Detection Of Phenolic Maturity In Vitis Vinifera L.cv. Cabernet Sauvignon Grape By Skin Texture Analysis

The detection of grape berry texture characteristics does not require any pretreatment,which leads to shorter detection time.It can also realize the continuous detection of a large number of samples,thus improving the representativeness and reliability of the detection.This method now is mainly used to detect the content and extractability of anthocyanin in wine grape,there are few studies on the detection of phenolic maturity in grape based on the texture characteristics of skin.The relationship between the texture characteristics of grape skins and tannin content and its characteristics has not been reported yet.In the present study,the relationship between the texture characteristics of Cabernet Sauvignon（Vitis vinifera L.）grape skin and the phenolic maturity indicators such as total phenol,anthocyanin and tannin was studied during maturation of grape from the four regions in 2016 and 2017,and the regression model between phenolic maturity and texture characteristics was established.Research results can provide theoretical and technical support for the establishment of detection method of wine grape phenolic maturity,and some reference for determine of optimal harvest time,formulation of reasonable maceration process,and vinification of high quality wine in the production.The main results of this study were as follows:（1）The reducing sugar content and maturation coefficient（M value）gradually increased,and titratable acid content decreased gradually during maturation of Cabernet Sauvignon grape from the four regions in 2016 and 2017.（2）There were annual and regional differences in the change of skin thickness and hardness during maturity of Cabernet Sauvignon grape.（3）During maturation,no significant change was found in the total phenol content for Jingyang,Shaanxi in 2016 and Xiaxian,Shanxi in 2016 and 2017.However,other years and regions showed an increasing trend.The skin tannin content from four regions in 2016 and2017 increased or remained stable,which was basically the same as total phenol content.The total anthocyanins and extractable anthocyanins content of grape skin from Jingyang,Shaanxi in 2016 dropped to 0.56 mg/g and 0.19 mg/g respectively at harvest.Jingyang,Shaanxi in2017 and other regions in different years showed a rising trend and reached to 0.43～1.45 mg/g and 0.19～0.57 mg/g at harvest,respectively.The total phenol content of Cabernet Sauvignon grape seeds from the four regions decreased gradually during maturation in 2016 and 2017.The seeds tannin content by protein precipitation assay remained stable for Wuwei,Gansu in 2016 and Jingyang,Shaanxi in 2016and 2017.However,other years and regions by protein and methylcellulose precipitation assays showed a decreasing trend.In 2016 and 2017,the cell maturation index of the four regions gradually increased,which increased to 54%～70.69%at harvest.The seed maturation index and total phenol indexs decreased gradually,which reduced to 6.22%～36%and 6.38～16.75 respectively at harvest.（4）There were interannual and regional differences in the change of the mean degree of polymerization（m DP）,molecular mass,proportion of extension subunits and terminal subunit of skin tannin during maturation.In addition,skin extension subunits were composed of catechin（C）,epicatechin（EC）,epicatechingallate（ECG）,and epigallocatechin（EGC）where EC and EGC dominated ranging from 41.23%to 60.93%.Skin terminal subunits consisted of C,EC and ECG where C was the main one ranging from 1.62%to 2.22%in this study.With regard to seeds tannin characteristics,m DP and molecular mass for the four regions were gradually increased during maturation in 2016 and 2017.There were interannual and regional differences in the change of proportion of extension subunits and terminal subunit during maturation.Extension subunits were composed of C,EC and ECG where EC dominated ranging from 51.25%to 65.36%.Terminal subunits consisted of C,EC and ECG where C was the main one ranging from 8.14%to 20.97%in this study.（5）Skin thickness and hardness expressed by skin break force（Fsk）,break energy（Wsk）and Young’s modulus(E_sk)showed a certain linear regression relationship with the phenolic maturity indicators such as the content of total anthocyanin and extractable anthocyanins,cell maturation index,skin tannin content by methylcellulose precipitation assays,and proportion of skin extension subunit EGC.It can thus be seen that skin texture characteristics might be useful for estimating phenolic maturity of Cabernet Sauvignon grape（R²=0.14～0.364,P≤0.05）.