatile compounds disappeared after milling,but 36 volatile compounds were newly generated after milling.The relative content of hydrocarbons increased significantly in the larval and adult samples,while the relative content of alcohols and aldehydes decreased significantly in the larval samples.The rest of the samples did not change significantly compared with the control samples.Esters The relative content increased to a maximum value of 3.37% when the Sitophilus zeamais developed to the adult stage.The relative content of ketones in the whole wheat flour samples at each insect stage was lower than that of the corresponding control samples.Compared with the corresponding control samples,acid substances were only detected in the whole wheat flour samples of the adult stage and the adult stage control group.The characteristic volatile compounds in whole wheat flour of different Sitophilus zeamais infested wheat were 2-methyl-tridecane,octadecane,dodecane,tetradecane,2,6,10-trimethyl-Dodecane,2-methyldodecane,2-butyl-1-octanol,2-ethylhexanol,decanal,trans-2-dodecenal and nonanal.The results of cluster analysis showed that the whole cycle of Sitophilus zeamais development in wheat grains and the prolongation of storage time would have a significant impact on the volatile components of whole wheat flour made from wheat.(4)A total of 79 volatile compounds were detected in the 9 groups of whole-wh eat flour steamed bread samples,and 48 volatile compounds were newly detected co mpared with the whole-wheat flour samples,but no acids were detected.The relative content of hydrocarbons and other substances increased with the growth and develop ment of Sitophilus zeamai,and the relative content of hydrocarbons in the whole wh eat flour steamed bread at the adult stage increased to about 3 times that of the orig inal sample,compared with the whole wheat flour sample,after the whole wheat flo ur was made into steamed bread,the relative content of hydrocarbons decreased,but the proportion was always the highest.The types of alcohols and aldehydes increased due to high temperature steaming,but their relative contents decreased with the gro wth and development of Sitophilus zeamais.The relative contents of esters and keton es also showed a downward trend with the growth and development of Sitophilus ze amais.The characteristic volatile compounds of whole-wheat flour steamed buns after different Sitophilus zeamais infested wheat were: decanal,trans-5-decenol,2,6,10-trim ethyl-dodecane,phenethyl alcohol,cocoaldehyde,6,10,14-Trimethyl-2-pentadecanone,D-terpene,hexadecane,tetradecane,ethyl palmitate,3,7,11-trimethyl-1-dodecane Alcohol,cyclotetradecane and(E)-3-methyl-2-undecene.The results of cluster analysis showed that when whole wheat flour was made into whole wheat flour steamed bread,the volatile components of each group of samples changed greatly,which may be due to the conversion of some compounds caused by high temperature.The sensory scores of whole-wheat flour steamed bread decreased with the growth and development of S itophilus zeamais,and the odor and color scores decreased most obviously,indicating that the damage of Sitophilus zeamais led to changes in the overall volatile matter of whole-wheat flour steamed bread,but the lipid hydrolysis and oxidation The odor produced is not eliminated by high temperature steaming.(5)The results of correlation analysis showed that the growth and development of Sitophilus zeamais inside wheat grains caused a significant negative correlation between linoleic acid and alcohols,and alcohol dehydrogenase and aldehydes,esters,ketones,and acids were all significantly positively correlated.There was a significant positive correlation between lipoxygenase and alcohols.It can be speculated that the changes of volatile compounds in wheat after being infested by different types of Sitophilus zeamais may be because the infestation of Sitophilus zeamais accelerated the hydrolysis and oxidation of wheat lipids.The alcohols and aldehydes that play an important role in the aroma are basically increased after the wheat that is not damaged by Sitophilus zeamais is fermented and steamed.After being attacked by different types of Sitophilus zeamais,a large number of irritating alcohols produced by lipid oxidation in wheat increased slightly after milling,but decreased to varying degrees after fermentation and steaming;and except for the adult samples,aldehydes,esters and ketones decreased after milling but increased after fermented and steamed into whole-wheat flour steamed bread.The aldehydes,alcohols,esters and Ketones were significantly reduced after wheat was milled into whole wheat flour,and the relative content detected after fermented and steamed into steamed bread was even lower. |
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