Effects of Crop Thinning on Pinot Noir Wine Composition

The production of high-quality wine is an important target worldwide, and crop level (yield) is an essential criterion for vineyard management which influences both output and quality of wine products. The crop level can be modified by cluster thinning which is used to achieve yield and quality goals through the removal of clusters on the grapevine. Previous studies have shown that cluster thinning enhanced grape maturity and color intensity. And the reduction of crop level increases wine quality and taste intensity by raising content of quality-important compounds like acetate esters and anthocyanins. However, some studies indicate that difference between wines made from crop-thinning vines and full crop vines are not always detected. And variations in vineyard climate and grape cultivar are found to override the outcomes of crop thinning practice. The results of the impacts of crop thinning on wine quality are still inconclusive.

In this study, Pinot noir wine chemical and volatile composition were investigated over three growing seasons (2013 to 2015) in twelve commercial wineries where crop level was managed using crop thinning management practices. Crop level was reduced by lag-phase cluster thinning using a cluster per shoot regime (e.g. 2, 1.5 and 1 cluster/shoot) and compared to a full crop control (non-thinned) while two sites used variable ton per acre treatments (e.g. 3.25, 2.5, and 1.75 ton/acre). After harvest, fruit from field replicates was combined to produce one wine for each treatment. And the data analysis was mainly based on statistical analysis. Results showed that (1) different wineries had different responses in their wines as there was some wines have consistency in wine composition over the years. (2) It was also found that crop thinning practice had the different impact on wine composition in 2014 and 2015 vintage year with that in 2013 vintage year, which could be that the 2013 season was different climatically than in 2014 and 2015 which were more similar. And some compositions like TMA increased from 2013 to 2015, which indicated the year effect overrode the crop thinning effect. (3) There were some significant differences found in Pinot noir wine composition with crop thinning treatments, but there were no consistent differences across all vineyard sites across three years. For non-volatile phenols, crop thinning practice had limited significant effects on total phenol (TP) and total monomeric anthocyanins (TMA) in most wineries over three vintage years, but sometimes increased TP and TMA content in Pinot noir wine in some wineries. For volatile aroma compounds, there had no consistent trend of specific compound concentration or group content of terpenoids, C13-norisoprenoids, higher alcohols, fatty acids, esters and volatile phenolic compounds influenced by crop thinning level in some wineries, depending on the winery and vintage year.

Thesis Committee: Michael Qian, Elizabeth Tomasino, Patricia Skinkis, Alexandra Stone