Research On Lipid Changes Of Paddy During Accelerated Ageing Process

Rice is one of the most important cereals and staple foods of over 70%of population in China.Paddy storage is by taking certain measures to control rice metabolic process,thereby delaying the ageing of paddy,so that it maintains good qualities during storage.Lipid hydrolysis and oxidation are usually recognized as essential important factors for influcing the storage proterties of paddy.Therefore,the study of paddy during the period of storage and its effects on paddy quality provides a theoretical basis for clarifying the mechanism of paddy quality deterioration during storage.In this study,indica paddy"Nong Xiang 32" and "Yu Zhen Xiang" were used as raw materials,artificial accelerated ageing test was used to study the changes of physiological and biochemical characteristics related to lipid metabolism and volatile components in the accelerated ageing process of paddy.Ultra performance liquid chromatography coupled with Q-TOF mass spectrometry(UPLC-Q-TOF-MS)based metabolomics approach was employed to investigate lipid metabolites during the storage of paddy.The main of this study was to provide a novel insight into the biochemical mechanisms of paddy grain at the lipid metabolomic level.Results can be summarized as follows:The changes of physiological and biochemical characteristics related to lipid metabolism in paddy during accelerated ageing were studied.During the accelerated ageing of paddy,the germination rate,crude fat content,catalase activity,POD activity,SOD activity,and APX activity decreased with the prolonged ageing time(P<0.05).The peroxide number,minimum viscosity,final viscosity,setback value and pasting temperature exhibited a rising trend along with the ageing time.During the accelerated ageing process at 42?,the fatty acid value,MDA content and lipase activity showed a tendency of increasing first and then decreasing.However,the fatty acid value increased in the ageing process at 30?,and the lipase activity increased rapidly in the early stage of ageing and increased slowly in the middle and late stages of ageing.During the ageing process,the catalase activity,SOD activity and germination rate of Nong Xiang 32 were higher than those of Yu Zhen Xiang,while the fatty acid value,MDA content,peroxide value,lipase activity of Yu Zhen Xiang were higher than those of Nong Xiang 32.Yu Zhen Xiang had a more severe degree of lipid peroxidation than Nong Xiang 32,and Nong Xiang 32 exhibitied the superior storability.The headspace solid phase microextraction-gas chromatography-mass spectrometry technique was used to study the changes of volatile metabolites in paddy during accelerated ageing.Volatile metabolites in the ageing process of paddy were hydrocarbons,aldehydes,esters,alcohols,ketones,acids,heterocyclics and others.The proportion of the hydrocarbon was the highest,followed by esters and aldehydes.The total content of hydrocarbons increased firstly and then decreased,the total ester content showed a tendency of increasing first and then decreasing,and the total aldehyde content showed an increasing trend.During the accelerated ageing process at 42?,217 and 213 species of volatile metabolites were detected in Nong Xiang 32 and Yu Zhen Xiang.During the accelerated ageing process at 30?,234 and 225 species of volatile metabolites were detected in Nong Xiang 32 and Yu Zhen Xiang respectively.The variety and relative content of volatile metabolites during the ageing process of paddy were not only related to varieties,but also related to ageing temperature.The UPLC-Q-TOF-MS technique was used to analyze the non-targeted lipid metabolite profiles of paddy after ageing 0 d and 15 d at 42?.The metabolite date were subjected multivariate statistical analysis(e.g.PCA and PLS-DA)for lipid metobolities markers selection and identification.After accelerated ageing at 42? for 15 d,Nong Xiang 32 and Yu Zhen Xiang identified 387 and 262 species of differential metabolites respectively.The differential metabolites were phospholipids,diradylglycerols,fatty acids and conjugates,sterols,fatty esters and so on.Pathway enrichment analysis showed that accelerating ageing accelerated the biosynthesis of unsaturated fatty acids,linoleic acid metabolism,and steroid biosynthesis in Nong Xiang 32,but the biosynthesis of unsaturated fatty acids,and cutin,suberine and wax biosynthesis were inhibited in Yu Zhen Xiang,resulting in the decreasing in eicosadienoic acid,docosenoic acid,eicosanoic acid,tetracosenoic acid,9,10,18-trihydroxystearine acid,10,16-dihydroxyhexadecanoic acid,and docosadicarboxylic acid.Metabolic pathways enrichment combined with volatile metabolites were used to analyze lipid metabolism pathways.The results indicated that the metabolism of linoleic acid and oleic acid were regulated significantly during the accelerated ageing of paddy.After 15 d of accelerated ageing of Nong Xiang 32,the linoleic acid undergoed lipid peroxidation,leading to 9(S)-HODE,9-OxoODE,13(S)-HODE,3-OxoODE,trans-2-octenal,heptanal,trans-2-nonenal,and 2-pentylfuran increased significantly eventually.Oleic acid undergoed lipid peroxidation,leading to Nonanal,Decanal,Octanal,trans-2-Decenal increased significantly eventually.The lipid peroxidation of linoleic acid occured after 15 d accelerating ageing of Yu Zhen Xiang,resulting in 9,10-epoxy-13-hydroxy-11-octadecenoic acid,trans-2-octenal,and trans-2-nonenal and 2-pentyl furan increased significantly.Oleic acid undergoed lipid peroxidation,leading to a significant increase in Nonanal,Decanal,octanal,and trans-2-Decenal.