The production of high-quality wine is an important target for wineries worldwide, and low crop level (yield) has been one vineyard management criterion long believed to achieve wine quality. Low yields are most often achieved by removing clusters from the grapevine. Some studies show that cluster thinning can enhance grape maturity and color intensity. The reduction of crop level also increased wine quality and taste intensity by raising the content of quality-important compounds like esters and anthocyanins. However, some other studies indicated that differences between wines made from crop- thinning vines and full crop vines were not always detected. Variations in vineyard climate and grape cultivar and rootstock may override the outcomes of cluster thinning practices. The impacts of cluster thinning on wine quality are still inconclusive.
In this study, Pinot noir wine composition was investigated over three growing seasons (2013 to 2015) in twelve commercial wineries in Oregon where crop level was adjusted using cluster thinning at lag-phase of berry development using a cluster number per shoot regime (e.g. 2, 1.5 and 1 cluster/shoot) and compared to a full crop control (non-thinned). Two sites used variable ton per hectare treatments (e.g. 1.31, 1.01, and 0.71 ton/hectare).
After harvest, fruits from field replicates were combined to produce the wine. The phenolic compounds and volatile aroma compounds were analyzed by HPLC, UV- spectrometry and GC-MS techniques. Results showed that the effects of cluster thinning treatments on Pinot noir wine composition were dependent on vineyard site and vintage year. For wine phenolic compounds, the influences of cluster thinning on major phenolic compounds were observed in six wineries but various with the vintages and wineries. Results also showed that cluster thinning increased total monomeric anthocyanins (TMA) and total phenolic content (TP) in wines from five of the six wineries. Certain volatile compounds were mainly influenced by cluster thinning treatments, depending on wineries and vintages. It was demonstrated that wines from one winery were insensitive to cluster thinning treatments. Further research is underway to confirm the influence of cluster thinning practices on volatile compounds in Pinot noir wines.