Physicochemical quality and aroma composition of Chardonnay berries changed by spraying BTH during veraison

The objective of this study was to find the effects of benzothiazole (BTH) treatment at the veraison on the physicochemical and aroma qualities of grapes. The test material was taken as the 17-years-old vines Chardonnay (Vitis Vinifera L.) in the East-West direction with the row spacing and plant distance of 3 m×1 m in the sandy soil, with the location in the Mogao Vineyard, HuangYang Town, Wuwei City, Gansu Province, China. The vineyard was managed, according to the standard viticulture practices of the area. The treatment consisted of the BTH at the middle stage of grape veraison (50% berries color changed, August 8) and the control group. The solution of BTH treatment was prepared at the concentration of 50 mg/L of water with 0.1% Tween 80 as the wetting agent. The control sample was treated with water with 0.1% Tween 80. A randomized block design was used for the experiment with three biological replications. The treatments were applied once to the clusters. A comparison was made on the physicochemical and aroma indexes of ripeness berries (September 20). The aroma components were analyzed qualitatively by the headspace solid-phase microextraction-mass spectrometry coupled with the gas chromatography-mass spectrometry (HS-SPME-GC-MS). The results showed that the BTH treatment significantly enhanced the 100 berries weight, the total soluble solids, and reduced sugar with 4.64%, 6.67%, and 8.63% (P<0.01), respectively, and the skin/berry ratio significantly increased with 8.31% (P<0.05), compared with the control. By contrast, the contents of total carotenoid, chlorophyll-a, and chlorophyll-b significantly decreased by 9.01%, 9.28%, and 13.42% (P<0.01), respectively. The titratable acid of berries decreased slightly, but there was no significant difference (P>0.05). Furthermore, there were no significant effects of pH and color index of grape (P>0.05). The total content of bound aroma compounds was detected by 481.68 μg/L in the control sample, and the BTH-treated berries contained up to 568.65 μg/L, which was 18.06% higher than the control. Moreover, the total content of free aroma compounds was as high as 2 331.41 μg/L in the control, and 1 948.22 μg/L in the BTH-treated berries which decreased by 16.44%, compared with the control. In addition, 72 bound aroma compounds were identified, including 62 in control and 64 in BTH berries. There were 54 free aroma compounds, including 42 in control and 51 in BTH. The types and contents of bound aroma compounds increased in this case. The types of free aroma compounds increased, but their contents decreased. The BTH inhibited the accumulation of free C6/C9, aromatic, branched-chain aliphatic compounds, and norisoprenoids in the berries, which decreased by 16.65%, 31.77%, 26.25 % and 21.05%, respectively, compared with the control. However, the BTH treatment promoted the accumulation of free linear aliphatic compounds and terpenoids, as well as the bound aroma except for the branched-chain aliphatic compounds in the berries. The content of free linear aliphatic compounds and terpenoids increased by 51.97%, and 60.12%, respectively. The contents of bound C6/C9, linear aliphatic, aromatic, terpenoids, and norisoprenoids were 18.14%, 1.13%, 31.56%, 24.39%, and 4.22% higher than the control, respectively. The principal component analysis showed that the correlation between the BTH induce and bound linear aliphatic compounds, C6/C9 compounds, terpenoids, aromatic compounds, and norisoprenoids was >0.5. At the same time, the correlation between the BTH-induced and free linear aliphatic compounds and terpenoids was also >0.75. In conclusion, the BTH-induced resistance treatment at the middle stage of grape veraison improved the fermentation characteristics of Chardonnay grapes by the increasing skin/berry ratio, soluble solids, and reducing sugar contents, while the higher bound C6/C9, linear aliphatics, aromatics, terpenoids, and norisoprenoids in the BTH-treated grapes was provided better support for the variety aroma of Chardonnay wines. The findings can provide the theoretical basis and technical support for the application of the BTH, in order to improve the quality of Chardonnay wine grapes for better wine raw materials.