Influence Of Bunch Exposure On Amino Acid Accumulation In Grape Berries And Volatile Compound Production In Wines

Grapevine bunch exposure is widely accepted as a canopy management strategy.It can improve the microclimate condition of grape bunch and grape quality.Free amino acids in grape are essential to the growth and metabolism of yeast during alcoholic fermentation.Amino acids composition can also regulate the formation of yeast volatile metabolites in a decisive way.Bunch exposure treatments have the potential to influence the accumulation of amino acids in grape berry,and ultimately the flavor of wine.To improve wine quality by bunch exposure treatment under the continental monsoon climate condition of China,it is nessesory to understand the specific effect of this cultivation method on the composition of amino acids in grape and aroma compounds in wine.Three consecutive years bunch exposure treatments were conducted at Yuanyi and Guangdong farms in Manasi country,Xinjiang Uygur Autonomous Region.The effect of bunch exposure on the accumulation of free amino acids in Vitis vinifera L.cv.Cabernet Sauvignon grapes at harvest and fermented volatile compounds in the wines were investigated.The main results are as follows.Half leaf removal after veraison of grapes at Yuanyi farm increased the accumulation of total free amino acids in grape berries.After alcoholic fermentation,the production of esters in the half leaf removal treated wines increased or showed no significant difference compared with the control wines,meanwhile the volatile acids production decreased.Leaf removal after veraison of grapes generally decreased yeast assimilable nitrogen content in grape berries,vintage and vineyard showed influence as well.The concentrations of branch chain amino acids(Val,Leu and Ile)increased in the grape berries of leaf removal treatment at veraison,after-veraison and pre-harvest of grapes.The concentration of total aromatic amino acids(Phe,Tyr and Trp)also increased in the grape berries of leaf removal treatments at each berry development stages at Yuanyi farm.Arg,y-aminobutyric acid,Trp,Leu and His were the most important contributors to the discrimination of amino acids composition in bunch exposures treated grape berries.After alcoholic fermentation,the wines from grapes of half leaf removal treatment showed increased production of higher alcohols and volatile acids.The wines from grapes of half leaf removal pre-harvest showed decreased production of higher alcohols and volatile acids,meanwhile the accumulation of esters increased,which could improve wine flavor quality.The concentrations of higher alcohols,acetate esters and ethyl esters in the wines after alcoholic fermentation from the grapes of bunch exposure treatments showed positive correlation with Glu and Ser contents in the musts.3-methylthio-propanol concentration in the wines showed negative correlation with Val and Phe contents in the musts.There was a negative correlation between the content of Phe in the musts and the concentration of 2-phenylethanol and total higher alcohols in the wines.Adding of branch chain amino acids in both grape must and chemically defined medium before fermentation showed no significant influence on the production of alcohol,glycerol,acetic acids and pH of the wines after alcoholic fermentation,whereas the production of branch chain higher alcohols and esters of medium-chain fatty acids significantly increased.In chemically defined medium the productions of 2-phenylethanol,phenethyl acetate and fatty acids increased with branch chain amino acids addition.But in the grape must with lower content of nitrogen,adding of branch chain amino acids showed negative influence on the productions of 2-phenylethanol,phenethyl acetate and isoamyl acetate.The influence of branch chain amino acids addition on the gene expression profile of yeast was investigated using microarray.Branch chain amino acids addition showed the most significant influence on yeast gene expressions in the exponential phase of yeast growth.Genes related in stress response,sexual reproduction and the biosynthesis of thiamine were upregulated when branch chain amino acids were added,which exhibited stronger fermentation vigor.In the stationary phase of yeast growth,the expression of GAP1 coding for general amino acids premease was upregulated,which may resulted in more effective assimilation of amino acids when branch chain amino acids were added.