| Green leaf volatiles,are also called the GLVs aroma because of most of them have injured leaves smell.They include short chain aldehydes,alcohols that are derived from the unsaturated fatty acid via LOX,HPL and other enzymes.GLVs are abundant in grape berries that can affect grapes and wine flavor.Sometimes,they can improve the taste of grape and wine,making grape aroma typically.Basically,they are considered to be a not mature flavor because their low threshold value and high concentration content can affect the quality of grape aromas.Lipoxygenase and hydroperoxide lyase are the two key enzymes in fatty acid metabolism.So LOX-HPL pathway is also called GLV pathway.This experiment was carried out in Zhihuiyuanshi vinery which is located in east of Helan Mountain,Yinchuan city,Ningxia province.This experiment time was from June to September,2015.Taking Carbernet Sauvignon?Vitis vinifera L.,cv.?as the experimental material,we used three Flim materials to cover setting fruit position that can pass different proportion of UV radiation?PE flim filter 30% UV out,PETG flim filter 30% UV-A and 70% UV-B out,PC flim filter 90% UV out?,using the method of SPME-GC-MS to study the effect of different UV radiation intensity on the aroma precursor substances?linoleic acid and linolenic acid?of Cabernet Sauvignon during ripening,changes of LOX,HPL activities and volatile aroma composition accumulation pattern.The results are as follows:1.UV reduction treatment significantly reduced the soluble solids content and pH value of mature berries,turned down the speed of converting acid to sugar process and delayed fruit maturing course.The lower level of UV radiation,the more significant effect.2.Unsaturated fatty acid?mainlylinoleic acid and linolenic acid?,were accounting for 70% of total fatty acids in grape skin.UV reduction treatment did not change the tendency of the fatty acids.Linoleic acid content(285-1846 mg·kg-1)and linolenic acid content(77-933 mg·kg-1)presented similar accumulation pattern in the process of fruit ripening: the content was very low,basiclly stable before verarison,then it accumulated a lot after verarison and finally significantly decreased when ripe.UV reduction treatment reduced the content of fatty acids and the proportion of unsaturated fatty acids.The extent of the reduction associated with UV exposure dose.3.UV reduction treatment did not change the trend of enzyme activity.LOX,9,13-HPL activity presented a trend of reduction after rise.Enzyme activity were basiclly stable before veraison,slowly rised during 7 to 9 weeks after flowering,rapidly rised after veraison,then significantly reduced during the mature period and the level was basiclly the same as early veraison level.Experiment results showed that enzyme activities were associated with a significant precursor substance of unsaturated fatty acid.Speculate that the enzyme activity is limited by the concentration of substrate.4.UV reduction treatment do not have significant effect on aroma components,however the effect on contents of aroma were significant.Amino acid derivatives rose during ripening,UV reduction treatment significantly increased the content of amino acid derivatives,making fruit aroma stronger in the grape berries.Terpene class were only detected before veraison,however the content was very low and it showed declining trends,so we cannot make a judgment.Fatty acids derivatives slowly rose in the process before veraison,rapidly accumulated after veraison,then decreased significantly during the mature period.Results showed that UV reduction treatment significantly reduced the GLVs aroma of grape and made the ratio of aliphatic aldehydes and alcohols lower.It had a positive effect on improving the green leaf flavor of grape and wine.Correlation analysis showed that the fatty acid derivatives,LOX,HPL enzyme activities,unsaturated fatty acid content,total aroma had very significant positive correlation. |
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